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Hi Carl, This will probably start a huge discussion but here is my 2 cents worth.
I would reccommend a 12nm Ha filter for your modded DSLR for a couple of reasons.
1 You have to get some stars to focus on without shooting 5 minute frames to focus.
2 Just the sheer exposure times required to get an image will heat up a DSLR quite a bit.
Explanation:
The 3nm Ha filters are very narrow and require exposures of 15 minutes or more to capture enough data to be usable and do not allow stars to shine through very much at all.
The 7nm being wider shorthen up the times a bit but still are short on focus stars with short exposures. Exposure times to collect data are probably 10 minutes or more.
The 12nm allow a much wider band spread but still capture Ha very well. A lot of the better imagers here use a 12nm to collect Ha data for blending purposes.
From memory Mike Sidonio uses a 12nm Ha and produces wonderful Ha and blended images with this filter.
You will certainly find it much easier to use and get great results.
It's also a lot cheaper than it's narrower mates.
Hope this helps.
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