What´s causing my focus problem? Tilted sensor? (ASI1600)

[MartinFransson]

*New tests performed March 26, 2018. See post further down*

 

I hope someone can help me.

I have a new ZWO ASI 1600MM-Cool with an 8 pos. filter wheel and ZWO original filters. I use it with a Samyang 135/2.0 lens.

My problem is that I can´t get an even focus plane even if I stop the lens down to f/4. If I get the stars pin sharp in the middle, I get odd stars in opposing cornes of the image. If I adjust it to be good in two corners + the middle, I get elongated (radially) stars in the other two corners. See attached image, autostretched in Pixinsight.

What could be causing this? It shouldn´t be a spacing problem since I use the camera + filter wheel + ZWO EOS adapter for Canon EF-mount lenses. Added up, that should work.

I have tested the lens itself using the Lonely Speck aberration test chart and it is pin sharp to the edges even at f/2 on a full frame camera.

Any ideas? I´m pulling my hair here...

Edited March 26, 2018 by MartinFransson

[MartinFransson]

I could add that the problem is somewhat reduced when using an Astrodon 3nm Ha instead of the original filters...

Could it be that the sensor in the camera is not mounted completely flat? Is there any way to investigate that?

Edited January 6, 2018 by MartinFransson

[vlaiv]

This looks familiar, I used to get star shapes looking like that, and problem is two fold.

First is sensor / lens distance - you need to get it spot on for flat field. What you see in image above is mostly astigmatism. Right side of the image is in focus but has astigmatism - cross shape, while left has out of focus astigmatism - elliptical shape - bad sensor / lens distance.

Second is that you have some sort of tilt in optical train - sensor is tilted in relation to focal plane. Right side in focus, left a bit out of focus.

Ha subs still display this sort of behavior but it is much less pronounced because star shapes in corners when using refractive optics depend on wavelength of light. So in this particular configuration Ha (red part of spectrum) is less affected, but green would be a bit more, and blue still more than that.

[MartinFransson]

14 minutes ago, vlaiv said:

This looks familiar, I used to get star shapes looking like that, and problem is two fold.

First is sensor / lens distance - you need to get it spot on for flat field. What you see in image above is mostly astigmatism. Right side of the image is in focus but has astigmatism - cross shape, while left has out of focus astigmatism - elliptical shape - bad sensor / lens distance.

Second is that you have some sort of tilt in optical train - sensor is tilted in relation to focal plane. Right side in focus, left a bit out of focus.

Ha subs still display this sort of behavior but it is much less pronounced because star shapes in corners when using refractive optics depend on wavelength of light. So in this particular configuration Ha (red part of spectrum) is less affected, but green would be a bit more, and blue still more than that.

Thanks for the analysis! Two questions then, considering the problem is two fold:

1. Why would the distance be anything but spot on since I am using the recommended gear for this. The ZWO lens adapter is specifically adjusted to get the right distance.

2. What could cause the tilt? Sensor not mounted properly? Everything else is just put together and the lens itself has a flat plane.

 

[thomasv]

I'm wondering how your camera / lens assembly is supported mechanically? Both camera and the samyang are fairly heavy. I have a geoptik EOS adapter with a feet that can be used for support, but I think the ZWO one hasn't got one. At F2 the optical train will be very sensitive to any flexture due to weight.

 

[Alien 13]

It could also be filter tilt or they not being suitable at f/2, easy to check by removing one..

Alan

[MartinFransson]

1 hour ago, thomasv said:

I'm wondering how your camera / lens assembly is supported mechanically? Both camera and the samyang are fairly heavy. I have a geoptik EOS adapter with a feet that can be used for support, but I think the ZWO one hasn't got one. At F2 the optical train will be very sensitive to any flexture due to weight.

 

I have a ZWO camera mount for the camera and a scope ring for the lens, so they should both be supported.

21 minutes ago, Alien 13 said:

It could also be filter tilt or they not being suitable at f/2, easy to check by removing one..

Alan

You mean, removing and shooting without any filter at all?

