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This section will list some extra reading material for now.
Specifications for Apochromatic Lenses
1. Optical component surface finish
Dimensions of scratches and points shall not exceed the following values:
Scratches Width, not more than 0.04mm.
Total length, mm, not more than 3mm.
Points Diameter no more than 0.4mm.
Quantity no more than 0.5 clear aperture.
Accumulation of defects diameter of a limited area, 5mm.
Total area of scratched and points no more, square mm 1
The total scratch length and the point quantity that are close on width and
diameter to the maximum permissible shall not exceed the following values:
Scratches Width, 0.02 mm.
Total length, mm, not more than 2mm.
Points Diameter, mm 0.3.
Quantity, no more than 0.2 clear aperture.
Scratches and points with the following dimensions are disregarded:
Scratch width, not more, 0.004 mm
Point diameter, not more, 0.01 mm
2. Anti-reflection coating requirements
Non-uniformity of coating interference coloration, which doesn't degrade
characteristics of coating optical parameters and doesn't affect output
characteristics of a lens, is permissible on anti-reflection surfaces of optical
elements.
3. Bubbles and inclusions requirements
Permissible dimensions of bubbles and inclusions:
In lens material bubbles and inclusions of no more than 0.3mm in size are
allowable , and a number of bubbles of diameter 0.03mm to 0.3mm in each lens
shall not exceed 3 pieces.
These do not affect optical performance in any way.
Survey of Refractive Systems for Astronomical Telescopes
A Tale of Two Astronomers
By Allister St. Claire
Once upon a time there lived two amateur astronomers, Alfred and Steve. Alfred
and Steve both entered the astronomy hobby 1 year ago. Both purchased their
first telescope from the same hobby store in their area. In fact, they purchased
the same first telescope, a 90mm f/10 achromat on a sturdy EQ mount with an RA
motor drive.
From here our budding astronomers diverge on separate courses through the hobby.
Alfred's Tale
Alfred grabs his observing book and owner's manual and is out observing the
first night. He uses the books to teach himself the names of constellations and
bright stars. The night sky transforms from a chaos of bright lights to an
ordered system of constellations and asterisms. Soon he is able to navigate the
skies like he navigates the streets of his town.
Alfred finds the telescope awkward to use at first. There are so many steps to
remember; find object in finder, find it in the eyepiece, tighten the RA access,
engage the drive and focus the eyepiece. However, as with any skill, practice
makes perfect. Soon the individual steps blur into unconscious movements of his
hands.
Alfred graduates to using a star atlas with an accompanying guide. He's now
taken to recording his evening observations in a simple log. Double stars are a
particular delight. Alfred eagerly scans each constellation on his atlas for
double star listings.
Time passes and the telescope becomes like a trusted friend. Alfred knows the
magnifications of his 2 eyepieces and barlow by heart. He's able to quickly find
any object plotted in his Atlas as long as it's within the light grasp of his
telescope. His familiarity with the telescope is now to the point where he
doesn't need a light to find the RA and DEC axis locks, motor drive engagement
clutch or centering the finder. The telescope has now become an extension of
Alfred's desire and skill to observe.
After 1 year Alfred is convinced of two things. First, astronomy is an amazing
hobby invoking feelings of awe about the universe. Second, his telescope is the
best telescope in the world. Looking back on 12 months of observing with it, the
telescope has never let him down.
Steve's Tale
Steve's been involved in a number of hobbies over the years. Like Alfred, he
makes an impulse buy. However, unlike Alfred, Steve's smart. He knows it's
important to read up on his purchase and the hobby in general. Steve logs online
and finds a telescope review site and reads the two reviews on his telescope.
With a sinking feeling Steve learns his scope will show secondary color, have an
upper magnification ceiling of 140x and is awkward to use due to it's long focal
length.
Steve spends the first month learning both the constellations and looking for
the issues with his scope that the reviews informed him about. With dismay Steve
views the secondary color on brighter objects. To determine the extent of the
secondary color, Steve views stars of dimmer magnitudes. From this he determines
where the threshold is for the secondary color. He also tests the upper
magnification of the scope and finds it falls apart at around 130x!
