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What's with the concentric rings in my Flats?

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#1 PhotonHunter1

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Posted 23 October 2017 - 09:31 PM

Below are two screen shots (one with and without STF Stretch) of an SII Flat from my ES 102 APO Triplet. I used SGP Flats Calibration Wizard, setting the ADU to 22500, +/-500. What is causing the concentric circles noticeable in the corners of the image?

 

These were shot by pointing my scope towards a white wall with a t-shirt over the fully extended dew shield. I am imaging with my ASI1600mm-c using the EFW-7 with 36mm filters. I have two 1" tubes extenders on my focusing tube and the tube itself extended to the 4th mark. This is the exact setup that I imaged my DSO with. Gain/offset are 200/50, temp -20. So many questions:

  • What is the cause of these? Is it the triplet lenses?? 
  • How do I eliminate? Crop these out, leave them in, or take some other approach to capture my flats?
  • Should I be using a different ADU setting with the Flats Calibration Wizard?

I should note that I get the same artifact with Ha and OIII, although not as pronounced. Help me Mr. Flats Wizard...

Attached Thumbnails

  • Screen Shot SII.jpg
  • Screen Shot SII with STF Stretch.jpg

Edited by mike8888, 23 October 2017 - 09:38 PM.


#2 Ares69

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Posted 23 October 2017 - 09:54 PM

seem to me to be some vignetting from the filter or reflection from the filter

Try taking a flat w/o any filter... if they disappear... you have confirmation

 

In reality, you take flat to remove such filter generated issue... so shouldnt be a big problem if they are there... the flat will calibrate them out from your lights



#3 Sean13

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Posted 23 October 2017 - 10:07 PM

I have those on that camera and slightly more with the 1.25" filters. Seems normal to me. The point of the flat is to calibrate them out. If they are there in flats, they are there in your lights, and you want them present in the flats so they calibrate out during the stacking process.

 

Also I have read that 12-18k ADU is best for flats for this camera, I use 18k for mine.


Edited by Sean13, 23 October 2017 - 10:08 PM.


#4 pfile

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Posted 23 October 2017 - 11:27 PM

i guess the question is if the flats do or do not correct the lights. if they do not, i would investigate internal reflections, for instance, off the extension tubes. you can buy material to flock them in case they are shiny...

 

rob



#5 cfosterstars

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Posted 24 October 2017 - 09:16 PM

I have the exact same issue and I know that it is not vignetting. Please see my other thread:

 

https://www.cloudyni...ing-wrong-here/

 

I believe that the filter is in facing the wrong direction. I have yet to test to be sure, but it is not vignetting in my case. I have the 36 mm filters and a 3 inch focuser. Also my LUM filter does not show the issue. 



#6 cfosterstars

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Posted 28 November 2017 - 02:49 PM

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.


Edited by cfosterstars, 28 November 2017 - 03:32 PM.


#7 cfosterstars

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Posted 28 November 2017 - 03:35 PM

seem to me to be some vignetting from the filter or reflection from the filter

Try taking a flat w/o any filter... if they disappear... you have confirmation

 

In reality, you take flat to remove such filter generated issue... so shouldnt be a big problem if they are there... the flat will calibrate them out from your lights

I have exactly the same issue and the problems is that the flats DO NOT calibrate this out for me. The artifacts show up in my images. 



#8 cfosterstars

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Posted 28 November 2017 - 03:43 PM

i guess the question is if the flats do or do not correct the lights. if they do not, i would investigate internal reflections, for instance, off the extension tubes. you can buy material to flock them in case they are shiny...

 

rob

Both the OP and myself which both use ZWO 36mm filter and both are seeing this issue on just there narrow band and not the LUM. We see just about exactly the same image and we both have different high quality OTA with different focal lengths from different vendors. There are other ZWO filter users out there, but very few are using the 36mm size. The common element is the 36mm filter size from the same vendor. The simplest explanation is that the culprit is the filters. I dont see the same issue with any of my LRGB filter in fact. For me, these artifacts done flat calibrate out with standard pixinsight process flows. I also have no issues with the same OTA on either of two DSLR cameras including a full frame canon 6D. One possibility is my WO flattener IV causing a reflection pattern. I wonder if for a smaller filter diameter, this ring might be outside of the aperture for the 1.25" filter so you dont see it. 



#9 PhotonHunter1

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Posted 29 November 2017 - 03:23 PM

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.



