Narrowband Filter Choice

[Greg Marshall]

I've been dissatisfied with my Astronomik brand O-3 and S-2 filters because they are too broad (12nm) to provide adequate contrast. More specifically, if I adjust exposure and curves to match the brightness of stars relative to the 6nm H-a filter, most targets have almost no oxygen or sulfur signal. So I'm planning to replace them with narrower Astrodon filters, but can't decide between 5nm and 3nm. I'll be replacing the H-a filter as well, since I need them to be reasonably parfocal.

My concern is that the 3nm filters are much more expensive and might be more of a pain to use (especially in terms of longer exposures). My interest is entirely in "pretty pictures", not science research, but need to use NB at my home observatory to deal with light pollution.

Any and all opinions are appreciated!

[Ian G]

I don't say that I'm an expert in this. But... when I was facing this question myself, and asked around, I found out that a major consideration in it is the f-ratio of your system. I even consulted with Dean at Starizona, since I image mostly with Hyperstar. He told me that I should be using not 7-10nm filters but much broader one. At f/2 the light cone is very steep and it actually shifts the band that is being filtered out. I ended up using Baader's 35nm H-Alpha. It is inexpensive, high-quality and easier on the exposure times. Just my 2c.
Ian

[Greg Marshall]

Ian, thanks for the feedback, even if it is late!

I didn't have any trouble with the H-alpha filter (I was using a 6nm filter for H-a). I'm sure it's true that you can't use a very narrow filter with a super fast scope because of the angle, but f/2 is pretty extreme. You must be using a hyperstar system, right? My primary scope is f/5.6 and other s I sometimes use are slower.

Some very knowledgeable people on another forum suggested I get a 5 or 6 nm filter for H-a and 3nm for OIII and SII. However, I ran across a good deal on a set of 3nm filters - all three wavelengths plus a filter wheel. If it turns out that the H-a is too narrow I'm sure I can find someone who will trade a 5 or 6, since those are much less expensive.