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FLATS ISSUES CONTINUED - FLOCKING MY FOCUSER DRAWTUBE

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#1 cfosterstars

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Posted 26 January 2018 - 01:43 PM

I have been posting on my efforts to improve my flat frames to eliminate artifacts in my imaging. This data is mostly in the following post:

 

https://www.cloudyni...ring-artifacts/

 

Most of this thread discussed edge of filter light leakage issue with the ZWO 36mm filters and all the work that was put into resolving that issue. I then purchased a set of 36mm Astrodon filters and did a comparison to the ZWO filters and that data was covered in this post.

 

https://www.cloudyni...ontrolled-test/

 

The net conclusion of these posts is that by either masking the edges of the ZWO filters or going to much better Astrodon filters that are fully coated to the edges you can eliminate the serious edge of image "vignetting-like" patterns in flats that is uncorrectable with out serious post processing of data. By masking the ZWO filter or using full-coated, better quality (and more expensive) Astrodon filters, I now can get quite clear results. What remained in the flats were very weak (~1-2% intensity variation) concentric ring patters only visible with an aggressive screen stretch. Without stretching the flat the flat was -- well -- FLAT. You cant seen anything in my current flats. This is probably good enough, but the ring pattern remains. 

 

Even with all that was done above there are still unanswer questions and maybe issues to fix. One issue that was raised was the reflections off my focuser draw tube. Although clearly visible in the pictures that I took through the OTA pointed a the bright sky, it is not clear what effect this source of reflection really had in my flats or imaging. I posted my first attempt to fix this using BLACK 2.0 paint. This was basically a failure with no noticeable improvement in this reflection source both visually through the OTA or in the flat frame ring patterns. This is what I am referring to:

 

reflections from focuser tube
 
This is an image taken with my i-phone through my OTA with the camera, flattener and filter wheel removed with the OTA pointed at a bright, but cloudy sky. The edge glow is due to reflection off the focuser draw tube. The draw tube on my ORION EON 115mm APO is fully painted black. I tried, but it is as black as you can get with paint.  The reflections are due to the threads:
 
Threads On drawtube

 

The draw tube is full threaded end to end. The reflection from threads and the roughness that they cause can be eliminated with paint. It is not clear what real effect that they have in my flats or images. However, I want to see if they could be eliminated with flocking. I purchased flocking material from Scopestuff a while back and had it on hand. I was not sure how to apply it since the draw tube is very long and my hands cant reach into the drawtube since it is too narrow. I called ORION and they showed me how to remove the entire focuser assembly. It was easily unscrewed from threads were it attaches directly to the OTA tube body. It came off in one piece:

 

Focuser And camera removed
 
With the focuser removed, It was easy to apply the flocking to the inside of the drawtube. The material had a sticky backing and I applied it in mostly one sheet. I then used another very narrow piece to cover over the seam. The net result was great: This is an image through the drawtube without the flattener while the focuer was still off the OTA:
 
Flocked drawtube
 
The reflections are basically gone. The exposed metal is where the flattener threads onto the OTA. This is through the flattener with the focuser assembly still off the OTA:
 
Flocked Drawture through reducer
 
This result was better than I hoped it would turn out. However, there is a clear ring of reflection from the flattener. I suspect that due to the much brighter reflections from the threads of the drawtube, I just could not see it before. More to come in the next post.

 

 



#2 mikefulb

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Posted 26 January 2018 - 01:47 PM

Nice post! I think many of us are dealing with similar problems due to internal reflections. I think we have good values now in imaging optics but bafflibg and blackening of internal surfaces can still leave much to be desired.

#3 bmhjr

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Posted 26 January 2018 - 01:54 PM

Just curious, have you seen any issue with the flaking off of the flocking and getting on the optics?  Nice job, BTW.



#4 cfosterstars

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Posted 26 January 2018 - 01:57 PM

I took the opportunity while the camera was off the OTA to clean the sensor. However, on inspections I really could not see any dust on the sensor cover itself, but a lot on the camera window. This is easily cleaned. I then remounted the focuser and the flattener on the OTA and took the companion image pointing at the bright, but cloudy sky: 

 

This was a previous image without the flocking through the OTA with the flattener installed:

 

filter edge reflections 1
 
And the same setup with flocking the drawtube:
 
Flocked through reducer At Sum

 

The sky brightness was not the same as before since it was different days also the camera will adjust the brightness of the image so it is easy to over interpret the results. The new ring is the same reflection off of the flattener and now it looks very bright. By checking with my eye through the OTA, it is not that bright, but is mainly a trick of the camera. However, it is still real. Before flocking it was totally swamped by the drawtube reflections.



