ED80 Coma and Tilt?

[Sabalias]

Hi all,

I wonder if someone could help me please? I've been experimenting with my new camera on my (second hand) Skywatcher ED80 and I'm getting some odd star shapes. I'm thinking that there is some tilt in there but I'm also wondering if I have a coma problem (top right corner seems more than just tilt). The picture below is a lights only stack through a luminance filter but all frames the same. Any thoughts and advice on correcting would be most gratefully received. 

 

Many thanks,

Stu

[alacant]

Hi

Given the mechanics of an 80ed, I'd say very good. Adjusting from here may make matters worse. For values as close as this, simply reseating the camera or any other part of the train would most likely result in different tilt.

If anything, maybe the focus is a little wayward. Best not to use a focusing mask as not all colours coincide in the focusing pattern. To get you close, choose a -not too bright- white star. Look for the minimum hfd value as you rack the focuser.. Now adjust focus -a tiny amount- until the blue halo just begins to disappear.

Edited October 3, 2022 by alacant
to reseat

[Stuart1971]

Those figures above show a bit of curvature, as the central figure is lower than the four corners…so a flattener may help, or if you have one fitted, then maybe adjust the spacing….you have an APC-s size sensor so with that scope, a flattener will help loads I would imagine…👍🏻

I dont think that is tilt at all….

Edited October 3, 2022 by Stuart1971

[Sabalias]

Thanks Alacant. I'm using a ZWO EAF  which I'm still getting to grips with so I'll work on that.

[Sabalias]

5 minutes ago, Stuart1971 said:

Those figures above show a bit of curvature, as the central figure is lower than the four corners…so a flattener may help, or if you have one fitted, then maybe adjust the spacing….you have an APC-s size sensor so with that scope, a flattener will help loads I would imagine…👍🏻

I dont think that is tilt at all….

Thanks Stuart. I'm using a flattener with recommended back-focus measured using a vernier calliper. I'll go back a check everything on that front in case I went wrong.

[Stuart1971]

Just now, Sabalias said:

Thanks Stuart. I'm using a flattener with recommended back-focus measured using a vernier calliper. I'll go back a check everything on that front in case I went wrong.

Sometimes the recommended figures are not exact, the actual distance can vary from one set up to another, best to experiment with 0.5mm steps, back and fourth to see if you can better the stars, but the Image actually looks very good, I have bad tilt on my camera and am in the process of sorting out…it’s been a real pain the the behind TBH..😮

[pipnina]

I am not sure that this is a spacing issue. In the top and bottom left you see one corner with saggital shaped stars and the other corner with tangential shaped stars. If it were a spacing issue, as I understand it, both corners should exhibit either tangential OR saggital distortion?

The opposite sidenon the right seems to be much rounder, but I am zooming in on a phone right now so it might be a trick of the eyes!

[Sabalias]

3 hours ago, pipnina said:

I am not sure that this is a spacing issue. In the top and bottom left you see one corner with saggital shaped stars and the other corner with tangential shaped stars. If it were a spacing issue, as I understand it, both corners should exhibit either tangential OR saggital distortion?

The opposite sidenon the right seems to be much rounder, but I am zooming in on a phone right now so it might be a trick of the eyes!

Thanks Pipnina. I’d be interested to read what you have to say if you get a chance to confirm. 

[Adam J]

7 hours ago, Sabalias said:

Hi all,

I wonder if someone could help me please? I've been experimenting with my new camera on my (second hand) Skywatcher ED80 and I'm getting some odd star shapes. I'm thinking that there is some tilt in there but I'm also wondering if I have a coma problem (top right corner seems more than just tilt). The picture below is a lights only stack through a luminance filter but all frames the same. Any thoughts and advice on correcting would be most gratefully received. 

 

Many thanks,

Stu

I would say that scope is out of colimation. Not sure what you would do about that in the case of a SW ED80 though as the main cell is not adjustable. You should do a star test to confirm. its also possible that the corrector itself is tilted, that con be confirmed by using a laser colimator (must itself be colimated) to check that the dot passes though the centre of the objective lens. What this is not is sensor tilt as that would not result in coma in the centre of the image. 

