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Yes, the simple red filter just doesn't have a high enough transmission to let the fainter features show up.

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Which is the Brand and model of your filter?

I have not been able to find transmission data for the Wratten 23A (light red) but I found these for the 25 (red) and 29 (deep red). At 656 nm they have transmisson of 88% and 90% respectively. The 29 is very interesting: it could be a very cheap substitute for a Ha filter for testing Ha visibility on nebulae.

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It is a very old inexpensive filter that was sold with a common small telescope about 20 years ago, and it is unmarked (I have no idea who made it). From the way it improves Mars, it looks to be similar to the Wratten #23a but is not extremely high in red transmission. Newer filters are significantly better in their red transmission, as one filter company markets one "Wratten #23a" equivalent with a near 90% H-alpha transmission, so that one might indeed make a good H-alpha filter.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info

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