#5 cfosterstars

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Posted 26 January 2018 - 01:59 PM

Just curious, have you seen any issue with the flaking off of the flocking and getting on the optics?  Nice job, BTW.

I used the flocking previously on my ST80 guide scope and never saw an issue, but I was not imaging through it. However, The material is actually quite tough and does not flake that I can see. It is important to clean up the edges so that they are not subject to wear, but I dont think this will be a problem. I will watch for it though.



#6 cfosterstars

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Posted 26 January 2018 - 02:22 PM

With the focuser and flattener installed, I then re-attached the ZWO filter wheel. The filter has the 36mm Astrodon e-series LRGB and 5nm Ha, OIII, and SII filters with the ZWO filter masks as I showed in the other thread. I then took images through the filters and flattener with the OTA pointed at the sky.

 

This is the image through the LUM filter prior to flocking the focuser draw tube:

 

Pre flocked thru LUM
 
This is the same image through the LUM filter after flocking the focuser draw tube:
 
Flocked through LUM
.
 
You can see that the reflections from the focuser drawtube is eliminated. What remains is the reflection from the flattener and a further reflection from the filter edge. This is not leakage since the Astrodon filters are fully coated. However, as I posted previously, the Astrodon LRGB filters do not have their edge bevels blackened: I was surprised by this when I was mounting them.
 
LRGB FILTER EDGE
 
However, the narrow band filters are blackend on the bevel edges:
 
Ha filter Surface EDGE
 
I believe that the remaining reflection at the filter edge is due to scatter off the bevel edges.
This is the BLUE filter:
 
Flocked through BLUE
 
It shows the same reflections on the edge of the filter.
The GREEN filter also show the same reflection:
 
Flocked through GREEN
 
The RED filter also showed this that I could see with my eye, but the camera image was not very clear.
 
However, the narrow band filters did not show this reflection. This is the SII filter:
 
Flocked through SII

 

The pictures from the Ha and OIII were poor due to the lighting, but also did not show the edge of filter reflection. My plan is to use a sharpie to blacken the LRGB bevel edges the next time I have the filter wheel remove to see if this will remove these reflections.

 



#7 cfosterstars

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Posted 26 January 2018 - 02:34 PM

I was hoping that this would effect the remaining ring patterns in my FLATs. Unfortunately, this was not seen. This is the flats data from the LUM filter after flocking: This shows the STF streched flat, the histogram and the image statistics from Pixinsight

 

LUM Flat after flocking
 
This is the flats analysis of the LUM filter flat using CCD inspector. The histogram does show some deviation from a symmetric distribution, but the total variation of the flat is less than 5%.
 
LUM flat CCD inspector
 
This is the same STF streched flat, the histogram and the image statistics for the BLUE filter:
 
BLUE Flat after flocking
 
This is the flats analysis of the BLUE filter flat using CCD inspector. The histogram does show some deviation from a symmetric distribution, but the total variation of the flat is less than 5%.
 
BLUE flat CCD inspector
 
And the STF streched flat, the histogram and the image statistics for the GREEN filter:
 
GREEN Flat after flocking
 
This is the flats analysis of the GREEN filter flat using CCD inspector. The histogram does show some deviation from a symmetric distribution, but the total variation of the flat is less than 5%.
 
GREEN flat CCD inspector

 

These results are basically the same as prior to flocking the drawtube - except for removing a lot of dust rings from cleaning the camera window wink.gif . There are still three dust spots that are directly on the sensor window itself that I will need to open the camera to remove. I will see if further reduction of the remaining reflections will remove the remaining ring pattern in the flats. If these countermeasure dont remove the rings, then they may originate from the lack of AR coating on the sensor itself.


Edited by cfosterstars, 26 January 2018 - 08:12 PM.


#8 pfile

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Posted 26 January 2018 - 03:06 PM

i went thru the flocking process you describe; what i need to do is blacken the edges of my LRGB filters... but i think overall my main source of reflections on all my OTAs must be the focal reducers i'm using. i think this is what your experience is showing. my panel flats never correct these reflections.

 

one thing i guess i should try one night is to take nighttime "super flats" and see if the reflections are better modeled using this technique. i think it could be hard to automate with my setup though.

