The Most from Your Scope

Jack Kramer

Have you ever been dissatisfied with the performance of your telescope? Probably every one of us can answer "yes". There are many reasons why a telescope doesn't perform quite as well as we might want. Some of this has to do with the size and design of the instrument, and certainly with its quality. But there are things we can do to improve the situation. The following hints are based on personal experience and discussions among various amateur astronomers.

 

Out of the Box

When you first receive a new telescope, it's logical to assume that it's in top shape and ready to start collecting photons. Even if the supplier pre-checks the scope as it comes from the manufacturer (something which most suppliers don't do) vibration that occurs in shipping can jar the optics out of alignment, or even damage parts. It's always good to check everything right away, then contact the supplier immediately regarding any discrepancies. Also, if you see a large amount of particles on the optical elements, especially in a closed tube telescope, contact the supplier; this may mean the manufacturer didn't properly clean the instrument before installing the optics. Optical re-alignment (collimation) of a reflector is something you can do yourself, but that's about all - too much messing around may void your warranty. However, if a refractor appears out of collimation, send it back to the supplier; refractors stay aligned very well, but re-aligning them is something best left to a pro.

Collimating an SCT

Schmidt-Cassegrain telescopes are popular because of their convenient size and adaptability to a wide range of accessories. One perception is that you get this at the cost of poorer image quality in comparison to Newtonians and refractors. But in many cases, really inferior image quality in an SCT is traceable to poor collimation. Newtonian users are constantly tweaking their collimation. Quite a few SCT owners never touch the collimation; they don't realize that their scopes, like other reflectors, periodically need some adjustment. In fact, as with all Cassegrain-type systems, collimation is especially critical because the average SCT has a very fast primary mirror - around f/2. The good news is that since collimation of an SCT is done by adjusting the alignment of the secondary mirror, they tend to stay aligned better than Newtonians. If you have Internet access, there are a number of web sites that provide info on collimating an SCT; here's one of them that specifically addresses the Celestron C-8:

http://www.america.net/%7Eerg/collimate.htm

You can also join an on-line chat group designed for SCT users. On the home page for this group is a "files" section that includes instructions for collimating and maintaining SCTs. Here's the URL:

http://www.egroups.com/group/sct-user

The Star Diagonal

The star diagonal is that gizmo on an SCT or refractor that re-directs the light at a 90o angle so you don't get a sore neck when looking through the eyepiece. Sometimes it's referred-to as a "right angle adapter". It's common for many telescopes to come equipped with a prism star diagonal. But a diagonal that uses a mirror instead of a prism will invariably improve the brightness and contrast of the image. This is easy to understand if you look at how much glass the light has to travel through when it's re-directed by a prism. Moreover, light encounters three surfaces in a prism, but only one with a mirror. And a good diagonal mirror has a surface that is ground much more precisely than the surfaces of a prism. A mirror type star diagonal is more expensive, but it's worth it. To check the quality of your star diagonal, remove it and, if possible, use an eyepiece straight through. Look at a high contrast object such as the terminator of the moon, then compare the view with and without the diagonal.

A Newtonian Diagonal Mirror Problem?

Since it is relatively easy to grind a flat mirror, most of us assume that a poor image in a Newtonian telescope is solely the fault of the primary mirror, rather than the flat diagonal mirror. But as I discovered myself many years ago, sometimes it's the diagonal that is at fault. Astigmatism is usually the only optical problem with diagonal mirrors. An astigmatic condition is usually indicated if star images have little tails on them that can't be corrected through collimation. The source of the problem is fairly easy to check. If you have astigmatism in the star test, you can rule out the eyepiece by rotating it relative to the scope and seeing if the astigmatism rotates with it. Once you're sure the astigmatism is in the scope (either the primary or secondary), note carefully the tube orientation and presentation angle of the astigmatism, then rotate the primary, and recheck. If the astigmatism hasn't rotated, then it's likely to be the fault of the diagonal.

