View Full Version : Imaging: Hydrogen Alpha and Stacking

Dennis Persyk
February 5th 04, 10:04 PM
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

Clear skies,

Dennis Persyk
Igloo Observatory Home Page http://dpersyk.home.att.net
Hampshire, IL