Analysis of the sub-second performance of a CEM70 mount

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timh
Posts: 515
Joined: Mon Aug 26, 2019 5:50 pm

Analysis of the sub-second performance of a CEM70 mount

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Post by timh »

When a set up is not delivering round images of stars then there is a long list of 'usual suspects' - and especially whenever a long focus Newtonian is involved - inter alia - collimation, guiding, coma correction, alignment, focuser tilt etc.

I was recently surprised to discover that something usually taken for granted - the smooth movement of the mount on the sub-second time-scale - i.e. the details of how the stepper motor stepping is converted to RA motion - can also be the culprit. Replacement of the mount by FLO solved the problem in my case.

I thought that it might be of value to describe this story in more detail because (for all I know) it might be quite common across mounts and potentially relevant to anyone seeking better resolution via short 'lucky imaging' exposures for deepsky at finer image scales. Don't just assume that 'seeing' is the limitation - it might in fact be the mount. Maybe rather likely to be the mount in fact when - as I was last year - trying to use a goto Dobsonian for second-scale lucky imaging?

I had been using the CEM70 equatorial mount for about a year. For guided exposures of usually a minute or so at an image scale of about 1 arcsec per pixel it had seemed to perform OK. On close examination stars were perhaps never quite round (except when the seeing was poorer) but given such a long list of possible causes I had always put it down to inequalities in RA v DEC guiding or problems with the telescope set up. However - when I switched to doing deepsky lucky imaging at exposures of just 2 seconds down at an image scale of 0.5 arcsec/ pixel the problem of ellipsoid stars became obvious. For 2s exposures uneven guiding could be ruled out as a cause. The fact that the long axis of extension always lined up with RA was a clue ..

So I then tried to devise a simple (if rather painstaking) method to analyse what was going on which involved 1) using extremely short (100ms) exposure images (captured as Sharpcap .SER files and converted to .FIT files using PIPP) to first prove that the problem didn't lie anywhere with the optical path and then 2) mapping star movement versus the pixels of the camera sensor with successive 100ms exposures for 20s or so. This indicated that the mount was 'wobbling' as it moved along in RA. Simulation based on the observed movement provided a good fit to the distorted star shapes seen in longer exposures.

The details are provided in the attached Powerpoint which I hope wil be fairly self-explanatory..

Tim
Attachments
Star shape. CEM70 wobble effect. before and after.pptx
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