After the first month Steve admits he was a "sucker" on his first telescope
purchase. He now reads the telescope review site with a vengeance. He reads and
digests review after review. Steve learns optical terms, company names and
simple methods for testing the optics of a telescope.
Another month passes. Steve orders his second telescope, a 4" apochromatic
refractor on an alt-az mount. Ahhh - this will be the perfect telescope.
The new Apo arrives. A proud Steve is out observing with it the first evening.
He hasn't learned the constellations or bright stars yet as he was field testing
the sucker telescope for his first month in the hobby. No matter, he will learn
them now.
He will as soon as he completes star testing his new APO. Steve's "smart" and
doesn't just drop $3000 on a new telescope without testing the optics. His first
night out and Steve realizes that to star test his new APO he will needed a
driven mount so he can compare the star images with those in his new star
testing book.
Back online and a week of reading mount reviews. Steve learns all about drive
error, PEC, dual axis drives and other telescope mount terms. An order is placed
and several days later his new mount arrives. Out comes his APO for it's second
night in the backyard (Steve's been busy reading mount reviews) and Steve
realizes he will need quality eyepieces for the high magnifications needed for
star testing.
Back online and 2 weeks of reading eyepiece reviews ......
Time passes and Steve's read reviews stating 4" of telescope aperture is too
small for "serious" observing. This kicks off not only another round of reading
reviews, but now he's intensely engaged in posting on the astronomy forums. Boy,
he won't make the mistake of buying a telescope that's too small. Steve's smart.
Over the next year Steve buys, tests and sells an 8" sct, 10" dob, 18" dob, 12"
GOTO SCT and a 6" APO. Given that the price commitment for each scope is
increasing, Steve reads and researches his potential purchases with greater and
greater thoroughness. Test sessions with each telescope take on the atmosphere
of a tax audit. With so much money riding on each purchase, Steve must be
certain the telescope is without flaws.
After 1 year, Steve is convinced of two things. First, astronomy is an expensive
hobby. Second, unless you are "smart", you will be suckered into owning and
using some 90mm beginner's scope.
Email Author
Star Testing Astronomical Telescopes
A Manual for Optical Evaluation and Adjustment
By Harold Richard Suiter *
DEFINITIONS
Highest Useful Magnification
This is the highest visual power a telescope can achieve
before the image becomes too dim for useful observing (generally at about 50x to
60x per inch of telescope aperture). However, this power is very often
unreachable due to turbulence in our atmosphere that makes the image too blurry
and unstable to see any detail.
On nights of less-than-perfect seeing, medium to low power planetary, binary
star, and globular cluster observing (at 25x to 30x per inch of aperture or
less) is usually more enjoyable than fruitlessly attempting to push a
telescope's magnification to its theoretical limits. Very high powers are
generally best reserved for planetary observations and binary star splitting.
Small aperture telescopes can usually use more power per inch of aperture on any
given night than larger telescopes, as they look through a smaller column of air
and see less of the turbulence in our atmosphere. While some observers use up to
100x per inch of refractor aperture on Mars and Jupiter, the actual number of
minutes they spend observing at such powers is small in relation to the number
of hours they spend waiting for the atmosphere to stabilize enough for them to
use such very high powers.
Therefore...... any time you see brand new $200 scopes advertising capabilities of 525x and the like, know full well that it is the mathematical capability of the combined pieces, not what will happen in the real world.
Limiting Magnitude This is the magnitude (or brightness) of the faintest star that
can be seen with a telescope. The larger the number, the fainter the star that
can be seen.
An approximate formula for determining the visual limiting magnitude of a
telescope is 7.5 + 5 log aperture (in cm).
