Quote:
Originally Posted by Startrek
Stephane
A couple of things I just thought of over and above previously mentioned tilt etc.... or even coma which it’s not
1/ You mentioned your guiding was 0.60 arc sec error total ?
What was the individual errors on Ra and Dec
If Ra and Dec are significantly different, especially Dec , this will affect your Star shape even though the guiding rms or average is good
Maybe check that out
For example, say Ra 0.52 and is Dec 0.78
2/ Also you rig could be suffering from some flexure in your guiding scope set up
Are you using a central axis mounted guide scope piggybacked on the OTA or just using the offset shoe bracket with a single bracket mounted finder / guide scope ??
Recommend the central axis piggybacked mount using a solid losmandy bar
3/ To prove your PA is ok ( forget relying on the software for the moment ) just find a bright star up in the east / north east ( say magnitude 1.5 to 1.0 ) at 40 to 60 degrees in Alt , centre it , start tracking , don’t use any guiding and take a 30 sec , 45 sec , 60 sec and 80 sec exposure
If your PA is good you should be getting a round Star to at least 60 sec unguided. If your only getting 30 to 45 sec exposures before the Star starts to morph then your PA is not good ( possibly +5 arc minutes ) I normally can take up to 60 to 80 sec exposures unguided on the same rig before Star shape is affected and my PA is usually around an arc minute or less
Even with guiding, poor PA can cause all your smaller stars ( not the big bright ones ) to be eggy or odd shaped usually in one direction , on subs longer than 3 minutes over a long period of tracking time
Just a few other things I thought of which may help !!
Cheers
Martin
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Martin, thank you for your detailed response. My guide scope is on a shoe bracket fitted where the finder scope was originally. I have had problems with it moving. And although I can’t pinpoint the exact issue, I believe it is the main cause of my problem (there may be multiple problems, but this one I think is the cause of star drift). I will consider getting a solid Losmandy bar and mounting it on solidly.
As for guiding, I will need to check in detail tonight weather permitting.
I will also try your PA test tonight. I wouldn’t be surprised if this also is an issue, although I wouldn’t expect an overall star drift throughout the night. I’ll take an unguided 60 sec exposure on a bright star and see if there is any drift. Anything to look out for in guiding graph for poor PA?
Thanks again, Martin. Your comments are much appreciated.
Regards,
Stéphane
P.S. Attached is a 2-minute sub from last night. Note that stars are much rounder than in my 5-minute image. (Sorry the compression is terrible but you can see the stars)