Glossary of terms
and brands
for Telescope Buyers and Owners
achromat
A type of refractor
objective made from lenses -- usually two -- of different
materials, selected to bring two colours of light -- usually blue and yellow -- to the same
focal point, thereby bringing the rest of the colors of visible light to very nearly the
same focal point, thus making a decent stab at being in focus for all colors of visible
light at the same time. That is, an achromat attempts to reduce longitudinal chromatic aberration.
An achromat will also have spherical aberration corrected at one wavelength of light
-- usually green.
actual
field of view
A characteristic of eyepiece/telescope
combinations. The actual field of view is the angle, measured on the sky, from one edge of
the eyepiece field of view to the other. It is approximately equal to the apparent field of view divided by the magnification.
altaz
altazimuth
mounting
A kind of telescope mounting in which one
of the two axes the telescope turns about is pointed vertically, and the other
horizontally. The first axis is the azimuth axis, and the second is the elevation axis.
A telescope on an altazimuth mounting must
usually rotate around both axes simultaneously, to follow celestial objects across the sky
as the Earth rotates. For telescopes not too large or too powerful, a human being can
usually operate an altazimuth mounting well enough for visual observations. Several modern
commercial amateur telescopes have altazimuth mountings which are motor-driven and
computer controlled.
aluminizing
Telescope mirrors used to be made of metal,
laboriously worked to the right shape. Metals tarnish, however, so that the labour -- or
most of it -- had to be repeated periodically: Polishing off the tarnish changed the shape
enough to matter. It was a big deal, in the nineteenth century, when new chemical
processes made it possible to deposit a film of silver on glass: A tarnished mirror could
be repaired simply by removing the remains of the old coating chemically, and depositing a
new one. In the 1930s, the new process of aluminizing allowed deposition of a coating of
aluminium on a mirror in a vacuum chamber, essentially by boiling aluminium vapor off a
heated piece of that metal.
Aluminium is much more tarnish-resistant
than silver, hence its advantage. Other metals and materials may similarly be deposited.
anastigmatic
Loosely, "corrected for astigmatism", but the more technically correct sense of this
term is to describe an optical system which is corrected for spherical aberration, coma, and
astigmatism. The point is, that in many common optical designs, spherical aberration and
coma tend to be more serious problems than astigmatism, so there isn't much point in
correcting astigmatism unless the other two are already dealt with.
aperture
Loosely, the working diameter of a lens or
mirror; the diameter of the largest beam of light that can get through it. Also see clear aperture.
aplanatic
Loosely, "corrected for coma", but the more technically correct sense of this term is to
describe an optical system which is corrected for both spherical
aberration and for coma. The point is, that in many common optical designs, spherical
aberration tends to be a more serious problem than coma, so there isn't much point in
correcting coma unless spherical aberration is already dealt with.
APO
Allegedly, an abbreviation for apochromat, but often merely a meaningless marketing term.
apochromat
A type of refractor
objective made from lenses -- usually three -- of different
materials, selected to bring three colours of light to the same focal point, thereby
bringing the colours that are in between to very nearly the same focal point, thereby
permitting a decent stab at being well-focused for all colors of visible light at the same
time. Thus, an apochromat attempts to reduce longitudinal
chromatic aberration.
An apochromat will also have spherical aberration corrected at two wavelengths of
light -- usually blue and yellow -- thus attempting to reduce chromatic variation of spherical
aberration. It will probably have coma corrected at one wavelength
of light -- usually green.
At any rate, what preceeds is the technical
definition. There is an increasing colloquial tendency to label any telescope which is
free, or almost free, from chromatic aberration an
apochromat. The problem with that is that advertisers have picked up on
"apochromat" as a high-tech term useful in suckering buyers: Thus as the years
go by, what gets labelled "apochromat", gets worse, and worse, and worse...
The problem is compounded because there are
telescopes which are apochromats in the technical sense, as discussed in the preceding
paragraphs, which in fact show a lot of chromatic
aberration when used visually. These early apochromats had one of their "three
colours" in the near ultraviolet, so that they could be focused visually to take sharp
photographs with early blue/ultraviolet-sensitive photographic emulsions.
apparent
field of view
A characteristic of eyepieces.
The apparent field of view is the angle through which your eyeball rotates when you look
through an eyepiece and transfer your gaze from one edge of the field to the other.
aspheric
Not spherical, as in "the primary
mirror of a Newtonian telescope has an aspheric surface".
astigmatism
An aberration of optical systems. If you
try to focus on a star with a telescope whose only aberration is astigmatism, you will
find that the best-focus image is notably larger than in a similar telescope with no
astigmatism. Moreover, you will find that when you move the focuser a little way one side
of best focus, the star image straightens out into a short straight-line segment. If you
then move the focuser the same distance the other side of best focus, you will find the
image to be a similar short straight-line segment, but oriented at ninety degrees to the
first one.
Pure astigmatism is relatively rare in
telescopes. Astigmatism is most often seen off axis, mixed with other aberrations -- often
with coma -- sometimes resulting in bewilderingly complicated images.
averted vision
When you look squarely at something, you
are using a part of the retina of your eye that is not as sensitive to low light levels as
the parts that are off to the side. Thus to see faint objects, don't look straight at
them. Centre them in the field of view of your telescope, but fix your stare part way out
to the edge of the field.
People sometimes ask which way to avert --
that is, which way away from the centre of the field to move their gaze. Try several.
back focal
length
Classically, the distance from the last
optical element of a system encountered by the light passing through it, to the focal plane. Opinions differ on whether flat mirrors and diagonal prisms count as "optical elements", for determining back focal
length. I think they do not.
With classical all-lens optical systems,
like refractor objectives and many
camera lenses, back focal length is approximately what you get if you put a measuring
stick against the lens cell and measure the distance to the focal
plane. But with things like Schmidt-Cassegrains, the
term is more confusing -- for that system, back focal length is measured from the
secondary, which is way up inside the tube.
There is an increasing tendency to measure
the back focal length of folded systems, like the various Cassegrain
types, from the point where the beam emerges from within the assembly of optical parts --
e.g., from where it comes out of the hole in the primary. Be careful of which meaning is
in use.
Barlow lens
A negative (diverging) lens that fits into
your telescope's focuser, a little way inside the focal plane. It acts as a focal-length
multiplier for the telescope. Thus, suppose you have a telescope with a focal length of
1000 mm. If you use a 10 mm focal-length eyepiece, you will obtain a magnification of
1000/10 = 100x. With a 2x Barlow lens used in front of the eyepiece, the telescope's focal
length will be increased to 2000 mm, and the same eyepiece will give a magnification of
2000/10 = 200x.
The multiplication ratios of commercially
available Barlow lenses vary from 1.75 to 5. Barlow lenses are sometimes also called
telextenders.
Not all Barlow lenses work with all
eyepieces and all telescopes. Try before you buy.
barrel
distortion
An aberration of optical systems, in which
magnification decreases with distance away from the optical axis. With such a system,
squares are imaged with their sides bulged out, looking sort of like barrels. (Actually,
the points of the squares are imaged a little bit less far out than where they ought to
be, because of the reduced magnification in the outer part of the field.) Also see pincushion distortion.
binocular
Two telescopes held side by side, so you
can look at something with both eyes at once. Binoculars make quite good instruments for
beginning astronomers, and even advanced amateurs find them fun and useful.
