Star Testing

What is star testing?

Observing moderately bright stars under good seeing with high magnification can tell you a lot about the quality and alignment of your telescope. You can, for example, identify an over corrected or under corrected objective, spherical aberration, turned down edge (for mirrors), and astigmatism errors just by examining out of focus star images. In fact, if you don't own a laser alignment tool, observing a star at high magnification is a good way to tweak up your instrument's alignment.

Star testing is done by examining a magnified star image with the eyepiece racked both inside and outside of focus, and comparing the resulting images.

If you are using a star test to align your optics, you might find it difficult with a telescope that doesn't have a clock drive. By the time you go to the business end of your telescope to make an adjustment and return to the eyepiece, the star may not even still be in your field of view. If you live in the northern hemisphere, you can resolve this issue by using Polaris as you alignment star.

A good website that shows the kind of star images produced by different optics errors is Star Testing Astronomical Telescopes.

Star aligning is a technique I've used a lot. It is sometimes a touchy procedure, however. Often I either bump my telescope or make too big an adjustment and cause the star to be moved out of the field of view. If you want a quicker, easier way, you probably want a laser alignment tool. I've seen them at: SHOP.COM . Just look for laser in the Camera and Optics department.

Double Star Observing

The stars may be locked together gravitationally, or they may be actually very far apart and independent and just happen to be along the line of view.

Double star observing is fun for a number of reasons. I use double stars (or binary stars as they are sometimes called) primarily to compare and evaluate optics.

Observing close doubles under good conditions gives some indication of optical quality. If a double separated just over the theoretical resolution of your telescope can be cleanly split, the optics are near perfect. If not, maybe doing some star testing can help identify the problem.

Doubles are also interesting objects to observe because of the variability. Often the component stars are different colors, or different magnitudes. These variations give each double its own identity.

Star Cluster, Galaxy, and Nebulae For more general star, galaxy, and nebula observing, seeing is not much of a concern, but transparency is. You'll be straining to see the faintest details of interesting objects, and nearly every photon matters. It's with this general star observing that aperture is of great importance. The bigger the aperture of the telescope you use, the brighter objects will be, and the more objects you will ultimately be able to see. However, if you get such a big telescope that it becomes too much of a bother to use, you've gone too far.

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You can check out the Telescope Tutorial for more details on how optical performance relates to aperture. For here I'll just mention that detail resolution is proportional to the diameter of a telescope, and limited by atmospheric phenomenon. Light gathering power, all important for star observing, is proportional to telescope diameter squared. So for star observing, diameter is especially important.

Star Observing Hints Most star observing is done at relatively low power. Open clusters, the Andromeda galaxy, and some nebulae are large enough in apparent size that low power (less than 100x) is best. For this observing, a clock drive isn't necessary.Planetary nebulae, globular clusters, and some galaxies will generally work best at moderate power, perhaps 100x to 150x. But even at this magnification, a clock drive isn't a necessity. Transparency is usually best when objects are not near the horizon. While not as critical as with planetary observing, you will probably get better views if you let your telescope cool down before observing.

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If you live in a large city, you may need to find an observing site that has better transparency. Another constraint caused by cities is all the lights. While not strictly affecting transparency, stray light definitely restricts the ability to see dim objects.

Let your eyes become fully dark adjusted for observing stars. You'll be surprised by how much more detail you can see with fully dark-adapted eyes.

If you are using charts between observations, use a red light to observe your charts. Eyes are less affected by red light, and you'll loose less of your night vision if you use a red beam. If you use a computer screen for star charts, see if it has a night vision mode, which will again display primarily with red light.

If you have a pesky street light or porch light that's giving trouble, consider using a towel or your coat to drape over your head when at the eyepiece. This will block out the troublesome stray light.

Consider using filters designed to increase the contrast of nebulae. A search for nebula filter in the Camera and Optics section at SHOP.COM will show what I'm talking about. These filters work by screening out wavelengths that interfere with seeing nebula features. Keep in mind, however, that since these filters screen out light, they don't work as well on smaller aperture telescopes. I don't think I would make the investment if my instrument was smaller than a 6 to 8 inch.

For very dim objects, learn to use averted vision to help discern faint details. Averted vision describes the technique of looking away from the dim details you want to see, and examining them as best you can with your peripheral vision. The center of your vision is best for details, but your peripheral vision is more light sensitive.

What Telescopes are Best for Stargazing? If you are new to amateur astronomy-- start with a decent pair of binoculars. You can get a good functional pair for under $100. Try for 50mm diameter or better. And don't get too high a magnification. You'll find you can't hold them still well enough over about 10x unless you mount them on some kind of tripod. Check out the Binocular Tutorial page for more information. You may find you can get by for some time with a couple of pair of binoculars, like a 7x50 for spotting objects and a 10x50 or 15x70 for a bit higher magnification. Or you can spend a bit more and get a zoom binocular. As to telescope type, there are a couple of considerations. Dobsonians can certainly give the most aperture for the dollar. An 8 inch Dobsonian can go for around $500, while an 8 inch Cassegrain will generally cost $1500 and up. Both telescopes and SHOP.COM carry telescopes of all kinds, so do some price comparison and see if you can't find what works for you.

That being said, I point out that there are two kind of popular Cassegrain telescopes available for the beginner astronomer. See the Cassegrain Tutorial for more details. Suffice to say, I would argue that for stargazing, a Newtonian or Schmidt Cassegrain would be better than a Maksutov. The Maksutov has a very large f ratio (around f/15), which provides too narrow a field of view for some star objects.

Purists will argue that you should learn the constellations, and then learn how to star hop to find objects with your telescope. Others will say it wastes too much time, and computerized mount telescopes allow you to see many more objects in an evening.

I think it all depends upon the observer and how he or she enjoys the hobby. You may find that the ability to see several objects in an evening because of the telescope's computerized drive is more important than a large aperture.

You may also find that portability plays an important part to you. If you are a youngster, elderly, or handicapped, you may find that the difficulty of working with a large telescope out ways its benefits. You may find that for good views you have to travel to a different site. For these and other reasons, you may want to temper the urge to get a behemoth telescope. I've been involved with astronomy for over 40 years, and I currently own nothing bigger than a 6 inch Newtonian.

You might find the following table useful for determining which telescope matches up to your anticipated observing. For finding deep space objects such as galaxies, you'll likely want to concentrate on the wide-field and general purpose telescopes, the larger aperture the better.

For small objects like globular clusters and double stars, you'd likely be best served by telescopes to the right side of the table. Again, the larger instruments will let you see dimmer objects.

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.

Star Photography

Star photography, as with star observing, is quite varied in technique. Constellations, rich star fields, and some extensive nebulae can be photographed with simply a guided camera. You can use either the barn door mount or the piggyback method. See the Observing Comets for descriptions of those types of photography.

For photos of clusters, globular clusters, and galaxies you'll need to photograph through your telescope. This is a most difficult kind of photography. To do it you need a clock driven telescope and either an auto-guider or a guide scope mounted along side your main instrument.

Time exposures are a must here, typically several minutes. While this kind of work can still be done with a standard film camera, if you have the money you may want to buy a specially designed CCD camera for telescope astronomy.

You can get into astrophotography cheapest with a standard SLR 35mm camera. If you're on a budget, check out new and used models at: Calumet Photo. Review the Choosing a Camera page to see what's special about a SLR (Single Lens Reflex) camera.

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