302 replies to this topic

[cfosterstars]

I previously posted a thread to discuss issues that I have been having with poor flat frame calibration with my ZWO ASI1600MM-C with EFW and 36mm ZWO filters. This is a link to the previous thread:

 

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

 

Let me just start by saying that this is going to be a long post with a lot of data and that I don’t have a fix for this problem. The issue is that with my ASI1600MM-C, ZWO EFW and 36mm ZWO filters, I have had continual issue with flat frame calibration not being effective or worse contributing artifacts to my stacked light frames that I have been unable to remove with the processing techniques that I am capable of current. There may be a way to remove them with processing, but my point is that they should not be there in the first place. I am also rather new to a mono camera with filter wheel. Most of my experience is with my Canon DSLR cameras and I am also new to pixinsight for processing.

 

I am also not the only user of these products that has either this exact issue or one that is very similar. I have seen threads on CN from at least four other posters that should the same issue that I am going to show. Three others have very different equipment than I do and one has very similar equipment with the exception of filter size than I do. However, the common factor is the camera, filter wheel and filters from ZWO. The effects are worse on narrow band filters but do show up to a lesser extent on the LRGB filters. The others that have reported this issue have Explore Scientific, Stellarvue and Celestron OTAs. The issue is also seen for all three ZWO filter sizes: 1.25” mounted, 31mm unmounted and 36mm unmounted.

 

I state emphatically that I have no agenda or bone to pick with ZWO. I really like their products and have so far had four cameras from them. I have had some of the best imaging I have produced with their equipment. I really just want to fix this issue so I can take better pictures.  I have posted to the ZWO forum on this issue, but so far, have had no response. Part of the issue is that it is not easy to explain and there are many possible causes. I am making this post (or posts since it is a lot of data and some I still have to process and some I am still collecting) so that others who have this same issue can help debug it and I can direct ZWO to the post for the data.

 

First - my equipment: the data that I am posting is from an ORION EON 115mm APO with a Williams Optics Flattener IV so the scope is at F/5.75. The camera is a generation 3 ASI1600MM-C with the 7 position EFW and the 36mm LRGB and narrow band filters from ZWO. My mount is a ORION HDX-110 EQ-G that I run with EQMOD, CdC, and Sequence Generator Pro. I have an ORION 70mm guide scope with an ASI290MM-mini guide camera and use PHD2 for drift alignment and guiding.

 

Second – how I take flats: basically, I have tried every way that there is and they all show the same issue. I will show data from using a Aniltak Flatman EM panel that I diffuse further with 1-2 layer of tee shirt fabric giving a LUM filter flat exposure of 0.5s and an SII filter Flat 15.9 seconds. I also have a home made light box with LED and diffusers that I also diffuse further with 1-2 layers of tee shirt fabric giving a LUM filter flat exposure of 0.002s and an SII filter Flat 0.25 seconds. I have also done sky flat with 3-6 layers of tee shirt fabric a LUM filter flat exposure of 0.03s and an SII filter Flat 1.3 seconds. So, this is three different types of light sources with different degrees of collimation, different methods of diffusion, and different exposure times.  The exposures times for all three light sources was determined the same way with SGP flat calibration wizard and the flat frame were collected with SGP to give a peak of the histogram in the middle of the response at 25K ADU using 16bit (I will show histrograms of the data). All the calibration was with dark flat frame taken immediately after the flats with the exact same exposure times. I did not use any bias frames. All flats methods basically have the same issue.

 

Third – exposure settings: For all exposures, light, dark, flat, dark flat, all used a gain of 76, offset of 40, USB of 60 and were all taken at -15C with my ASI1600MM-COOL (ver. 3) camera.

 

Fourth – image and flat processing methods: The flats were mostly processed with Pixinsight using the method from Warren Keller book but with flat dark specified and not bias frame or scaled dark frames. Light frames were also calibrated, registered and integrated with Pixinsight following Warren Keller book – basically nothing fancy here. Just as a sanity check, I also used ImagesPlus 6.0 and did a full image processing with that package with exactly the same results. I really don’t think that this is a processing issue. My method is basically standard and it is highly unlikely that multiple users would be making the exact same settings errors – not impossible, but I think highly unlikely. If, however, someone thinks this is the issue, I will post all the details of my calibration method with all the setting or screen shots if necessary.

Edited by cfosterstars, 01 December 2017 - 08:46 PM.

[cfosterstars]

I have uploaded most of the data and screen captures to my cloudy night image gallery. I am creating separate galleries for different sets of flats and lights so that they don’t get confused. This is why this is taking so long to get to this post. Also, I am still collecting data and I still have lots of data already collected that I have not processed or posted. I am just limited by time and patience. I would much rather be taking imaging. I have two galleries started. The first un-stretched flat, stretched flat, histogram and a calibrated, registered and integrated light frame from each of the seven filters from the original configuration of my filter wheel when I started debugging this problem. This is the original data:

 

Album: Flat frames from ASI1600MM-C - original orientation

35 images

0 comments

 

The issue is best illustrated with the SII filter from this configuration. What I mean by configuration is that I did not make any changes to the hardware or software to address the problem at this point.

 

This is the SII master flat frame - STF stretched in Pixinsight - this was taken with the original filter orientation. This was 40 flat frames at 15.42 seconds taken using Sequence Generator Pro. Integration time was determined using SGP flats calibration wizard. Illumination was using an Alnitak Flatman with two layers of tee shirt fabric. No bias frames used only Flat Dark Frames - 40 flat dark frames.