Binoculars are traditionally labelled with
two numbers, separated by the letter "x", as in "7x50". The
"x" is read as "by"; hence a binocular labelled "7x50" is
called a "seven by fifty". In such a pair of numbers, the first one is how many
times the binocular magnifies, and the second is the clear
aperture of its front lenses, in millimetres. Thus a 7x50 magnifies seven times, and
has front lenses 50 mm in diameter.
Note that what most people call "a
pair of binoculars" is in fact just one binocular: That is, a binocular is sufficient
for two eyes; a pair of binoculars will do for four.
bino-viewer
An optical gadget that splits the single
beam of light coming out of a telescope into two parts, so you can observe with both eyes
at once. Bino-viewers generally do not work with all telescopes or all eyepieces.
Good bino-viewers are quite expensive.
Cass
Cassegrain
In the strictest sense, a Cassegrain
telescope is a reflecting telescope that uses two curved mirrors of particular shapes to
form the image: The primary mirror is a concave paraboloid,
just as in a Newtonian. The secondary
mirror is a much smaller convex hyperboloid, positioned a little way toward the
primary from the primary's focal plane. The secondary reflects the converging beam of
light back toward the primary, which typically has a hole in the center, so that the focal
plane of the combined mirrors is easily accessible behind the primary.
More loosely, the term
"Cassegrain", or "Cassegrain configuration", or "Cassegrain
system", has come to be applied to any telescope in which a large primary mirror and
a small convex secondary mirror are among the elements used to form the image. Common
Cassegrain systems include the Schmidt-Cassegrain, in
which a corrector plate which appears flat to the unaided eye is mounted just skyward of
the secondary mirror, and several varieties of Maksutov-Cassegrain,
in which a correcting lens that resembles a thick watch glass is similarly mounted. In
some Maksutov-Cassegrains, the secondary mirror is an aluminized
spot on the correcting lens.
There are many other Cassegrain systems
besides the ones just mentioned. In some, the only difference from the classical
Cassegrain is that the two mirrors are not the traditional paraboloid/hyperboloid
combination.
catadioptric
A catadioptric optical system is one that
uses both lenses and mirrors to form the image. Flat mirrors, panes of flat glass, and
prisms which are used to reflect the light beam about, do not count in making this
determination. Common catadioptric systems include Schmidt-Cassegrains
and Maksutovs, but there are lots of others.
Celestron
cell
The mechanical support that grips a lens or
mirror, and which is in turn fastened to the telescope tube, or to some other piece of
telescope structure.
cemented
doublet
chromatic
aberration
chromatic difference of magnification
An aberration of optical systems, in which
the magnification is different in different colours of light. One might think of the image
of an object seen with such an aberration, as composed of superimposed red, green, and
blue images of slightly different sizes.
Chromatic difference of magnification is
often detectable as coloured fringes at the edges of objects of different colours or
brightness, near the edges of the field of view. It is particularly common in binoculars.
Chromatic difference of magnification is
also called lateral color.
chromatic variation of spherical
aberration
An aberration of optical systems, in which spherical aberration is different in different colours of
light.
Chromatic variation of spherical aberration
is also called spherochromatism.
clear aperture
The diameter of the beam of light that goes
into a telescope. In most cases, that is the unobstructed diameter of the front lens, or
the corrector, or the primary mirror.
clock drive
sidereal
drive.
coated
"Coated" is most commonly used to
describe a lens or prism in which the surfaces where the beam of light goes from air into
glass, or from glass into air, have been coated with something that reduces the amount of
light that gets reflected. That reflected light is lost to the image, and may cause glare,
so reducing it is desirable. The coatings that do this are more correctly called low-reflection coatings -- for after all, mirrors are
coated, too, but with something that is supposed to increase reflectivity rather than
reduce it. But the term "coated" is common usage for "coated with
low-reflection coatings."
There is another meaning, that has to do
with marketing terminology. If you bought a binocular that had "coated optics",
you might naively think that all the surfaces where a beam of light went from air
into glass, or from glass into air, were coated with low-reflection coatings, right?
Wrong! The common usage in optical advertising is that "coated optics" merely
means that some of those surfaces are so coated -- perhaps only one. To get what
you thought you were buying, you would have to seek a binocular that was "fully coated". Also see multicoated
and fully multicoated.
collimation
A telescope is said to be collimated when
all its optical parts are correctly aligned with respect to one another -- everything is
square, centred, and spaced correctly. Some instruments have collimation set at the time
of manufacture, and with luck, the collimation will be correct for the life of the
instrument; these instruments include binoculars and many small refractors. Others require
frequent tweaking, which is sometimes a vexing task; these include most Newtonian reflectors.
coma
An aberration of optical systems. The image
of a star, in a telescope whose only aberration is coma, resembles a short, stubby comet
with a broad tail, or a side view of a badminton birdie.
Pure coma is relatively rare in telescopes.
Coma is most often seen off axis, mixed with other aberrations -- often with astigmatism -- sometimes resulting in bewilderingly complicated
images.
compound
telescope
contrast
I include this common word because the
amateur astronomy community often uses it in two very different senses without being clear
which is which. To someone concerned with the measurement of light, contrast is merely the
ratio of brightnesses of two parts of an image. You can determine it by actually measuring
the brightnesses with a photocell, or a light meter, or some such thing, and dividing. But
to someone concerned with the processing which the brain and eye do when light falls upon
the retina, contrast might more often mean how well two areas of different brightness are
distinguished; for example, can you see such-and-such a faint galaxy on the black sky
beyond it?
The relationship between contrast as
measured by light meters and contrast as reported by visual observers, is complicated and
poorly understood. How well two areas of different brightness are distinguished by the eye
and brain has to do with lots more than the ratio of brightnesses.
corrector
plate
Crayford
focuser
A fancy type of telescope focuser in which the focus tube is moved by a roller, and may well
slide on rollers, as well. Crayford focusers are noted for smooth, slack-free motion, and
for precisely adjustable friction.
Criterion
Former United States manufacturer of
amateur telescopes and accessories, mid twentieth century. Criterion was particularly
noted for quality Newtonians, notably the relatively inexpensive
Dynascope line. Criterion also produced Schmidt-Cassegrains,
though not as successfully as Celestron and Meade.
curvature
of field
An aberration of optical systems, in which
the surface on which images in sharp focus are formed is not flat, but curved. Curvature
of field is also called field curvature.
dark adaptation
Your eye does not reach its full power of
detecting low light levels -- its full dark adaptation -- until you have been in a dark
environment for quite a long time -- perhaps several hours. To obtain full dark
adaptation, the darkness of the surroundings must be very deep. Many observing sites have
enough stray light from nearby towns and cities to prevent full dark adaptation.
Do not confuse this hours-long process with
the opening of the pupils of your eyes to their maximum diameter: That takes at most tens
of seconds. Dark adaptation has to do with slower changes in the sensitivities of
light-detecting cells on the retinas of the observer's eyes.
