Welllllllll….this is a post I was really hoping NOT to write, but I should have known better. 🙂 And, frankly, if this series is to be at all “accurate”, it really ought to include problems…this is AP, there are always problems. 🙂
So…our target is NGC 1491, and interesting NB region in Perseus. I’d planned to start shooting on 9/19. Wife and I were attending a concert that evening, but the magic of SGP, remote desktop, and a son who can open/close TinyObs for me allow me to manage/execute sessions from my phone if desired. So that was the plan!
“The plan” went sideways about 9:30pm. I was asking the rig to autofocus for the first filter, Hydrogen Alpha, and getting nowhere. It simply couldn’t find any stars! I was frankly much more interested in Heart than the session, so after a few aborted attempts, I killed the session, texted the kid to close the obs, and wrote the night off. I do hate wasting a clear night, but so be it.
On 9/20, back at home, I planned to babysit things, and get a really good look at why the Ha filter wouldn’t focus. I found out pretty quick when i started taking 30s test frames…
There’s just not that many stars! There’s certainly more than you see here, but one of the downsides to NB imaging is that, by definition, the filters are very restrictive…takes quite a few photons to punch through that filter and register a signal…and we have a region here that simply doesn’t have very many large, bright stars.
Now…eventually I can get enough stars to get a good focus reading on…but it takes a good 2-3 minutes, which is simply impractical for focus frames, since we’d be looking at 15-20 minutes just to autofocus!
SO…I decided to try a little SGP trick. SGP will allow me to set “offsets” for filters…meaning, I can tell it “Hey…the Ha filter’s focus position is always ___ far away from Red’s”. Red, being a much broader filter, can find more than enough stars for a clean autofocus run on short (10s) exposures :
So, I autofocused for Red, then manually found (what I thought was) ideal focus for Ha, noted the offset, and punched it into SGP. I then told SGP “When you autofocus tonight as the temp changes, use Red, then use the offset to move Ha”
Aaaaaaaaaaaand…that failed miserably…
Juuuuuuust enough out of focus to be distractingly soft.
SO…back to the drawing board (I hope) tonight, 9/21. I think my next attempted “fix” will be to see if I can tell SGP to move the scope to another star-rich area, focus there, then move back and resume imaging our target.