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Polar Alignment For Normal Camera Lenses

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Posted by Daniel Johnson on October 26, 2002 17:34:35 UTC

The shorter the focal length, the less accurate the alignment needed for a given exposure. For exposures of 5 to 10 minutes with a 50mm lens, polar alignment need be only approximate (Polaris is close enough). You can catch some decent exposures in 10 minutes. When I started astrophotography 15 or 20 years ago, I used ordinary lenses in the 50 to 135 mm range, and I tracked the stars with a hand-made platform driven by a screw that I turned once per minute by hand (such "barn door hinge" mounts were popular then and are still easy to make). It worked, but not for lenses over 135 mm, and not for exposures much beyond 5 or 6 minutes (I had fast lenses). All I did was aim the hinges of the device at Polaris. For 50 mm lenses, you do not have to track at all for exposures under 15 seconds or so, but you won't see much at f/3 in 15 seconds; with 135mm, stars start to streak after only 5 to 10 seconds if you use no tracking at all. If you want to catch red nebulosity, Kodak Supra 400 film is fairly fast. Since the human eye is not very sensitive to the wavelength of red nebulosity (657 nm, the hydrogen alpha wavelength), and since many films understandably are made to mimic the human eye's color sensitivity, a lot of films will show stars and the Milky Way well but miss the beautiful reds (of, say, the Orion nebula or the North American nebula) entirely. Kodak Supra 400 is highly sensitive at 657 nm. This particular film is not a universal choice, just the one I'm using lately after frustrating experiences with other films. You may have to special order it (try adorama.com to get it mail order if your local pro photography shop doesn't have it).
But back to lenses. If you stop down your camera to, say, f/5, you'll probably need 10 minutes or longer, but even 3 to 5 minutes will show some delightful detail. If you ever try to shoot through the scope itself, the much longer focal length magnifies tracking errors, and accurate polar alignment is a must.
Experiment with your lens. At the corners, you'll see some aberrations (spherical aberration/coma, and maybe chromatic aberration). These become less severe when you stop your lens down. Any given lens will have a tradeoff point that you need to find between having fast pictures and having pinpoint stars across most of the frame. Changing your zoom may change how much you need to stop down. Do try one shot wide open--you may be lucky and have a really good lens.
If you have any way of digitally manipulating your photos, it is well worth the minor cost to have a professional photo shop scan your best negatives. It will show more detail than scanning the prints at home, though even scanning them at home works if your photo shop knows how to print night sky photos. If you use a one-hour photo shop, they'll develop the negatives OK but their automated systems may not PRINT those negatives if their programming detects "underexposure"--of course, your negatives will look underexposed to their computer.

--Dan Johnson

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