C11 edge diffraction spikes

[Xsubmariner]

Hi,

Please can anyone help with suggestions on the likely source of some diffraction spikes I am experiencing when imaging with my C11 edge/0.7 reducer/S.L. maxi wheel with OAG/Baader2” filters/SX-36 setup.

The spikes can clearly be seen in the uncalibrated picture below. They were present only fainter in some Atik 383 images I took earlier in the month.

I removed and examined the reducer and it looked fine. 

Any advice would be greatly appreciated before I attempt a strip down of the scope.

thanks Martin

 

 

[vlaiv]

First culprit on my list would be prism from OAG.

However, to have that much diffraction, there should be also shadow on the image, since for OAG to produce this kind of thing it would need to be inside light path towards the sensor.

Check it anyway - best way to do it is take OAG + camera assembly and look down inside it, is OAG overlapping with the sensor? If it is - try moving it further out.

Another way to check it would be to take short exposures of very bright star - diffraction spike should be present only in one part of the image - like bottom, or side and not on the other - prism is located on only one part of the field and in others light cone should not hit it.

[The Admiral]

5 minutes ago, vlaiv said:

Another way to check it would be to take short exposures of very bright star - diffraction spike should be present only in one part of the image - like bottom, or side and not on the other - prism is located on only one part of the field and in others light cone should not hit it.

Can't offer an explanation, but most of the brighter stars all over the field seem to show some evidence of it.

Ian

[vlaiv]

Just now, The Admiral said:

Can't offer an explanation, but most of the brighter stars all over the field seem to show some evidence of it.

Ian

Yes, and most puzzling part is that there is no shadow of OAG in the image.

However, it looks like image, although not calibrated, was manipulated in some way - at least stretched, but maybe also DBE applied - at least it looks so because background around galaxy and bright stars is a bit darker.

Maybe DBE removed evidence of the shadow?

@Xsubmariner, what does your flat file look like? Does it show shadow from OAG prominently?

[Xsubmariner]

Hi Vlaiv & Ian,

Thanks for your feedback. The OAG is clear of the sensor and to double check I removed the prism and guide camera completely, yet the spikes were still present.

I have fits files available, unfortunately they are 32Mb.

Martin

[Xsubmariner]

Vlaiv, a friend was with me that night and was introducing me to Pixinsight. Will it help if I post a Lu file. What file format could I forward that will meet the 2Mb limit. Thanks 

Martin

[Stargazer33]

As there is a large defraction spike in the 4 O'clock/10 O'clock position, I would try taking an image, then rotate something and see if the large dif spike moves. If not try something else until it does move. If nothing makes it move then it must be the ota. Is the 'scope new?

[vlaiv]

37 minutes ago, Xsubmariner said:

Vlaiv, a friend was with me that night and was introducing me to Pixinsight. Will it help if I post a Lu file. What file format could I forward that will meet the 2Mb limit. Thanks 

Martin

No need to upload file if you are certain that OAG prism is not the cause of diffraction spikes.

Next thing to do is to examine your imaging train to see if you have any straight edges what so ever - this can be anything, like wire that you run across corrector plate for dew heater on secondary - anything with a straight edge.

If you can't find anything - there is another possibility to examine - are there any sort of wires in direction where you image? Like overhead power lines - no matter how distant or out of focus - as long as they are directly in light path.

One thing that might be helpful is to figure out direction of spikes in your images - straight edge that is causing it will be perpendicular to this. Do they rotate as your imaging session progresses, or are they always oriented the same? In first case - it is external to imaging train, in second - it is related to imaging train (it remains "stationary" with respect to scope).

[Xsubmariner]

Thanks Vlaiv Hi Vlaiv & Bryan,

Thanks for your feedback.  Bryan: no the edge is several years old although I have only had it for 6 months, installed in ROR observatory in January.

Vlaiv: there are no overhead obstructions and I image with a heated dewshield where cable is away from scope plate. I will check earlier Sbig images to see if they were present.

thanks again for your continued support, it is much appreciated.

Martin