This image was taken with my 6 inch f/5 Newtonian and a modified Quickcam Express webcam. Check out the 6 Inch Newtonian Astrophotos page for more Newtonian/webcam photos, and check out the Webcam Astrocamera page for illustrations of my modified webcam.

Yet another tutorial on telescopes

The holy grail, the perfect telescope

I'll preface my discussion with a few opinions that I've settled on after some 40 years of amateur astronomy. First, I'm a believer in the oft made statement that every telescope has its sky, assuming that its optics are reasonably good. I also haven't been able to find that "perfect" telescope. It seems, after all this time of telescope use, that all telescopes are a compromise in some way or another.

There are many tradeoffs, including viewing interests, tolerance for maintenance, portability, and price. It is the interplay of all of the factors that makes different telescope users have different requirements. So I caution you, don't let anyone tell you that you need a certain telescope because it's the best. You're the only one who will know how all of the factors combine in guiding your telescope preference.

If You Know What You Want To View ...

Shown below is a chart to help you decide what type of telescope you might be shopping for if you already know what kind of observing you find interesting. From left to right the chart lists wide-field to high resolution telescopes.

From bottom to top the chart goes from portable to large and non-portable telescopes.

You'll notice that going from bottom to top yields larger apertures, and from left to right yields higher f ratio (longer focal length) telescopes in general. Going from the lower left in any direction tends to also mean higher price.

So if you're interested in a portable telescope for star gazing, start in the lower left corner. If you're a planet hunter and portability isn't an issue, start in the upper right, etc.

The chart doesn't state hard and fast rules for telescope use, but presents the general features amateurs often consider.

Telescope/Observing Preference Table

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What's Cheezy, Scope-wise?

If you're not yet sure enough to use the shopping chart, here's some more information to help you understand the type of telescope that might be right for you.

The mistakes new buyers make are usually consequences of not knowing the factors I've cited. You may buy a telescope that is far to cheesy for you to see what you desire. You might just as well error on the other extreme and buy a behemoth that is so bulky and difficult to handle that it ends up in a garage sale in just a few months. But if you choose well, you'll find the hobby of astronomy enormously enjoyable, if not a little humbling.

There are many good brands out there, including Meade, Celestron, Vixen, Tele Vue, Orion, Zhumell, Questar, and others. Stick with these brands or similarly priced brands and you'll likely come out well.

A oft stated general rule, and one worth considering, is don't by a telescope from a department store. That rule isn't quite as cut and dried today, since some major manufactures such as Meade and Celestron market through places like Sams or Walmart.

Stay away from telescopes that are on flimsy mounts. In fact, in todays market the mount is usually what suffers on a department store telescope -- not necessarily the optics. The cheap mount may work at low magnification, but at higher power, or with the slightest breeze, you'll see an image maddeningly jumping around as you try to see details. Sometimes this can be remedied with some bracing, but it's best to avoid telescopes with undersized mounts. Either that, or consider making your own Pipe Fitting Tripod.

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Steer away from scopes that are advertised by their magnifying power, like 500 x 60mm, suggesting a 60mm diameter telescope that magnifies 500 times. These telescopes usually do include a Barlow lens and a very short focal length eyepiece (4mm or so) that actually produce a magnification of 500x. The problem is, the listed magnification is way beyond what is useful for that size of telescope even if the optics were superb, and these telescopes have sub par eyepieces.

The general rule of thumb is that the maximum useful magnification for any instrument is about 50 times the objective diameter expressed in inches. The objective is the main image forming lens or mirror. So a 60mm telescope is about 2.4 inches in diameter, yielding a maximum useful magnification of 120 times.

This common stated rule is a bit simplistic. It applies more to midsize and larger telescopes. Good quality, smaller refractors can often tolerate up to 75x or even 100x per inch of objective diameter and still give good images if the target object is bright enough.

That might not seem like much, but it is enough to see countless moon craters, cloud bands on Jupiter, Saturn's rings, and some detail on Mars during favorable (close) oppositions. The following calculator presents the maximum useful magnification for some popular sized telescopes.

There is a caveat here. The maximum useful magnification is a number based upon the telescope alone. In actual use, the atmosphere places further restrictions on the maximum useful magnification. This is particularly true for bigger instruments, say 8 inch and larger. It's a rare occasion for an 8 inch to be actually be useful at 400x. A factor of 30x to 20x per inch of objective diameter better applies to telescopes 8 inches and over.

For you older folks (like me), you may find that the higher magnifications present another problem. With higher magnifications, the light from the target object is being spread over a greater and greater area, making the target appear dimmer. When this happens, you may find that the floaters in your eye start to be visible and quite distracting when straining for those fine details. A little less magnification will brighten the image and cause considerably less distraction from floaters.

How many telescopes is enough?

I caution you that if you choose well and really get into the hobby of astronomy, you'll likely either migrate through a number of telescopes as your interests change, or end up owning more than one at a time. To the consternation of my spouse, I own five.

Why five, you ask? In my defense I want to point out that many amateur astronomers own much more than that. And the reasons vary. For some, it's just that there are so many types, and they want to experience them all. Many telescopes have specific uses, such as a planetary telescope, a light bucket (star telescope), a travel telescope, etc.

My telescopes and reasons for them are as follows:

Do you get the idea? Different telescopes have different advantages, and we'll go over many of them here.

Some important parameters

For those low contrast targets, a pretty good rule of thumb is that subtracting the diameter of a reflector's secondary from the diameter of its primary gives the size of refractor that would deliver as good a view. So when looking at Jupiter with an 8 inch Schmidt Cassegrain that has a 3 inch secondary, a 5 inch refractor (8 minus 3) would give a similar contrast result. The image in the 8 inch would be brighter, but the contrast would be similar to that in the 5 inch.

Ln high contrast targets, the 8 inch would deliver better, and certainly would deliver better on dim objects. This topic is discussed in detail at The Effects of a Central Obstruction.

That brings up Light gathering power, another telescope attribute. Think of a telescope like a funnel. If you put a test tube out in the rain, it will collect some water, but will take awhile to fill up. Now put a funnel in the tube, and the wider opening of the funnel will collect water over a much greater area and fill the test tube much quicker.

That's essentially what a telescope does for the eye. It collects light over a much bigger area than the eye pupil and funnels it into the eye. Just like a funnel, the light gathering power of a telescope is a function of area, which is proportional to the diameter squared. Thus a telescope of twice the diameter will collect four times the light (make things four times brighter).

The following calculator illustrates the relationship between telescope diameter, resolution, and light gathering power.

In the preceding calculator, note that the resolution is a function of diameter. Twice the diameter will resolve something 1/2 as big.

Note that star magnitude is a logarithmic scale, with each magnitude being about 2.5 times brighter than the next larger magnitude. That is, a magnitude 5 star is 2.5 times brighter than a magnitude 6. Incidentally, under dark skies the naked eye is limited to about magnitude 6.

The following table shows the parameters and best uses of some common telescopes.

Telescope Characteristics Chart

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Another useful chart standardizes on the 6" size to better give the price differential of the different telescope types.