 

rob



#9 calypsob

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Posted 26 January 2018 - 03:09 PM

wht does the inside of your filter wheel look like?  Is there electric tape on each filter? 



#10 calypsob

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Posted 26 January 2018 - 03:12 PM

i went thru the flocking process you describe; what i need to do is blacken the edges of my LRGB filters... but i think overall my main source of reflections on all my OTAs must be the focal reducers i'm using. i think this is what your experience is showing. my panel flats never correct these reflections.

 

one thing i guess i should try one night is to take nighttime "super flats" and see if the reflections are better modeled using this technique. i think it could be hard to automate with my setup though.

 

rob

I have noticed that every reducer I have owned had visible rough ground edges.  Apparently there is an ink made specifically for darkening the edges of optics. Unfortunately taking apart a reducer is risky if there are no markings indicating where the optics were aligned.  



#11 cfosterstars

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Posted 26 January 2018 - 03:20 PM

wht does the inside of your filter wheel look like?  Is there electric tape on each filter? 

This was shown in the post link shown above. All the details and lots more pictures, but here is the filter wheel as I as changing to the AD - no tape any more. This show only the first filter being change. All the paper and tape were removed and all the filters look like #7:

 

ZWO FILTER EDGE MASKS
 
Curious: why do you ask?


#12 Kaos

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Posted 26 January 2018 - 04:03 PM

I was hoping that this would effect the remaining ring patterns in my FLATs. Unfortunately, this was not seen. This is the flats data from the LUM filter after flocking: This shows the STF streched flat, the histogram and the image statistics from Pixinsight

 

 
 
This is the flats analysis of the LUM using CCD inspector. The histogram does show some deviation from a symmetric distribution, but the total variation of the flat is less than 5%.
 
 
 
This is the same STF streched flat, the histogram and the image statistics for the BLUE filter:
 
 
 
This is the flats analysis of the LUM using CCD inspector. The histogram does show some deviation from a symmetric distribution, but the total variation of the flat is less than 5%.
 
 
 
And the STF streched flat, the histogram and the image statistics for the GREEN filter:
 
 
 
This is the flats analysis of the LUM using CCD inspector. The histogram does show some deviation from a symmetric distribution, but the total variation of the flat is less than 5%.
 
 

 

These results are basically the same as prior to flocking the drawtube - except for removing a lot of dust rings from cleaning the camera window wink.gif . There are still three dust spots that are directly on the sensor window itself that I will need to open the camera to remove. I will see if further reduction of the remaining reflections will remove the remaining ring pattern in the flats. If these countermeasure dont remove the rings, then they may originate from the lack of AR coating on the sensor itself.

The only differences between your system and mine are that you use the 36mm and I use the 1.25 mounted filters and we have different filed flatteners. I use the Hotech Self-Centering FF. I had hard rings at the edges with my ZWO filters and get a small amount with the Astrodon Gen. 2 Series E filters (I think the flats will correct, but have not verified this yet - but my light frames look really clean of rings with the AD filters). The AD filters are barely noticeable. Sam told me that he thought it was reflections off the filter housing. My point is that I think that your FF is giving you some reflective issues as my EON does not have the pattern yours does. In other words, eliminate the OTA and difference in filter size that leaves only the FF (and spacers), as we both have the same camera. I use the Baader VariLock 46 T2 Variable Extension Tube - 29 mm - 46 mm turned all the way in to get to the required spacing from the Hotech FF to the camera. After all the testing you have done I am starting to suspect your FF/spacers. Have you tried removing the FF and retesting for the rings? I would also test it without any filters to see if you get a clean flat. Mine, with no filter looks really flat with no ringing at all. I would probably do the no filter test first and then remove the FF if the issue is still present. Is it possible to flock your FF? The Hotech is accessible at one end, so it may be flocked if you can get your fingers in enough to do it.

 

Kaos


Edited by Kaos, 26 January 2018 - 04:05 PM.


#13 cfosterstars

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Posted 26 January 2018 - 08:11 PM

And the STF streched flat, the histogram and the image statistics for the OIII filter:

 

OIII Flat after flocking
 
Even through the filter bevel edge reflection is absent from the OIII filter, the concentric rings are still present in the STF stretched flat. This is the flats analysis of the OIII using CCD inspector. The histogram does show some deviation from a symmetric distribution, but the total variation of the flat is less than 5%.
 
