Focusing problem with ASI1600 - what could be wrong?

martinf

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...

martinf

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

acab

I'm not sure about focusing. I know that the 1.25" filters are too small for fast lenses. They can cause some vignetting and perhaps some focusing problems on the edges. To be sure try imaging without the filter wheel.

Good luck

martinf

acab wrote:
I'm not sure about focusing. I know that the 1.25" filters are too small for fast lenses. They can cause some vignetting and perhaps some focusing problems on the edges. To be sure try imaging without the filter wheel.

Good luck

I use the 31mm filters and the lens was stopped down to f/4 without improvements.

I can see elongated stars on one side and if I try to correct that with refocusing I get "banana shaped" stars in the opposite corners.

Sensor tilt?

Will try without filters when I get a chance.

Sam@ZWO

I think the lens is very heavy

you'd better add one support ring to support this lens

martinf

Hi Sam!

Sorry for the late reply. The lens was supported with a ring. And the same problem occured also when using a telescope.

I´ve had the camera sent in for service. Let´s see if it works better when I get it back :)

Support@ZWO

great, hoping this time the perfromance will be perfect!

martinf

Sean.Wang wrote:
great, hoping this time the perfromance will be perfect!

Hi Sean!

I sent you an email regarding this. The camera is back from service and it is better but not good enough. I´ve so far tested with two different lenses, both supported with rings.

Support@ZWO

Hi Martin

I think we has done the best adjustment. your lens is very sensitive with sensor alignment.

martinf

Sean.Wang wrote:
Hi Martin

I think we has done the best adjustment. your lens is very sensitive with sensor alignment.

Hi Sean,

The lens might be sensitive but I have tried with two different lenses and a Newtonian telescope. All with the same result.

To really rule out that the problem is other than the camera, I have asked a friend to send me his camera (the same model as mine). If everything works with his camera then there must still be something wring with mine. If it doesn´t work, I have other problems in my imaging train.

Support@ZWO

OK, if need my help, leave a message here.

martinf

Just wanted to follow up on this. After testing back and forth with two cameras I think I have the focus issues sorted out. I can now get sharp images out to the corners with very careful fine tuning of the focus :)

Support@ZWO

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