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Film Speed, Etc.

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Posted by Daniel Johnson on November 28, 2002 02:58:04 UTC

It's not just a question of film speed, but also spectral sensitivity. Many films are poor at capturing red nebulosity (Hydrogen alpha wavelength, about 657 nm) because the human eye is not very sensitive at that wavelength, and films are intentionally made to mimic the eye. One film I've enjoyed a lot is Kodak Supra 400, a professional film that you may have to special order (try ,where I bought a pack of five rolls mail order). This film captures both red and blue nebulosities well, and fast. Most fast films become grainy. This one isn't bad. Other photographers might suggest other films. I know I've tried a lot that flopped on red nebulosities, or that gave ridiculous sky colors.
Regarding the sky looking like day, that may be from overexposure, or it may just be from the film processor using computerized automatic correction to make your picture look "right"--after all, you want all your pictures to look bright as day, right? Ha ha. I've managed to rescue some such photos by having the negatives scanned. I then use a computer to process the resulting file for the proper night sky darkness. Look at the negatives. Are any stars visible? Any meteor streaks? If so, computers may save the day.
As for your streaked photos, I assume you are polar aligned and tracking with a smaller scope on the side of your main scope? If you're doing long exposures in altazimuth mode, it doesn't work. You track the central star fairly well, but the others rotate around it. Polar alignment is a must (technically, using a field derotator can work too, but it has many, many drawbacks). Also, you guide scope needs to have long enough focal length. I'm still experimenting with "long enough," but I think my ETX 90 will make a good guide scope for my 10-inch LX200 GPS with its f/6.3 photoreducer. A friend of mine just gave up on using an Orion Short-tube 80 as a guide scope on his 10-inch LX200--not long enough focal length.
There's no substitute for careful polar alignment and guiding well (either with a reticle eyepiece, or with an electronic autoguider). Wind will streak your exposures. Bumping the scope will. So tell us the details of your setup (Wedge? Guide scope? Counterweights?). I use a Meade 12.5mm illuminated reticle eyepiece, but I've had to replace the red LED (Waaaaaay too bright) with the Rigel Systems Pulseguide. Very nice.

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