OIII flat CCD inspector
 
Basically even though there are still some reflections and there are the concentric rings in the flat, these issue do not seem to result if issue with the actual light frames. The intensity of the reflections my just not be high enough to really cause an issue, but I will still work on getting rid of them - they simply can't be a good thing.

 



#14 freestar8n

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Posted 26 January 2018 - 09:00 PM

If you look closely enough at anything you will see anomalies. Try calibrating a light and see how well it works.

If you are doing aesthetic imaging all that matters is if you see artifacts in the final result.

You have had serious issues with leakage but they have been addressed. These new issues are on a much smaller scale.

If you can’t solve them you can try making a superflat.

Frank

#15 calypsob

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Posted 27 January 2018 - 12:20 AM

 

wht does the inside of your filter wheel look like?  Is there electric tape on each filter? 

This was shown in the post link shown above. All the details and lots more pictures, but here is the filter wheel as I as changing to the AD - no tape any more. This show only the first filter being change. All the paper and tape were removed and all the filters look like #7:

 

 
 
Curious: why do you ask?

 

TBH that whole filter wheel looks pretty shiny to me.  maybe you could print some sort of scale out onto paper and place that over the filter without touching the glass to determine where the strange donuts are coming from.  In your pixinisght example, it almost looks like the screws are causing the glass to bow outward. 



#16 calypsob

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Posted 27 January 2018 - 12:21 AM

If you look closely enough at anything you will see anomalies. Try calibrating a light and see how well it works.

If you are doing aesthetic imaging all that matters is if you see artifacts in the final result.

You have had serious issues with leakage but they have been addressed. These new issues are on a much smaller scale.

If you can’t solve them you can try making a superflat.

Frank

Frank, what is a super flat? 



#17 Jon Rista

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Posted 27 January 2018 - 02:00 AM

I  am with Frank on this one. I think you had some fairly significant reflections before, but I think you have solved the most serious of them. '

 

I would like to see how well the flats calibrate the lights. If the remnant pattern in your flats is now consistent, then that is really what matters. Consistent flats that don't change night over night should reliably calibrate  your lights night after night. As long as they do that, then how they look doesn't really matter. In fact, if the concentric rings and other patterns in your flats are always there and reliably reproduced, then you WANT the flats to replicate those patterns, otherwise you wouldn't be able to correct the same pattern in the lights. That is the entire purpose of a flat! :p



#18 freestar8n

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Posted 27 January 2018 - 05:17 PM

Many of us can see rings in our flats yet they calibrate just fine. They aren’t ideal to have but what matters is if the ring pattern or amplitude is object dependent or sensitive to how the flats are taken. And you can tell that by looking for the pattern in a calibrated light. This is where blue sky flats may work better than light box flats.

Superflats are used professionally to create a flat based on highly dithered images of an empty part of the sky. They are using actual sky background to capture the true flat. I have never needed to make one but you can find references in the web. One example is to remove fringing from airglow interference.

Frank

#19 cfosterstars

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Posted 30 January 2018 - 03:42 PM

I  am with Frank on this one. I think you had some fairly significant reflections before, but I think you have solved the most serious of them. '

 

I would like to see how well the flats calibrate the lights. If the remnant pattern in your flats is now consistent, then that is really what matters. Consistent flats that don't change night over night should reliably calibrate  your lights night after night. As long as they do that, then how they look doesn't really matter. In fact, if the concentric rings and other patterns in your flats are always there and reliably reproduced, then you WANT the flats to replicate those patterns, otherwise you wouldn't be able to correct the same pattern in the lights. That is the entire purpose of a flat! tongue2.gif

 

 

Many of us can see rings in our flats yet they calibrate just fine. They aren’t ideal to have but what matters is if the ring pattern or amplitude is object dependent or sensitive to how the flats are taken. And you can tell that by looking for the pattern in a calibrated light. This is where blue sky flats may work better than light box flats.

Superflats are used professionally to create a flat based on highly dithered images of an empty part of the sky. They are using actual sky background to capture the true flat. I have never needed to make one but you can find references in the web. One example is to remove fringing from airglow interference.

Frank

John and Frank, 

 

At the very beginning of this post I stated:

 

"By masking the ZWO filter or using full-coated, better quality (and more expensive) Astrodon filters, I now can get quite clear results. What remained in the flats were very weak (~1-2% intensity variation) concentric ring patters only visible with an aggressive screen stretch. Without stretching the flat the flat was -- well -- FLAT. You cant seen anything in my current flats. This is probably good enough, but the ring pattern remains."

