Quote:
Originally Posted by gregbradley
Just curious, I am not implying its flexure but its not mentioned in your posts what you exact setup is so it leaves it open. Do you have a photo of your setup.
I take it you've double checked your connections and everything is seated properly, the pick off prism is firm and does not wiggle etc etc.
Greg.
|
Hi Greg,
Sorry I jumped the gun on your question, but I thought I knew where it might be heading. Of course I'm not saying my system wouldn't have any flexure. Everything does for sure, and so would my adapters and focuser, etc. It's just that the way I'm doing the tests I don't believe it is possible for ordinary flexure to explain the consistent results of drift after a slew east. For all these tests I've completely eliminated the ONAG. The camera is fitted to a Baader Click Lock that is clamped into the 2" Feathertouch. No, not ideal & not screwed together, but flex in this would show up in declination since I've got X=RA. And, it sure would be odd that flex would only exist after a slew in a particular direction, wouldn't it? I'd reason that something loose would move in both directions. Here is a quick view of the business end (camera is not RA=X in this photo!):
http://www.pbase.com/prejto/image/149159999
I certainly cannot feel anything moving in this setup!
The issues I've dealt with mostly concern two problems that I've never been able to properly rectify. One is the issue of Protrack and unguided. My supermodels have told me that I've got excellent PA, and I've used large models with 100s of points, yet unguided is all over the place. One time it will seem good but the next run terrible. I think I now realize that this has been slew dependent. Usually I'd just slew to a different part of the sky and start an exposure. From what I've been demonstrating I now think that if I slewed west I would get a good result, but that if I'd slewed east, and not waited at least a full minute, I would get a lousy result. The mount would be drifting between 5-10 arcsec over that first min and Protrack has no way to deal with that at all. Same thing with guiding I believe. If I slewed west, took a guide camera photo, select star, etc, the star will show up reasonably centered on the guide graph. But if I've done an east slew, the mount is "running away" during that first minute while selecting a guide star and plotting the first point on the graph. Often the first point is completely off the guide graph. Of course, if I wait a minute+ everything will be fine! Unfortunately I didn't understand this and thought I must be picking the wrong aggression settings, etc. You see the guide star escaping and think you need a higher setting to get it back centered. Then of course the drift ends and the setting is too aggressive. It also explains an issue that has made me nuts. I might have perfect guiding established. I finish with one filter and slew away to refocus with a different filter, slew back to the target and not be able to reestablish guiding with settings that worked just minutes before....all because I probably slewed east returning to the target.
For sure one can live with this if its understood. But, it's far from ideal and I sure didn't plonk down $10k for a mount that needs to be pampered in this way. These issues have all come into focus and demand resolution when I upped my imaging scale to .63 from 1.14. Probably for most people using the MX and doing wide field, or imaging scales greater than 1 arcsec you wouldn't really see this issue as much, but it gets amplified at long focal lengths for sure.
If I'm correct about this I bet there is a software fix that could deal with this by adjust how slews are done. Perhaps the mount needs to apply the brakes sooner and/or more gradually, or overshoot the target and slew back (west) by a couple of degrees. It could all happen automatically and transparently.
Peter