Keep in mind that this formula does not take into account light loss within the
scope, seeing conditions, the observer’s age (visual performance decreases as we
get older), the telescope’s age (the reflectivity of telescope mirrors decreases
as they get older), etc. The limiting magnitudes specified by manufacturers for
their telescopes assume very dark skies, trained observers, and excellent
atmospheric transparency – and are therefore rarely obtainable under average
observing conditions. The photographic limiting magnitude is always greater than
the visual (typically by two magnitudes).
Focal Length This is the length of the effective optical path of a telescope
or eyepiece (the distance from the main mirror or lens where the light is
gathered to the point where the prime focus image is formed). Focal length is
typically expressed in millimeters.
The longer the focal length, the higher the magnification and the narrower the
field of view with any given eyepiece. The shorter the focal length, the lower
the magnification and the wider the field of view with the same eyepiece.
Focal Ratio This is the "speed" of a telescope’s optics, found by dividing
the focal length by the aperture. The smaller the f/number, the lower the
magnification, the wider the field, and the brighter the image with any given
eyepiece or camera.
Fast f/4 to f/5 focal ratios are generally best for lower power wide field
observing and deep space photography. Slow f/11 to f/15 focal ratios are usually
better suited to higher power lunar, planetary, and binary star observing and
high power photography. Medium f/6 to f/10 focal ratios work well with either.
An f/5 system can photograph a nebula or other faint
extended deep space object in one-fourth the time of an f/10 system, but the
image will be only one-half as large. Point sources, such as stars, are recorded
based on the aperture, however, rather than the focal ratio – so that the larger
the aperture, the fainter the star you can see or photograph, no matter what the
focal ratio.
Resolution
This is the ability of a telescope to separate closely-spaced binary stars into
two distinct objects, measured in seconds of arc. One arc second equals 1/3600th
of a degree and is about the width of a 25-cent coin at a distance of three
miles!
In essence, resolution is a measure of how much detail a telescope can reveal.
The resolution values on our website are derived using the Dawes’ limit formula.
Dawes’ limit only applies to point sources of light (stars). Smaller separations
can be resolved in extended objects, such as the planets. For example, Cassini’s
Division in the rings of Saturn (0.5 arc seconds across), was discovered using a
2.5” telescope – which has a Dawes’ limit of 1.8 arc seconds!
The ability of a telescope to resolve to Dawes’ limit is usually much more
affected by seeing conditions, by the difference in brightness between the
binary star components, and by the observer’s visual acuity, than it is by the
optical quality of the telescope.
Aperture This is the diameter of the light-gathering main mirror or
objective lens of a telescope.
In general, the larger the aperture, the better the resolution and the fainter
the objects you can see.
A Better Method of Measuring Optical Performance
Astronomy Technologies Astro-Tech Ritchey-Chrétien Optics
Optical features of this Astrograph . . .
Ritchey-Chrétien Optical Design: The Astro-Tech
astrograph is a true Ritchey-Chrétien (RC) reflector optical system. Unlike a
Maksutov-Cassegrain or Schmidt-Cassegrain catadioptric scope (that uses simple
spherical mirrors and corrector lenses), or Newtonian reflectors (that use a
coma-producing parabolic primary mirror), the Astro-Tech RC is a Cassegrain-type
two-mirror optical system that uses a concave hyperbolic primary and a convex
hyperbolic secondary mirror to form its images. These sophisticated and
difficult-to-make mirrors combine to produce images at the Cassegrain focus at
the rear of this Astro-Tech scope that are free from coma and spherical
aberration, with a smaller spot size, over a much wider field than conventional
Newtonians or catadioptrics. The images are likewise free from the chromatic
aberration found in refractors and some catadioptrics. Because of this wide
coma-free field, small spot size, and relatively fast focal ratio, the
Ritchey-Chrétien design is particularly well suited to wide field
astrophotography, rather than visual observing. For imaging, the RC is the
optical system of choice for most of the major professional observatory imaging
telescopes built in the last half-century. For example, the Hubble Space
Telescope, the twin 10-meter Keck telescopes in Hawaii, and the four 8.2 meter
telescopes of the Very Large Telescope array in Chile are all Ritchey-Chrétiens.