 

 

The very predominant radial ring is the problem. This pattern leads to very bad artifacts in the light frames that I have been unable to remove with post processing. I have tried both gradient exterminator in Photoshop, DBE and ABE in Pixinsight to no avail. This artifact is clearly visible even in the unstretched image.

 

 

This screen capture shows the intensity histogram of the SII master flat frame from the histogram transformation in pixinsight. There are distinct nodes in the histogram. I spent a lot of time think about the physics here and neither interference or diffraction effects make any sense to me. They appear to be caused by some sort of reflection issue. This flat frame did not calibrate out these artifacts from the SII light frames and lead to color artifacts in the combined image.

 

 

This is the processed SII master from from NGC7000  - SII filter Master Light Frame - 30 X 240s exposures. I calibrated using 60 DARKs, the flat frame master above, 60 Dark Flats for calibration. Calibration, Star Alignment and stacking with PixInsight. The ring in the flat leave a clear defect in the SII light frame master.

 

 

This is the flats analysis from CCD Inspector of the SII master flat frame:

 

 

It is showing more than 30% intensity variation across the image, but it is not a typical vignetting fall off but a dark ring. This issue is not vignetting. The OTA has a 3" focuser, I am using 36mm filter diameters and the smallest aperture is the t-threads on the William Optics flattener IV. 

 

This issue appears in the Ha filter flat frame equally to the SII. It is visible in the OIII flat master but much weaker. Similar data to the SII master I show above for the Ha and OIII filter is posted in the gallery but the SII illustrates the problem. 

 

Now on to the LUM filter. It shows to first glance a completely normal flat. This is the LUM master flat frame - STF stretched in Pixinsight -  taken with the original filter orientation - the radial interference pattern is barely noticeable and but does seem to be present to a very small extent. This was 40 flat frames at 0.5 seconds using Sequence Generator Pro. Integration time was determined using SGP flats calibration wizard. Illumination was using an Alnitak Flatman with two layers of tee shirt fabric. No bias frames used only Flat Dark Frames - 40 flat dark frames 

 

 

The unstretched LUM flat looks completely flat with no obvious evidence of any vignetting or reflection artifacts.

 

 

This screen capture shows the intensity histogram of the LUM master flat frame. There are no obvious nodes in the histogram as seen in the SII and Ha. The histrogram looks rather Gaussian with a bit of tailing. This is likely due to slight vignetting but also due to the very slight ring pattern.

 

 

This is the processed NGC7000 LUM filter Master Light Frame - 60 X 90s exposures. I used 60 DARKs, 60 flats by filter, 60 Dark Flats for calibration. Calibration, Star Alignment and stacking with PixInsight. There appear to be no obvious artifacts in the LUM filter masters that appear in the SII, Ha and OIII master frames.

 

 

This is the flats analysis from CCD Inspector of the LUM master flat frame:

 

 

This shows that the maximum amount of physical vignetting is ~3% intensity variation. The ring artifact in the SII, Ha and OIII filter flats is not vignetting since they have the same physical optical path. 

Edited by cfosterstars, 01 December 2017 - 09:32 PM.

[mistateo]

Not sure I can be of much help, but I have had much better success with flat frames on the ASI1600MM-C and my 1.25" zwo filters shooting for about 15k ADU in SGP (16 bit?).  When I was going for 30k, it would take a long time, AND the flats had HARD lines between the bright and dark areas.  I must say though, your vignetting looks significant for 36mm filters.  I have one scope shooting at f/4.8 and the other at f/6.3 and the vignetting is much less severe on my 1.25" threaded filters.  I removed the 11mm spacer on the camera, and screw the camera directly into the filter wheel, on the side with the writing on it, which orients the filters the same direction as if they were screwed directly into the nosepiece (no insults intended here, yours are probably facing the same direction).  I also noticed that if the humidity is really bad, my flats aren't completely even, so I take the scope into the garage to warm up in that case and shoot my flats.  FWIW, I am using a white t-shirt, with 2 layers, front and back of shirt, and an Ipad with notes on full brightness.  The exposure times calculated for 15k ADU with my setup at f/4.8:

 

L-.05s

R-.09s

G-.-07s

B-.09s

 

Ha-1.49s

Oiii-1.05s

Sii-2.05s

 

Edit:  Forgot to mention those times were all at unity gain

Edited by mistateo, 01 December 2017 - 08:35 PM.

[cfosterstars]

Not sure I can be of much help, but I have had much better success with flat frames on the ASI1600MM-C and my 1.25" zwo filters shooting for about 15k ADU in SGP (16 bit?).  When I was going for 30k, it would take a long time, AND the flats had HARD lines between the bright and dark areas.  I must say though, your vignetting looks significant for 36mm filters.  I have one scope shooting at f/4.8 and the other at f/6.3 and the vignetting is much less severe on my 1.25" threaded filters.  I removed the 11mm spacer on the camera, and screw the camera directly into the filter wheel, on the side with the writing on it, which orients the filters the same direction as if they were screwed directly into the nosepiece (no insults intended here, yours are probably facing the same direction).  I also noticed that if the humidity is really bad, my flats aren't completely even, so I take the scope into the garage to warm up in that case and shoot my flats.  FWIW, I am using a white t-shirt, with 2 layers, front and back of shirt, and an Ipad with notes on full brightness.  The exposure times calculated for 15k ADU with my setup at f/4.8:

 

L-.05s

R-.09s

G-.-07s

B-.09s

 

Ha-1.49s

Oiii-1.05s

Sii-2.05s

 

Edit:  Forgot to mention those times were all at unity gain

One conclusion is that this is not vignetting. The LUM data show that the vignetting is at most ~3% and the ring pattern is 10X that level. Also my Canon T5i with the same WO flattener and an APS-C sensor show no vignetting either. You only start to see vignetting with the flattener on my Canon 6D full frame sensor. 

