Elongated Stars - Poor Guiding EQ6-R Pro

[Tommy Lease]

Hello All,

I need some assistance help with my guiding. Im currently averaging around 1" tot error and getting egg-shaped stars. I cant go over 120 sec exp time without getting elongated stars. I recently upgraded to an 8" f4 Newt 805mm FL, coming from a 350mm FL refractor. With the longer FL I've been really struggling. At 350mm, I could get 8min exposures with round stars. My mount is an EQ6R-Pro and is only a few months old. Also, Im doing pulse guiding via EQMOD. I've attached log files for your viewing pleasure. 

Thanks in advance

https://openphdguiding.org/logs/dl/PHD2_logs_pNvG.zip

[Bruce Waddington]

Hi Tommy.   I think the problem is with your guide scope setup and you’re getting differential flexure.  You can read about that in an appendix of this document:

 

https://openphdguiding.org/tutorial-analyzing-phd2-guiding-results/

 

Your guiding is as good as it was before and you should be getting reasonably round stars – and they shouldn’t become worse with longer exposure times.  There’s a simple test described in the above document that will confirm differential flexure.

 

The guide scope arrangement you have now is not up to the job of imaging at the longer focal length.  It probably employs a wimpy, stalk-mounted finder scope with endless places that will allow unwanted movement relative to the main scope.  You can move to an OAG arrangement or you can try to substantially improve the guide scope arrangement.  That would have to include much more rigid mounting arrangements without Delrin-tipped thumb-screws and preferably a longer focal length to reduce the coarseness of your current image scale (which is 6.5 arc-sec/px).   Keep in mind that your guide camera will “see” movements of 4 microns, less than 10% of the thickness of a human hair.

 

Hope you can get this resolved easily,

Bruce

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[Tommy Lease]

Thank you so much for the detailed feedback! Differential flexure makes sense. The stock finder bracket doesnt seem very stable. I will try and figure out a different method to mount the guidescope or take the plunge into OAG(which I was trying to avoid lol). Ive heard horror stories trying to get an OAG to work and just not sure I want that headache. But, if it will get me round stars, I may just have too.

So, in the logs, do you see any backlash that would indicate bad guiding? When I looked at my PHD graph I would notice a spike every once in a while and figured it was backlash.

Thanks,

-Tommy

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[Bruce Waddington]

Your Dec guiding performance is fine, it’s uniformly better than RA.  I didn’t see any evidence of backlash but have you used the Guiding Assistant to measure it?   Personally, I would go looking for problems when there’s no evidence for them. J

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[Tommy Lease]

I did run the guiding assistant, but it didn't find any issues with backlash. It just recommended to make some minor adjustments. The more I think about it, maybe my guidescope came out of alignment with my main scope?

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[Bruce Waddington]

The alignment of the guide scope and main scope is largely irrelevant, it’s not your problem.  Your problem is with the cheesy guide scope/guide camera assembly. J   if you don’t believe me, work through the simple procedure in the log analysis document I pointed to.

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[Tommy Lease]

Thanks Bruce. I will read through that document.

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[George Shoup]

I'd like to see that picture again of Hubble with his guide assembly.

No cheese there.

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[Tommy Lease]

George, huh? I don't get your comment..

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