Dawes limit
The Dawes limit is an empirically
determined standard of how well an excellent small telescope can be expected to perform,
in excellent conditions, when observing double stars that are not too faint, in which the
two components are of the same brightness, or very nearly so. The empirical result is that
double stars can be distinguished as separate when they are as closely separated as 4.56
arc-seconds, divided by the telescope clear aperture in
inches. Thus a four-inch telescope can expect to resolve such double stars down to a
separation of 4.56 / 4.00 = 1.14 seconds of arc. In metric units, the formula is 116
arc-seconds divided by the telescope clear aperture in
millimetres. To be just barely distinguished means there will be a barely perceptible dark
line between the images of the two components. Note that the ability to perceive such a
dark line may vary from observer to observer: The Dawes limit has to do with the
physiology of human vision as well as the optical quality of telescopes. (And Dawes was
noted for having particularly acute vision.)
Dawes performed his experiments, on which
the formula is based, using refractors. It turns out that telescopes in which a secondary mirror or diagonal
mirror obstructs part of the incoming beam, can do ever so slightly better than the
limit, for stars of equal brightness, but they don't do as well as refractors when the
brightness of the stars is considerably different.
declination
One of two coordinates for the celestial
sphere, which are analogous to latitude and longitude for the Earth's surface. The
declination of an object is how many degrees it is north or south of the celestial
equator. The other coordinate is called right ascension,
and it is measured eastward from a somewhat arbitrary "prime meridian" on the
sky.
The "prime meridian" passes
through the position of the Sun at the time of the vernal equinox. Thus its position
changes slowly over the years, due to the precession of the equinoxes. The position of the
celestial poles also changes with precession. Thus, to locate an object from its right
ascension and declination, you must also know the date for which those coordinates are
valid; that date is called the epoch of the coordinates.
deep sky
Deep sky astronomical objects are generally
considered to be those which lie beyond the solar system, except that some people do not
include double stars and variable stars. That's not because these objects do not lie
beyond the solar system -- they certainly do -- but because the term "deep sky"
evolved to describe what many people do with large-aperture telescopes at low to medium
magnification. What they do is look at galaxies, star clusters and nebulae. It is a
mouthful to say "I look at galaxies, star clusters and nebulae", and much easier
just to say "I look at deep-sky objects".
Veteran deep-sky observers can routinely
detect extremely faint objects in circumstances when less experienced observers see
nothing. They use terms like "faint fuzzy nothing", "lumpy darkness",
and "the elusive Elvis Nebula", a lot.
dew cap
A cylindrical extension of the telescope
tube beyond the upper end, whose purpose is to retard the formation of dew on optical
elements at or near that end. Dew caps are common on refractors, but often are useful on Schmidt-Cassegrains and Maksutovs.
Many Newtonians also might benefit from extensions of the tube,
to keep dew off the diagonal mirror.
diagonal
mirror
There are two meanings. The small flat
mirror used near the upper end of a Newtonian telescope, to
direct the converging beam of light over to the side of the tube, is called the diagonal
mirror. (Now and then you see a Newtonian that uses a prism
instead of a mirror.)
Alternatively, star
diagonals, that use mirrors and not prisms, are sometimes also called diagonal
mirrors.
diffraction
limited
A perfect telescope, used in perfect
conditions, does not deliver a perfectly sharp image. A detailed explanation of why
requires a modest amount of physics, and has to do with the wave nature of light. The
relevant physical phenomenon is sometimes called "diffraction" (not quite
correctly, if you are a purist). Thus an optical system is said to be "diffraction
limited" when it is so good, that when used in excellent conditions (seeing is
steady, telescope has cooled down to the local air temperature, telescope is well
collimated...), then only the wave nature of light itself prevents the telescope
from delivering perfectly sharp images.
To say that a telescope is diffraction
limited, is to say that no conceivable improvement in its optical quality can make its
images noticeably sharper. (You are not allowed to increase the aperture or change the
telescope design, however.) That is an extremely strong statement: Think "a
perfect ten" -- or maybe a 9.8; not merely "good enough", but
"superb".
Marketeers frequently abuse the term
"diffraction limited". To read some ads, you'd think it meant "any
telescope manufactured by us". But in my opinion, only a very small proportion of the
astronomical telescopes produced for the commercial market are truly diffraction limited,
and only a few limited-production manufacturers produce such systems for their entire
product line.
Also see Rayleigh
criterion and Dawes limit.
digital
setting circles
Electronic apparatus to achieve the
function of setting circles; that is, to indicate where in
the sky your telescope is pointed. These devices are generally capable of performing
trigonometry to correct for imperfect polar alignment of
your telescope, or to allow them to be used with an altazimuth
mounting.
distortion
An aberration of optical systems, in which
magnification changes systematically with distance away from the optical axis, causing
distortions of images. See barrel distortion and pincushion distortion.
Dob
An atrocious abbreviation for the last name
of noted telescope maker and inventor John Dobson. See Dobson
mounting, for example.
Dobson
mounting
An ingenious modern altazimuth mounting, invented by San Francisco telescope
maker John Dobson, which uses such slippery plastics as Dupont's "Teflon" to
achieve very smooth bearing operation. Dobson-mounted telescopes generally require no
slow-motion controls: They are guided by hand, even when observing with substantial
magnification. Telescopes as large as a meter in clear aperture
have been so mounted. I personally have operated a 45 cm Dobson-mounted Newtonian at a magnification of 1200x, which was pushing the limit
for hand guiding, but which did work, and which provided a view of the lunar surface
normally reserved for astronauts.
Dobson-mounted telescopes are almost always
Newtonians, but other telescope types have been so mounted.
Dobsonian
Another name for a telescope with a Dobson mounting.
Dobsonians are most commonly Newtonians, but strictly, the term applies only to the mounting
type, not the optical system.
doublet
Two simple lenses used in combination,
placed close together or in contact. If they are cemented together, they constitute a
"cemented doublet". If they are merely closely adjacent, they are a
"separated doublet".
DSCs
dust cap
A lid for one end of a telescope tube, to
keep dust from settling on the optics.
ED doublet
An achromat made
with ED glass. The most common commercially available ED doublets
are better than achromats made with conventional glass, by a
factor of about four, at bringing all colours of visible light to the same focal position,
but not as good as fluorite doublets.
ED glass
epoch
The coordinates commonly use for the
celestial sphere, which are analogous to latitude and longitude for the Earth's surface,
are called right ascension and declination.
The "prime meridian" of this system passes through the position of the Sun at
the time of the vernal equinox. Thus its position changes slowly over the years, due to
the precession of the equinoxes. The position of the celestial poles also changes with
precession. Thus, to locate an object from its right ascension and declination, you must
also know the date for which those coordinates are valid; that date is called the epoch of
the coordinates.
equatorial
mounting
A telescope mounting in which one of the
axes about which the telescope rotates is pointed parallel to the Earth's axis of
rotation. Such a telescope can be made to follow celestial objects across the sky, as the
Earth rotates, merely by turning this one axis at a constant rate of one revolution per
day. A simple, geared-down motor can be used to do so. See sidereal
drive.
Erfle eyepiece
A specific eyepiece design. The Erfle has a
rather wide apparent field of view -- perhaps 68
degrees or more. The image quality at the edges of the field, at small focal ratios, is not as good as for more modern wide-field
eyepieces.
It used to be said that Erfles were
"Erfle big and Erfle expensive", but no longer -- modern wide-field eyepieces
are more massive and more expensive, by far.
Erfles are generally composed of five or
six simple lenses, grouped into two doublets and a singlet, or three doublets.
exit pupil
More or less, the beam of light that comes
out of the eyepiece of a telescope. (More nearly correctly, the
image of the objective formed by the eyepiece.)