Edited by cfosterstars, 01 December 2017 - 08:50 PM.

[cfosterstars]

 One suggestion from my previous thread is that this could be due to the orientation of the filters relative to the camera to OTA direction. ZWO instructions state that the orientation is important for the narrow band filters and shows pictures of how to determine the proper orientation. I tried their instruction and found that they were difficult to reproduce. I did my best and that is how I got to the original configuration that I have shown the flats data for. The first change I made with to flip my filter wheel around. I did not remove the filter and flip them in the filter wheel carousel. I un-mounted the camera from the scope, unscrewed the filter wheel from the camera and adapters, turned the filter wheel around and remounted the adapters in the exact configuration that I had previous. I made sure to keep as many variables constant. I then re-ran flat frame for all the seven filters using exactly the same method as I showed previously - small illumination, setting, times, etc. - everything constant.  This change the orientation of all filters and the direction of the filter wheel and the internal screws and spacing of the filter wheel only.  I have posted the data so far for the LUM, Ha, OIII and SII filters in this gallery:

 

Album: Flat frames from ASI1600MM-C - flipped orientation

22 images

0 comments

 

These are the results for the SII filter: This is the SII master flat frame - STF stretched in Pixinsight - this was taken with the flipped filter orientation. This was 40 flat frames at 15.42 seconds taken using Sequence Generator Pro. Integration time was determined using SGP flats calibration wizard. Illumination was using an Alnitak Flatman with two layers of tee shirt fabric. No bias frames used only Flat Dark Frames - 40 flat dark frames.

 

 

The heavy edge ring is gone and the frame is much flatter. There is now a different pattern of three concentric rings. This is un-stretched flat master and it looks much improved and has no obvious pattern.

 

 

This screen capture shows the intensity histogram of the SII master flat frame that was taken after flipping the entire filter wheel assembly to reverse the orientation of the filters relative to the OTA vs. camera. There are only weak nodes in the intensity histogram showing significant reduction in reflection artifacts but not complete elimination of them.

 

 

This is bad data, but it is what I have for the SII light frames with this orientation that I have had time to process. This is part of the Heart Nebula mosaic with SII filter Master Light Frame - 30 X 240s exposures. I used 60 DARKs, 60 flats by filter, 60 Dark Flats for calibration. Calibration, Star Alignment and stacking with PixInsight. There is still a radial gradient pattern as seen in the SII Master Flat did not calibrate out from the light frame. The same artifacts are visible in each light frame. However, the image is better than the SII master with the unflipped filter wheel.

 

This is the flats analysis from CCD Inspector of the SII master flat frame from the flipped orientation:

 

The intensity variation has be cut from over 30% in the original orientation to ~6%. If we believe that the maximum physical vignetting drop off is about 3%, then we have cut the reflection artifact by an order of magnitude - not eliminated it - or we simply have a different source of reflection in this orientation. Since nothing was change accept the filter wheel, it is unlikely that the source of reflection causing the heavy edge ring is coming from the OTA forward. This also further proves that the issue is not vignetting since that should not have changed substantially by flipping just to orientation of the filter wheel.

 

Now for the LUM filter: This is the LUM master flat frame - STF stretched in Pixinsight - this was taken after flipping the entire filter wheel assembly to reverse the orientation of the filters relative to the OTA vs. camera. Compared to the un-flipped orientation, there is more artifacts and a decided top-to-bottom gradient. There is still some vignetting and there is a somewhat stronger radial ring pattern. This was 40 flat frames at 0.5 seconds using Sequence Generator Pro. Integration time was determined using SGP flats calibration wizard. Illumination was using an Alnitak Flatman with two layers of tee shirt fabric. 

 

 

The unstretched LUM master flat still look rather unremarkable, but if you stare at it you can see hints of the issues seen in the stretched version.

 

 

The histogram, however, clearly shows that the reflection issue has degraded the LUM flat significantly with the appearance of an intensity node:

 

 

This is the flats analysis from CCD Inspector of the SII master flat frame from the flipped orientation:

 

The intensity uniformity has clearly degraded for the LUM flat from ~3% in the original configuration to ~8% in a mostly top-to-bottom gradient, but the ring issue is also present.

 

 

I have not said anything about the RED, GREEN and BLUE filter data. For those filters, they basically tracked the LUM filter as far as the effect of flipping the filter wheel - the new concentric ring pattern appeared but more strongly than for the LUM filter and much weaker than the SII, Ha, and OIII filters. For all these effect, they have followed from best to worse: LUM, G, B, R, OIII, Ha, SII. with RGB not all that different but with OIII much better than Ha and SII. You can see all the data in the gallery links.

Edited by cfosterstars, 01 December 2017 - 11:09 PM.

