Take H-alpha images without an H-alpha filter

If you have an IDAS LPS filter (sold by Hutech) and an IR-blocking Red filter, you too can enjoy imaging in (or about) H-alpha! 

UPDATE: As Don Goldman pointed out, I misinterpreted the transmission chart for IDAS LPS. The reality is that a red filter + IDAS LPS combination provides two transmission peaks: one around H-a, the other around 613nm, which is color orange. Not quite, H-a, but for emission sources this may be just as well :-)

Details:
IDAS LPS filter
Custom Scientific (SBIG) or another Red filter with IR-block
Peak transmission: 87% at 656nm
Pass-band: 25nm  (peak and pass-band will be different with other red filters)

A lot of imagers are starting to us narrow-band filters in order to improve contrast, capture new details and to combat light pollution. Hydrogen-alpha filters have become very popular and are available from a variety of vendors.  H-a filters work particularly well on emission nebulae, but can also be very effective in  reducing the effects of light pollution and in reducing star saturation and bloat.

When comparing the transmission curves of the IDAS LPS filter and the SBIG Red filter with IR-block, I noticed that the combination of both curves produces a single notch centered at the H-alpha line. Furthermore, with the peak transmission of the IDAS filter being around 95% and the SBIG Red filter at about the same point is 92%, the combined filters have a transmission of about 87% at the peak. When combined, the two filters produce a nice notch that is just about 25nm wide centered on the H-alpha line. This is a bit wide for true narrow-band imaging, but seems to be good enough for removing the effects of light pollution and for highlighting H-alpha emission lines. 

Although I had only night with this filter combination (LPS + Red), the results are very encouraging. Here are the two images taken with this filter combination. The Orion Nebula image was acquired with the full moon being less than 30 away and a couple of 15 minute long exposures. Click on the image to see a larger version:

If you get a chance to try this filter combination, please drop me a line, or better yet, post a picture and send me the link!

Copyright 2002 Paul Kanevsky