Quote:
Originally Posted by gregbradley
That's a bummer. Careful about removing the focuser as its shimmed to make it square. They didn't tell me that and the shims fell out. I put them back where I thought they were and everything seems fine.
I looked at this image in CCDInspector and it says there is quite a bit of tilt and collimation is off 3.1 arc seconds (not much). I don't know how much to believe CCDInspector although I noticed it can occasionally give inconsistent readings but its a guide.
At your focal length I would recommend a large Tpoint model and use the Protrack corrections if you are able to. I find it can take stars just that bit more to round.
Also try an infrared 750nm 25mm filter in front of your guide camera. Sometimes the guide stars are not bright enough but other times they are and I think that can help sometimes as well.
Greg.
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Hi, Greg,
Thanks for the carefully thought and helpful reply, and the warning about the shims on the focuser.
I've attached a pic of the brace that we've added to stop the focuser flopping in RA as we track across the sky. The brace is extremely lightweight but very strong east-west. It reduces the amount of flop by a factor of at least five. I did of course do the maths to make sure that the focuser could still move in and out a good 5 mm (which is heaps) before exceeding the elastic limit of the brace! Colder weather should help further.
We use a reasonably elaborate pointing model. I think you've hit the nail on the head with camera tilt being the culprit. As long as the camera can flop about under gravity, fiddling with the tilt won't help. The current brace just prevents (most) east-west flop. We plan to replace it with a system that braces both north-south and east-west, and even more firmly.
The new system should also have fine adjustments to help with camera tilt.
To make this a bit more general for other readers, we note that we've found this sort of anti-flop arrangement to be useful on other scopes, even a Celestron C11, and on guidescopes and the like.
Very best,
Mike