Evoguide 50ED Odd Star Shapes

[Andyb90]

Hi everyone,

I was using the Evoguide 50ED last night with my ZWO ASI290 camera for guiding and a new IR cut filter. I'm using PHD2 and noticed the star shapes seemed to have a 'tail'. The stars were affected across the field. It almost looked like coma. I removed the IR cut filter to see if that was causing the problem and the shapes did not change.

Has anyone else seen this with an Evoguide 50ED? I'm not sure if coma is even possible with a doublet. Or could there be some other cause?

Andy.

[vlaiv]

This is due to poor lens figure. I had this with TS 60mm guide scope - fast achromat at F/4. Stars looked like coma sign. Not true coma, but mixture of aberrations - oddly shaped central star with small "curly" tail to one side.

For guiding it really did not matter, and I accepted it as being aberration from fast achromat optics. However, I would not be happy to see such thing on ED scope, although designated as "guide" scope - I would expect lens figure to be good enough to do some widefield imaging with it.

[paul mc c]

Thats interesting,although not an Evoguide,but wy 50mm guide scope with qhy cam started showing the same thing,and i hav'nt touched it,but only on the right of the screen.

[vlaiv]

I guess it could be issue of decentered / tilted lens. One of two lens elements not seated properly or something like that?

[spillage]

I wouldn't worry too much. My stars in phd are really awful but I still manage around 0.6-0.8.

[Andyb90]

3 hours ago, vlaiv said:

This is due to poor lens figure. I had this with TS 60mm guide scope - fast achromat at F/4. Stars looked like coma sign. Not true coma, but mixture of aberrations - oddly shaped central star with small "curly" tail to one side.

For guiding it really did not matter, and I accepted it as being aberration from fast achromat optics. However, I would not be happy to see such thing on ED scope, although designated as "guide" scope - I would expect lens figure to be good enough to do some widefield imaging with it.

Thanks for the response.

The guiding seemed fine, but I think I'll return the scope for a replacement as I don't think I could ever sell it with this issue.

[Andyb90]

On 15/02/2019 at 13:29, vlaiv said:

This is due to poor lens figure. I had this with TS 60mm guide scope - fast achromat at F/4. Stars looked like coma sign. Not true coma, but mixture of aberrations - oddly shaped central star with small "curly" tail to one side.

For guiding it really did not matter, and I accepted it as being aberration from fast achromat optics. However, I would not be happy to see such thing on ED scope, although designated as "guide" scope - I would expect lens figure to be good enough to do some widefield imaging with it.

I managed to get a screenshot from PHD2 of the guide star. Do you think it still looks like a lens issue from the screenshot?

I guess I'll also need to rule out an issue with the ASI290mm mini too.

[happy-kat]

Was that star in the centre of the frame?

[vlaiv]

That looks like coma, regular coma - it should not be there in refractor.

Do stars over whole field have this same aberration? It is likely that lens are not properly seated in lens cell.

[Andyb90]

1 hour ago, happy-kat said:

Was that star in the centre of the frame?

 

58 minutes ago, vlaiv said:

That looks like coma, regular coma - it should not be there in refractor.

Do stars over whole field have this same aberration? It is likely that lens are not properly seated in lens cell.

I just found an individual guide image frame. It looks like the issue is on stars throughout the frame.

This image is the circled stars from the centre of the frame zoomed in. The same 'tail' is visible on the larger star.

 

[happy-kat]

This is a 4 second exposure from the middle of an image I took with mine using a DSLR and noisy ISO3200, the stars in the middle are pretty round.