[cfosterstars]

Both myself and the other posters who have discussed this issue in various threads have had a lot of input from many people out there on CN. However, I have not seen any solution to the issue. Some have just cropped out the edges of the frame. I have just tried to remove as much as I can with either DBE in pixinsight or gradient exterminator with a rather poor degree of success. This is the soul nebula narrow band partially processed that show how bad this can get:

 

 

For me, this ruins a lot of good data. This is currently the very weak link in my chain. I have tried to find this issue for quite a while now and have only come so far. From the data that I have shown so far, I am planning the following next steps:

 

1) I am retaking the soul nebula data right now will all 7 filters in the flipped orientation. I am almost complete with that. But ave not started processing the data

 

2) I took flats in the flipped orientation with three different methods to compare with the histograms. This needs a lot of processing also.

 

3) I will process the data from the soul nebula both with and with flat to see if the flats are actually making the data worse.

 

4) I think that the improvement from flipping the filter wheel is from some internal component of the filter wheel itself that is the source of the reflection causing the heavy dark ring in the original orientation. By flipping the filter wheel, that source is now downstream of the OTA and is reflecting much less or not at all. I think that the new, weaker multi-ring pattern is interference from the filters themselves since all filter seem to have the same pattern. I plan to darken the filter edges and to flip the orientation of the filters in the filter wheel directly and leave the filter wheel orientation in the flipped configuration.

 

5) I bit the bullet and bought a 5nm Ha filter from Astrodon. I will install that and compare directly to the ZWO filter. 

 

Any other suggestions, discussion, analysis or even guesses would be welcome. This has been a very tough nut to crack.

 

Thanks,

 

Chris

 

[bratislav]

 

 

Second – how I take flats: basically, I have tried every way that there is and they all show the same issue. I will show data from using a Aniltak Flatman EM panel that I diffuse further with 1-2 layer of tee shirt fabric giving a LUM filter flat exposure of 0.5s and an SII filter Flat 15.9 seconds. I also have a home made light box with LED and diffusers that I also diffuse further with 1-2 layers of tee shirt fabric giving a LUM filter flat exposure of 0.002s and an SII filter Flat 0.25 seconds. I have also done sky flat with 3-6 layers of tee shirt fabric a LUM filter flat exposure of 0.03s and an SII filter Flat 1.3 seconds. So, this is three different types of light sources with different degrees of collimation, different methods of diffusion, and different exposure times.  The exposures times for all three light sources was determined the same way with SGP flat calibration wizard and the flat frame were collected with SGP to give a peak of the histogram in the middle of the response at 25K ADU using 16bit (I will show histrograms of the data). All the calibration was with dark flat frame taken immediately after the flats with the exact same exposure times. I did not use any bias frames. All flats methods basically have the same issue.

 

Have you tried to take the camera out and produce flats in isolation? That is, just camera, FW and off axis guider (if used). Add some extension tubes (no optics), cover with white T-shirt/tissue and expose flats. Do you still see same artifacts? If not, it is something in your imaging train that produces those. A grazing reflection from somewhere can do all sorts of things, including "ring of fire" artifacts. How do you connect camera/OAG to the flattenner? M48? M42? Something else (2" tube, 1-1/4" tube) ? If you just take the camera out and look towards the brightly lit objective end (point the scope towards blue sky), do you see any reflections anywhere ?

[cfosterstars]

 

 

 

Second – how I take flats: basically, I have tried every way that there is and they all show the same issue. I will show data from using a Aniltak Flatman EM panel that I diffuse further with 1-2 layer of tee shirt fabric giving a LUM filter flat exposure of 0.5s and an SII filter Flat 15.9 seconds. I also have a home made light box with LED and diffusers that I also diffuse further with 1-2 layers of tee shirt fabric giving a LUM filter flat exposure of 0.002s and an SII filter Flat 0.25 seconds. I have also done sky flat with 3-6 layers of tee shirt fabric a LUM filter flat exposure of 0.03s and an SII filter Flat 1.3 seconds. So, this is three different types of light sources with different degrees of collimation, different methods of diffusion, and different exposure times.  The exposures times for all three light sources was determined the same way with SGP flat calibration wizard and the flat frame were collected with SGP to give a peak of the histogram in the middle of the response at 25K ADU using 16bit (I will show histrograms of the data). All the calibration was with dark flat frame taken immediately after the flats with the exact same exposure times. I did not use any bias frames. All flats methods basically have the same issue.

 

Have you tried to take the camera out and produce flats in isolation? That is, just camera, FW and off axis guider (if used). Add some extension tubes (no optics), cover with white T-shirt/tissue and expose flats. Do you still see same artifacts? If not, it is something in your imaging train that produces those. A grazing reflection from somewhere can do all sorts of things, including "ring of fire" artifacts. How do you connect camera/OAG to the flattenner? M48? M42? Something else (2" tube, 1-1/4" tube) ? If you just take the camera out and look towards the brightly lit objective end (point the scope towards blue sky), do you see any reflections anywhere ?

 

As I stated at the beginning, I am not the only one with this same issue. I have not done as you suggest, but another poster on CN with the same issue did exactly as you suggested and the edge artifact for the original configuation was present even in the "camera-only" flat. However, I have not tried this myself. I will add it to the list of things to do. I have no OAG but use a guide scope. The camera has M42 threads. I have a 10mm M42 spacer between the camera and the filter wheel so its all M42. Between the filter and the flattener is a M42 spacer to set the total backfocus to 55mm for the flattener optimization and is directly threaded to the WO F4. I had a custom flange made by PreciseParts Build-An-Adapter to directly mount the WO F4 to 3" focuser on the ORION EON APO that replaced the nose piece of the WO F4. If I hook up my Canon T5i to the rig in place of the ASI, there is no vignetting.