 

This was meant to convey that although the flats still have ring patterns, I have no uncorrectable issue with these flats calibrating my light frames. With the ZWO filters with all the masking or with the AD filter fully coated, my calibrated light frames show no edge color rings that I could not correct "easily" with Pixinsight. I am sorry if that was not clear from what I wrote. These FLATs work. Are they perfect? Well DBE is a wonderful tool. I had good results prior to this post with all my ZWO filters and my AD filters - they were good enough. However, I wanted to see if I could fully eliminate them. I was convinced that the ring patterns - as weak as they were - were still the effect of the remaining reflections in my optical path. This post was my first attempt to determine where they were coming from and whether I could fully eliminate them. 

 

Do I need to do this - probably not. Good enough is good enough. However, that said, the previous reflection and filter issues show that reflections are non-linear effects. And even if weak - too weak to really notice - they are still there. I dont think it will hurt anything to fix them. The less I have of these artifacts, the less I have to do with DBE. At least that is the theory. My next post will show how I think I fully eliminated them.


Edited by cfosterstars, 30 January 2018 - 03:44 PM.


#20 freestar8n

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Posted 30 January 2018 - 05:17 PM

I don't use PI and I rarely use any kind of gradient removal software - and if I do it's only for bad light pollution gradients.

 

If you are using some kind of post-processing to remove flat artifacts that show in a calibrated stack - then I would say that is definitely a problem that should be addressed.  I'm not sure if your reference to DBE includes removing ring artifacts.  Light pollution gradients cannot be removed by good flats because they represent faithfully captured glow from the sky - but the rings would be artifacts.

 

So - if you are in fact seeing rings in calibrated stacks - I would say it's worth solving the problem.  But if you are using DBE just to clean up sky gradients then I would ignore the rings in the flats - as I have done for years because they are I think around 0.5% variation in my case - and that's in the flats themselves.  What matters is the residual variation in the calibrated light - and that should be much smaller and hard to measure.

 

I forget if I mentioned it in these threads, but this topic did prompt me to try to reduce the rings in my system and I applied flock paper to one of the adapters in my EdgeHD imaging system - but looking in the OTA from the imaging side I'm afraid the main remaining culprit is in the field corrector lens system of the SCT itself - and not accessible.  Fortunately, as I said, the rings aren't a problem for me - but I don't think I can do anything about them.

 

Another thing to note is that if your flats have a good deal of vignetting then you won't be able to stretch them enough to make the rings visible.  So it requires a low vignetting system to see potential rings in the first place - and a strongly vignetted flat may not show the rings - just because you can't stretch it enough to see the small variation represented by the rings.

 

Frank



#21 cfosterstars

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Posted 30 January 2018 - 06:24 PM

Frank,

 

Even the best I could do with the ZWO filters required some DBE to remove the ring artifacts. THe difference was that prior to making the improvements, PI DBE or any other method used by me or others applied to stack frames with the issue was unable to fix the issue. Now these tools CAN fix them. They were still there to some extent - more with ZWO filters and less with astrodon filters.

 

My system has basically no or very very little vignetting. I will be posting a quantification of that amount in my next post. The rings are ONLY visible with a stretch. The unstretched flats are flat - as I stated in the first post on this thread. 


Edited by cfosterstars, 30 January 2018 - 06:26 PM.


#22 pfile

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Posted 30 January 2018 - 07:20 PM

 

i went thru the flocking process you describe; what i need to do is blacken the edges of my LRGB filters... but i think overall my main source of reflections on all my OTAs must be the focal reducers i'm using. i think this is what your experience is showing. my panel flats never correct these reflections.

 

one thing i guess i should try one night is to take nighttime "super flats" and see if the reflections are better modeled using this technique. i think it could be hard to automate with my setup though.

 

rob

I have noticed that every reducer I have owned had visible rough ground edges.  Apparently there is an ink made specifically for darkening the edges of optics. Unfortunately taking apart a reducer is risky if there are no markings indicating where the optics were aligned.  

 

 

i should take a look again - i flocked the inside of my orion flattener almost a year ago. but given the quality/price of that flattener i would not be surprised to find such edges.

 

as for the filter wheel, they are astrodon 36mm filters and IIRC the 'aperture' of each filter position is slightly smaller than the filter itself. so not sure where i could tape the filters...

 

rob




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