Stars Distorted if I use the 1.25 filter (8 position wheel) - NOT

technobill

You can ignore this thread. There is nothing wrong with the 1.25 filters. After much experimentation eliminating the causes for these strange + shaped stars. It was the lens collimation. I did not think that could cause such a strange result. Basically when stars were defocused one direction I would get the airy disk but with 4 small notches cut out top, bottom, left, and right. If I defocused the other way I would get 4 tiny spikes in the same locations.

As it is a refractor, I decided to rotate the front lens and I found that the anomalies rotated with the Front lens. I carefully removed some felt tape that is around the outside of the lens assembly and it revealed 4 small screws. I held my breath and tested each screw and one was slightly loose, so I gave it a gentle turn. Instantly I saw the defocused star begin to lose the silly artifacts. a slight turn more and it is gone.

WooHoo.

wvreeven

Hi,

Could you add an example image that shows the problem? Is this issue present in all the raw images or only in the stacked ones? I assume it is present in all raw images but I'd like to be sure.

Does your telescope have a flattener or reducer? If yes, are you sure you have the proper back focus between the camera sensor and the flattener/reducer? Even if you are exactly at what the manufacturer of the flattener/reducer has specified, you may be off and the back focus may need some adjustment. Does the issue also happen when you do NOT use the filter wheel? What happens when you remove the filters and simply take an image like that? Please note that filters add about 1.5 mm length to the light path which you need to take into account when computing the back focus distance.

That's a few questions that popped into mind. I'll add more depending on your answers :)

Wouter

katherine.tsai

You can upload photo to show us the problem, and yes I agree with wvreeven, please check the back focus of your telescope or reducer if you have.

technobill

Hi,

Thanks for taking an interest in my issue.

I don't think I have kept any images. I was so tied up and focused on trying to solve the problem, I never once thought of capturing an example :-( . Of course we have about 5 days of cloud now.

I will see if I can replicate the issue in the house with an artificial star. But I will only include that image if it is exactly the same issue as what I have been seeing with real stars, I don't want to muddy the waters.

I originally thought it may have been overly aggressive tracking but I think I have discarded that theory, as even on the shortest exposure I can see the same artifacting and it is very symmetrical.

Originally no flattener, but I then did try both a flattener that was at the correct spacing and separately a reducer, but no change.

I tried adding spacers behind the filter wheel to actually push the camera back further away, as a test and I think that made the problem worse so that is what makes me think it is the filter wheel.

Things I want to try next:

- No filter but very short exposure so stars do not bloat (I have already tried this and I think it works OK, but will confirm and save image)

- I have that small flat round disk adaptor that came with the 1600mm that lets me screw a 1.25 filter directly to the front of the camera. I will try that. as that is as close as the 1.25mm filter can get to the sensor.

- I have a 2" baader narrowband HA filter. I will try that.

katherine.tsai

Hi,

I will wait for your info.:)

technobill

Hmm!

Looks like I was mistaken. It looks like it is caused by my telescope. I had a clear night and did some testing. I did another test in the Eon 110 refractor and got the distorted plus shaped stars. I then removed the filter wheel and still had distorted stars.

So I grabbed my old ED80 and set it up on top of the Eon and plugged the camera and focuser into the ED80 and presto, round stars.

When I plugged it back into the Eon, I can now see that the defocused stars have a strange distorted airy disk. There is something majorly wrong with my Eon. I do remember ages back before I retired from astrophotography where the focuser jammed and had to be dismantled. I think the reassembly of the focuser has resulted in a major collimation issue.

If this is the case we will need to remove this thread or change the heading as I do not want to spread erroneous issues in regards to the ZWO gear. (a lot of peeps tend read a title and not the details)

I will report back after trying to resolve the issue in the scope. :

katherine.tsai

I am happy that you found the problem and thank you for the info!:)