Edited by cfosterstars, 01 December 2017 - 11:45 PM.

[bratislav]

I wouldn't say it is vignetting, but grazing reflections can do a lot of damage, and it is best to make sure they are not at play. 

I understand the frustration and that some people also have same problem, but I would also say there seems to be hundreds of users that do not have same type of issues. So it cannot be generic type problem, but combination of things, a very specific situation that procures these "ring" flats. Do your filters have blackened edges? Black marker can sometimes do wonders. But I'd first check for reflections inside imaging train before moving further.

Good luck!

[mikefulb]

Interesting I'm having a similar problem but I have a 3nm Astrodon OIII so go figure.

[bratislav]

Another thing I would do if I were you is take the camera out (leave EFW in place), point the scope towards something very bright, place your eye where chip would be and look.

Simple and almost silly, but believe me you can solve a lot of problems this way. You can see reflections, vignetting, obstructions, the lot. 

[freestar8n]

Some patterns in the flats are common and only a problem if the flats don’t calibrate well.

The best flats I know are sky flats with no diffuser or t shirt. It looks like all your flats involve a diffuser - and that can produce illumination different from the light.

So I would try blue sky or twilight flats with no diffuser or t shirt or anything. You may still get rings and streaks but if they calibrate well then all is fine.

Frank

[cfosterstars]

Another thing I would do if I were you is take the camera out (leave EFW in place), point the scope towards something very bright, place your eye where chip would be and look.

Simple and almost silly, but believe me you can solve a lot of problems this way. You can see reflections, vignetting, obstructions, the lot. 

I will definitely do this also. I am just now trying to complete some data collection with the system in tack first. I have some dark flats to take and then tomorrow - assuming nothing else comes up - I should be able to get to this.

[Kaos]

Viking,

 

Here are the results of my comparison between the ZWO filters and Meade filters. I am only posting the Lum and Blue as the Red and Green results are the same as the blue. I created master flats in Maxim DL for my ZWO L & B filters and I removed the Luminance filter from the wheel because I use the Hutech LPS (48mm attached to the front of my field falttener) which should block the IR so there is really no need for the IR Luminace ZWO filter.. See here for Hutech LPS  http://www.sciencece...ch/idas/lps.htm

 

I use the Hotech 2" SCA Self-Centering Field Flattener. Here is the order I tested things with the ZWO filters:

 

1. I removed the Hutech LPS - No change to the rings

2. I removed the filed flattener - No change to the rings

3. I removed the Lum filter from the housing and blackened the filter edges and the corners (as I could see a reflection from filter corners - This helped, but the ringing was still present.

4. I removed the ZWO Luminance filter - Rings problem resolved

5. Removed the ZWO RGB filters and replaced with Meade RGB filters - Ring problem resolved.

 

I am not sure about the unmounted filters, but the coating on the mounted filters only extends to NEAR the edge of the glass. This, I believe is causing a reflective issue with the mounted filters.

 

I replaced the ZWO filters with my old Meade RGB filters. In order to get the filters to fit the wheel, I had to make a gasket for the EFW housing out of painters tape. The Meade filters scraped ever so much. With the gasket, they clear for the filter housing and the wheel turns smoothly. I hope this does not cause me to not be able to reach focus...lol.

 

The flats were taken using an Aurora Flatfield Panel. and I used the Flat Caibration Wizard in SGP to se the ADU value at 23000 +- 500.

 

Here is a side-by-side comparison of the Luminace master flats.

 

 

Here is the comparison of the Blue filters (ZWO is on the left and Meade on the right).

 

 

Obviously I cleaned the optics so the Meade filters look cleaner with few dust bunnies.

 

I am not sure whether the ZWO filter curvature is slightly off or the coating is causing relections, but either way, at least for the 1.25 filters, they are just not good filters.

 

Kaos

Edited by Kaos, 02 December 2017 - 05:16 PM.

[Jon Rista]

Interesting about the coating on the filters not going to the edge. That would certainly cause some problems. Even without reflecting, right at the edge of the coating there could be odd refractive properties. There would also be higher reflectivity off the outer rim of the filter. I have a set of ZWO 1.25" LRGB filters...going to check them out now and see if they, too, are not coated to the edge.

[bratislav]

Bingo! This is the first plausible explanation I have seen so far that might cause these artifacts. I have looked and my (ZWO) filters also have coating "almost to the edge" but those areas are well under the lip of mounting hardware in the EFW (so light simply doesn't reach that area). This would also explain why some people have success in fixing this problem by creating additional mask in front of the filters. It also explains why reversing the filter somewhat alleviates the issue (having this ring closer to the camera hides it better, when it is closer to the sky, more of the light hits it). 

It is possible that some of the filters were coated with a mask that was a tad too small (filters have to be supported somehow in the chamber during coating process), so this uncoated/not well coated area falls into the light path. If this is the case, I think you would be entitled to ask for a replacement (I certainly would).

BTW 36mm filters are so oversized that I would suggest it is quite possible to blacken that area (say 1mm from the edge) and not worry about vignetting at all.

But again, you as a customer should not be expected to do that, so see above.

