I have the same problem with my NB 36mm ZWO filters, with or without FR. Skywatcher 120 ED Pro, ASi1600mm-c, 7 pos FW. I haven't been able to track this down yet either.
Mike,
Have your resolved your issues and do they effect your image calibration for your lights? I will be posting on a lot of work that I have done to try and nail this down shortly. Some of this is due to the filter orientation. One thing I have noticed is that both you and I have 36mm filters and most other ZWO filter users have 1.25" filters and they dont show a problem. This looks like an Airy or Fraunhofer diffraction pattern or some other sort of thin plate interference pattern to me and I am looking up the physics on this.
Also are you using any sort of flattener or reducer between your filter wheel and the telescope? I have a williams optics flattener IV that has an 87mm back focus. Roughly, if this is due to a reflection, then it should be about 57mm space. For me this is close to the spaceing between my flattener and filter wheel.
cfoster I have not. I started the calibration/integration process in PI and was not sure how it integrate my Flat Darks. Another helpful CN post and responses has me on the right track - I just need the time which hopefully will come this weekend. I'll let you know the results.
I was also thinking of posting this on the ES Forum and see if Sam would be able to provide some insight since my entire imaging train is ES gear minus the Baader VariClick field flattener - which you correctly diagnosed above. You're spot on too - the 36mm filter is lone common variable. What I find frustrating is the whole point of me purchasing the 36mm over the 31mm was to avoid any vignetting or issues with the filters being smaller than the sensor - current config or if I wanted to upgrade the camera or filters. I would find it aggravating if indeed the 36mm filter is the culprit.
I'm interested in what you find and will share any feedback from Explore Scientific.
It seems way too coincidental that three different users of the 36mm filters with three different vendors of OTA at different focal lengths some with focal reduces and some without show - to as well as we have compared - exactly the same artifacts in their flat frames. That seems to me to state that it cant be the bulk of the optical trains. The only things in common between the three of us are the camera, filter wheel and filters.
Mike and POV365,
I am in the middle of uploading a TON of data from my imaging with all the flats and analysis. It is just taking a very long time to upload all the data to CN. I will have two galleries with data from the filter wheel in both orientations. I would like us to compare notes and maybe we can figure this out.
This is a reflection issue. It may be due in part to the filter, but it may not be solely due to the filter. Whenever I have had flat issues like this, I always check my imaging train to see if anything else is reflecting. Usually, there is, and once I diffuse that other reflection, the issue goes away.
Something to keep in mind about flats, with the potential exception of twilight sky flats, is the collimation of the light will differ. The collimation of light from a flat box or flat panel will usually be quite different (i.e. non-collimated) than the light from deep space. This can lead to changes in reflections, particularly diffuse reflections, so that they show up differently in the flat than in the light.
I would not rule out the filters, however I would look for other sources and see if you have a compound source.
Jon,
I agree that this is a reflection issue. The physics is clearly wrong for diffraction or interference. I was going this route since the issue with quasi monochromatic light due to the narrow band filters only showing the issue. I think it is more to do with the high reflection from filters instead. But I think it is the filters that are the issue since by simply flipping the orientation or the filter wheel the pattern can be drastically reduced. I will show that in my data. Also the pattern is not dependent on the flat light source. I have used a light box, EM panel and sky flats and they all show the same issue.
The pattern of the ring on the Ha and SII indicates about a 57mm space that is reflecting. I was suspecting reflection between my filter wheel and focal reducer, but POV365 has the same pattern without a focal reducer.
Please be patient with me to get all my data uploaded and then it will be easier to reach a conclusion since I believe I have tried almost everything.
All,
I also bit the bullet and ordered an Astrodon 5nm Ha filter to test. That will be definite in determining if it IS the filters. My biggest issue now is bad weather for the next 2 weeks or more.
Edited by cfosterstars, 29 November 2017 - 09:06 PM.