This image was taken with my Meade ETX 90 and a modified Quickcam Express webcam. Check out the ETX page for more ETX/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 it's 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.

My Astro T-Shirt Designs

If You Know What You Want To View ...

Below is shown a chart to help you decide what type of telescope you might be shopping for if you already know what kind of observing you are interested in. 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 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

One online shopping mall that has telescopes in all of these categories is SHOP.COM . They carry very good prices on instruments and telescope accessories from Celestron, Meade, Tele Vue, Vixen, and Rigel Systems, to name a few.

Just enter astronomical telescopes, or the specific type of telescope or accessory in their search engine.

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 may 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. You can check out prices and types of telescopes by these and other vendors at one convenient shopping mall, SHOP.COM . Simply type astronomical telescope in their search window and you'll see a large selection from their many member stores.

A oft stated general rule, and one worth considering, is don't by a telescope from a department store.

Also, Stay away from telescopes that are on flimsy mounts. The 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.

Also, 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 do amount to a magnification of 500x. The problem is, that magnification is way beyond what is useful for that size of telescope, aside from the fact that such telescopes usually have sub par objectives and eyepieces.

The general rule of thumb is that the maximum useful magnification for any instrument is about 50 times the objective (main lens) diameter expressed in inches. So a 60mm telescope is about 2.5 inches in diameter, yielding a maximum useful magnification of 125 times. This rule of thumb is a bit simplistic. It applies more to mid to 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, especially on brighter objects. 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 tends to apply 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, you are spreading the light from the target object over a greater and greater area, making it 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 likely 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. The reasons vary. For some, it's just that there are so many types, and they want to experience them all. Others 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

Regardless of telescope type, there are a few parameters you should be aware of as they pertain to what you'll see through the telescope you choose.

Resolution is a term used to describe a telescope's ability to deliver detailed views. The larger the diameter of an instrument, the better the telescope's theoretical resolution. Prove it to yourself by poking a small hole through a piece of paper with your ball point pen. Now view a few things through that hole, like your computer screen for example. You'll find that through the hole you must be closer to the screen to read it than with your normal vision. The small hole made by your pen reduces the detail that you can see, as it is much smaller in diameter than your eye lens.

The main image forming element in a telescope is called the objective. As I'll discuss, it is a lens in some instruments, and a mirror in others. Resolution is related directly to telescope objective diameter. A lens of twice the diameter has twice the resolution capability -- that is, if you ignore the atmosphere. As it happens, for the typical backyard observer, the atmosphere creates a resolution limit of about one arc-second (1/3600 of a degree). That relates to about a one mile diameter crater on the Moon as seen from Earth. It occasionally gets better, and often gets worse.

Light gathering power is another telescope attribute. Think of it 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.

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

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