SharpStar 20032 and Canon 6D mk2 artefact

[CrispyCod]

Hi,

First off, hoping I have posted this in the right section and secondly really hoping someone can help me resolve an issue...

Bit of background, been doing astrophotography for at least 5 years now, with a Skywatcher Explorer 200p (originally bought for visual) and Qhy8l, EQ6Pro which have all served me well. got some nice images (and some bad ones) and learnt a lot. Took the plunge recently and upgraded the scope to a Sharpstar 20032 Astrograph, with a view to future proofing for a different camera at a later date. I also own a Canon 6D mk2 for normal photography.

First night out I thought I would try out the full frame capability and attached the Canon 6D mk2 via a Baader canon adaptor (the one that allows fitting of filters) and added my IDAS D2. which in all gave me the required ~55mm back focus. All was going well until I viewed the resulting image in Pixinsight and found the artefact shown in the attached images. At first I thought it was just vignetting and did a strong DBE to see and ended up with the second example (image is from a 'bench top' set of tests). I have attached with and without coma corrector and with and without a DBE pass (division) in Pixinsight. one on the night and the remaining on the bench with a t shirt over the end and a dim light source.

I am at a loss as to what the cause is and how to resolve I am concerned that there is something wrong with the scope itself, I can use the QHY8l no problem, but dont want to find out in a years time when I buy a lovely large mono camera that it still has an issue. Things I have tried:

Removing the IDAS filter from the image train, covering the viewfinder with the useful rubber thing on the camera strap, removing the CC, (made the artefact larger), recollimating, flocking the threads of the CC at the scope end (helped contrast but nothing else) manually playing around with back focus (no discernible difference), manually reducing the aperture at the M48 adaptor (this worked, but I did shrink by around 50% so not a practical solution) and a huge amount of internet searching.  And as it goes with these things no one else appears to have the issues that I can find.

I will be trying to see if Flats get rid of it, manually using the images I have are inconclusive at this point.

As well as suggestions or pointers as to the cause, I would love to hear from anyone who has used a full frame DSLR with this scope to put my mind at ease, and even more so the same combination of camera and Scope!

I am in touch with the Vendor who has been and continues to be very helpful and also with Sharpstar, less so.

 

Hope someone here can help!

 

Look forward to everyones thoughts...

[tooth_dr]

It could be reflections of the mirror box from the DSLR, with the sensor set far into the body. It’s a problem with my Nikon D800E FF DSLR on my 180mm F2.8 newt, and is difficult/impossible to correct fully with flats.  I suggest that it wouldn’t occur with a dedicated FF astro camera as there is no mirror box and the sensor is usually much closer to the front of the camera  

 

Edited March 9, 2021 by tooth_dr

[CrispyCod]

Hi Tooth_dr,

 

Thanks for the quick response, I thought it could have been this, damn. To be sure when you say 'mirror box' am I right in saying you mean the rectangular 'aperture' housing the sensor and Mirror etc?

dedicated camera is a long way away at the moment, bit of a shame as the manufacturer states suitable for full fram DSLR, and if a Nikon and Canon dont work, suggests a problem?

 

Any other means of addressing, maybe an tiny aperture adjustment?

[tooth_dr]

55 minutes ago, CrispyCod said:

Hi Tooth_dr,

 

Thanks for the quick response, I thought it could have been this, damn. To be sure when you say 'mirror box' am I right in saying you mean the rectangular 'aperture' housing the sensor and Mirror etc?

dedicated camera is a long way away at the moment, bit of a shame as the manufacturer states suitable for full fram DSLR, and if a Nikon and Canon dont work, suggests a problem?

 

Any other means of addressing, maybe an tiny aperture adjustment?

I'll get back to your PM directly when I get home.

I didnt pursue the issue, but it is indeed due to the position where there mirror folds down during the exposure.  Mine wasnt as bad as yours though, and presented as a band along the bottom of the image only.   I am using a mono APS-C sensor with the scope now instead of the DSLR

[tooth_dr]

This is my master flat, evidence of similar pattern but a lot less severe.

 

[CrispyCod]

Yep, Can see it in the middle there. Interesting indeed. Thanks.

Big thumbs up for the forum, a week of searching, nothing, one post on Stargazerslounge and I have some progress!

Not sure any flocking inside the camera is going to be a good idea. I wonder if anyone else has any suggestions as to a means of resolving?

In the meantime I am going to try very slight aperture adjustments, so will lose a some of the focal ratio or 'speed' but may get me somewhere.

As I said not the end of the world, even with the Qhy8l I am going to be getting better results than previous. but it would be nice to use the Canon.

[nfotis]

I suppose that you covered the camera eyepiece before taking the photos?

N.F.

 

[GalaxyGael]

Reviving this thread just in case you see it and maybe check this longish video (use auto translate if you don't have German like me). It seems like a similar issue for the 15028 HNT version, and I am looking at one of the other of these myself.

About 22 minutes in: