Is ZWO new OAG-L adapted to full-frame format sensors?

alainzu

Hello, I just received the new OAG-L and tested it with an ASI 2400 using a Sony full-frame format sensor.
The first results are very disappointing.
On this M31 astrophoto, there is a large shadow of the prism of the OAG on the upper large size with a wavelet-deformity of the image around the external part of the field.
This is not acceptable for me.
Is there any solution to get a clear image as I used to get before using the OAG?
Thanks for your comments

AstroDude42

alainzu so you rotated the OAG 90 degrees as Chen pointed out in https://bbs.astronomy-imaging-camera.com/d/13596-new-oag-l-prism-position-sensor right?

i don’t own that large OAG but looking at the following picture there seems to be a screw (see arrow pointing at it). On the small/old OAG, if you loosened the screw at that same position you could move the metal block (the one where the OAG‘s mirror is attached to) inside or outside. I guess it’s the same for the large OAG. Move it away from the main camera’s sensor until the mirror doesn’t obstruct it anymore. Then tighten the screw again. Take care not to scratch or break the mirror when moving it! Good luck.

w7ay

AstroDude42 @alainzu so you rotated the OAG 90 degrees...

Yes, from the frame that is posted, it looks like the prism is already on the long axis side, Olli. The OP just needs to retract the prism out from the light cone.

I am not sure that OAG's opening is large enough to fully handle short focal ratios. You will need to study the diagrams. Given the FOV, I suspect that the OP is using a pretty short focal length; so it depends on the aperture whether there will be vignetting of the prism.

If you want an OAG that has a large prism and handles up to 54mm thread, you will find an arguably better OAG from Askar (I had grabbed one but have not used it yet; I still prefer the flexibility and configurability of a guide scope :-).

Chen

AstroDude42

w7ay even if the prism can not be moved completely out of the sensor’s frame, there is still a chance to remove the vignetting with flat frames during post processing. Of course it’s better if there is no vignette at all. It would be interesting to know which scope the OP has.

Olli

w7ay

AstroDude42 there is still a chance to remove the vignetting with flat frames during post processing.

Hi Olli,

Intensity drop off can be removed by using flats, but the Fourier ("diffraction") effects cannot. Any vignetting will cause spikes to appear on stars; and no flats will remove them, no matter how much the Facebook crowd claims. For example, see the Fourier effects of the more typical Aperture Vignetting here (last post and a couple other posts on that Cloudy Nights thread have the correct answer, but read the entire thread for entertainment -- there is a lot of old wives' tales out there :-):

https://www.cloudynights.com/topic/547028-anyone-know-what-causes-this-artifact-on-bright-stars/

Chen

AstroDude42

w7ay Hi Chen,

point taken 🙂

Btw: i asked for the scope’s details because i can see heavy vignette in all corners of the image in the first post of this thread in addition to the shadow caused by the OAG‘s prism.

Olli

w7ay

AstroDude42

As you know, Olli, I had written a program on macos to do some Fourier Optics with (I called it "Airy" :-). So I just made a quick test to see what a notched circular aperture (to simulate the prism shadow) looks like.

Notice that the nice Airy rings have fractured, and there is a strong serrated spike in the vertical direction. So, instead of stars looking like an Airy disk, it looks like the point spread function on the right panel above.

Notice too, that in addition to a vertical spike, a bright star would not even be round!

This is sort of an extreme case. Most people do not have such a bad shadow as I modeled on the left panel. But it is just a matter of degree of error.

Chen

AstroDude42

w7ay As you know, Olli, I had written a program on macos to do some Fourier Optics with (I called it "Airy" :-). So I just made a quick test to see what a notched circular aperture (to simulate the prism shadow) looks like.

Impressive! I was aware that on optics like the Celestron RASA, the cables connecting to the camera in the front if the optics produce spikes, when they're not shaped as an arch, but i did not know that image quality can be degraded like this. Although it might not be as extreme as in your test.

Olli

alainzu

Hi, I could successfully retract the prism far from the sensor field and the preliminary results are quite better. I have to find the best compromise between a clear image and the ability to find stars with the guiding camera. I am looking forward a cloudy night to make new astro pictures. My scope is a Taka FSQ106ed.
Thanks for your replies.

AstroDude42

alainzu Hi, I could successfully retract the prism far from the sensor field and the preliminary results are quite better

Glad you resolved the problem. Clear skies!

Olli