By the way, the lens is stopped down to f/4 but the problem is still there.

[wxsatuser]

Which EOS adapter have you got, adjustable or short?

[MartinFransson]

2 hours ago, wxsatuser said:

Which EOS adapter have you got, adjustable or short?

The short one, specific for use with the filter wheel. No adjustment possible.

Starting to suspect the sensor has a tilt...

[wxsatuser]

What else is in the image train, originally you said " Camera, EOS adapter and filter wheel ".

[Astrobug]

I have a similar problem on mine and also I suspect a tilt on the sensor. When rotating the cam the deformation of the stars stays in the same corner. This was done with threaded connections. A tilt adjuster should solve the problem but this can't be used on a les.

Do you feel some play in the bayonet?

Edited January 7, 2018 by Astrobug

[vlaiv]

18 hours ago, MartinFransson said:

Thanks for the analysis! Two questions then, considering the problem is two fold:

1. Why would the distance be anything but spot on since I am using the recommended gear for this. The ZWO lens adapter is specifically adjusted to get the right distance.

2. What could cause the tilt? Sensor not mounted properly? Everything else is just put together and the lens itself has a flat plane.

 

Do you have exact matching lens adapter for ASI1600? Not all ZWO cameras have same T2/2" thread - sensor distance, so lens adapter should be adjustable for flange distance (which for Canon EF / EF-S is 44 mm).

If adapter is adjustable is it lockable (like with screws?) it can cause tilt if not tightened properly, well, anything can cause tilt if not tightened properly (and sometimes even if tightened properly but manufactured to wrong specs).

[MartinFransson]

31 minutes ago, vlaiv said:

Do you have exact matching lens adapter for ASI1600? Not all ZWO cameras have same T2/2" thread - sensor distance, so lens adapter should be adjustable for flange distance (which for Canon EF / EF-S is 44 mm).

If adapter is adjustable is it lockable (like with screws?) it can cause tilt if not tightened properly, well, anything can cause tilt if not tightened properly (and sometimes even if tightened properly but manufactured to wrong specs).

Yes, I have the one for use with a filter wheel and cameras with 6,5mm back focus (like the ASI1600). This is the one I have:

https://astronomy-imaging-camera.com/products/accessories/eos-lens-adapter-efw-asi1600/

It´s not adjustable, is it?

[MartinFransson]

14 hours ago, wxsatuser said:

What else is in the image train, originally you said " Camera, EOS adapter and filter wheel ".

That´s it. And the lens of course. Samyang 135/2.0, Canon EF mount.
The filters in the wheel are all ZWO filters except for the Astrodon 3nm (which works the best)

[wxsatuser]

Allow an extra 1mm for Astrodon filters.
Don't know about ZWO filter thickness but add one third the thickness to the image train.
This may or may not be that critical but you need everything on your side.

[MartinFransson]

6 hours ago, wxsatuser said:

Allow an extra 1mm for Astrodon filters.
Don't know about ZWO filter thickness but add one third the thickness to the image train.
This may or may not be that critical but you need everything on your side.

I might be a bit thick but if the results are better with the Astrodon than with the ZWO filter... then I should take away 1mm? Unfortunately that´s not possible... there is nothing to remove.

[wxsatuser]

The Astrodon filter will shift the focus point further behind the sensor image plane, it's thicker glass than ZWO filters.
It's not by much but like I said every little helps.

Having looked around it seems a few have had problems and it points towards the EOS adapter, seems it's a bit flimsy.
I cannot verify that as I don't have one, I use a TS/Geoptik clone that is quite robust and no real problems here.

[MartinFransson]

On 2018-01-08 at 05:58, wxsatuser said:

The Astrodon filter will shift the focus point further behind the sensor image plane, it's thicker glass than ZWO filters.
It's not by much but like I said every little helps.

Having looked around it seems a few have had problems and it points towards the EOS adapter, seems it's a bit flimsy.
I cannot verify that as I don't have one, I use a TS/Geoptik clone that is quite robust and no real problems here.

