![]() It’s worth the time to learn to collimate your telescope properly. If you see a comet-shaped blob, the problem could be several things, including severe miscollimation or coma. ![]() If you see a broken diffraction ring, it probably means your scope is slightly miscollimated. Make sure you keep the star centered in the field of view to avoid eyepiece edge aberrations creeping in and confusing things. You may have to use slightly lower or higher power depending on conditions. Your eyepiece must show the Airy disk and a diffraction ring, otherwise you can’t do the test. A little experimentation goes a long way. Just how bright a star you’ll need depends on your scope’s aperture. If your mount doesn’t have a drive, you’ll want to pick a star close to the celestial pole. In a perfect system with no obstructions (like a secondary mirror), the bulk of the light is concentrated in the Airy disk, with only a small amount spilling out to form a ring. That disk is the Airy disk, the best focused spot of light that a telescope with a circular aperture can make. When you look at a star at high magnification, you should see a small disk and a diffraction ring (or set of rings) surrounding it. And remember, if your observing eye has some astigmatism, you’ll see that in the star test. I typically test optics the same way I observe. Remember, you’re looking at the entire optical train, starting with the star and ending with your eyes and brain. This makes any sort of test pointless.įinally, if you use a star diagonal, do the test with it both in and out of your scope. If the scope’s optics are out of alignment, you’re not getting the best performance. So, for an f/5 scope, pick a 5mm eyepiece.įourth, be sure your telescope is collimated. Alternatively, choose one that matches the focal ratio of your telescope. For example, if you are using a 4-inch scope, pick an eyepiece that magnifies between 160 and 240 times. Choose an eyepiece that provides moderate to high power, typically one that magnifies 40 to 60 times per inch of aperture. You’ll want either a simple design like a Plössl or something well corrected, like a modern wide-field design. And when that happens, you’ll see a laundry list of issues. If the temperature keeps dropping through the night, your optics never will stabilize. If you can, store your scope somewhere where the temperature will be the same as outside. At least an hour, maybe longer depending on the temperature change you’re putting it through. Next, allow enough time for your telescope to cool down. Here’s a tip: If the stars are twinkling wildly, don’t bother testing your telescope that night. In my home state of Michigan, it’s rare to get conditions that are steady enough to test any scope with an aperture over 8 inches. The atmosphere must be calm to perform a valid test. As a last resort, you can have the optics checked professionally.įirst, check your seeing. There are also software programs that help you identify exactly what you’re looking at in the defocused patterns, artificial stars that allow you to test your scope during the day, and products that make testing even easier, like Ronchi gratings. If you’re looking for a more in-depth approach, you can find numerous books and web articles on this topic. I’ve never seen perfect optics that said, most of the issues I have seen have been either easy to correct (collimation, tube currents, retaining ring too tight, not enough cool-down time) or really don’t affect the view much. A noted optical designer once told me, “You pick what aberrations you want to correct for and what you can live with.” So remember, nothing’s perfect. Nearly all telescopes have some peculiarity. Testing a telescope under the stars is a critical examination. It requires you, your telescope, and a night of good seeing (atmospheric steadiness). To get you through this trauma, I’ve created a simple, easy-to-follow procedure to evaluate your optics. ![]() A clear night comes, you set it up under the stars, and … things don’t look quite right. Finally! The telescope you’ve wanted HAS ARRIVED! Maybe it’s your first maybe it’s your fourth.
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