 

PS if you removed the CCD and looked through the filters in the EFW toward bright source, most definitely you would see those uncoated areas light up.  

Edited by bratislav, 02 December 2017 - 08:06 PM.

[Kaos]

Bingo! This is the first plausible explanation I have seen so far that might cause these artifacts. I have looked and my (ZWO) filters also have coating "almost to the edge" but those areas are well under the lip of mounting hardware in the EFW (so light simply doesn't reach that area). This would also explain why some people have success in fixing this problem by creating additional mask in front of the filters. It also explains why reversing the filter somewhat alleviates the issue (having this ring closer to the camera hides it better, when it is closer to the sky, more of the light hits it). 

It is possible that some of the filters were coated with a mask that was a tad too small (filters have to be supported somehow in the chamber during coating process), so this uncoated/not well coated area falls into the light path. If this is the case, I think you would be entitled to ask for a replacement (I certainly would).

BTW 36mm filters are so oversized that I would suggest it is quite possible to blacken that area (say 1mm from the edge) and not worry about vignetting at all.

But again, you as a customer should not be expected to do that, so see above.

 

PS if you removed the CCD and looked through the filters in the EFW toward bright source, most definitely you would see those uncoated areas light up.  

That is the way I first noticed the reflection in the filter was by holding at an angle to the window. The edges underneath the locking clamp that holds the filter in glowed kind of a gold color different from the rest of the filter. I removed the filter and that is when I noticed that the filters are not coated all the way to the edge and the edges have no dark coating. So, I suspect that it is the coating that is causing the light to reflect off the edges of the filter housing creating the ringing artifacts.

 

Kaos

[cfosterstars]

Well I think we are all making progress here. I did a lot of testing myself today and have some interesting results that add to the discussion. This is a picture of my mount and rig showing my ASI1600MM-C mounted on my ORION EON 115mm APO refractor

 

 

This is a close up of my camera optic train with the ASI1600MM-C, 7 position EFW, spacers, Williams Optics flattern IV and EON115mm 3" focuser interface. All connections are M42 fully threaded adapters and a direct threaded coupler for the flattener to the 3" focuser.

 

 

This shows the orientation of the filter wheel in the flipped orientation. The access screws are pointing toward the camera so the screws that hold the filters are facing toward the camera also. This the orientation that has produced the flattest flat frames without the heavy edge ring pattern.

 

 

This is a close up of the BLUE filter mounted inside the filter wheel from the perspective of the camera. The screw heads and washers are not black and can reflect. The filter coatings do not go all the way to the edge of the filter. The filter bevel is also rough and is scattering light.

 

 

This is a close up of the edge of the BLUE filter showing the uncoated filter edge and the rough bevel. This uncoated edge is present on all the filters and has the highest contrast on the narrow band filters since these filter blocks the most light generating the greatest contrast to the uncoated edge/bevel.

 

 

After removing the camera and filter wheel, this is the view through the EON 115mm OTA and Williams Optic Flattener IV. The diffuse reflections is from the focuser draw tube. It is already blackened and roughened, but still shows a fair amount of reflections.  This is a source of at least some reflections in the optical train but it is not clear how much this is causing the flats issues. I have previously used this OTA many times with DSLRs and have seen no such issues with flat frame artifacts, but the reflectivity of the draw tube could and should be improved.

 

 

After putting the filter wheel back in place, this is the view through the LUM filter through the filter wheel, William Optics Flattener IV and the EON 115mm OTA. In addition to the reflection from the focuser draw tube, there is a very bright halo of multi color reflection coming from the filter edge and filter bevel.

 

 

Close up of the edge reflection from the LUM filter edge looking down the OTA through the filter wheel from the camera position. The filter edge reflection shows as a bright rainbow colored ring around the edge of the filter. you can also see the reflection from the focuser draw tube.

 

 

For my own case, the reflection from the draw tube was unexpected and annoying. I would have expected the internal flocking to be better, but this is a relatively easy fix. I have ordered some BLACK 2.0 super blackening paint from Amazon to recoat the draw tube. The tube was roughened and painted black but it is clearly not completely effective. I saw no issue with the Williams Optics Flattener IV as a source of reflection. 

 

My pictures don't do it justice on just how bright the reflection is from the filter edge and bevel. It is actually much brighter than the reflection from the draw tube. I believe that for me this is the source of the heavy edge ring artifacts. I really dont have a good way of addressing this issue. I think that making making rings for these filter, while doable, is not what the customer should have to do. I can do this in a precise way with my wife's cricut printer out of vinyl or thin plastic. One other issue are the screw heads and the washers - these should also be black and not just stainless steel and shiny - I can pain them as well. 

 

I have ordered an Astrodon 36mm Ha 5mm filter. It cost as much as the full set of narrow band filters from ZWO. I believe that they are specifically coated to the very edge and the edge comes blackened. I will do a direct comparison with ZWO filter in the original orientation of the filter wheel as a  worst case. 

[Jon Rista]

The drawtube reflection isn't all that bad. It might be causing a small part of the problem, but I don't suspect it is a huge issue. If you get it fully blackened it should be fine. 

 

That reflection off the lum filter, though, is definitely a problem. That is exactly the kind of thing you want to look for.  That is probably the bulk of the problem. I wonder if adding a small mask around the edge of the filter will help. If you can keep the light from glancing off that side surface of the edge of the filters, it might resolve the issue.

