Some advice about elongated stars

[Snoani]

Evening all

Since putting together my rig earlier this year, I feel that my images have improved massively and continue to improve as I gain experience and learn from my mistakes.  However, one thing that I have been plagued with from the start is elongated stars in the corners of my images.  I had honestly believed that these were due to a back focus spacing issue but I have taken the opportunity tonight to run an experiment and it appears that my field flattener and flattener/reducer are both spaced and working perfectly, so I am now left with the conclusion that it is another issue.  I am therefore looking for a little advice.  

My setup consists of a Meade 6000 Triplet APO at 480mm, a ZWO mini guide scope at 120mm focal length and a ZWO ASI 120MM mini guide camera which are guiding an HEQ5 (Rowan belt modified).  I am current imaging with an ASI 1600MM and filter wheel or DSRL when I want to shoot in colour.  I have both a field flattener and an 0.8x flattener/reducer depending on what focal length I want to use.  

To give an idea of the problem the image below the corners of a stack of 6 images for the other night which show that I have elongated stars in all corners, but that these are predominately worse on one side, although sometimes all corners are affected more equally.  

As I say say I believed this to be a spacing issue but tonight I have run an experiment where I created a field of illuminated dots and focused the telescope on these to see if the dots became elongated in the corners, but the result of this was that they generally were not affected.  The dots are slightly wonky because I had to capture each corner separately but they stayed round, showing that the flatteners are doing their job.  

I am therefore left to consider that the issue could be down to three things, or a combination of factors.  

Mount/guiding accuracy:  When I first started with this mount I was getting a total RMS error on PHD2 of about 1.5-2 seconds.  However, over time I have taken more time to level the mount, improved polar alignment (currently using Sharpcap) and I have installed a Rowan belt modification to reduce backlash, all of which means that I now see a RMS guiding error of between 0.5-1 seconds.  The ground that the tripod is setup on is solid and there shouldn't be any vibrations that I can think of, so I am not confident that this is the issue but I am happy to be told differently.     

Differential flexure - I therefore wonder if the issue is related to differential flexure.  As you will see above I have a guide scope attached via the finder shoe, which I assume leaves the rig open to flexure of the guide scope in relation to the imaging scope.  If people think that this is likely to be the issue, would an off axis guider be the best solution for this, although I am conscious I wouldn't be able to use this with the DSLR due to the back focus.  

Balance - I am happy that the setup is balanced well at the beginning of each session.  I used to try to make the balance slightly east heavy, but I have stopped doing this since installing the Rowan belt mod to the mount.  Should I still be balancing east heavy even with a belt driven mount?

If anybody has any other suggestions for this problem they would be more than welcome if it helps me eradicate the issue.  

Thank you for taking the time to read this. 

Jem

[Davey-T]

Have you tried rotating the camera to see what effect it has if any, do the stars look the same in really short and longer exposures.

Maybe tilt somewhere, try focusing on the stars that appear worst.

Dave

 

[Snoani]

1 hour ago, Davey-T said:

Have you tried rotating the camera to see what effect it has if any, do the stars look the same in really short and longer exposures.

Maybe tilt somewhere, try focusing on the stars that appear worst.

Dave

 

Thank you for your reply.  I don't think it would be tilt because I think this would have shown up on the experiment that I ran earlier.  

However, I hadn't considered rotating the camera or testing the exposure length.  I am forecast some clear skied on Thursday and hopefully these will materialise.  I'll give your suggestions a try then.

[Davey-T]

It can be very time consuming trying to sort this sort of thing.

If you've got some money to spare you could try CCD Ware to analyse images although it can be tricky interpreting results.

I used to live on the IOW when I was a boy and went to their Star Party a couple of times, not sure if it's still going.

Dave

[jambouk]

Tilt was the thing which came to my mind. As above, worth trying to image with the camera rotated 90 degrees.

James

 

[Snoani]

Thank you both for your replies.  It is still looking like I shall have some clear sky time tomorrow night, so I'll start the night by taking a few images at 90 degrees before returning to my normal rotation and undertaking an image run.  I hope that it will be that simple to identify, albeit maybe not to resolve the problem.  

23 hours ago, Davey-T said:

I used to live on the IOW when I was a boy and went to their Star Party a couple of times, not sure if it's still going.

I've not attended a star party myself, although I believe that it is still running, albeit not in the last 10 months for obvious reasons.  

[Owmuchonomy]

It’s some time since I battled with this problem but a radial elongation in each corner is generally considered to be a spacing issue. Is the set up you have specified to fill your chip with a flat field? If not it could also be field curvature.