#10 Jon Rista

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Posted 29 November 2017 - 06:51 PM

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.

#11 pointofview_365

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Posted 29 November 2017 - 07:36 PM

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.

#12 cfosterstars

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Posted 29 November 2017 - 09:03 PM

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.


#13 Jon Rista

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Posted 29 November 2017 - 09:18 PM

You can easily determine if there are other things reflecting by popping the camera off the scope, setting the FW to the L filter, and looking through the scope while pointing it at something bright white. If there is anything reflecting in the train, you'll see it. Just because flipping the filter mitigates the issue does not mean the issue is just the filter. The filter could be further reflecting light from something else that is reflecting. In any case, looking through the scope will tell you. It will usually be quite bright, but diffuse (i.e. not a specular reflection, not a caustic reflection.)



#14 cfosterstars

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Posted 29 November 2017 - 09:24 PM

You can easily determine if there are other things reflecting by popping the camera off the scope, setting the FW to the L filter, and looking through the scope while pointing it at something bright white. If there is anything reflecting in the train, you'll see it. Just because flipping the filter mitigates the issue does not mean the issue is just the filter. The filter could be further reflecting light from something else that is reflecting. In any case, looking through the scope will tell you. It will usually be quite bright, but diffuse (i.e. not a specular reflection, not a caustic reflection.)

For the light source - I assume you mean something artificial and not looking at a bright star like Alnitak? Why would this be specific much worse with narrow band filters and significantly less to non-existent with the LRGB filters? 



#15 Jon Rista

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Posted 29 November 2017 - 09:29 PM

As in a computer or TV screen with a white background. 

 

It can be worse with NB because it is a small range of wavelengths, which will reflect and scatter in a more consistent manner. With an L filter, you have the entire spectrum, which can reflect and scatter in a much different way, one which may cancel or hide the issue. 



#16 cfosterstars

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Posted 29 November 2017 - 09:31 PM

As in a computer or TV screen with a white background. 

 

It can be worse with NB because it is a small range of wavelengths, which will reflect and scatter in a more consistent manner. With an L filter, you have the entire spectrum, which can reflect and scatter in a much different way, one which may cancel or hide the issue. 

That make sense - thanks



#17 cyber

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Posted 30 November 2017 - 05:00 AM

I have the 31 mm unmounted ZWO filters and they showed a similar problem. It was definitely a reflection issue and it drove me nuts.

In my case, it was very easy to see: just remove the camera from the filter wheel and point it to a bright light source. You will be able to see the reflections.

 

The fix was actually pretty simple.

The edges of these filters are blackened, but this is not enough to completely prevent reflections. So I placed a small paper ring on top of the filter edge, which blocks light.

Documented in this post: https://www.cloudyni...ions/?p=7963691

 

Should be easy to test this with the 36 mm filters, because your flats will look completely different if it works.

 

However, I read that someone reversed his filter wheel and the problem persisted, so it might not work after all. Forgot where I read this though...


Edited by cyber, 30 November 2017 - 05:13 AM.


#18 pointofview_365

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Posted 30 November 2017 - 06:44 AM

I usually just read these discussions. I follow/try all advise given in here. Basically, the only source of reflection in my optical train is the screw heads that hold the filters in place. From what I can tell on mine, I don't even see a reflection/light scattering from that gold ring on the outside edge.

I've done the suggestion with cutting rings out of paper/plastic suggested on the ZWO forum. Maybe that will solve the issue. I've done the blacking of the edge of the filters. Painted the screw heads. Haven't had a chance to get it out. Doing some upgrades with rigrunner, Intel NUC, feathertouch, etc.

#19 ChrisWhite

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Posted 30 November 2017 - 07:41 AM

Something else that can help, is to take your flats against a diffuse light source.  Bright panels or daytime sky flats can amplify these reflection issues.  Taking flats against a uniform and diffuse light source such as an evenly illuminated wall with a diffuser (t-shirt or something) over the OTA works very well for me.  My flats generally end up in the 3 to 10 second range depending on NB or Broadband, and I use dark flats to calibrate. 

 

My flats don't exhibit these artifacts and correct perfectly as far as my eyes can tell.



#20 PhotonHunter1

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Posted 30 November 2017 - 09:15 AM

I have the 31 mm unmounted ZWO filters and they showed a similar problem. It was definitely a reflection issue and it drove me nuts.

In my case, it was very easy to see: just remove the camera from the filter wheel and point it to a bright light source. You will be able to see the reflections.

