How Large a Telescope?

Jack Kramer

A primary issue for anyone considering a new telescope boils down to the question "How large a telescope should I get?" Cost considerations aside, there just aren't any simple answers. Each different telescope design has unique assets and drawbacks. Some of these are related to their size. So here are a few thoughts on two key factors that relate to you yourself:

  • the site from which you will be doing most of your observing, and
  • how easy the telescope will be for you to use.

First, a bit of history: Charles Messier, the comet hunter and chronicler of deep sky objects, used a variety of instruments, but his favorite was said to be a refractor of about 31/2-inches diameter. In his day, the optics were not as good as those in present scopes. If you've ever encountered difficulties in your pursuit of the Messier list of deep sky objects, you may have been amazed how he was able to locate them all in the first place with such a relatively small and optically-inferior telescope. Of course, back in the 18th Century he didn't have to contend with the light pollution that we experience today. Skyglow makes or breaks a site.

The critical nature of your observing site is shown by the following experience. On a trip to New Mexico, I was checking out a few objects with my 4-inch refractor. This scope doesn't have enough light gathering to dramatically show off deep sky objects, but I had brought it for other purposes and wanted to see what it could do in a dark sky. From my backyard, it shows the globular cluster M13 as a circular fuzzball with some stars at the edges just resolved. To my amazement, in New Mexico the refractor showed M13 well resolved right to the core. And from down there, the Whirlpool Galaxy (M51) was visible with faint traces of the spiral arms instead of only the two fuzzy hubs that I was used to seeing in that scope from around home.

The lesson is clear: a telescope of any size will perform better in a dark sky, the farther away from a metropolitan area the better. A good 41/2-inch or 6-inch reflector will do a fine job on the planets, and if you can get away to a really dark site, then galaxies, star clusters, and nebulae will show up surprisingly well. And still larger telescopes in a dark sky give truly awesome views.

However, larger optics really need a dark sky. If you won't be getting out to darker observing sites, consider that light pollution has an especially adverse effect on light buckets. That large telescope that so beautifully shows off deep sky objects in a dark sky will also do a great job of gathering the glare from light pollution. This means that while a large scope will give some fine views from your backyard, you may not see much more than you would have seen with a somewhat smaller instrument. (The moon and planets, of course, are not as affected by light pollution.)

In addition, certain sites are naturally better for observing by virtue of local atmospheric conditions, which have a greater effect on large telescopes. One of the advantages of placing an observatory at high altitude is that you're looking through a reduced amount of air, plus the air is often drier. For someone living in the desert Southwest or in the mountains, it would be easier to decide in favor of a large scope.

The effects of atmospheric conditions was reinforced with a visit to the web page of an astrophotographer in a dark sky area of Upstate New York who has a roll-off roof observatory equipped with 16-inch f/5 and 10-inch f/6 reflectors, and a 6.1-inch Astrophysics refractor. One of his comments was that the 10-inch often out-performs his larger scope. I suspect that if he were in a drier climate and/or at a higher altitude, the 16-inch would almost always give the better view. This was underscored by a picture of his observatory on the web page. The photo was rendered more dramatic by the sun rising through a layer of morning fog. This web site URL is:

http://www.frontiernet.net/~gregoryt/index2.htm

It's a given that in a dark sky the larger the telescope the more you will see. Then that should inspire more enthusiasm for observing, right? Perhaps. This initial enthusiasm may be short-lived, however, if using the telescope proves to be too daunting, especially in the case of someone who is fairly new to astronomical observation. Experienced observers usually know enough to make informed choices as to the size of their next telescope. But how does the beginner know that he or she is sufficiently motivated to warrant the investment in a large telescope? The same thing applies to high-tech scopes, such as Schmidt-Cassegrains equipped with sophisticated drive systems. As a somewhat extreme corollary, a person who decides to take up the violin doesn't spend thousands of dollars on a Stradivarius right off the bat. Yet I suspect that a similar situation applies in many cases where there's an ad placed by someone selling a large, well-equipped telescope advertised as "seldom used".

For beginning observers, the first task is to learn about the sky. If they also have to do battle with an awkward or complex instrument, then there's an added element to be overcome. Even the simple Dobsonian may prove unwieldy in the larger sizes. Novice observers who immediately go for the big time often come to realize that learning the sky would have been a lot easier with a more modest instrument. You then have the makings of a perfectly good, but underutilized scope that gathers dust instead of starlight. There's an adage that says, "The frequency of observation is inversely proportional to the size of the telescope". I don't know who coined that, but it still rings true.

When it comes to transporting, setting up and taking down your telescope, you are the only one who can decide how much time and effort you're willing to expend. To some, a six-inch Dobsonian is as much scope as they care to haul around. Others think nothing of packing out an 18-inch light bucket. The Dobsonian mounting is a great step forward in that its light weight and manageable size has eliminated what was formerly the most awkward part of a large telescope. Note that any size Schmidt-Cassegrain usually takes longer to set up than even a 121/2 or 13-inch Dobsonian. Of course, there is still the telescope tube itself, which may be quite heavy. Scopes larger than about 13-inches generally have truss tube structures that are disassembled into a number of parts. But then that involves assembly and disassembly at the observing site. A nice feature of the older Orion catalog was that it indicated how much effort is required to set up each of their larger-sized telescopes. Can you do it by yourself or should you have an extra pair of hands? When someone says a large telescope is "portable", that may simply mean it's smaller than the average refrigerator!

For many people it makes very good sense to own more than one scope. (That just might require a really good sales pitch to your spouse!) Experienced observers who have a large instrument quite often own a second one - a smaller, easy-to-use telescope for their backyards or when they don't feel like hauling out the behemoth or when observing conditions are not the best. In some cases, they've hung onto their "starter scopes" long after they've graduated to larger instruments.

This leads to a final thought. It may seem reasonable to regard a large or high-tech scope as an investment for the future, for eventual trips to the boondocks or a venture some day into CCD imaging. That sounds like a good, forward-thinking plan. But how far ahead are you planning? For the present, you just might be depriving yourself of the real enjoyment of observing. Ultimately, you could lose interest before the future ever gets here.

You've probably concluded that I have a bias against large or high-tech telescopes. Actually, that's not the case at all. Such telescopes entice all of us, seasoned observer and beginner alike. They've made it possible for amateur astronomers to enjoy an expanded view of the universe on a scale unimagined only a generation ago. If you've ever viewed through one in a really dark sky, then you don't need to be convinced about the capabilities of a large telescope. But as you ponder the alternatives, it's important to bear in mind that these telescopes do require a higher level of commitment on your part. Sadly, I've seen several large telescopes put up for sale because their owners couldn't maintain that commitment. The decision as to telescope size depends not just on what you can afford, but on what you will actually do with that scope once you have it in hand.

Will you eagerly look forward to each opportunity to get out under the stars? Or will the telescope itself be a disincentive? In the final analysis, the best telescope is the one you will use most often. That depends on you.

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Say there...do you have a telescope? Would you care to share some information about it? A frequent topic of conversation at our club meetings is the different types of telescopes and the different brands, and what is a good scope for certain uses. As a follow-up to the above article, it would be helpful to many of our members if you could let us know about your personal experiences with certain telescopes by writing a review for the newsletter. First-hand info is far more valuable than the advertising hype seen in the astronomy press. If you'd like some help putting the article together, let me know.

Published in the February 2000 issue of the NightTimes