Image issues despite good tracking

[Questor (Jon)]

I've been having some issues with my images since getting the dedicated astro cam and filter wheel setup

I'm finding a much larger proportion of my images showing elongated stars. Normally I would think this was a tracking issue but I am getting a tracking RMS error of 0.6-0.8" most nights, which is well within my pixel scale of 1.15"/px

I am wondering if it's a balance issue - I am using the standard WO 245mm dovetail for my Zenithstar 73 and I have definitely found that I am heavy on the focuser end, and I can't move the scope any further up due to the length (or lack of) on the dovetail.

Anyone have thoughts?

[oska (John)]

A longer dovetail
Since it was a result of changing your image train I would first look at back focus and tilt. You'll probably have to add a bit (0.5-2.5mm) depending on your filters and other considerations/assumptions. Do plenty of test shots of a dense starfield paying attention to your corners and centre.

[AdamJL]

Care to share an image?

[Startrek (Martin)]

Type and model of mount
Sub exposure lengths
Polar alignment error ?
Are you balanced in Ra slightly east heavy ?
Are you balanced in Dec , even centre of mass
When you say tracking error of 0.60 to 0.80” I assume you mean “guiding” error ?
Your guiding program ? PHD2 ?
What’s your Ra error
What’s your Dec error
Any periodic error
If PHD2 you can send your guide logs including Guide Assistant data to the PHD2 forum and Bruce can analyse and reply with possible issues and recommendations
The reasons behind eggy or elongated stars is a process of elimination and could be multiple causes both mechanical in the mount or optical like camera tilt etc….
Like Adam mentioned some full size ( not cropped ) images may give some hint of the issues

[The_bluester (Paul)]

I would ask a similar question to what Martin did, what are your RA and DEC RMS figures? If the RA is great and the Dec just so-so you might get a reasonable looking overall RMS but elongated stars in Dec.

That said, can you pop up a pic of what a full frame sub looks like? That might give a hint as to where to start, guiding/tracking issues will show one sort of characteristic error, polar alignment another and backfocus/tilt a different one again.

[Questor (Jon)]

Sorry, have been at work all day.

Here's an example of the issues I have been having - some of the subs look ok, others seem to come out like this even with similar guiding numbers.
I am using an HEQ5 via EQMOD with phd2 for guiding

2 minute subs also seemed to be fine, hence my thinking it's a balance issue.

This is obviously just stretched through irfanview and saved as a jpg

https://ibb.co/LzK36dX

[Startrek (Martin)]

There’s a million things to check but I would firstly check to see if EQMOD and PHD2 are causing this.
Disconnect PHD2 and just track unguided on a Star field with firstly a few 10 sec , then 30 sec , then 60 sec exposures and see if you have round stars. Then switch on PHD2 and guide with say 60 sec exposures and check your stars
Also check your settings in EQMOD for PHD2 ( see attached)
The image has roundish stars on the far LHS but centre and RHS definitely elongated.
I don’t think balance would affect Star shape at different sections of the image , this more typical of camera tilt or something in the image train. Worse case differential flexure but this normally occurs on longer scopes like newts
When guiding are your Ra values similar to Dec values ?

Hope others can chime in to help

Cheers
Martin

[alpal]

Originally Posted by Questor

Sorry, have been at work all day.

Here's an example of the issues I have been having - some of the subs look ok, others seem to come out like this even with similar guiding numbers.
I am using an HEQ5 via EQMOD with phd2 for guiding

2 minute subs also seemed to be fine, hence my thinking it's a balance issue.

This is obviously just stretched through irfanview and saved as a jpg

https://ibb.co/LzK36dX

Looks like you have more than 1 problem.


You'll need to answer all the questions from Startrek at - Today, 10:13 AM
to get anywhere with this one.
You could have a faulty mount -
too much play in one axis?
Are you using an OAG?




cheers
Allan

[AdamJL]

Looks like some tilt as well. You can run the aberration inspector in NINA to check what your tilt is like to correct it out.