Telescope aperture: How Much Do You Need and How Much Can You Stand?

By Chuck Hawks

At a recent staff star party, Astronomy and Photography Online Editor Gordon Landers mentioned to me that he thought 4" of clear aperture represented a sort of "breaking point" among good quality astronomical telescopes. (Without excellent optics, of course, nothing else matters.) What he meant was telescopes with four or more inches of clear aperture significantly outperformed smaller telescopes and represented the threshold for what we might call "serious" amateur telescopes. I was inclined to agree with Gordon about 4" marking a significant plateau in clear aperture and asked Chief Technical Advisor Jim Fleck his opinion on the subject. Jim strongly agreed with Gordon that scopes below 4" are just too small and added that six inches of clear aperture was a significant plateau for deep sky observing. Thus, we all set the minimum acceptable clear aperture at 4".

This sentiment is reflected in our giant "Astronomical Telescope Comparison" article, in which we gave 4"-5" scopes a one point bonus (scopes below 4" received no bonus points), 6"-7" scopes a two point bonus and 8"-9.25" scopes a three point aperture bonus in our comparison rating matrix. Of course, our comparison matrix is subjective, based on our personal experience and preferences. Note that this "aperture bonus" is irrespective of telescope design (refractor, reflector or catadioptric).

In the real world, ED doublet and triplet (APO) refractors generally deliver more information to the eye and therefore an APO refractor can have a somewhat smaller clear aperture than a reflector (including catadioptrics) and still offer comparable performance. According to the results published in the article, "Aperture Comparison" by Jared Willson on Cloudy Nights, which compared 41 reasonably common astronomical telescopes, a 70mm APO refractor should provide similar low contrast resolution to a 90mm Questar 3.5 Mak-Cass catadioptric, which is about as good as mirror scopes get. Several 4" APO or ED refractors substantially out performed a Celestron C5 (5" CAT) and 6" APO refractors out performed the Celestron C6 XLT, a very good Schmidt-Cassegrain (SCT) catadioptric. A TEC 140 (5.5" APO refractor) achieved a low contrast number nearly identical to the very sophisticated Vixen VC200L 8" catadioptric.

Judging by their low contrast resolution numbers, the refractor's advantage probably averages between 30% and 40% over reflectors of various types, depending on the subject, viewing conditions and the individual telescopes compared. To generalize, a mirror based telescope needs to have about 1/3 more clear aperture than an APO refractor to deliver comparable visual performance. That figure correlates well with my personal experience, as I find the overall performance of my 4.5" Stellarvue refractor about equal to a good 6" SCT or Newtonian and Gordon's 6" Stellarvue about equal to a good 8" CAT. Of course, APO refractors are the most expensive telescopes for any given aperture, especially above 4" diameter, and reflectors 1/3 larger than an APO refractor are typically much less expensive to purchase. (As I write these words, an SV115T costs about eight times the price of a C6 XLT.)

Of course, increased aperture with the same focal ratio (f/number) necessarily means increased physical size and for a telescope that is supposed to be portable, that is a bad thing. A larger telescope requires a bigger mount and that means increased set-up time in the field, another negative factor. There is a practical limit to portable telescope size and weight and refractors usually reach that limit before Newtonian reflectors, which are in turn typically larger in size and weight than catadioptric telescopes of the same aperture. To reduce their physical length, refractors and Newtonians are typically built with considerably shorter focal lengths than CAT's, yet they are still physically longer.

Speaking for myself, a 4.5" refractor, 6" Newtonian reflector or 8" CAT represent the largest portable telescopes I am willing to drag into the field. I have owned all three types, in the form of a Stellarvue SV115T (f/7.0) refractor, Celestron SP-C6 (f/5.0) Newtonian and Celestron C8 (f/10) SCT. They all require about the same size mounting system for visual astronomy, typically a German equatorial in the Celestron CG5 size range. The 8" CAT has the fattest main tube, but is also the shortest and lightest, despite its significantly longer focal length. This explains the popularity of Celestron, Meade and Vixen CAT's. Substantially larger and heavier telescopes, such as a 6" refractor, 8" Newtonian or 11" CAT, are monsters well beyond my inclination to set up in the field. The old generalization is true: telescopes that are easy to use get used!

Even the very compact Schmidt-Cassegrain telescopes rapidly become difficult to manage as the clear aperture exceeds 8". Some amateur astronomers find 9.25" SCT's and their attendant mounts sufficiently portable and some do not. Most amateurs will draw the line somewhere at or below 9.25" for an SCT and smaller for other designs.

An argument for portability can be made for larger, truss type Newtonians on Dobsonian mounts, since when disassembled each piece probably weighs no more than an 8" CAT or its equatorial mount head. However, truss type scopes require extensive assembly and collimation in the field before each use and once in operation the simple Dobsonian mount lacks slow motion controls to compensate for the earth's rotation or accurately aim the telescope. The big telescope must be horsed around the night sky, a generally unsatisfactory arrangement. If you want to spend your time in the field looking through your telescope, rather than working on it, a Newt/Dob is not for you.

To summarize, for the intermediate or advanced amateur astronomer who uses his or her telescope in the field, 4" of clear aperture probably marks the practical minimum. 8" to 9.25" is the practical maximum aperture in the case of a CAT and, due to physical size constraints, less for other designs. (A typical 6" Newtonian or 4.5" refractor are longer and every bit as heavy as an 8" CAT.) If your sights are set on an APO refractor, you can probably get by with less aperture than if you favor a reflector, but you will pay considerably more than if you bought a reflector of comparable optical performance.

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Copyright 2011 by Chuck Hawks. All rights reserved.