The diameter of the exit pupil is equal to the clear aperture
divided by the magnification in use. If that diameter is greater than the diameter of the
pupil of the observer's eye, then not all of the light entering the telescope will reach
the observer's retina: Some will be wasted.
eyepiece
Eyepieces are glorified magnifying glasses
used to view the image formed by the objective of a telescope.
That image could in principle be cast onto a sheet of paper, placed at the right place in
the telescope's optical path (or perhaps captured on a piece of photographic film at the
same position). If you will imagine using a magnifying glass to examine the image, on that
same piece of paper, you will get an idea of what an eyepiece does -- except, of course,
that it does it without the paper.
Eyepieces come in various types. Every
eyepiece has a focal length. The magnification that results
when a given eyepiece is used with a given telescope, is equal to the focal length of the
telescope divided by the focal length of the eyepiece. Thus if the telescope has a focal
length of 1000 mm and the eyepiece has a focal length of 25 mm, the magnification will be
1000 / 25, or 40.
eye relief
The distance from the surface of the
rearmost lens of the eyepiece, to the exit
pupil. When the eyepiece is in use, that distance should be the distance from the
rearmost lens of the eyepiece to the iris of the observer's eye. Note that the iris is a
few mm behind the front surface of the eye. The remaining distance is the space between
the observer's eye and the eyepiece: It is the clearance available for moving the
observer's head without bumping the telescope, and is also the place where the observer's
spectacles must fit, if they are worn while observing.
f-number
field
curvature
An aberration of optical systems, in which
the surface on which images in sharp focus are formed is not flat, but curved. Field
curvature is also called curvature of field.
field
flattener
Field curvature
can be corrected by a simple lens placed at or close to the focal "plane" of the
instrument in question. Such a lens is called a field flattener. If the field curvature is
concave toward the incoming beam of light, the field flattener must be a negative lens. If
the field curvature is convex toward the incoming beam of light, the field flattener must
be a positive lens. The size of the field flattener must be equal to or slightly greater
than the size of the desired flat field.
field glass
field glasses
An instrument rather resembling a binocular, but which uses a special eyepiece
design, instead of prisms, to achieve an upright image. Field glasses
are thereby generally lighter and less expensive than binoculars,
but -- a disadvantage of the particular type of eyepiece used --
they have an extremely narrow apparent field of view.
In essence, this instrument consists of two
Galilean telescopes mounted side by side, as for a binocular. However, contemporary usage does not apply the term
"binocular" to two Galilean telescopes so mounted.
It is uncommon to see field glasses of
sufficient optical quality to use for astronomy, though their light weight may make them
desirable for use by very young beginners. Field glasses are also in common use as opera glasses, for concerts and the theater.
Note that although two of any other kind of
telescope mounted side by side are called a "binocular" -- not a "pair of
binoculars"; nevertheless, two Galilean telescopes mounted side by side are called
"field glasses" (plural). That is because "field
glass" is another name for Galilean telescope.
It is perhaps best not to think about what you would do if you needed a short phrase to
describe a collection of Galilean telescopes.
finder
A small, low-magnification telescope or
sight, equipped with a cross-hair or similar device, to be mounted upon a larger telescope
as an aid to locating objects for viewing.
flourite
Library paste used as an optical material.
I'm sorry, that was a joke: "Flourite" is a common misspelling of "fluorite". I do not think that there are any commercial optics
made from library paste, but some times I wonder.
fluorite
fluorite
doublet
An achromat in
which one lens is fluorite. Fluorite doublets are better than achromats made with conventional glass, by a factor of about eight,
at bringing all colors of visible light to the same focal position.
focal length
The focal length of a thin, simple lens is
the distance from the lens to the image it forms of an object that is very far away. For
more complicated optical systems, the focal length generally does not correspond to any
easily measurable distance. It is instead the focal length of a simple lens which would
give the same size image of an object that is very far away.
focal plane
The surface on which a lens or objective
forms an image: Where you would have to put a piece of paper to catch an image in sharp
focus. The term "focal plane" is often a misnomer, for the focal surface is
often not flat, but curved.
focal ratio
The ratio of focal
length to clear aperture, generally written as the
letter "f", followed by a slash, followed by the ratio, like this: f/8. Such an
"f/8" system (pronounced "eff eight") has a focal
length equal to eight times its clear aperture.
Focal ratios which are large numbers are
said to be long or slow; those which are small numbers are said to be short
or fast. There are no sharp lines for determining when to use these terms, but
among contemporary astronomical telescopes, any telescope with a focal ratio slower
(bigger number) than f/12 would probably be said to be slow, and any telescope with a
focal ratio faster (smaller number) than f/6, would be said to be fast.
Fast systems often have more aberrations
away from the centre of the field of view, than do slow ones. Fast systems are generally
more demanding of eyepiece designs, than are slow ones: In
practical terms, fast systems generally require more expensive eyepiece
designs than do slow ones.
focuser
The part of a telescope into which you put
the eyepiece, together with whatever arrangement it has for
focusing the image.
fork mounting
A telescope mounting in which the optical tube assembly is carried between the tines of a
stubby, two-armed fork. The OTA rotates about the bearings which attach
it to the arms, and the "handle" of the fork also rotates, thus providing the
telescope with the two motions it needs to point in any direction.
If the handle of the fork points parallel
to the Earth's axis of rotation, the system is an equatorial fork mounting. If it points
vertically, it is an altazimuth fork mounting. A Dobson
mounting is a particular type of specialized altazimuth fork mounting, made of
specific materials.
fully coated
fully
multicoated
A marketing term for an optical system in
which every optical surface where light passes from glass into air, or from air into
glass, is coated with one of the newer, high-tech low-reflection
coatings, which transmit more light than such earlier ones as magnesium fluoride. Also
see coated, fully coated, and multicoated.
Galaxy
Galilean
telescope
The kind of telescope built by Galileo
featured a singlet objective and a singlet eyepiece. The eyepiece -- a Galilean
eyepiece -- consisted of a so-called "negative" lens, which is what most
people think of as the opposite of a magnifying glass. This kind of lens gives a very
narrow apparent field of view, but it gives an image
that is upright and has the left and right sides correctly positioned; most astronomical
telescopes give an image that is upside down, or has left and right reversed.
In Galileo's original telescope, neither of
the lenses was an achromat, so the instrument had considerable chromatic aberration. By extension, however, any
telescope which uses a negative lens for the eyepiece is called a
Galilean telescope, even if it uses achromatic lenses.
Because of their simplicity, Galilean
telescopes are very inexpensive to manufacture. They are thus common as toys. Two Galilean
telescopes mounted side by side, as for a binocular, constitute field glasses.
Note that although two of any other kind of
telescope mounted side by side are called a binocular -- not a
"pair of binoculars"; nevertheless, two Galilean telescopes mounted side by side
are called "field glasses" (plural). That is because "field glass" is another name for Galilean telescope. It is
perhaps best not to think about what you would do if you needed a short label for a
collection of Galilean telescopes.
German
equatorial mounting
A telescope mounting in which the optical tube assembly is attached at right angles to one
of two long, rotating shafts, which are also at right angles to one another. The first
shaft -- the right ascension axis or polar axis -- points
parallel to the Earth's axis of rotation. The first shaft carries the bearings within
which the second shaft rotates. The second shaft -- the declination
axis, which carries the telescope -- points toward the celestial equator. Rotation of the
polar axis, and of the declination axis about its attachment point to the polar axis,
allows the telescope to point in any direction. The end of the declination axis opposite
to the telescope carries a counterweight.
ghost images
Ghost images are spurious images caused by
unwanted reflections in an optical system. The are sometimes in focus, or nearly so.
