Astronomical Telescope Tutorial

The image below is of the Lunar crater Tycho, and was taken with my 6 inch f/5 Newtonian and a modified Quickcam Express webcam. It is a stack of about 20 frames, selected for their clarity. More images taken with that setup are at 6 Inch Newtonian Astrophotos.

Such images are very reachable with any quality small telescope using either a purchased web cam astrocamera such as the Celestron NexImage or a homemade web cam conversion like that on the Webcam Astrocamera web page.

Tycho With 6 inch Newtonian and Quickcam Express Get This Image On A T-shirt Or Mug

Yet another tutorial on telescopes

Why another tutorial? Because:

The holy grail, the perfect telescope

As the two charts taken from the Amateur Astronomer Survey suggest, what constitutes the perfect telescope certainly varies from user to user. The chart on the left shows what telescope survey takers listed as telescope types they owned. The chart on the right shows what survey takers listed as the telescope types most often used. It's an instructive pair of charts, with some expected results and some confusing ones.

Both charts agree that refractors seem a popular favorite, but there's a lot of big Dobsonian users out there also. More curious is the indication that while slightly more survey takers own Newtonian telescopes vs Dobsonian telescopes, in actual use they seem to prefer the Dobsonian telescopes nearly 2 to 1 over the Newtonian telescopes. I would speculate that the ease of use of the Dobsonian over the Newtonian, especially in sizes over 6 inches, accounts for this result.

I'll preface my discussion of what constitutes perfection 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 ...

There are many different types of targets that draw people into the hobby and profession of astronomy. The most obvious are the sun (with proper safety equipment), the moon, and the planets. There are also many deep space objects (known as DSOs), such as galaxies, nebula, and star clusters. The Amateur Astronomy Survey shows what the survey takers up to now like to view most. These targets make different demands upon the user and his or her equipment. That's certainly part of the variance in the types of telescopes preferred -- the targets of most interest.

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?

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 mid-size 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.

Another option for dealing with floaters, should you have the funds, is to purchase a binoviewer accessory. The binoviewer goes into the focuser and directs the incoming light to two matched eyepieces, making the view available to both eyes. Since the floaters aren't the same in both eyes, a binoviewer will help eliminate them from your view. A binoviewer will also greatly reduce fatigue during long viewing sessions. Just be sure to purchase one of good quality.

A excellent site with more telescope buying considerations is Which Scope To Buy.

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

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. Strictly speaking, resolution is a function of the telescope diameter and the wavelength(s) being observed. Telescope resolution in arc-seconds is approximated by the following formula:

Resolution = 4.5/diameter(in)

This equation inherently assumes that the wavelength is that of the middle of the visible spectrum. If wavelength was included in the equation, it would be in the denominator. Thus, longer wavelength would give less resolution for any given instrument. Or conversely, for any given resolution, receiver size must increase with wavelength. That explains the enormous size radio telescopes must be to give good resolution at millimeter wavelengths.

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. A lens of twice the diameter has twice the resolution capability -- if you ignore the atmospheric limitations. 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. Seeing occasionally gets better, and often gets worse.

For a given diameter, a refractor is generally considered to give higher contrast views, especially on low contrast objects such as Mars, Jupiter, or low contrast nebula and galaxies. On high contrast objects, like the moon with its stark shadows or Saturn with its rings, refractors have no particular advantage over a reflector of similar size.

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.

On 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.