Buying a Telescope
prices as at 2003:
Choosing a telescope:
- no perfect compromise to suit portability, planetary viewing &
photography as well as deep sky photography and terrestrial
viewing
- budget (<$1000):
- mainly terrestrial viewing plus moon & large planets =>
3" refractor on alt-azimuth mount
- no terrestrial or photography but maximum bang for bucks for
resolution would appear to be the 8" Dobsonian
- learn principles of astronomy plus moon & large planets =>
4-6" Newtonian on equatorial mount
- intermediate ($1500-$2500):
- planetary viewing with terrestrial capability: 5"-6"
refractor on equatorial mount with computerised motor drive
- astrophotography of nebulae, comet searching: 8" F/4
Schmidt-Newtonian on equatorial mount with computerised motor drive
- portability with good planetary viewing and deep sky:
6"-8" Schmidt-Cassegrain
- moderate high end ($2500-$4000):
- planetary viewing with terrestrial capability: 5"APO
refractor on equatorial mount with computerised motor drive
- astrophotography of nebulae, comet searching: 10" F/4
Schmidt-Newtonian on equatorial mount with computerised motor drive
- good planetary viewing and deep sky photography: 8" Schmidt-Cassegrain
with advanced mount
- high end: ($4000-$10,000):
- 11"-14" Schmidt-Cassegrain with advanced mount
- super-high end:
- 12" Schmidt camera - photographic only
- large ED APO
refractors: 6" $US7,000; 7" $US12,000; 8" $US 20,000;
10" $US 40,000
- the perfect scope:
- The "perfect scope" is inexpensive but composed of high
quality components, small but robust of construction, provides
superb image quality of great contrast and illumination, is easy to
use, transport, and setup and does not exist - anywhere.
- the closest we might come to such a scope would be a fast, seven
inch apochromatic refractor of no more than 1000mm focal length.
Such a scope would cost no more than $A13,000 to purchase and mount.
- such a scope would:
- possess 95% strehl ratio optics. (95% of a
star's light would end up in its airy disk.)
- hold stars
direct down to magnitude 13.5 at 2mm exit pupils and reveal details
on the Gas Giants to the limit of atmospheric seeing conditions
found most places on the earth - outside the grounds of the world's
great observatories.
- show the cores of
1 arc-minute sized galaxies to near magnitude 14.
- resolve globular
clusters to magnitude 9.
- elongate matched pair doubles to .5
arcseconds and 1.5 magnitude disparates to .7 arcsecs.
-
be able to achieve 2 degree "rich fields" at 30x.
- be designed to be broken down quickly and re-assemble with minimum
fuss and re-alignment.
- dew shield, optics tube, visual back and
focuser could be handled as "carry on" baggage
- the mount
needed to support it, could fit in a robust travel case of no more
than 60 inches diagonal length.
- there is no comparison between such a scope and a 4 inch F10
achromat of similar focal length. Nor is there any comparison
between such a scope and a 6", F12 MCT. Nor could a 4" APO
refractor stand much of a chance given the limitations of some 12.5
square inches of light collecting area. Meanwhile, even the finest
SCTs of twice 7 inches in aperture would fall down in such a
comparison. Why? Because of limited lunar-planetary performance due
to oft-questionable optics, challenging sky conditions, and that
large "plug" in the middle of the optical train.
What
to look for on a telescope:
- will it be suitable for your needs:
- portability eg. Maksutov, SCT
- ease of use eg. Maksutov, SCT
- price eg. Newtonian/Dobsonian is cheapest
- terrestrial viewing:
- upright image, close focus
- eg. refractor, Maksutov, SCT
- general astronomy for the beginner with limited
astrophotography (moon, sun, Jupiter, Saturn & Mars; starfields
& comets by piggyback):
- best option may be a 6-10" Newtonian (on Dobsonian mount if
cannot afford the extra $800-$1000 for a sturdy equatorial mount -
but this can be added later) eg. 8" Newtonian on Dobsonian
mount = $A800;
- this will show you 80% of what you will ever be able to see
- to get the remaining 20%, you will need to fork out lots of cash
and perhaps in this case the best option may be a Meade 10"
LX-200 SCT with equatorial wedge $A8000 which will allow excellent
astrophotography capabilities whilst being the biggest scope 1
person can transport & set up (the 8" is only $1000 less so
probably not worth it, the 12" needs 2 people whilst the
14" will need 3 people and preferably a permanent mount in dark
skies on a mountain!)