Several common eyepiece designs are plagued by ghosts. To look at Jupiter, for example,
with such an eyepiece, is perhaps to see a second, dimmer, image of the planet in the
field of view, changing its relative position with respect to the real image as the
telescope is moved.
glasses
Goto
Japanese optical manufacturer whose
amateur-sized telescopes and accessories were imported into the United States in the mid
20th century. Goto is perhaps better-known for planetarium projectors. There is occasional
confusion between "Goto", the manufacturer, and "goto",
meaning a computer-controlled telescope with a command interface in which the observer
tells the telescope what object to "go to" next.
goto, or go-to
Shorthand for the kind of command interface
to a computer-controlled telescope, in which the observer tells the telescope what object
to "go to" next. Not to be confused with the optical manufacturer, "Goto".
Gregorian
In the strictest sense, a Gregorian
telescope is a reflecting telescope that uses two specific shapes of concave mirrors to
form the image: The primary mirror is a concave paraboloid, just as in a Newtonian. The secondary mirror is a much smaller concave ellipsoid,
positioned a little way beyond the primary's focal plane. The secondary reflects the beam
of light diverging from the focus, back toward the primary, which typically has a hole in
the center, so that the focal plane of the combined mirrors is easily accessible behind
the primary.
More loosely, the term
"Gregorian", or "Gregorian configuration", or "Gregorian
system", has come to be applied to any telescope in which a large primary mirror and
a small, extra-focal, concave secondary mirror are among the elements used to form the
image. Gregorian systems are uncommon among contemporary astronomical telescopes: Celestron briefly manufactured a 66 mm aperture Gregorian Maksutov,
but I cannot think of any other recent commercially available units.
It is common to confuse Gregorian
telescopes with the Maksutov designs by John Gregory. The latter
are correctly called "Gregory Maksutovs", but
they are not Gregorian configurations; they are Cassegrain
configurations.
Gregory
Maksutov
Any one of several Cassegrain
configuration Maksutov designs by John Gregory. These all feature
a secondary mirror which is merely an aluminized spot on one surface of the Maksutov corrector.
It is common to confuse Gregory Maksutovs
with Gregorians: They are not the same.
Intes
Russian manufacturer of telescopes and
components. Actually, there are two closely related businesses, Intes and Intes Micro -- I
lump them here together because of the similarity in names. Both are noted for
high-quality Maksutov telescopes.
helical
focuser
A telescope focuser
in which adjustment of focus is made by screwing in and out either the eyepiece itself, or
a focus tube which contains it. Many binoculars have a helical focuser for one or both
eyepieces.
Huygenian
eyepiece
A specific eyepiece design. Same as Huygens eyepiece.
The Huygenian is among the oldest of
eyepiece designs. It has a narrow apparent field of view
-- perhaps only 30 degrees -- and works well only with telescopes which have quite long focal ratios. Many inexpensive small refractors come equipped with
Huygenian eyepieces, which not only have the faults just listed, but are also often not
very well made. However, a decent Huygenian -- if you can find one -- will work reasonably
well with focal ratios of perhaps f/15 or more.
The simplest form of Huygenian eyepiece is
composed of two simple lenses, each flat on one side and convex on the other. The lens
nearest your eye is smaller and has a shorter focal length than the other. The flat sides
of both lenses are toward your eye. Even though neither of the lenses is an achromat, Huygenian eyepieces do an excellent job of correcting lateral color.
Huygens
eyepiece
Kellner
eyepiece
A specific eyepiece design. In essence, the
Kellner is an achromatized Ramsden. It has a slightly
larger apparent field of view than the Ramsden, and
works at slightly faster focal ratios. Kellners tend to have
rather prominent ghosts.
Kellner eyepieces consist of a small achromat -- a cemented doublet --
near your eye, and a simple lens at the far end of the eyepiece.
kidney-bean
effect
A popular name for an image defect seen
when using an eyepiece which (1) has an exit pupil almost as
large as the pupil of the user's eye, and (2) is afflicted with an uncommon aberration
called "spherical aberration of the exit pupil". The presence of that
aberration, in essence, means that you must hold your eye a distance from the eyepiece
that varies, depending on whether you want to look at things near the center of the field,
or near the edge. Having your eye in the wrong place, and a little off center, as well,
results in the appearance of a dark, kidney-bean shaped patch, part way out to the edge of
the field. The only common eyepiece type in which this effect is noticeable is the early
version of the Tele Vue Nagler.
Koenig
eyepiece
A family of eyepiece designs. Koenigs have
a rather wide apparent field of view -- perhaps as
much as 70 degrees.
The various eyepieces commonly labeled
"Koenig" contain anywhere from four to seven simple lenses, grouped into various
combinations of cemented doublets and singlets.
"Koenig" is sometimes spelled
"Konig".
Lanthanum
LV eyepiece
A specific eyepiece design. Lanthanum LV is
a "house brand" of the large Japanese optical manufacturer, Vixen.
The Lanthanum LV eyepiece series is particularly noted for having a long eye relief of 20 mm, even for the shortest focal lengths.
At least some of the Lanthanum LV eyepieces
are composed of seven simple lenses, cemented together into three doublets
and a singlet. The frontmost doublet
consists in essence of a built-in Barlow lens. The rest of the
eyepiece comprises two doublets separated by a singlet.
lateral colour
light-pollution
reduction filter
A filter specially prepared to reflect
wavelengths which are prominent in light pollution, and transmit other wavelengths. Such a
filter improves the contrast of deep-sky objects, when light
pollution is present.
What makes good light-pollution reduction
filters possible is that much light pollution comes from mercury-vapor and sodium-vapor
street lights, which emit preferentially in narrow spectral ranges.
limiting
magnitude
The magnitude of the faintest stars which
can be seen with a given optical system. It depends on the aperture,
magnification, sensitivity of the observer's eye, sky transparency, light pollution, and
steadiness of the air.
longitudinal
chromatic aberration
An aberration of optical systems, in which
the focal length of a system is different for different colors of light.
If you were to try to focus the image of a
white star, in a telescope in which the only aberration was longitudinal chromatic
aberration, your eye -- being most sensitive to green light -- would lead you to position
the focuser so that green light was in focus. At that point, the red and blue light from
the star would not be in focus, but would merge to form an out-of-focus violet halo about
the star.
longitudinal
colour
low-reflection
coating
Any of several coating materials whose
purpose is to reduce unwanted reflections from optical elements which are supposed to
transmit light. These optical elements include lenses and prisms.
When a beam of light passes from air into
glass, or from glass into air, most glasses transmit only about 96 percent of the light,
and reflect the rest. The first coating material that was widely used commercially,
magnesium fluoride, increased the transmission to 98 percent. More modern coatings,
comprised of multiple layers of material (see multicoated),
transmit 99 percent or more.
There is a great deal of marketing
gibberish obscuring the kind and quality of coatings in use on commercially-available
optics. See coated, fully coated, multicoated, and fully multicoated.
LPR filter
MA eyepiece
A specific eyepiece design. MA is a
"house brand" of Meade. "MA" perhaps means
"Modified Achromatic". These inexpensive eyepieces work reasonably well with
longer focal ratios.
