Non circular/egg shaped stars

[David Crane]

Hi

I have developed non circulars that I can't find the reason for.

My equipment is an HEQ5 out, Atik414EX imaging camera, QHY5-ll guide camera, 80mm TS Photoline APO triplet scope.

Everything has been checked for tightness and flex. in the optical path. The crayford focuser has been checked over and is tight.

P1 v2.bmp

P2 v2.bmp

P3 v2.bmp

P4 v2.bmp

[vlaiv]

This is not related to either flex or poor guiding.

It looks more like astigmatism related issue. Are you using focal reducer / field flattener with TS 80 Apo?

I have the same scope and I noticed that x0.79 FF/FR produces this effect - in my scope it's in the very far corners and it's related to tilt and spacing. Although everything looked tight with focuser, it was actually clamping of FF/FR that was not straight - for that reason I decided to go with threaded application - something that I solved last autumn, but still awaits for first light to see if it's been solved.

[david_taurus83]

Do you have a field flattener fitted? Also, are the 4 frames you've posted individual frames or drops from a bigger frame? Might be easier to post jpegs as .bmp don't show up on the forum.

 

[Thalestris24]

Yes, post jpegs please! Can't see the bmps too clearly. However, it could be a combination of things. How long were your exposures? Are you using the fw with the scope? You may have some flex/tilt/sag in the image train which is complicating things. It can be useful to take some short exposures of a bright target to help eliminate some causes. I don't think you need a ff/fr with the Atik414 - it's a small sensor (11mm diag). What guide scope are you using - how is it attached?

Louise

[David Crane]

Sorry, Here are the files as jpg's

[David Crane]

9 hours ago, david_taurus83 said:

Do you have a field flattener fitted? Also, are the 4 frames you've posted individual frames or drops from a bigger frame? Might be easier to post jpegs as .bmp don't show up on the forum.

 

Hi , no there is no flattener fitted , its a case of looking at the asymmetrical location of the distorted stars i thought a flattener would only correct a uniform problem eg all stars a egg shaped towards the corners . I needed to make sure the objective lens was not causing the defect

[David Crane]

6 hours ago, Thalestris24 said:

Yes, post jpegs please! Can't see the bmps too clearly. However, it could be a combination of things. How long were your exposures? Are you using the fw with the scope? You may have some flex/tilt/sag in the image train which is complicating things. It can be useful to take some short exposures of a bright target to help eliminate some causes. I don't think you need a ff/fr with the Atik414 - it's a small sensor (11mm diag). What guide scope are you using - how is it attached?

Louise

Hi , The images are stacks of 6 min exposures in LUM , the eggy stars are visible in a single 6 min sub I'm using a TS photoline super APO 80/40 , I understand some fast scope do suffer from elongated stars without a flattener, the issue for me is its not a uniform distortion. This scope would need a similar correction in 2 corners and different ones for the other 2 corners

[David Crane]

9 hours ago, david_taurus83 said:

Do you have a field flattener fitted? Also, are the 4 frames you've posted individual frames or drops from a bigger frame? Might be easier to post jpegs as .bmp don't show up on the forum.

 

The images posted are a 4 panel mosaic before combining with MS ICE.

[swag72]

You say in the original post that you 'developed' these egg shaped stars....... can you shed some light in that? Was it all working OK at some stage? Has this come on just recently?

[Thalestris24]

10 minutes ago, David Crane said:

The images posted are a 4 panel mosaic before combining with MS ICE.

I'm pretty sure the atik414 sensor is too small To need a flattener. You need to post some single subs (As jpegs!.) You didn't answer about guide scope?

Louise

[David Crane]

Just now, swag72 said:

You say in the original post that you 'developed' these egg shaped stars....... can you shed some light in that? Was it all working OK at some stage? Has this come on just recently?

It may be better to say they probably were all the time , it was that location ie unable to see polaris led to terrible tracking issues and focus related problems meant few nights with subs of any value

[David Crane]

Just now, Thalestris24 said:

I'm pretty sure the atik414 sensor is too small To need a flattener. You need to post some single subs. You didn't answer about guide scope?

Louise

Sorry , the guide scope is a QHY mini guider with a QHY5 2 mono , the single subs exhibit the same problem , here is one of the subs

Thanks

[swag72]

In my opinion, if this was a guiding issue then the stars would be elongated in the same direction across the entire frame.

[Thalestris24]

2 minutes ago, swag72 said:

In my opinion, if this was a guiding issue then the stars would be elongated in the same direction across the entire frame.

David says it was a 4-panel mosaic....

[David Crane]

Just now, swag72 said:

In my opinion, if this was a guiding issue then the stars would be elongated in the same direction across the entire frame.

I agree , Its a question of if a flattener is not required due to the size of the sensor then an objective issuse ??

 

[swag72]

1 minute ago, Thalestris24 said:

David says it was a 4-panel mosaic....

But he has shown the individual panes, each one showing elongation and if it was guiding then it would be the same across each pane shown and there wouldn't be the difference in the top right corner (as it's been shown)

[Thalestris24]

1 minute ago, David Crane said:

I agree , Its a question of if a flattener is not required due to the size of the sensor then an objective issuse ??

 

I don't think you have an optics problem.

[David Crane]

Just now, Thalestris24 said:

David says it was a 4-panel mosaic....

Yes it is the seperate stacks ... each of the 4 show eggs with differing directions , ruling out guiding maybe . The kit has also been on a different mount with no improvement  

[David Crane]

Just now, Thalestris24 said:

I don't think you have an optics problem.

What would your next step be.

so far  :

Track eliminated via a different mount.

Light path tight.

Filter wheel filters checked to make sure they are seated correctly , 

There is one lump of glass left that may need correcting image wise or has one of its elements causing the problem

 

But if I've missed something less expensive ? 

[Thalestris24]

I think you just need to run some methodical tests. First, arrange your camera so that it's in line with RA and DEC. Then take take some short single subs of, say, a bright open cluster. Then analyse/post the results.

[David Crane]

12 minutes ago, Thalestris24 said:

I think you just need to run some methodical tests. First, arrange your camera so that it's in line with RA and DEC. Then take take some short single subs of, say, a bright open cluster. Then analyse/post the results.

Ok , need some clear skies . thanks for you advice.

[Thalestris24]

ps it's almost certainly not a problem with the scope objective optics (unless you dropped it on something hard!). It could be some tilt in your optical train but you need the RA/DEC camera alignment (which you should have anyway) so that you can be sure what part(s) of the image are being affected.

Louise

[michael8554]

Strange.

9/10ths of the frame looks like a guiding problem, except top right is going the other way. ......

As Louise says, take a 20sec unguided shot, then guiding artefacts will be eliminated.

Michael 

[swag72]

To eliminate guiding as the cause I would take shorter exposures than 20 secs and do about 5 sec maximum..... that way you can be SURE that any elongation isn't due to guiding. I personally would feel that 20s is too long unguided. Then I'd rule out the focuser........ Put your telescope to zenith so that it's pointing straight up and there's no gravity effects on the focuser. Tighten it all up in that position..... again gravity will do the best work to ensure that it is square in the draw tube etc..... Then take some shots. 

The next test after this would be rotating your camera by 90 degrees and seeing if the elongation changes position. I can't ever remember for the life of me what it means though...... There's two options, the elongation moves or it doesn't (in line with the camera rotation). One option points to the camera sensor and the other to the optics, but I can't work out which option relates to which issue!!!

Let us know how you get on.

[michael8554]

I agree Sara, I realise my suggestion of 20secs was too high.

If you rotate 90 and the problem moves 90, then problem is not the camera.

Michael