Poor Star Shapes

[Richard Sauerbrun]

Hello group

I am seeking assistance solving a troubling issue with my images. Rather than round, all my stars seem to have a triangle shape with the point at the bottom. I am using an 8" SCT on a Celestron AVX mount with a 240mm guide scope and camera. The mount has been carefully polar aligned and balanced, and I have a good 3-star alignment. I use the CPWI app to control the mount and have trained it for PEC correction. The imaging train is tight and does not seem to flex.

PHD2 seems to perform well, and is doing a dither every other frame (using NINA for my sequencer). Here is the link to my guide log ... https://openphdguiding.org/logs/dl/PHD2_logs_q77m.zip

I am also attaching a zoomed in view of one of my subs ... so you can see the star shapes.

Any help on what to do next will be very much appreciated ... thank you.

[bw_msgboard]

That looks like an optical problem to me, the star shapes look comatic - something that can be made worse by collimation error.  I recommend you take some short, unguided exposures (e.g. 5 sec) of a star-field and see what all the stars in the field look like.  You will want to consider the shape of the stars relative to where they are in the field of view to sort out issues like coma and field curvature.  You should do this testing when the scope has reached thermal equilibrium, something that may take a long time if you are working in a hot location.

 

Good luck,

Bruce

---

From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Richard Sauerbrun
Sent: Friday, June 18, 2021 7:27 AM
To: Open PHD Guiding
Subject: [open-phd-guiding] Poor Star Shapes

--
You received this message because you are subscribed to the Google Groups "Open PHD Guiding" group.
To unsubscribe from this group and stop receiving emails from it, send an email to open-phd-guidi...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/6a7c5a48-881d-4b77-adf0-b40ecdff53ddn%40googlegroups.com.

[Michael Freeberg]

Hi Richard,

What is the focal length that you are guiding?  You may want to consider possible using a oag instead of a guide scope with the sct.  It is possible that you are getting some mirror flop in the sct.  I would also check the collimation of your sct.  I had a sct and used a oag , that seemed to work well.  

Mike

[Brian Valente]

I agree with Bruce it looks like coma that would be addressed via collimation

you didn't mention which part of your image that subframe was from. a full resolution full frame would help to better evaluate the image

Look across the frame and see if the start shapes are different in the middle vcs. at the corners

--

You received this message because you are subscribed to the Google Groups "Open PHD Guiding" group.
To unsubscribe from this group and stop receiving emails from it, send an email to open-phd-guidi...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/6a7c5a48-881d-4b77-adf0-b40ecdff53ddn%40googlegroups.com.

--

Brian 

Brian Valente

portfolio brianvalentephotography.com 

[Richard Sauerbrun]

Thank you to Bruce, Mike & Brian ... all great comments.

These stars were located just to the right of center in my image, so they were not near the edge. I am in Scottsdale, AZ, so the temps at midnight are still above 100F and get no better all night. I am using an f6.3 focal reducer, so the C8 is at 1280mm. I have tried an OAG, but had all kinds of trouble with the pick off mirror and getting good focus for the guide camera. Tonight I will check my collimation.

Thanks again for the feedback.

[bw_msgboard]

If you're using a focal reducer, you have to be sure the camera/fr spacing is accurate or you will create more optical aberrations.

 

https://agenaastro.com/articles/guides/focusers/focal-reducers-guide.html#working

 

In temperatures like you're talking about, the OTA is never going to reach thermal equilibrium, so there will be an unstable layer of hot air over the mirror that acts like an unwanted lens. You might be able to mitigate this a bit if the OTA has fans that can be left on.   But this may not be a good time to be pursuing these problems.

 

Bruce

---

From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Richard Sauerbrun

Sent: Friday, June 18, 2021 8:18 AM
To: Open PHD Guiding
Subject: Re: [open-phd-guiding] Poor Star Shapes

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/3b164aa1-f867-479b-afd8-4f3ea143ceabn%40googlegroups.com.

[Richard Sauerbrun]

Thanks again, Bruce ... more great info to absorb

I am running my ASI294MC-Pro with the Celestron f6.3 reducer at a 105mm back focus.

Maybe I will not spend a whole lot of time on this until it is cooler.

[Brian Valente]

>>> I am in Scottsdale, AZ, so the temps at midnight are still above 100F and get no better all night.

  >>>Maybe I will not spend a whole lot of time on this until it is cooler.

that sounds like a great idea :) 

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/2ea4d691-f3eb-41df-b331-1444e594f4dan%40googlegroups.com.