 

I've had even brighter reflections off of StellarVue spacers, which sadly are neither blackened nor have internal microbaffling, they are machined smooth and reflect like mad. I had serious issues with those spacers. Once I blackened the spacer surface, though, all my issues went away. 

[cfosterstars]

The drawtube reflection isn't all that bad. It might be causing a small part of the problem, but I don't suspect it is a huge issue. If you get it fully blackened it should be fine. 

 

That reflection off the lum filter, though, is definitely a problem. That is exactly the kind of thing you want to look for.  That is probably the bulk of the problem. I wonder if adding a small mask around the edge of the filter will help. If you can keep the light from glancing off that side surface of the edge of the filters, it might resolve the issue.

 

I've had even brighter reflections off of StellarVue spacers, which sadly are neither blackened nor have internal microbaffling, they are machined smooth and reflect like mad. I had serious issues with those spacers. Once I blackened the spacer surface, though, all my issues went away. 

Jon,

 

Sincerely, thanks for all your help. I really mean that. I think this is the root cause of the issue and is mostly fixable. I am just not sure that I want to kluge a solution. I am seriously considering the investment in higher quality filters, but that is a lot of bucks. I may give the mask a try and see if it works.

[Jon Rista]

Can't go wrong with higher quality filters. We may also be able to get ZWO to resolve the issues with the current filters. A beveled edge is clearly a bad idea, and if they aren't blackening the sides, that should be an easy fix as well. 

[Kaos]

 

The drawtube reflection isn't all that bad. It might be causing a small part of the problem, but I don't suspect it is a huge issue. If you get it fully blackened it should be fine. 

 

That reflection off the lum filter, though, is definitely a problem. That is exactly the kind of thing you want to look for.  That is probably the bulk of the problem. I wonder if adding a small mask around the edge of the filter will help. If you can keep the light from glancing off that side surface of the edge of the filters, it might resolve the issue.

 

I've had even brighter reflections off of StellarVue spacers, which sadly are neither blackened nor have internal microbaffling, they are machined smooth and reflect like mad. I had serious issues with those spacers. Once I blackened the spacer surface, though, all my issues went away. 

Jon,

 

Sincerely, thanks for all your help. I really mean that. I think this is the root cause of the issue and is mostly fixable. I am just not sure that I want to kluge a solution. I am seriously considering the investment in higher quality filters, but that is a lot of bucks. I may give the mask a try and see if it works.

 

I agree with Jon on the OTA reflections. Remember, you are pointing at a really bright object so any internal reflections will be magnified significantly. My EON 115 has internal baffles that are supposed to cut down on internal OTA reflections. I would expect that your flats will correct the internal reflections almost totally once you get some new filters. Man, I am really PO'd that ZWO sold me a bill of goods with these filters. I hope Sam does view this thread and can see the results of both my and your analysis work on this issue. Thanks for pointing it out to begin with. The rings have always been an annoyance and I was simply just fixing them in PS. Now, after changing my filters I will not have to take that step!

 

Kaos

[SandyHouTex]

Well I think we are all making progress here. I did a lot of testing myself today and have some interesting results that add to the discussion. This is a picture of my mount and rig showing my ASI1600MM-C mounted on my ORION EON 115mm APO refractor

 

 

 

This is a close up of my camera optic train with the ASI1600MM-C, 7 position EFW, spacers, Williams Optics flattern IV and EON115mm 3" focuser interface. All connections are M42 fully threaded adapters and a direct threaded coupler for the flattener to the 3" focuser.

 

 

 

This shows the orientation of the filter wheel in the flipped orientation. The access screws are pointing toward the camera so the screws that hold the filters are facing toward the camera also. This the orientation that has produced the flattest flat frames without the heavy edge ring pattern.

 

 

 

This is a close up of the BLUE filter mounted inside the filter wheel from the perspective of the camera. The screw heads and washers are not black and can reflect. The filter coatings do not go all the way to the edge of the filter. The filter bevel is also rough and is scattering light.

 

 

 

This is a close up of the edge of the BLUE filter showing the uncoated filter edge and the rough bevel. This uncoated edge is present on all the filters and has the highest contrast on the narrow band filters since these filter blocks the most light generating the greatest contrast to the uncoated edge/bevel.

 

 

 

After removing the camera and filter wheel, this is the view through the EON 115mm OTA and Williams Optic Flattener IV. The diffuse reflections is from the focuser draw tube. It is already blackened and roughened, but still shows a fair amount of reflections.  This is a source of at least some reflections in the optical train but it is not clear how much this is causing the flats issues. I have previously used this OTA many times with DSLRs and have seen no such issues with flat frame artifacts, but the reflectivity of the draw tube could and should be improved.

 

 

 

After putting the filter wheel back in place, this is the view through the LUM filter through the filter wheel, William Optics Flattener IV and the EON 115mm OTA. In addition to the reflection from the focuser draw tube, there is a very bright halo of multi color reflection coming from the filter edge and filter bevel.

 

 

 

Close up of the edge reflection from the LUM filter edge looking down the OTA through the filter wheel from the camera position. The filter edge reflection shows as a bright rainbow colored ring around the edge of the filter. you can also see the reflection from the focuser draw tube.

 

 

 

For my own case, the reflection from the draw tube was unexpected and annoying. I would have expected the internal flocking to be better, but this is a relatively easy fix. I have ordered some BLACK 2.0 super blackening paint from Amazon to recoat the draw tube. The tube was roughened and painted black but it is clearly not completely effective. I saw no issue with the Williams Optics Flattener IV as a source of reflection. 

