photo:telescope_buying
buying a telescope
(prices as at 2003 - please note I DO NOT SELL telescopes or cameras)
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):
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:
optics:
scratches
malalignment:
-
does it easily go out of alignment by bumping it or in travel?
does it need collimation?
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
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:
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:
photo/telescope_buying.txt · Last modified: 2013/02/08 01:08 by gary1