Reflector Afflictions

If star images have flares, spikes or tails on them, the source of the problem still may not be related to the optics themselves. There could be any number of causes for this problem, but here are a few items to check. Spider vanes and mirror clips will both throw diffraction spikes; if one is overlapping the other, that spike may appear particularly prominent. Also, the bottom of a focuser tube that protrudes into the light path can throw a big spike. Perform a star test by racking in and out a short focal length eyepiece until you see the concentric rings of light called the "diffraction pattern". Mirrors on three point supports that are flexing will usually present diffraction rings that are somewhat triangular shaped, rather than perfectly round. If so, then the mirror cell may be at fault. An oddly shaped diffraction ring may indicate that the mirror is being held too tightly - pinched - at one or more spots. Some users of lower priced Dobsonians have found that mirrors are being distorted by too many large globs of glue that are used to hold the primary mirrors in place (particularly so on Meade products). If there is one big spike or a bright lane of distortion reaching out across an otherwise round diffraction pattern, you could have air currents in the tube. However, if there is an elliptical diffraction pattern, it is more likely to be an optical astigmatism that's ground into the glass itself (assuming the scope is well collimated).

Telescopes come and go, but an eyepiece is forever

Good eyepieces make a significant contribution to image quality. If your telescope came equipped with nondescript eyepieces of the Ramsden or "Modified Achromat" type, you can improve the view by replacing them. Don't go for the very cheapest eyepieces - they won't give you edge-to-edge sharpness, image contrast will be poorer, and there are often internal reflections that throw glare into the field of view.

There are five desirable qualities of an eyepiece:

1) Image sharp to the edge of the field

2) Wide apparent field of view

3) Long Eye Relief

4) Good contrast

5) Reasonable cost

Some very good eyepieces meet only two or three of these, but if cost is no object, then you can easily find an eyepiece that meets the first four criteria. Eyepieces that provide very large fields of view and long eye relief are popular with many observers; they are especially well suited for deep sky observing. But they don't show any more detail than some simpler and less expensive designs, particularly the Orthoscopic, which has been around for many years and is often regarded as the best choice for planetary observing. Because there are just four optical elements, the throughput of light is greater than in some complex designs with up to nine elements. Although it won't give you a wide field of view and long eye relief, a new Orthoscopic with modern day multicoated lenses will generally give you a brighter image with more contrast. The word "orthoscopic" is really a generic term that means the image quality is undistorted across the entire field of view, but the term has become synonymous with the Abbe Orthoscopic design of an eye singlet and field triplet. A quality Plossl, Orion's Ultrascopic and the Celestron Ultima are reasonably priced eyepieces of basically similar design that also do a fine job. Tele Vue eyepieces are expensive, but you can't go wrong with any of their products. It may seem odd to spend hundreds of dollars for an eyepiece collection; however, they are a worthwhile investment that can be used in any telescope you may acquire in the future. Although a wide field of view makes an eyepiece more pleasant to use, be aware that in order to get good images you don't necessarily have to spend a huge amount of money on an eyepiece the size of a hand grenade!

Final Thought: Theoretical Limits

What can you really expect from your telescope? While much depends on the size, design and quality of the optics, there are certain theoretical limits. However, many so-called theoretical limits are not theoretical at all, but derived via experimentation. Dawes' limits on resolution, Rayleigh's criterion for diffraction limited optics, and limiting magnitude for a given aperture are not based on theory, but on what some people were able to see with real telescopes. While they do a fair job of defining certain limits of performance, they assume a set of other conditions, which often do not apply. The amateur astronomer with a telescope out in the night encounters a range of circumstances for which no controlled environment can account. One factor is certainly the skill of the observer. Another is the condition prevailing at the observing site. As a result, the limits may not be achieved, and for the same reason, those limits may sometimes be exceeded. Perhaps that's why they're called "theoretical".

Published in the December 2000 issue of the NightTimes