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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):
    • 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. collimating a telescope)
      • 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.

 

 

 

photo/telescope_buying.txt · Last modified: 2013/02/07 14:08 by gary1