These eyepieces are similar in construction
to Kellners.
magnification
The magnification that results when a given
eyepiece is used with a given telescope, is equal to the focal
length of the telescope divided by the focal length of the eyepiece.
Mak
Maksutov
Maksutov
corrector
A particular kind of lens used in Maksutov optical systems. Maksutov correctors are thick, and have
strongly curved, nearly concentric, surfaces. They resemble large, thick watch-glasses, or
certain styles of art-deco ash trays.
Masuyama
eyepiece
A five-element eyepiece sometimes described
as a "hybrid Plossl", with better performance
away from the center of the field. I believe the design consists of two doublets
with a singlet in between, but I am not sure.
Meade
Major United States manufacturer and
importer of telescopes and accessories. Meade got its start as an importer of small
telescopes and components in the 1970s, then expanded into Newtonians,
and finally took off big-time as a competitor to Celestron in the
Schmidt-Cassegrain market. Current product line includes
a variety of Schmidt-Cassegrain, Newtonian,
Maksutov-Cassegrain, and refractor
telescopes, as well as numerous accessories.
mirror
diagonal
monocentric
eyepiece
A class of related eyepiece designs.
Monocentrics tend to have very narrow apparent fields of
view, and prominent ghost images, but they have only two
air/glass interfaces, hence very little scattered light. Good ones therefore make nice
eyepieces for Lunar, planetary, and double-star work.
All the types of monocentric eyepiece that
I know of consist of three simple lenses, cemented together into a triplet.
monocular
A spotting telescope, which resembles half
a binocular. Monoculars are often hand-held, and are used in much
the same way, and for the same purposes, as binoculars.
mounting rings
Circular clamps which fasten around an optical tube assembly and are threaded, machined, or
otherwise prepared to fasten to a mounting. Also called tube rings.
multicoated
"Multicoated" describes a lens or
prism in which the surfaces where the beam of light goes from air into glass, or from
glass into air, has been coated with a high-tech, many-layered coating that decreases the
amount of light that gets reflected. That reflected light is lost to the image, and may
cause glare, so reducing it is desirable. The coatings that do this are more correctly
called low-reflection coatings -- for after all,
mirrors are coated, too, but with something that is supposed to increase reflectivity
rather than reduce it.
There is another meaning, that has to do
with marketing terminology. If you bought a binocular that had "multicoated
optics", you might naively think that all the surfaces where a beam of light
went from air into glass, or from glass into air, were coated with fancy low-reflection
coatings, right? Wrong! The common usage in optical advertising is that "multicoated
optics" merely means that some of those surfaces are so coated -- perhaps only
one surface! To get what you thought you were buying, you would have to seek a binocular
that was "fully multicoated". Also see coated and fully coated.
Nagler
eyepiece
A specific eyepiece design. Nagler is a
"house brand" of Tele Vue. These eyepieces are noted for
a very wide apparent field of view -- 82 degrees --
and for excellent correction at fast focal ratios. Naglers are
big, heavy, and expensive, and consist of seven or eight simple lenses grouped together
into four singlets or doublets.
Newtonian
A simple and common design of reflector.
The objective is a concave paraboloid at the lower end of the
tube. A flat diagonal mirror near the upper end of the tube
reflects the converging beam over to the side of the tube, where the focuser
is mounted.
Nikon
Japanese manufacturer of optical goods.
Products for amateur astronomers include several lines of high-quality eyepieces.
Nova
object glass
objective
Loosely, the big mirror or big lens of a
telescope. Some times it is a little hard to tell what optical components make up the
objective and what are accessories or auxiliaries. Is a Schmidt
corrector plate or a Maksutov corrector part of the
objective?
obstruction
ratio
In telescopes which have a diagonal mirror or a secondary
mirror so positioned as to obstruct part of the incoming beam of light, the presence
of that obstruction reduces the contrast of the image somewhat. The amount of contrast
degradation depends on the relative size of the obstruction, compared to the clear aperture of the telescope. The question is, what number
do we use to quantify that proportion? The big issue is, do we report the ratio of areas,
or the ratio of diameters?
In a certain sense, it doesn't matter.
Thus, a telescope with a clear aperture of 150 mm, having an
obstruction that is 50 mm in diameter, experiences the same loss of contrast whether we
report the obstruction as 0.33 (ratio of diameters) or 0.11 (ratio of areas). The
important thing is to make sure to specify which of the two ratios the reported number
actually is.
Historically, the usual quantity to report
was the ratio of diameters. Then various manufacturers started reporting the ratio of
areas, as a marketing gimmick: After all, an obstruction ratio of 0.11 sounds smaller than
one of 0.33, even if we are describing the same 150 mm aperture telescope with its 50 mm
obstruction, and particularly if we forget to specify which ratio we are talking about.
It is probably simpler to report the ratio
of diameters, if only to save readers the possible effort of doing a square root if they
are given the ratio of areas and want to know the ratio of diameters. Furthermore, most of
the technical literature that describes the effect of obstructions on contrast uses the
ratio of diameters as a parameter in things like equations and graphs.
off-axis
Said of a telescope whose optical elements
(not counting simple flat mirrors and prisms) are not mechanically
symmetric about a single optical axis. The common types are reflectors, in which the primary mirror is tilted, so as to deflect the reflected beam
off to the side, so that other optical components, which might otherwise partially obscure
the incoming beam, will not do so.
Many off-axis telescope designs are hard to
make.
See, for example, the Schiefspiegler or the tri-Schiefspiegler.
opera glasses
optical
tube assembly
The tube that holds the optics, also called
an OTA. An optical tube assembly generally consists at minimum of all
the optics it takes to form an image (but not necessarily any eyepieces
or star diagonal), plus mechanical parts to hold them in
place, correctly positioned with respect to one another, plus some sort of focuser. Optical tube assemblies often also come with finders, tube rings, and perhaps other
gadgets.
The optical tube assembly for a refractor of the traditional style resembles a spy-glass. For a Newtonian, it resembles a water heater. For a Schmidt-Cassegrain or a Maksutov-Cassegrain,
a waste basket.
orthoscopic
eyepiece
A specific eyepiece design. Orthoscopics
have moderate apparent fields of view -- 40 or 45
degrees -- and work pretty well at fast focal ratios. Many
consider them the best eyepieces for Lunar, planetary, and double-star work.
There are actually several designs called
"orthoscopic". The most common kind has a simple lens nearest your eye, and a
cemented triplet further away. Another kind resembles a Plossl.
OTA
pincushion
distortion
An aberration of optical systems, in which
magnification increases with distance away from the optical axis. With such a system,
squares are imaged with their sides curved inward, looking sort of like pincushions.
(Actually, the points of the squares are imaged a little bit further out than where they
ought to be, because of the increased magnification in the outer part of the field.) Also
see barrel distortion.
Plossl
eyepiece
polar
alignment
The process of getting the axis of an equatorial mounting that is supposed to point parallel to
the Earth's axis of rotation, to do so.