For serious amateur astrophotographers without NASA’s optical budget, an
Astro-Tech RC is very likely the imaging system of choice.
Fully Multicoated Quartz and BK7 Mirrors: The primary mirror of the 6”
Astro-Tech is first-quality BK7 optical glass, while the 8” and larger
Astro-Tech RCs use primary mirrors of low thermal expansion quartz for maximum
focus stability during long exposure imaging sessions. Both 6” RC mirrors are
vacuum-coated with enhanced aluminum for high reflectivity and over coated with
a durable layer of silicon monoxide (quartz) for long life. The 8” and larger
mirrors are dielectric coated for long life and reflectivity approaching 99%+.
Computer Designed and Fabricated Optics: To keep the cost of each
Astro-Tech RC so reasonable when compared to competitive RC scopes, the
computer-optimized Astro-Tech hyperboloid mirrors are automatically ground and
finished to very high tolerances using custom-made computerized mirror grinding
machines. This precision computer control guarantees an exact repeatability of
figure from mirror to mirror that is difficult to achieve using more costly
conventional hand figuring. After grinding and polishing, each mirror is
individually tested using a Zygo interferometer to assure that it meets or
exceeds its specified surface accuracy.
Frill-Free Design: To further keep its cost reasonable, an Astro-Tech RC
does away with most of the bells and whistles found on competitive scopes that
add little to their performance (but much to their cost). For example,
Astro-Tech front and rear cells are first die-cast, then CNC machine-finished,
rather than completely CNC machined from raw stock at considerably greater
expense but no significant improvement in performance as is the case with other
RCs. Glare stops in the optical tube are a molded insert, rather than machined
aluminum, resulting in a significant savings in cost at no appreciable
difference in performance. The Astro-Tech scopes use an external manual
dual-speed Crayford focuser, rather than the considerably more complicated and
much more costly motorized movable secondary mirror system that other
manufacturers use for focusing. The result of the Astro-Tech no-frills approach
is genuine Ritchey-Chrétien wide-field performance at a fraction the cost of
other commercial RC systems. While the mechanical bells and whistles may be
limited in an Astro-Tech RC, an Astro-Tech scope still has the high precision
flat field/coma-free true Ritchey-Chrétien optics that are the most important
reason for buying an RC scope.
Mechanical Features Of This Telescope’s Optical System . . .
Fixed Primary Mirror with Computer Optimized Primary and Secondary Baffling:
Unlike traditional Cassegrain designs that move the primary mirror fore and aft
along the central baffle tube in order to achieve focus (which can lead to image
shift as the mirror position is adjusted) each Astro-Tech RC primary mirror is
fixed. The Astro-Tech is focused externally by means of a dual-speed 2”
Crayford-style focuser on the rear cell, thereby eliminating a Cassegrain’s
moving mirror image shift during focusing. Molded field stops are installed
along the interior of the optical tube to effectively prevent stray off-axis
light from reaching the image plane, resulting in improved contrast. In addition
multiple glare-stop microbaffles on the inner surfaces of the primary mirror
baffle tube and the secondary mirror light shield further prevent off-axis light
from reaching the image plane, resulting in still further improved contrast.
Collimatable Secondary Mirror: Since the primary mirror of an Astro-Tech
RC is fixed in position, only the secondary mirror can (or needs to) be
collimated. This makes it easy to keep the Astro-Tech RC optics aligned for peak
performance. Collimation adjustments to the secondary mirror are made by
adjusting the three collimating screws in the back of the secondary mirror
holder.
Cooling Fan: The open tube RC design allows for fast cool-down of the
primary and secondary mirrors. Built-in fans on the rear cell of the 10” and
larger scopes increases the air-flow around the optics to achieve still quicker
“cool down” times of the larger primary mirrors. The 6” and 8” scopes do not
have primary mirror cooling fans, as their mirrors are small enough to cool down
quickly without any external aid.
Last modified: 06/26/09 |
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