Focusing Tips and Tricks for Astrophotography
From SkyInsight
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Most of these tips and trick are from the AstroPhoto Insight Newsletter, a free quarterly newsletter about astrophotography.
Michael Covington's Digital SLR (DSLR) Focusing Tips
- Make sure you can see clearly into your camera's eyepiece! Almost all modern SLRs have eyepiece diopter adjustments. People go around with these terribly out of adjustment and wonder why nothing ever looks perfectly sharp. Check your camera instruction manual and adjust the SLR eyepiece so it fits your eyes.
- The Canon Angle Finder C is a dual-magnification right-angle finder that fits almost every SLR with a rectangular eyepiece frame, not just Canons. (It comes with adapters for more than one size of frame.) It is similar to the classic Olympus Varimagni Finder but gives a brighter image and longer eye relief. I've used it with Canon, Nikon (rectang-ular frame), Olympus, and Minolta cameras.
- If your camera is digital, there's no substitute for actually taking a test exposure and then viewing it on the camera's LCD screen, magnified as much as possible. That's how I focus everything with my digital SLR - I take several short trial exposures and compare them. This doesn't rely on the accuracy of the SLR groundglass or eyepiece; you're seeing exactly what the CCD sees.
Focusing the DSI
Fred - Prairie Creek Observatory, Charleton, AR
To get a sharp image, I slowly decrease the gain on my DSI as I focus in. This causes the star of choice to appear as the defocused 'doughnut' everyone is so familiar with. Even at the end of focus, you can still see a ring until the focus is dead on. You can leave your dew shield in place because you don't have to place or remove a focusing mask.
Yes, this takes longer but the final result is well worth the effort.
Max Corneau, Rockwall, TX
Precise focusing is the number one challenge an astrophotographer faces after attaching a camera to a telescope. Regardless of the quality of your optics and other equipment, precise focus is essential. The Meade DSI comes with a 1.25�? ring, known as a Parfocal ring or parfocalizing ring. This is a handy device, that when used properly will get you “in the ballpark�? every time.
- Step 1. Focus your telescope as precisely as possible with a DSI attached. Once you’ve got it, try one more time to ensure the best focus possible.
- Step 2. Choose an eyepiece to make “parfocal�? to your DSI. Slide the ring onto the barrel of an eyepiece. Slide the ring as far back as it will go and gently tighten the retaining nut.
- Step 3. Carefully remove the DSI and Insert the eyepiece into the barrel. The image will be out of focus. Using both hands, slide the eyepiece out until you achieve best focus and hold the eyepiece at that point. Slide the ring down till it is flush to the outer barrel and tighten the retaining ring. To check your work, let go of the eyepiece and ring and ensure the object is still in focus.
How well will this work? Parfocal eyepiece performance depends on several factors. First, use a decent eyepiece. Second, parfocal rings are always susceptible to slippage. Third, our eyes automatically compensate for images that are out of focus by up to 3%. In step 1 above, I was not joking about going back to ensure proper initial focus on the DSI.
Using the Parfocal approach gets you immediately “in the ballpark�? and allows the DSI to begin it’s “autocontrast�? function immediately so you can finish the precision focus.