By extension, the process of orienting a
telescope with a computer-controlled altazimuth mount to the sky around it. Such a
telescope doesn't need to have particular mechanical parts pointing in particular
directions, but it does need to know which way is north and which way is up, and can
usually figure those things out by being pointed to several known and specified stars in
succession.
polar-alignment
telescope
A small telescope permanently attached to
an equatorial mounting, aligned with the mounting axis that is supposed to point parallel
to the Earth's axis of rotation. The polar-alignment telescope probably has markings
visible through its eyepiece which can be aligned with stars near the north or south
celestial poles, to aid in performing a polar alignment.
primary mirror
prime focus
The meaning of this term is changing. It
used to apply only to reflecting telescopes in which the primary mirror was a paraboloid, and thus capable of forming an
image all by itself. Some such telescopes were built so that film or some other equipment
could be placed inside the tube, or perhaps a bit beyond the upper end, to capture light
that had reflected only off the primary mirror. That
location was called the "prime focus".
In some very large professional telescopes,
there was room inside the tube for an enclosed space where an observer could sit, to guide
the telescope when the prime focus was in use. Yet even much smaller telescopes were
sometimes equipped to use the prime focus, using extra optics and what-not to allow
guiding from the side of the tube.
Other focal positions were named by the
design of the optical system that illuminated them -- the Newtonian focus for a Newtonian, the Cassegrain focus for a Cassegrain,
and so on. One also heard the term "prime focus" used for refractors, when film
or equipment was placed in the focal plane of the objective.
In many modern amateur telescopes, the primary mirror is not capable of forming an image all by
itself, or even if the Schmidt corrector plate or Maksutov corrector is included, assuming there is one.
Besides, "Gregory-Maksutov-Cassegrain focus", and the like, is a mouthful.
Perhaps for such reasons, the term "prime focus" is increasingly being used in a
more general manner: In systems in which there is only one place where there is a focuser -- where an image is formed and you can get at it with an
eyepiece or some other equipment -- that location is called the "prime focus".
Note that there are systems in which the
image can be accessed at more than one location. For example, reflecting
telescopes are still built with paraboloidal primary mirrors,
which have interchangeable flat diagonal mirrors and
hyperboloidal secondary mirrors, with a focuser
installed both at the Newtonian focus and the Cassegrain focus. For these systems, the term "prime
focus" often indicates the Newtonian focus.
prism
An odd-shaped hunk of glass, often more or
less triangular in one cross-section, used to bend or reflect light internally.
prism diagonal
Quantum
Line of Maksutov
telescopes, manufactured by OTI in the late 1970s and 1980s. Noted for aesthetics, fit and
finish.
quarter-wave
optics
quartz
In telescope optics, quartz
generally does not refer to the crystalline material found in nature, but to a glassy
substance, fused quartz, formed by melting down lots of quartz crystals (e.g., very
pure sand) in an oven. Quartz has thermal properties that make it valuable for lenses and
mirrors alike, and optical properties that are of particular use in certain kinds of
systems, even those which don't need its thermal properties. In particular, very fine achromats can be made from quartz and fluorite.
Questar
United States manufacturer of Maksutov telescopes, founded in the 1950s. The 3.5-inch Questar has
been known for high mechanical and optical quality, immaculate fit and finish, and elegant
aesthetics, for some fifty years.
rack
and pinion focuser
A focuser in which
the tube containing the eyepiece has a straight row of gear teeth (the rack) running along
one side of it, which are engaged by a more conventional-looking gear (the pinion),
attached to a knob, in order to adjust the focus.
Ramsden
eyepiece
A specific eyepiece design. The Ramsden is
a very old design, with a rather narrow apparent field
of view -- perhaps as little as 30 degrees. Ramsdens do not work well at focal ratios shorter than about f/9, but good ones make
surprisingly nice eyepieces for Lunar, planetary, and double-star observation, at longer focal ratios. Ramsdens often have prominent ghosts.
The simplest form of Ramsden consists of
two identical simple lenses, each flat on one side and convex on the other, with convex
sides facing each other, spaced apart by a distance equal to or slightly less than their
focal length. Even though neither of the lenses is an achromat,
Ramsden eyepieces do an excellent job of correcting lateral colour.
Rayleigh
criterion
The famous physicist, Lord Rayleigh,
asserted that it would be difficult to tell that an optical system was less than perfect,
if (1) it was well-enough figured that as a converging wavefront of light approached
focus, at most a small part it departed from the theoretically correct shape, (2) the
maximum difference in deviation from perfection, measured between any two points on the
wavefront, did not exceed a quarter of the wavelength of light in question, and (3) the
deviation was smooth.
Rayleigh was a smart man: Optics which meet
this criterion are indeed difficult to distinguish from optics which are truly diffraction limited. However, optics good enough to meet
the Rayleigh criterion are far less common than many marketers of telescopes would have
you believe. The business about "only a small part", often gets forgotten, as
does the business about "smooth", and whereas Rayleigh specified a measurement
of error which in modern terms is called "peak to valley, on the wavefront",
optical accuracies are often reported on the optical surfaces, or as plus/minus (+/-) or
root-mean-square (RMS) deviations. All of these other kinds of report result in a smaller
numerical value of the deviation, than would be measured peak-to-valley, on the wavefront.
The discrepancy can be as much as a factor of six. Thus the common term "quarter-wave optics" means exactly nothing -- and
probably, not what you hope it means -- unless it also includes some indication of what
measurement is reported.
reflector
A telescope in which only mirrors are used
to form the image. The eyepiece doesn't count, and if there are any prisms used to
redirect the beam of light, they don't count either.
reflex finder
reflex sight
refractor
A telescope in which only lenses are used
to form the image. If there are any mirrors or prisms used in the system, whose sole
purpose is to redirect the beam of light, they don't count: It's still a refractor.
resolution
Loosely, the ability of a telescope to show
detail. Also known as resolving power. One common way to
describe the resolution of a telescope is to state the minimum angular separation at which
a double star, whose two components are fairly bright and have very nearly the same
brightness, can be distinguished as two separate stars. See Dawes
limit.
resolving
power
right
ascension
One of two coordinates for the celestial
sphere, which are analogous to latitude and longitude for the Earth's surface. The right
ascension of an object is how far it lies east of a rather arbitrarily chosen "prime
meridian" in the celestial sphere.
Because the Earth rotates in twenty-four
hours, right ascension is not measured in degrees, but in hours and fractions thereof: An
hour of right ascension corresponds to fifteen degrees of "longitude" upon the
celestial sphere. The other coordinate is called declination.
The "prime meridian" passes
through the position of the Sun at the time of the vernal equinox. Thus its position
changes slowly over the years, due to the precession of the equinoxes. The position of the
celestial poles also changes with precession. Thus, to locate an object from its right
ascension and declination, you must also know the date for which those coordinates are
valid; that date is called the epoch of the coordinates.
Ritchey-Chretien
A telescope closely resembling a classical,
two-mirror-only Cassegrain, except that the primary mirror is an
hyperboloid -- slightly more strongly figured than the Cassegrain's paraboloid -- and the
secondary is a slightly stronger hyperboloid than that of the Cassegrain.
Ritchey-Chretiens are corrected for coma as well as for spherical
aberration; thus they can deliver relatively sharp images across a wider field of view
than can Cassegrains. For that reason, many of the large reflectors used by professional
astronomers have been made as Ritchey-Chretiens, and at focal
ratios suitable for photographic work.
Schiefspiegler
A particular type of off-axis
compound telescope: Specifically, an off-axis
Cassegrain.
There are actually several kinds of
Schiefspiegler, but they are usually described with additional adjectives or nomenclature,
as in the tri-Schiefspiegler.