- planets and binary stars:
- needs high magnification, good contrast with quality optics with
focal length > 1000mm and aperture > 100mm
- eg. refractor, Maksutov, SCT although Newtonian > 6" may
be OK
- comet searching or deep sky viewing:
- needs large apertures to detect faint objects, galaxies &
nebulae (>8") and to resolve globular clusters (>
10")
- NB. eye cannot detect much colour in deep sky objects unless
aperture > 16" so don't expect to see them in colour without
a camera
- eg. Newtonian/Dobsonian
- astrophotography:
- needs small f ratio and sturdy equatorial mount with motor drive
eg. fast f/4 Newtonian or Schmidt-Newtonian or f/8 SCT
- definitely NOT a Dobsonian or slow f/ratio scope such as Maksutov
or slow refractor
- if you are really serious about astrophotography and wish to take
long exposures of nebulae, etc then you need a system that will
provide a stable telescope with facility for RA & Dec input from
a guiding CCD - probably the choice here is the Meade LX200 range
of SCT's (not the LX90's as these cannot be guide-corrected
automatically)
- wide field views of the Pleiades, Beehive, The
Two Great Nebulae nor The Cygnus Veil Complex:
- 3" wide field refractor or a telescope with ultra-wide
eyepiece and short focal length
- optics:
- scratches
- malalignment:
- how easy is it to re-align it (ie. collimation)
- some such as Maksutovs must be sent overseas to manufacturer, but fortunately
should be well aligned
- does it easily go out of alignment by bumping it or in travel?
- does it need collimation?
- refractors and Maskutov's should not need alignment unless
badly bumped
- all other's may need collimation after travel or being bumped
- NB. collimation is a particularly important problem for fast (low
f/ratio) reflectors as these exaggerate the effects of poor
alignment
- aperture size
- quality of optic system:
- if refractor is it achromatic & even better but unlikely,
apochromatic
- if reflector:
- what is the wave error of the mirror - ie. how accurately was
it made eg. 1/6th wave, 1/8th wave
- open mirrors usually need re-aluminising after 15yrs, closed
as in SCT should last 25-30yrs
- if hold mirror to a light globe & can see the
filaments through it, will need re-aluminising
- what quality eyepieces are supplied and are they damaged and
appropriate for system
- "wave error":
- The values above mean the measured maximum error on the
wavefront at the focus of the scope and are conventionally
measured as a fraction of a wavelength (which leaves it open to
interpretation on which wavelength to use but usually, for
visual scopes, it is intended to be green at 555 nm)
- it's generally considered that a telescope has to be at least
1/4th wave in total in order to produce acceptable quality
images. This is called "diffraction limited".
- the wave error for a telescope system is the sum of the
individual wave errors for each mirror or lens - eg. a Newtonian
with 2 mirrors each 1/10th wave results in a scope of 1/5th
wave.
- mount:
- if it has a motor driven mount:
- how accurate is it
- are there any problems with the drive - eg. worn out cogs, motor
- is it noisy
- does it slip
- is it free of vibrations
- equatorial vs alt-azimuth
- if it has a computerised motor drive:
- can the software be updated online to add positions of new comets,
etc
- can it connect to laptops & if so which port
- how accurate is it
-
Some
internet forum discussions on the pros & cons of which telescope:
- 4" achromat refractors:
- An F10 achromat represents a compromise between the unwieldy
F15 units of the past and the fast achros of the present. It is
probably the best that can be done with crown and flint glass in
the present. A fine F10 instrument makes for an acceptable
lunar-planetary-doubles scope while also supporting use as a 30x
sweeper. (This even with use of 1.25 inch eyepieces.)
- not all F10 achromats are the same. To get the most out of one
requires diffraction limited optics and careful spacing of the
doublet pair to minimize chromatic aberrations.
- as at 2002, it is common knowledge among optophiles that the
current batch of Chinese-sourced 102mm refractors distributed by
Celestron, Meade, and other resellers do not meet the low-
chromaticism, diffraction-limited requirements of a good
lunar-planetary-double star scope. Although the Chinese do a
fair job with the fabrication of their doublets, they are unable
to "fine-tune" spacing well-enough to minimize
chromatic aberration. Nor do they take great care in collimating
the optical-train. Thus the task of doublet-spacing and
collimation - especially in faster models - falls squarely into
the hands of the amateur who owns one. OR in the hands of those
resellers (such as StellarVue) which tackle the task either by
"rejecting" scopes or implementing the required
adjustments - for a price. Thus you could probably purchase a
"well-tempered" chinese-sourced F10 4 inch scope for
around $1000 new and improved. OR you could take the time to do
the work yourself!
- But Celestron did not always purchase scopes manufactured in
China for resale as the C102-HD. Before the
"Chinese-invasion", the model was manufactured by the
Vixen Optical Company of Japan. As such, it featured all-metal
construction (dew shield and objective cell included). And was
also quality tested and tuned - at the factory. A process which
adds a great deal to production cost and soon sent the price
well beyond Celestron's $1000 resell cap. (A cap which includes
CG4 mount and a number of low-cost accessories.) Thus Vixen
Optical versions of the C102-HD are no longer sold and along
with so passed the C102-HD model's reputation for "good
optics, solid mechanical fit, and attractive finish". So
today you can purchase a used Chinese-sourced C102-HD for around
$200, or a Vixen-sourced model for three times as much. But even
among Vixen Optical sourced units there is room for diversion of
quality.