After taking everything apart I´m beginning to suspect you´re right! There seems to be room for some play between the lens and EOS adapter. I thought I had that eliminated by securing the lens with a scope ring. See attached photo. However, when tightening the screws on the scope ring, I´m able to "point" the lens to different directions and something has to flex to do that.

I took everything apart, put the lens on the camera and placed it vertically with the camera down. That way there should be no flexing of the adapter. Then I gently tightened each of the screws until they barely touched the lens. Once they were all in contact I made a final tightening. Now I hope everything is without tilt. I also removed one of the filters to be able to do some tests without filter. The third test will be with the camera on my newtonian instead of with the lens.

Now all I need is some clear skies to be able to test it all...

[wxsatuser]

Hope that cures your problem because it's a very nice lens when it works.

BTW
Mine is not perfect, one corner has slight problems but I can live with it.
One can never say it's the lens or it could be the mount, after all they were meant to fix to a camera body.

[MartinFransson]

So, now I've done some more testing and the problem remains. Maybe not as pronounced, but still.

I tested:

- With and without filters. 

- Adjusting the tilt of the lens.

- Using my Newtonian instead of a lens. 

- Rotating the camera in the focuser.

None of the above helped. The problem is slightly less pronounced with the Newtonian but it's still there. Always in the same opposite corners even when rotating the camera 90 degrees in the focuser.

So, should I send the camera back to check it for sensor tilt?

I can post new images here for analysis in a while. 

Edited March 26, 2018 by MartinFransson
Spelling

[MartinFransson]

Here are a few test shots:

https://www.dropbox.com/sh/scdggkuzkz9lkfl/AAAb-XKj2w6D_pROG92anSvNa?dl=0

The single image is with the Samyang 135 mm lens, the ones in the folders are with my Skywatcher 150-PDS newtonian and Baader mkIII MPCC.

In the "Refocused" folder I have two different focus settings, to try to make the bottom left and the top right corners good. Can´t get them both in proper focus at the same time. In the "Rotated" folder I have left the focus alone but rotated the camera in the focuser, to rule out that the effect is from focuser sag or similar.

In all of my tests, it is always the bottom left versus the top right corner. Regardless of optics. Doesn´t that suggest problem being the sensor?

[Kyuzumaki]

Is this not just cosign error? The stars in all corners are slightly oval, if this is the problem and you have a different camera with a smaller sensor it should disappear.

Only other suggestion is possibly the sensor is slightly off axis? So closer to one edge than another. You could spin the sensor around and see if it rotates on the centre pixel or slightly off centre

Edited March 29, 2018 by Kyuzumaki

[MartinFransson]

On 2018-03-29 at 22:44, Kyuzumaki said:

Is this not just cosign error? The stars in all corners are slightly oval, if this is the problem and you have a different camera with a smaller sensor it should disappear.

Only other suggestion is possibly the sensor is slightly off axis? So closer to one edge than another. You could spin the sensor around and see if it rotates on the centre pixel or slightly off centre

I´m not sure what this cosign error is, but I´ve used the same telescope with other cameras (with larger sensors) without this problem.

The problem stays the same (bottom left corner vs top right corner) when rotating the camera in the focuser.

[Thalestris24]

1 hour ago, MartinFransson said:

I´m not sure what this cosign error is, but I´ve used the same telescope with other cameras (with larger sensors) without this problem.

The problem stays the same (bottom left corner vs top right corner) when rotating the camera in the focuser.

Hi

I'd suggest fitting one of these - but not sure if you have room in your imaging train? I guess you could check any adjustments with ccd inspector.

Louise

 

[MartinFransson]

1 hour ago, Thalestris24 said:

Hi

I'd suggest fitting one of these - but not sure if you have room in your imaging train? I guess you could check any adjustments with ccd inspector.

Louise

 

No, there is no room. And if there is something wrong with the camera, I´d prefer to have it fixed instead of adding extra components.

How does CCD Inspector work? I´m not familiar with that software.