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Imaging: Hydrogen Alpha and Stacking
When the moon is bright, most deep-sky observers and imagers give up
because of moon light pollution. This need not be the case for imaging if one uses a narrow-band Hydrogen-alpha (H-a) filter and images emission nebulae. Emission nebulae contain vast volumes of ionized hydrogen which emit a very narrow emission line in the red. If one utilizes a matching narrow-band filter, the camera sees predominantly just the red emission and the light pollution from the moon and everything else ceases to be a problem. Hydrogen-alpha imaging does have some downsides: Focusing is much more difficult because starlight contains little H-a light. I normally focus on a magnitude 8 star using 0.1 second exposures, giving me essentially real-time focusing. With the H-a filter, I need 10 second focusing exposures. The exposure times have to be quite a bit longer due to the weak signal flux incident on the camera via the H-a filter. Finally, stacking (co-registering) many images is a powerful tool to decrease noise. In my example below I show the effect of a single two-minute exposure versus a stack of 22 two-minute exposures. Images at http://home.att.net/~dpersyk/new.htm Clear skies, Dennis Persyk Igloo Observatory Home Page http://dpersyk.home.att.net Hampshire, IL |
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