stars look like rice grains

[Somnium (Aidan)]

i have been back to imaging with my NEQ6 pro, 10" f4 newt and QHY9 after a significant absence while i built my observatory. this was more of a system check to iron out the bugs in my remote system, but i came across a strange issue and i can't work out what the problem is. my stars, when viewed up close look like grains of rice, the width of the rice shape is the size the star should be when perfectly focused but it is about 1.5 - 2 times longer than it is wide. this appears across the field and is not shaped like coma or other aberrations that i am familiar with, i have investigated the following

• focus - not the issue, i have stepped through the focus of the stars extremely slowly and doesn't resolve the issue
• collimation - i have collimated and re collimated and this doesn't impact the issue, my laser collimator could be out of whack, but i wouldn't expect this type of issue due to collimating problems either.
• tracking - i thought it was a tracking issue which would make sense but the polar alignment is perfect and the issue remains consistently with 1 - 2 second exposures (taken during focus)

This has me stumped and is very frustrating, i will post a picture of the issue tonight

Thanks for the help

[peter_4059 (Peter)]

Flexure between the guider and the imaging train?

[Somnium (Aidan)]

Originally Posted by peter_4059

Flexure between the guider and the imaging train?

Possible, I have had issues with that in the past but I wouldn't have thought it would show up on 2 sec exposures and the issues doesn't get worse over 5 or even 10 mins ...

[billdan (Bill)]

Hi Aidan,

Very unusual to get elongated stars at 2 secs exposure.

You may have to go through a process of elimination.

Are the longer lengths aligned with the RA axis? If so this would suggest a tracking issue and not collimation.

You can eliminate the guidecam and track unguided with good polar alignment.

You can eliminate the QHY9 as the problem by looking through an eyepiece.

Otherwise see if the problem disappears in different areas of the sky which may suggest a balance problem.

It could also be vibration, make sure the camera is bolted down tight.

Regards
Bill

[raymo]

Hi Aidan, I had the same problem, which I thought was a tracking problem, but the elongation didn't get any worse with longer exposures.
Turned out that the camera wasn't precisely perpendicular to the light
path. Took the camera off, put it back on, problem gone. If you are
getting elongation after only 2 secs, it's not going to be a tracking problem unless the motor is not running, or running very slowly.
raymo

[billdan (Bill)]

Good thinking Ray, reckon you nailed it.
Bill

[Somnium (Aidan)]

that may explain a lot ... the issue is repeatable, if i mount the camera from scratch it is likely to happen again. i wish i had ccd inspector, that would help ...

[raymo]

Is there nowhere between the camera and the focuser draw tube that you can randomly insert a small piece of thin cardboard that will tilt the camera a little to one side? By moving the piece of cardboard between
2 sec subs you could find out which way the camera is leaning [assuming that this is the problem.] Just a thought.
raymo

[Somnium (Aidan)]

Originally Posted by raymo

Is there nowhere between the camera and the focuser draw tube that you can randomly insert a small piece of thin cardboard that will tilt the camera a little to one side? By moving the piece of cardboard between
2 sec subs you could find out which way the camera is leaning [assuming that this is the problem.] Just a thought.
raymo

yeah, i will give it a go when i head down there again this weekend. i suspect it is down to the way the mounting of the filter wheel to the camera is done with the QHY9. i will have a play around and see how i go