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How long before star trails?


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Hello all,

what is the longest exposure you have managed with a CPC 9.25 scope on a HD Pro wedge?

I was trying to image M51 last night and was trying 120 second subs at ISO 1600 and had short star trails.

is an exposure of 120 seconds an unrealistic expectation with my setup?

Many thanks,

 

David

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All rather a question of how good your polar alignment is, I suspect.  I was going to get a wedge for my C9.25 before I saw quite how chunky they are!  Instead I went for an HEQ5-Pro which was OK, but I never pushed its limits and moved on to an Avalon from there.

I assume you’re not guiding?

Tony

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Without guiding you are going to be limited.  I suspect that even with good polar alignment you'll struggle to get to two minutes.  There's other problems inherant in all telescopes, namely periodic error.    You might be able to train the drive so that it take this into account, which may help to extend the time.

Beyond that, guiding is the key to sorting out most problems.

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Yep. Two minutes is fairly ambitious without guiding unless your mount has low periodic error and you've drifted or Pole-Mastered or some such to really dial in your polar alignment.

For example my CEM-25P is touted to have low PE: guaranteed to have less than 10" over the Turn of the Worm. But that's 2" in a two-minute exposure. Minimum. Depending on the angular size of your pixels (probably that or less with a 9" cat!), that might be already objectionable.

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As above, good polar alignment will help and then it's down to your mount.

Choosing a target closer to the pole will also mean less star movement, but you're still likely to see some.

I have an NEQ6 and one night imaging M1, there were some clouds rolling in, so I switched off guiding for around 15 minutes.  When the clouds had gone, I went to look for the guide star, but to my utter amazement it was in exactly the same place!  I think I must have just got lucky with the polar alignment and the mount was working through a part of it's gearing where there was low periodic error.

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