The original Schiefspiegler has a primary mirror with a relatively long focal
ratio. Schiefspieglers are awkward and odd-looking -- some common ways to mount the
optical parts result in telescopes that resemble "washtub bass" musical
instruments, or perhaps certain kinds of hand-held garden sprayers. (For that reason, they
have occasionally been waggishly referred to as "Sheepsprinklers".) They are
sometimes hard to keep in collimation, too. Notwithstanding, they can deliver excellent
images.
Schmidt camera
One of several related types of
photographic telescope. The simplest design consists of a concave spherical primary mirror with a Schmidt
corrector plate at its center of curvature. Schmidt cameras usually have extremely
fast focal ratios, and provide very high-quality images. The focal "plane" of the simplest design is not flat, but
strongly curved, and is furthermore positioned well inside the body of the camera, where
it is difficult to reach with an eyepiece. Hence the system is almost always used as a
camera.
Schmidt
corrector plate
The key piece of any Schmidt-type optical
system, such as a Schmidt camera, a Schmidt-Cassegrain, or any of many others. To the naked
eye, a Schmidt corrector plate looks like a flat piece of glass, but its surface is
actually figured in a subtle manner, to affect the spherical aberration of the system in
which it is used.
Schmidt-Cassegrain
A Cassegrain
configuration telescope whose optics consist of a concave primary
mirror, which is spherical in most of the common commercial designs, a full-aperture Schmidt corrector plate mounted near the focal point
of the mirror, and a small, convex secondary mirror positioned in the converging beam, as
in a true Cassegrain.
The development of techniques to
manufacture Schmidt corrector plates of reasonable
quality (not everyone would call the quality "reasonable"), in the 1960s,
revolutionized amateur astronomy, by making available at not too high a price telescopes
which were were relatively compact and portable: A Schmidt-Cassegrain optical tube assembly is lots more compact than that of
a Newtonian or refractor of the same
aperture.
SCT
secondary
mirror
In telescopes which use more than one
curved mirror to form the image, the one which the light beam reaches second is called the
secondary mirror. It is usually smaller than the primary mirror.
semi-APO
semi-apochromat
Marketing term intended to convey the
impression that a telecope is particularly well corrected for longitudinal chromatic aberration. There is
no generally accepted technical definition for semi-apochromat. It is probably wisest to
assume that the phrase is meaningless.
separated
doublet
setting
circles
sidereal drive
"Quick, Chewie, the jump to
lightspeed!" Sorry, that was a joke. Actually, a mechanism used to make a telescope
follow stars across the sky as the Earth rotates.
Modern sidereal drives for small telescopes
are generally electrically powered and electronically regulated; older variants have used
(a) synchronous motors, with regulation provided by the number of cycles per second of
alternating-current line voltage, (b) clockwork mechanisms with a pendulum or governor for
regulation, (c) clepsydra-style devices, with regulation provided by the slow leakage of
water, sand, or compressed air from a container, and (d) hired help, turning a crank on
the sidereal drive in precise synchronization with the ticking of a clock. And that last
was not a joke.
singlet
SMA eyepiece
A specific eyepiece design. SMA is a
"house brand" of Celestron. "SMA" perhaps
means "Super Modified Achromatic". These inexpensive eyepieces work reasonably
well with longer focal ratios.
These eyepieces are similar in construction
to Kellners.
spherical
aberration
An aberration of optical systems, in which
the focal length of a lens or mirror varies with distance sideways from its center. That
is, if you were to place different annular masks -- discs of paper with a ring cut out --
squarely in front of such a lens or mirror, you would find that the focal length you
measured depended on the size of the ring.
spherochromatism
An aberration of optical systems, in which spherical aberration is different in different colours of
light. That is, if you performed a knife-edge or star test of a system with
spherochromatism, you would get different results in different colours of light.
Spherochromatism is also called chromatic variation of spherical
aberration.
spider
star diagonal
A gadget resembling an plumbing
"elbow" fitting, containing a mirror or prism, attached to a downward-pointing focuser to redirect the beam of light upward for more convenient
viewing. That is, a star diagonal fits into a focuser, has an eyepiece fitted into it, and bends the light path through a right
angle.
star test
Super
Wide Angle eyepiece
A specific eyepiece design. Super Wide
Angle is a "house brand" of Meade. These eyepieces are well
corrected, with large apparent fields of view (67
degrees).
Super Wide Angle eyepieces are reported to
be generally similar in design to Panoptics or Erfles.
symmetrical
eyepiece
A specific eyepiece design. The optics of a
symmetrical eyepiece comprise two cemented doublets
oriented face-to-face. Many designs sold as Plossl eyepieces
are actually symmetrical: True Plossls are not symmetrical.
telecompressor
The opposite of a telextender
or a Barlow lens. A telecompressor multiplies the focal length of a telescope by a factor which is less than one,
thus reducing the focal length. Its purpose is generally to
improve the photographic speed of a system, so that the time exposures required to take
pictures are not so long.
telextender
tri-Schiefspiegler
A common name for several related designs
of off-axis telescope, all related to the Schiefspiegler, but using three mirrors instead of two.
triplet
Three simple lenses used in combination,
placed close together or in contact.
truss-tube
Dobson
A Dobson-mounted
telescope in which the telescope tube is a truss assembly, perhaps looking like part of a
bridge, or a bird cage, or the skeleton of a rather baroque telephone booth. Truss tubes
are often designed for quick and easy assembly and disassembly in the field, and the
disassembled parts are not very bulky. Thus a quite large telescope may be transported in
modest space.
Other types of telescopes than ones with Dobson mountings can, and do, have truss tubes, but the
truss-tube Dobson has become sufficiently popular -- and big ones are sufficiently
eye-catching -- to deserve a separate entry in this glossary.
truss-tube
Dobsonian
tube rings
Ultima
eyepiece
A specific eyepiece design. Ultima is a
"house brand" of Celestron, who use it to label many
other products besides eyepieces. Ultima eyepieces have moderate apparent fields of view, of about 50 degrees.
Ultima eyepieces are reported to consist of
four to seven lenses, grouped into various combinations of singlets
and cemented doublets.
Ultra
Wide Angle eyepiece
A specific eyepiece design. Ultra Wide
Angle is a "house brand" of Meade. These eyepieces are well
corrected, with very large apparent fields of view,
of 84 degrees.
Ultra Wide Angle eyepieces are reported to
be generally similar in design to Naglers.
UltraScopic
eyepiece
A specific eyepiece design. UltraScopic is
a "house brand" of Orion. These eyepieces have moderate apparent fields of view, generally of 52 degrees.
UltraScopics are reported to consist of
five or seven simple lenses, grouped into a singlet and either two
or three cemented doublets.
unit-magnification
finder
A finder which
operates without magnification, as do many rifle 'scopes. Such a device is in essence a
miniature head-up display: It projects a cross-hair, illuminated bulls-eye, or similar
pattern on the sky, where the observer may view it easily with both eyes open.
Unit-magnification finders make it very easy to find objects which are bright enough to be
seen with the naked eye, or which are close enough to naked-eye stars.
Vixen
Zeiss
zero-power
finder
zoom eyepiece
Any eyepiece whose
focal length can be varied, perhaps by twisting a knurled ring on the barrel. Many do not
work well at all, though several recent models are notable exceptions to this rule.
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