Hi Astropixel,
this was intended !
If I use the astronomik ha-alpha clip filter system it is quite hard to focus the lens on a star at all (it is a Canon 70-200 EF/4 "white series" lens) as the transmission is quite low. So I can focus only on the brightest(!!) red(!) stars (i.e. Beteigeuze, Arcturus, ....). Therefore I stop down the lens to have also some smaller stars sharp for sure.
If I don't use the clip filter system there is no notable difference in the performance between f/5.6 or f/6.3. But compared to open aperture (f/4) it is better to have it at f/5.6. Although this Canon lens is quite good at 70mm and 200mm focal length you sometimes see loss of sharpness at the edge of the field.
This can be increased by stopping the lens down.
Taking H-alpha fotos with a DSLR is not as comfortable as a cooled CCD, of course (but for me a
DSLR is more affordable
)
You have to take exposure times > 15 Minutes for 1 shot (even better 20-30 minutes) and I found
that debayering with the super pixel method gives somewhat besser results. Also take enough
dark frames as the Canon DSLRs produce lots of noise (esp in summer).
If you are interested, I could send you some example raw files to experiment with ....
Christoph
