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

Dennis,

I shoot all of my Ha images when the Moon is present. Then I have allot of catching up to do on the color though when the Moon is gone.
Sometime I do not even do the color for the Ha depending on the image. Though if you play it right you can get 20 or so 15min images in Ha per night per object and get a really nice smooth image.

Great post and not sure how many realize that with a simple Ha imaging filter you can image any clear night if the Moon is up or not

Mark