- 4" f/10 refractors: Vixen achromat $A800 vs $5000 APO,
whilst the Vixen achromat gives about 80% of the view quality of
an APO
- A 4" Vixen achromat is just about optimal for Jupiter.
Optimal because such scopes incorporate less turbulence in the
image, while providing just the right balance of illumination
and contrast. Could you time transit events with it? Sure but
not as many. Would you get those once in a lifetime blow out
views when the sky is incredibly steady? Probably not. Solid
consistent performers them unobstructed diffraction-limited four
inchers...
-
- small scopes are more easily pressed to service:
- a 400mm focal length, three inch diameter rich field
achromatic refractor on altazimuth mount will be used
with far greater frequency than a 24 inch dobson mounted
newtonian. Small scopes give larger fields of view. Our 400mm
focal length scope - using an eyepiece that admits light to the
full 7mm of pupil size - can be used at 11x. At 11x, an eyepiece
stopped to 60 degrees of apparent field, can pull some 5 angular
degrees of sky into the retina at one time. Meanwhile, our 24
inch scope must bear a magnification of 87x to ensure that all
the light collected by the 24 inch primary fits through that
same 7mm eye pupil. Because of this, only 41 arc minutes or
roughly 2/3rds of one degree of sky can be seen fully
illuminated in most common eyepieces.
- As more and more of the retina receives an image, flaws in the
eye (astigmatisms) may become problematic. For some, below 10x
per inch aperture, bright stars no longer appear perfect points.
In fact they flare astigmatically while passing through focus.
Thus to see perfect stars through a 24 inch scope, some 288x is
needed. This reduces the field of view to 12.5 arc-minutes!
- a certain minimum exit pupil is needed to show stars against a
"dark" background sky. 0.5x per millimeter of aperture
is the minimum magnification needed to experience this under
average skies (where stars can be seen unaided down to magnitude
5.0 direct). Thus, our 24 inch scope must be used either under
the darkest or at 300x plus under average skies to get a sense
of skydark in the eyepiece. Meanwhile, the 3" accomplishes
this same aesthetic at a mere 40x and 6" at 70x.
- Larger apertures also collect more atmospherically aberrated
light and make this type aberrations more apparent in the
eyepiece. Generally, the 3" is able to experience
8/10 conditions where 6" experiences 7/10 skies. Thus, a
rule of thumb might be that if a 3 inch scope has 9/10 seeing
once every two weeks. A 6 inch scope may have 8/10 seeing that
often. On that same night a high quality 12 inch scope might get
7/10 stability. While a 24 inch scope would, at best, have 6/10
seeing. At such an aperture, 8/10 seeing - even if the optics
could support it - might be experienced only once or twice a
year. Is it any wonder that the world's "great
telescopes" were installed in locations where only the very
finest seeing conditions prevail?
- this is why scopists, are attracted to smaller apertures. High
quality scopes in the 3 to 6 inch aperture range can be found
relatively inexpensively and the kind of skies needed to enjoy
them come with far greater frequency.
- There is one area however, where a deepsky 3" is valued -
this lies in the realm of large open clusters, a few nearby
galaxies, and certain over large, bright and obscuration
nebulae. For here it is essential to get the "wide
field" perspective - To gather as much of the night sky in
the field of view as possible. And as we have seen, larger
apertures and especially longer focal length scopes are
disadvantaged when it comes to studies of this kind... It is in
this area that the 3" is without peer, while 6"
must go wanting. For neither perfect collimation, nor absolute
color correction is needed to enjoy the Pleiades, Beehive, The
Two Great Nebulae nor The Cygnus Veil Complex.
- 8" of aperture provides the additional light grasp needed
to enjoy every class of celestial studies. There are no
exceptions here. Face on galaxies and extended bright nebulosity
are all susceptible to a quality 8" scope. But
unfortunately, few 8" scopes possess the optical quality
needed to give the kinds of views that a 6" MCT gives of
Moon, planets and disparate double stars. (These are doubles
whose magnitudinal delta is 1.5 or greater and are found are at
or near Dawes limit of resolution.) But such scopes do exist (or
can be made possible by simply collimating the optical train to
the highest degree of perfection possible). Even though 8"
of aperture is ideal for all classes of studies, the difference
between 8" and 6" - even 10" and 6" is not
enough to justify acquiring a larger scope in this aperture
range. Rare is the night sky when 12" of aperture can be
put to full advantage on Moon and planets.
-