Telescope Aperture Considerations

Often in astronomy we hear the expression that "There is no substitute for aperture." I use to believe that the only way to see more detail was to get a larger aperture telescope as well, but over time have come to realize that there are ways to see more with telescopes that we already own. Much of it I believe comes from training our eyes to see more detail, and by being a patient and persistent observer, regardless of the aperture of the telescope.

When I first got involved in astronomy, I didn't understand the importance of training the eye to see more detail, or the importance of being a patient and persistent observer. Therefore, I couldn't understand how some long time observers such as Walter Scott Houston, the author for many years of The Deep-Sky Wonders column in Sky and Telescope Magazine, and John Mallas, who co-wrote and provided the drawings used in the book The Messier Album, An Observer's Handbook could use a 4" aperture telescope for most of their observing. Another more recent example of someone who uses a 4" aperture telescope for much of their observing is Stephen James O'Meara, who wrote the book The Messier Objects , and produced some very fine drawings using it. Slowly, over time, I began to realize that there are a number of important factors in how well a telescope performs, and how much can be seen with it, besides its aperture. This includes optical quality, mechanical quality, portability and ease of use, and probably most importantly of all, the person using the telescope.

Optical Quality

Optical quality is important because it impacts what you see at the eyepiece. If you have two telescopes, a larger one that has only fair optics at best, and a smaller one that has excellent optics, the larger one may show a brighter image, but not necessarily more detail. Case in point was Henry Kater's observation of the Encke Minima and Encke Division back in the 1820's. He could clearly see this detail in his smaller 6-1/4" reflector, but not at all, or not as well, in his larger 6-3/4" reflector.

I have found the same thing in telescopes I have owned, or used at star parties. Telescopes with very good to excellent optics on nights of good seeing and transparency can indeed outperform smaller well made ones with the amount of detail they show. A 20" reflector I owned had such optics, and the views I had with that telescope stay with me to this day. On nights of steady seeing and good transparency M42 was a mass of red, blue, green, and purple colors. The detail I was seeing in the M31 reminded me of a long-exposure photograph I had taken of the Andromeda Galaxy a number of years before using a C8 at f/5.8 using hypered film. With an OIII filter portions of the Veil Nebula look like a twisted rope.

I had similar experiences with telescopes with very good to excellent optics at star parties where I had a chance to use telescopes with apertures up to 32". Other telescopes that did not have very good optics would show a brighter image than a smaller well made ones but often would not show more detail. Some times the telescope had good optics but the mechanical quality wasn't very good. The mount was too shaky to get much of a view, or the focuser was too loose or too stiff so it was difficult to obtain sharp focus. On nights of falling temperatures sometimes larger aperture telescopes would never quite settle down and produce good views of the planets.

Portability and Ease of Use

Portability and ease of use are important as well in any telescope. A friend of mine owns a 10" SCT. For him it is a big telescope, and a hassle to set up, so although he used it a lot when he first got it he began to use it less and less as time went on. It got to the point that he only used it a few times a year. I suggested he purchase a small 3.5" aperture high quality telescope. With its portability and high quality optics he observes very often and is really enjoying himself. If there is no substitute for aperture, then he should always see more with the 10". However, since he only observes with the 10" a few times a year, and observes with the 3.5" telescope often, he actually sees more with his smaller telescope because he is outside using it. Which is really the important thing in astronomy - to be outside using what we have and enjoying it. The old saying that the best telescope for you is the one you use most often is very true.

The Observer

What is more important than aperture, or type of telescope, is the person using it, how well trained their eye is, and how much of a patient and persistent observer they are.

As an example, William Dawes is given credit for co-discovering along with William C. Bond and George P. Bond Saturn's Crepe Ring in 1850. Yet Dawes was using a telescope of 6-1/3" while they were using a 15". Dawes was reluctant to mention his observation to his friend William Lassell because Lassell had not seen it in his 24" aperture telescope. People say, well of course, he was "eagle-eye" Dawes. But he became this way by carefully training his eyes.

Stephen James O'Meara's has produced some remarkable drawings of the Messier objects using a 4" aperture telescope. True, he has an excellent observing site, and is considered one of the world's best visual observers. However, he would spend hours on each drawing carefully recording what he was seeing. The more he observed and sketched, the more he trained his eyes to see.

In the Fall 2001 issue of Amateur Astronomy Tom Clark, who made large aperture telescopes for a number of years, has an article on an 8" aperture telescope in which he writes:

"According to one friend of mine, 6" and 8" scopes are good only for using as ashtrays - but that is certainly not my opinion - at least not anymore. I have to admit that at one time I was an aperture bigot. Starting out with a 13" as our first scope, followed quickly by a 17", my untrained eyes could not see very well, and aperture seemed to be the only way to increase what you could see. I had a pretty low impression of small optics, but I had only looked through them a couple of times, for just a two-second glance. Looking back, that now seems rather dumb. I guess we all have been guilty of being short-sighted at times... A 6" telescope will show a lot of detail in galaxies and nebulae under good dark skies."

Does this mean that people shouldn't use large aperture telescopes? Of course not, but what they can learn to do is train their eyes to see all of the detail that they can see with the telescope that they already own. There is so much that can be seen with even modest sized instruments. I didn't understand this at first. When I first got involved in astronomy, I went through a pair of 2.4" achromatic refractors, a 4-1/4" and 6" reflectors in less then 5 years. All I could think was, "Gee, M42 looks great in this aperture, it must look even better in a bigger aperture telescope!" So I kept selling the telescope I owned to buy a bigger one.

Then an older friend of mine had an 8" SCT that he was thinking of selling. I helped him from time to time with an astronomy class he was teaching at a nearby high school. His 8" telescope was a full 2" larger than the 6" telescope that I owned at the time. Would he sell it to me, I asked? No, he said, because you haven't seen all that you could with the telescope you already own was his answer. Looking back many years later, I understood what he meant. If I didn't appreciate the telescope equipment that I owned, and didn't get the most out of it, what good would it be to buy a larger aperture telescope?

These days my main telescopes are 4.1", 5.1" and 7" aperture. For many observers these may sound on the small side, and wonder how can I see anything with them. My answer is there is plenty to see, because I observe as often as I can and train my eye to see as much as I can. Sketching certainly helps to train the eye. Also, I appreciate what my telescopes can show me. When I go out to observe, I don't worry about what I may or may not be able to see with them. I simply go out and see as much as I can. I realize also that I will never be able to see all of the objects visible in these 4.1" - 7" aperture telescopes. Or as Walter Scott Houston put it:

"It's difficult to imagine anyone viewing every object within reach of a 6-inch telescope, not to mention a 12-inch".

Anyone can do this. If you want to see more detail, it doesn't mean you have to run out and buy a larger aperture telescope. Instead, with your current telescope equipment, take the time to study each object that you observe. Try different eyepiece magnifications, and filters, to bring out detail you may not have noticed before. It is easy to locate M13, look at it briefly, move on quickly to M57, then to M27, M11, and so on. By the end of the night, a person may have seen a bunch of deep-sky objects, but have they really observed them? Can they recall in their mind's eye what these objects look like? The ghostly star chains of M13, or the softly mottled green appearance of the hour glass shape M27, and the "ears" on each side of it that give it a football shape? Train your eye to see more detail. Take notes, or make sketches. Even simple ones will help you see more detail.

The universe is a wondrous place. We can enjoy it more simply by appreciating what we already own and training our eyes to see more.

Article 2000 - 2014, Eric Jamison, All rights reserved.


Home