Getting Set-up at Long Focal Length

[Ramius]

Hi all
Still very inexperienced but getting the hang of short to medium focal length astrophotography so thought I might try planetary for variety. I am using a C8 SCT at about 4500mm with an ASI533MC Pro (not ideal but workable i think). I did eventually get some footage but it did not go smoothly.

I asssume I need a decent PA to support location and tracking of the target, but at that focal length I could not get Nina 3 Point Polar alignment working. With a rough PA at that focal length stars trailed and I could not get a plate solution. Reducing exposure lost too many stars. In short I needed better alignment to use the alignmment tool - classic Catch 22.

Eventually I gave up and manually located the target using the C8 finder scope and recentred manually as the target drifted. Got some video this way but extremely tedious.

Is PA really this difficult at long focal length or am i missing something obvious - or do I need to adopt a different appproach? Perhaps a different polar alignment method or a different scope configuration (such as removing the barlow for alignment or adding a guide scope purely to get alignment?).

I cant find any references to this specific issue so a steer in the right direction would be much appreciated.

Cheers
Bill

[Drac0]

Hi Bill,

Certainly lose the barlow for alignment, adding it later won't change it. You could use a different scope if you like, as long as you don't physically move the mount itself after it's complete. Or use the guide scope for PA as you suggest. Being able to use the guide scope as an electronic finder scope is also an advantage going that way. Or you could go the old fashion way & learn how to drift align... :P

Note that with plate solving it can also depend on the database you're using as to whether you get a result or not. As the focal length increases, the field of view reduces so you need a database with more stars per square degree to get a result. I use ASTAP with it's larger D80 database to cover longer focal lengths.

Cheers,
Mark

[Ramius]

Thanks for response Mark
Then its not unexpected that I am having difficulty with my current approach - that's good to know.
I have the large database installed so that should not be an issue. I think I will try aligning without the Barlow. It means finding focus twice but....
Hmm - as you suggest might be easier to set PA with the refractor.
Couple of workable options there -thanks.
Bill