H-alpha filter question

Hi all,

I tried asking this question on the CCD forum, but did not get a responce, so maybe someone here can help.

I am looking at getting an H-alpha filter for CCD imaging and I am trying to decide which one to get. From looking at what is available, there seems to be a choise of 4.

Custom Scientific 10nm $170
Custom Scientific 10nm/with AR coatings $220
Custom Scientific 4.5nm/with AR coatings $275
Astrodon 6nm $275

I understand that the narrower the bandpass, the better the light pollution rejection and the fewer stars in the image, but I gather that longer exposures are required, so my gut feeling is to go with the Astrodon, but I would like to hear from other users as to their recomendations.

For example, what difference is there between the 10nm and the 4.5nm? Is the extra cost worth it? What about the anti-reflection coatings, are they worth the extra cost?

Thanks in advance

Maurice

Longer exposures should not be required with narrow filters so long as the emission line is not cut by the filter.

This is going to be the case with these wider bandpass filters. For research we use filters with even narrower band passes than these and still get all of the Ha.

Now you will contaminate the image with NII emission at 654.8nm and 658.3 if the filter covers these. The only filter to block this to an extent will be the Custom Scientific. But consider that this emission is usually ~4% of Ha and will be a minor contribution.
Also please consider the F ratio of you telescope. The filter transmission was calculated at a given f. By using a lower f ratio you shift the peak of the curve and with the 4.5 nm you may cut some Ha. With the 10nm you are safe, as I believe with the 6. However with the 4.5 you may have a problem if you are using a very different f ratio. I believe that the filters are usually calculated around f10.
Eric

I am in no way an expert in filters so I just want to share my experience with you.
I have been using an "Astrodon Schuler" filter for a year now with great success. It's a 9nm 1.25" dia H alpha filter which you can find a description of on Astrodon's web site. I have mounted it in a filter wheel but before that, I have been using it straight screwed on an adapter. I am imaging with a ST2k camera and WO 80ED refractor. At $199, I think this filter is doing a great job.
Francois