Creating an Astro Image Part IV – The Problems

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…

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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 :

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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…

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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.

Fingers crossed!

 

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