Tilt and coma

[v4169sgr]

Results of a few little experiments Saturday evening. All measurements taken interactively as I went, informing me of sensible next steps.

All single frames, without calibration or any other processing. Aim is to understand a little more about how tile and aberrations affect star images towards the edges of the frame.

The assembly is: Canon 450D + WO 'Copper' T-adapter + 2mm spacer + Baader MPCC + Baader twist lock clamp, fitted to a Baader Diamond Steeltrack focuser on my OO UK CT8 Newt. The whole arrangement designed to minimise the effect of uneven or unequal tightening of grubscrews etc.

Images shown are the results of applying the 'tilt' command in Siril, that "computes the sensor tilt as the FWHM difference between the best and worst corner truncated mean values ... expressed by the difference in FWHM between the stars in the center and the stars on the edges of the sensor." Worth bearing in mind that "the drawn quadrilateral has its proportions exaggerated, in order to be more visible on the screen."

Order of images shown is top to bottom, left to right, in order of images taken. I tried rotating the camera by 90 degrees (refocusing each time between images), then repositioning it back to the original orientation, then taking it out of the clamp and replacing it, then simply taking another frame changing nothing to understand the consistency of the measurement.

After that, I disassembled the camera and coma corrector assembly, took out the 2 mm spacer, re-assembled, then took the final two frames. All the images shown are screenshots.

My impressions:

1. Taking the camera + CC out of the clamp and replacing it, however carefully done, introduces noticeable tilt. This seems unavoidable. Getting a good 'un seems to be more luck than judgement.

2. I'm not sure I can see much of a difference in aberration between two frames of approximately equal tilt before and after taking out the spacer - more details in a reply to follow.

Happy to hear how others approach the whole issue of non-circular stars in their frames. I think I've done what I can short of replacing the coma corrector.

 

 

 

 

 

 

 

[v4169sgr]

Next two images are a corner of the first and sixth frames, magnified by 208%. Remember there's been no processing.

[michael8554]

Take a number of short exposures with your full assembly : Canon 450D + WO 'Copper' T-adapter + 2mm spacer + Baader MPCC + Baader twist lock clamp.

Taking out and replacing each time with no rotation.

If results aren't consistent then look for a screwed connection instead of the twist-lock.

Has your Canon been astro modded, did the sensor get returned with the sensor tilt correctly shimmed ?

Michael

[v4169sgr]

Thanks for the reply, Michael.

The results above already show that they aren't consistent over the test you are suggesting. I therefore agree a screw fitting will be more consistent, but am mystified how one is supposed to screw in a Baader MPCC into a BDS focuser?

The camera has not been modded in any way.

[michael8554]

I read that the focuser has:

"S58 Dovetail ring-clamp built into focusing tube for accepting optional T-2 / M48 / M68 and other S58 direct thread adapters to enable direct thread connection of accessories and cameras without 2" clamp"

And the MPCC has M42 threads.

Should be possible to screw them together ?

Michael