 

The fix was actually pretty simple.

The edges of these filters are blackened, but this is not enough to completely prevent reflections. So I placed a small paper ring on top of the filter edge, which blocks light.

Documented in this post: https://www.cloudyni...ions/?p=7963691

 

Should be easy to test this with the 36 mm filters, because your flats will look completely different if it works.

 

However, I read that someone reversed his filter wheel and the problem persisted, so it might not work after all. Forgot where I read this though...

I reversed my camera and field flattener on the filter wheel, and while I couldn’t achieve focus (ff would protrude into filter wheel) I still had the concentric rings. I’ll try the Lum recommendation tonight with a bright object and see if I can determine more. Very interested in your data cfoster. I’ll share my findings later tonight. 



#21 cyber

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Posted 30 November 2017 - 09:44 AM

 

 

I reversed my camera and field flattener on the filter wheel, and while I couldn’t achieve focus (ff would protrude into filter wheel) I still had the concentric rings. I’ll try the Lum recommendation tonight with a bright object and see if I can determine more. Very interested in your data cfoster. I’ll share my findings later tonight. 

 

I also did the following test to check for reflections inside my telescope, reducer and spacers:

- remove all optical elements from the setup, use just the camera + FW.

- point it at a plain surface, e.g. a white wall, for a reasonably even illumination of the filter.

- take a test exposure with your usual flat frame ADU.

 

In my case, these artificial 'flat' frames showed the same edge reflection in my NB filters, but not with LRGB, so I was sure that the filters caused it.



#22 CP Kuiper

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Posted 30 November 2017 - 10:05 AM

Newton's Rings caused by the filter?

https://en.wikipedia.../Newton's_rings



#23 PhotonHunter1

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Posted 30 November 2017 - 12:36 PM

Something else that can help, is to take your flats against a diffuse light source.  Bright panels or daytime sky flats can amplify these reflection issues.  Taking flats against a uniform and diffuse light source such as an evenly illuminated wall with a diffuser (t-shirt or something) over the OTA works very well for me.  My flats generally end up in the 3 to 10 second range depending on NB or Broadband, and I use dark flats to calibrate. 

 

My flats don't exhibit these artifacts and correct perfectly as far as my eyes can tell.

I’ve tried multiple configurations; a light box, the wall, and the ceiling. all with and without a t-shirt, and when using a t-shirt, I tried it when the dew shield was fully retracted, fully extended, and in-between.

Interesting comment regarding the screw heads - something I haven’t considered. Good suggestions with the paper ring as well - hmmm, near the full moon - a perfect time to try these various approaches. Still waiting on a response from ES as well...



#24 cfosterstars

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Posted 30 November 2017 - 12:55 PM

 

 

 

I reversed my camera and field flattener on the filter wheel, and while I couldn’t achieve focus (ff would protrude into filter wheel) I still had the concentric rings. I’ll try the Lum recommendation tonight with a bright object and see if I can determine more. Very interested in your data cfoster. I’ll share my findings later tonight. 

 

 

I also did the following test to check for reflections inside my telescope, reducer and spacers:

- remove all optical elements from the setup, use just the camera + FW.

- point it at a plain surface, e.g. a white wall, for a reasonably even illumination of the filter.

- take a test exposure with your usual flat frame ADU.

 

In my case, these artificial 'flat' frames showed the same edge reflection in my NB filters, but not with LRGB, so I was sure that the filters caused it.

 

These are big clues. This basically narrows down the issue quite a bit I think.

 

As I stated above:

 

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.

 

This is likely in this part of the hardware. There have been three suggestions: edge of filter darkening, filter screw head darkening, paper ring around the filter edge. 

 

One other thing that occurred to me was that I think the 1.25" filters from ZWO are in filter rings and not unmounted. I know that the 36mm filters are unmounted and I believe that the 31mm filters are unmounted. This has two effects: 1) it eliminates the issue about which is the proper orientation of the filter with respect to the image train since they only go into the filter wheel one way. You could flip around the whole wheel assembly, but that the only way to change which orientation of the filters. 2) the edge of the filters are in the ring so they may have less reflection and there are no screws to hold the filters to cause reflections.


Edited by cfosterstars, 30 November 2017 - 12:56 PM.


#25 ChrisWhite

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Posted 30 November 2017 - 11:10 PM

Could you upload your masterflat as well as a single flat frame to DB?


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