Help understanding what looks like field rotation

[GraemeH]

I managed to get a good amount of time imaging the Double Cluster on Friday evening, and at first glance the processed image on my laptop screen looked quite good.  When I took a closer look today on my desktop with a much larger monitor it was obvious that there was something that looked very much like field rotation around the centre of my image, i.e. fairly round stars in the centre with some obvious trailing getting worse closest to the corners of the frame.

This is one individual sub straight from the camera.  Most (but not all) of the subs look similar.

My confusion comes from the fact that I was off-axis guiding, and whilst my guiding performance wasn't brilliant with RMS values from PHD2 around 2" in both RA and Dec there is clearly a difference in star shape in different parts of the frame.  Now, if I'm understanding things correctly then any true field rotation as a result of PA error should be centred around the guide star - please tell me if I'm wrong on this.

Can anyone offer any suggestions of what could be causing this?  For reference this was taken on a Canon 6D at prime using a Celestron OAG on a wedge mounted Nexstar Evolution Edge HD 8 - I know this isn't an ideal platform for imaging, but it's what I have.  Effective focal length based on plate solving results is 2142mm.

Thanks for any pointers that anyone may be able to offer.

Graeme

[alacant]

Hi. Try moving the camera closer to the back of the telescope and/or get the focus as perfect as possible. Is the OAG pushing it too far away perhaps? HTH and clear skies. 

[GraemeH]

Thanks alacant.  I'm actually using all the spacers and connectors as recommended by Celestron to ensure that the distance from the back of the scope to the camera sensor is precisely 133mm as required for the EdgeHD 8.  It's perfectly possible that my focus wasn't perfect, but I don't understand why that would cause the issue I saw.  Also, I've imaged a number of other targets recently with exactly the same setup and haven't seen this before.

[knobby]

Guiding at 2000mm is going to be a real challenge, I'd suggest possible pa not good enough or maybe the mirror moving ... Did you use the locks ?

[GraemeH]

I know I'm going to struggle with guiding with the setup. The ASPA error reported by the mount was around 27" in RA and around 70" in Dec, PHD2 guiding assistant indicated closer to 5' PAE. Either way, it's not the fact that I'm seeing some tracking errors that's confusing me - more the nature of the error, i.e. why am I not rotating around the guide star? 

[newbie alert]

Sounds more like a spacing issue than a guiding one...mirror flop isn't really a issue as the imaging and guiding camera's are in the same imaging train.. Sara put a picture up a while back to do with spacing..

[alacant]

3 hours ago, GraemeH said:

133mm as required for the EdgeHD 8

But if you're not in focus, it's not gonna be 133mm. In your case I'd say it's more than 133mm. Focus by moving the mirror -I think- toward the corrector. HTH.

[Filroden]

3 hours ago, GraemeH said:

I'm actually using all the spacers and connectors as recommended by Celestron to ensure that the distance from the back of the scope to the camera sensor is precisely 133mm as required for the EdgeHD 8.

Manufacturing tolerances and not overly tightening each connector will often result in under or over shooting the desired spacing. This does look like a spacing issue and the best advice I can give is to get your spacing to be just a little too short, so the any star elongations all point towards the centre of the image, then gradually add spacers until your stars are round to the edge. It is easier to add spacers then to work back from being a little too long on the spacing (which is what your image looks like).

[GraemeH]

OK, I think I understand now. If I'm a bit outside focus I'll get the effect shown in Sara's 'too much spacing' picture, which is pretty much what I'm seeing. 

Thanks for the replies folks.