Adam 

Edited October 3, 2022 by Adam J

[Sabalias]

Hi All,

 

I've done a star test using my highest power eyepiece and all seemed good (though it wasn't easy to see for certain). I thought I'd try a star test using a camera and got the following result:

Does this mean anything?

Many thanks,

Stu

[Stuart1971]

2 hours ago, Sabalias said:

Hi All,

 

I've done a star test using my highest power eyepiece and all seemed good (though it wasn't easy to see for certain). I thought I'd try a star test using a camera and got the following result:

Does this mean anything?

Many thanks,

Stu

Yes, it’s out of focus….

Sorry…😀

Edited October 12, 2022 by Stuart1971

[pipnina]

48 minutes ago, Sabalias said:

Hi All,

 

I've done a star test using my highest power eyepiece and all seemed good (though it wasn't easy to see for certain). I thought I'd try a star test using a camera and got the following result:

Does this mean anything?

Many thanks,

Stu

Looks like the central white pip is off center to the left in all of the images. That could possibly be a sign of miscollimation but I'm sadly not experienced enough with refractors to say for sure 😕

[Stuart1971]

8 minutes ago, pipnina said:

Looks like the central white pip is off center to the left in all of the images. That could possibly be a sign of miscollimation but I'm sadly not experienced enough with refractors to say for sure 😕

Do you think so…looks pretty well collimated to my eyes…🤔

 

[alacant]

9 hours ago, Sabalias said:

mean anything

Remove/disable the EAF and go again.

They're great on reflectors and top end refractors, but will most likely need a carefully calibrated offset with an 80ed.

Cheers 

[Sabalias]

34 minutes ago, alacant said:

Remove/disable the EAF and go again.

They're great on reflectors and top end refractors, but will most likely need a carefully calibrated offset with an 80ed.

Cheers 

Thanks Alacant. I’ve managed to get good focus but the example above was deliberately out of focus to see if a star test would work with a camera (you probably realised that). 
 

Stu

[alacant]

3 hours ago, Sabalias said:

managed to get good focus

Yes, that's fine. You can't really do a star test with just a camera. An afocal 10mm may get you there, but perhaps the best method is by examining single frames of a star field. Anywhere in Cygnus is good for this ATM.

It's just our philosophy when troubleshooting to strip to the bare minimum before replacing items singularly, especially in this case where focus is critical and there's a focuser in the mix.

But hey, you've a relatively good example, so our initial recommendation remains: leave it as it is;)

Cheers 

Edited October 12, 2022 by alacant
ortografía - spell

[Clarkey]

1 hour ago, alacant said:

But hey, you've a relatively good example, so our initial recommendation remains: leave it as it is;

I would agree. The ED80 is quite good optically, but the focuser is barely acceptable. If you are having to pixel peep to see the problems - probably best left alone.

[Adam J]

11 hours ago, Sabalias said:

Hi All,

 

I've done a star test using my highest power eyepiece and all seemed good (though it wasn't easy to see for certain). I thought I'd try a star test using a camera and got the following result:

Does this mean anything?

Many thanks,

Stu

The outermost ring of the airy disk is not even in its illumination so collimation is most likely out. It's most even to the top right of the image indicating the likely center of the optical axis is out in that direction. Of course in the case of the ED 80 it's likely that the focus tube is not running parallel to the optical axis and less likely that your optics are not aligned to the center of the OTA tube. 

Perform a test with a collimated laster collimator and see if the dot passes through the center of the objective. If not then the focuser is your issue. 

Edited October 12, 2022 by Adam J

[Sabalias]

Thanks everyone. It’s reassuring to read that things aren’t as bad as I thought they might be 😁. I bought the scope with an old version Baader Steeltrack focuser added so comments about the focuser are particularly interesting. I’ll investigate that possibility. 
 

Stu

[Adam J]

19 minutes ago, Sabalias said:

Thanks everyone. It’s reassuring to read that things aren’t as bad as I thought they might be 😁. I bought the scope with an old version Baader Steeltrack focuser added so comments about the focuser are particularly interesting. I’ll investigate that possibility. 
 

Stu

Could be the replacement focuser is not aligned to the tube, those are solid so unlikely focus sag. Could still be the optics. Only way to know is to test.