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Ha filter

Started by james515, 02/03/2005 05:51AM
Posted 02/03/2005 05:51AM Opening Post
Can some one tell me what this filter is, i keep seeing images taken with an Ha filter, but not sure which one it is. Is it the same Ha solar filter or a much less expensive kind of Ha filter?

James
Posted 02/03/2005 06:25AM | Edited 02/03/2005 06:26AM #1
Hi James,
It's a narrowband filter that isolates and lets pass just light emanating from Hydrogen Alpha, a thin slice of the red spectrum. Emission nebula are exceedingly rich in this area--it's what gives them their glow--so you see Ha filters used on these targets, often replacing the luminance. They come in both 1-1/4 and 2-inch sizes, just like other RGB, LPR and visual color filters. (I've never used a solar filter, so can't speak to them.)I used one for the first time the other night on B33 (or IC434) and can tell you this: without the filter I couldn't see the Horsehead on my monitor at all, even after a 30 or 45-second integration. With the filter in place--wow!--there it was in great detail. (Image posted previously on this list.) After just one use I already feel like it's a must-have item in my arsenal.
I hope this clarifies more than confuses (and that I haven't botched the science too badly).
Have fun,
Larry Durst
Posted 02/03/2005 07:20AM | Edited 02/03/2005 07:23AM #2
James,

H-alpha filters for deep sky imaging are similar to H-alpha solar filters in that they both isolate the very narrow Hydrogen-alpha portion of the spectrum. However, the filter that is designed for deep sky imaging SOULD NOT BE USED for solar viewing!! It will not block enough sunlight and severe eye damage can occur.

As far as the deep sky Ha filter, there are essentially three types or "Narrowness". These are 10nm, 6nm, and 3nm. The 10nm has a broader window and lets in more light so exposures can be shorter and guide stars can be found. The 3nm gives the most contrast between the object and the sky, but at a cost of longer imaging times and much fewer guide stars. A good compromise is a 6nm. Even though these filters can isolate the Ha bandwidth of the spectrum and give amazing results (like Larry pointed out in his post) you still need long exposure times or many shorter exposures stacked to overcome the signal to noise ratio. Short exposures times may show the object, but there will be a lot of noise when processed. Best to get a lot of exposure time.

I agree with Larry, if you are going to image emission nebulae then a high quality Ha filter is very necessary. These filters are available from Custom Scientific filters, Astronomik, Don Goldman Tru-Balance filters(my favorite), Lumicon and others.

Tom Davis
Inkom, ID

Starsearch Observatory
Lat 42N 47' 00" Long 112W 10' 47"
Elev 1752m
www.tvdavisastropics.com