Aperture Fever

Aperture is (not) Everything

“Everybody knows” that the most important thing about your telescope is to get the biggest aperture possible. Read any Internet discussion group and they’ll tell you that.

I disagree.

That’s certainly true if everything else is the same; but everything else is rarely the same. In particular, what a beginner to astronomy needs more than anything else is experience using their telescope. A huge telescope that doesn’t get used enough to build up experience is a poor choice.

Aperture is important, but it upsets me when I see beginners being given extremist advice as though there were no options and as though it applied to everyone regardless of circumstances. In this article I try to give the beginner some additional tools to use to think through their first purchase.

Typical Internet Advice

I was prompted to write this little essay by several responses in recent beginner posts in Astronomy discussion groups. I’ve edited the text but did not make up these examples.

Example: A Good Answer

Good answer! We told the beginner “it depends”, encouraged them to consider their situation, and gave them good classic advice with an explanation of why that would be a good choice.

Example: Stupid Answers

What They Say

If we summarize responses to beginner questions, we see generally:

• “Buy the largest telescope you can afford”; or
• “Buy the largest telescope you can transport and use” .

What They Mean

Taken in context, this is not bad advice. Especially that second point, and especially if we take it to mean the entire system: optics, mount, eyepieces, etc.

When we say “large” for a telescope we are referring to aperture — the diameter of the optical tube and the main lens or mirror. All else being the same, a larger aperture is better for two reasons:

1. Light Gathering

Many galaxies and nebulae are quite large, they are just too dim to see. When you are observing such objects, your telescope’s job is not to make them bigger but to make them brighter.

The amount by which your telescope makes objects brighter is a function of the area of the main lens or mirror.
So, a small increase in the diameter of your lens or mirror will result in a large increase in your telescope’s light-gathering ability. Here are some examples, where the “area” figure is a measure of light-gathering ability:

So you can see that, for light-gathering purposes, a 200 mm8-inch telescope is not twice as good as a 100 mm4-inch telescope — it is about four times as good. To get twice as good as a 100 mm telescope you need only move up to a 150 mm6-inch.

2. Resolution

If you are interested in looking at the Moon and bright planets like Jupiter and Saturn, light-gathering ability isn’t very important because those objects are already very bright. Instead, magnification is important, since you may want to “zoom in” on lunar details, and since planets like Saturn and Jupiter appear very small in the sky. To have a pleasing view of Saturn, for example, you need to use a magnification of 150x or more, with 250x giving great views.

See here for an article on how magnification is calculated. For now, it’s enough to know that the maximum magnification your scope can provide without the view losing detail (“going all fuzzy”) is a function of aperture.

A general rule of thumb is that your scope’s maximum magnification will be about 2x per millimetre50x per inch of aperture in very good observing conditions. So a 100 mm4 inch scope would be limited to about 200x magnification, and a 150 mm6 inch to about 300x.

Does this mean that a 360 mm14 inch scope can magnify 700 times? No, not in any practical sense. Other factors like our atmosphere limit the practical magnification of any amateur telescope not located on a mountain top to about 400x, and it’s rare to be able to use that much.

Not Every Factor is Important for Everyone

Those sound like good reasons to prefer a larger diameter. However, as a beginner, not all of those factors might be important to you.

Light-gathering ability is very important for looking at dim deep space objects (DSOs) like galaxies. But it’s not important for looking at the moon and bright planets – they are so bright that even a small telescope will gather enough light. If you are interested only in these, a smaller telescope will be adequate.

Young children, for example, may not find looking at dim DSOs interesting – even in large telescopes they are quite faint, and nothing like the photos they have seen in magazines. The Moon, Jupiter, and Saturn, on the other hand, are fascinating for children. So a smaller telescope that they can handle themselves might be a better choice. On the other hand, you should consider that, if they become truly fascinated by astronomy, they may eventually outgrow the small ‘scope and want something larger.

Magnification is important if you are interested in observing planets and, as you can see above, a larger scope can support higher magnification, up to a point. A 150 mm6 inch scope, for example, will support significantly more magnification than a 80 mm3 inch scope, all else being the same.

However, there are diminishing returns. Atmospheric conditions limit available magnification to about 400x even under perfect conditions, so going beyond about 200 mm8 inch in aperture for planetary viewing may not be worth the substantial additional investment and size. Also, the higher the magnification, the faster a planet will drift out of your field of view, requiring frequent adjustment or investment in a motor drive.

A larger scope would, of course, be more versatile for DSOs even if it was beyond the point of diminishing returns for planetary use.

There are Costs to Large Aperture

If larger apertures were as inexpensive and easy to use as smaller apertures, there would be no question that bigger is better. However, this isn’t the case. Larger apertures come with costs, including:

• Your telescope is a system. It’s important to invest in optical tube, eyepieces, and mount in appropriate balance. A superb optical tube on a poor mount is a poor telescope. Ensure you don’t spend so much of your budget on aperture that you end up with poor-quality components for the rest of the system. (This is one reason Dobsonians are so popular for large scopes – the mount is inexpensive yet very stable.)
• Obviously you need a scope that will fit in your storage and observing locations. (I’ve read reports from beginners trying to use big scopes on an apartment balcony where cramped quarters limit them to pointing in only one direction; then complaining that there is nothing interesting to look at in that direction.)
• You also need to be able to transport your scope, from storage to observing location and possibly to dark sites. Ensure you can handle the largest and heaviest pieces by yourself, and that everything you need will fit in your vehicle.
• As a beginner you need practice more than anything else. Get a scope you can set up and be using within 20 minutes so you can use those 1-hour opportunities. With practice, nearly any scope can be set up quickly, but a beginner will find simpler setups less frustrating.

So I Should. . . ?

The above points are not an argument against buying a large aperture scope, but there are additional factors you should consider in this important decision. You should:

• Balance your scope budget for scope, mount, eyepieces, maps, etc.
• Start with simple bright objects that don’t require huge aperture, to gain experience with the sky and with operating a scope.
• Within your scope budget, buy the largest scope that you can store, transport, set up quickly, and operate, using your situation, not someone else’s.
• Seek a scope that you can use frequently, when you have a spare hour.
• Resist peer pressure.
• Plan to trade up, or add scopes of different sizes & configurations, as you gain experience.
• Ensure that this applies to the intended operator if that’s not you.