 

My pictures don't do it justice on just how bright the reflection is from the filter edge and bevel. It is actually much brighter than the reflection from the draw tube. I believe that for me this is the source of the heavy edge ring artifacts. I really dont have a good way of addressing this issue. I think that making making rings for these filter, while doable, is not what the customer should have to do. I can do this in a precise way with my wife's cricut printer out of vinyl or thin plastic. One other issue are the screw heads and the washers - these should also be black and not just stainless steel and shiny - I can pain them as well. 

 

I have ordered an Astrodon 36mm Ha 5mm filter. It cost as much as the full set of narrow band filters from ZWO. I believe that they are specifically coated to the very edge and the edge comes blackened. I will do a direct comparison with ZWO filter in the original orientation of the filter wheel as a  worst case. 

I don’t see a big difference between the Astrodon and ZWO unmounted Ha filters on the OPT website.  Both seem to have edge chamfers, and I would expect that there will be a small area that is uncoated around the periphery for both.  You have to hold them in the fixture when coating them somehow.

 

I do agree this is probably what is causing the issues.  But recall there were flares around the Astrodon filters in a previous post.  The ONLY filters that didn’t cause the rings around the stars were the Astronomics.

 

Maybe using mounted filters would solve this issue.  I looked at my Orion narrowband 1.25 inch mounted filters and I can’t see any uncoated area.  The retaining ring would cover it.

Edited by SandyHouTex, 03 December 2017 - 12:22 AM.

[Kaos]

 

Well I think we are all making progress here. I did a lot of testing myself today and have some interesting results that add to the discussion. This is a picture of my mount and rig showing my ASI1600MM-C mounted on my ORION EON 115mm APO refractor

 

 

 

This is a close up of my camera optic train with the ASI1600MM-C, 7 position EFW, spacers, Williams Optics flattern IV and EON115mm 3" focuser interface. All connections are M42 fully threaded adapters and a direct threaded coupler for the flattener to the 3" focuser.

 

 

 

This shows the orientation of the filter wheel in the flipped orientation. The access screws are pointing toward the camera so the screws that hold the filters are facing toward the camera also. This the orientation that has produced the flattest flat frames without the heavy edge ring pattern.

 

 

 

This is a close up of the BLUE filter mounted inside the filter wheel from the perspective of the camera. The screw heads and washers are not black and can reflect. The filter coatings do not go all the way to the edge of the filter. The filter bevel is also rough and is scattering light.

 

 

 

This is a close up of the edge of the BLUE filter showing the uncoated filter edge and the rough bevel. This uncoated edge is present on all the filters and has the highest contrast on the narrow band filters since these filter blocks the most light generating the greatest contrast to the uncoated edge/bevel.

 

 

 

After removing the camera and filter wheel, this is the view through the EON 115mm OTA and Williams Optic Flattener IV. The diffuse reflections is from the focuser draw tube. It is already blackened and roughened, but still shows a fair amount of reflections.  This is a source of at least some reflections in the optical train but it is not clear how much this is causing the flats issues. I have previously used this OTA many times with DSLRs and have seen no such issues with flat frame artifacts, but the reflectivity of the draw tube could and should be improved.

 

 

 

After putting the filter wheel back in place, this is the view through the LUM filter through the filter wheel, William Optics Flattener IV and the EON 115mm OTA. In addition to the reflection from the focuser draw tube, there is a very bright halo of multi color reflection coming from the filter edge and filter bevel.

 

 

 

Close up of the edge reflection from the LUM filter edge looking down the OTA through the filter wheel from the camera position. The filter edge reflection shows as a bright rainbow colored ring around the edge of the filter. you can also see the reflection from the focuser draw tube.

 

 

 

For my own case, the reflection from the draw tube was unexpected and annoying. I would have expected the internal flocking to be better, but this is a relatively easy fix. I have ordered some BLACK 2.0 super blackening paint from Amazon to recoat the draw tube. The tube was roughened and painted black but it is clearly not completely effective. I saw no issue with the Williams Optics Flattener IV as a source of reflection. 

 

My pictures don't do it justice on just how bright the reflection is from the filter edge and bevel. It is actually much brighter than the reflection from the draw tube. I believe that for me this is the source of the heavy edge ring artifacts. I really dont have a good way of addressing this issue. I think that making making rings for these filter, while doable, is not what the customer should have to do. I can do this in a precise way with my wife's cricut printer out of vinyl or thin plastic. One other issue are the screw heads and the washers - these should also be black and not just stainless steel and shiny - I can pain them as well. 

 

I have ordered an Astrodon 36mm Ha 5mm filter. It cost as much as the full set of narrow band filters from ZWO. I believe that they are specifically coated to the very edge and the edge comes blackened. I will do a direct comparison with ZWO filter in the original orientation of the filter wheel as a  worst case. 

I don’t see a big difference between the Astrodon and ZWO unmounted Ha filters.

 

Sandy...do you have the ringing issue? What size filters are you using? Are the Astrodon filters coated fully and the edges darkened? If you do not have the ringing issue with the ZWO filters, it is pretty hard to conclude that there is little difference. In other words, there may not be much difference if you do not see the ringing issue with the ZWO filters in your flats and lights. My old Meade LRGB filters make a huge difference with this problem.I suspect that the Astrodon filters would not exhibit this issue but I cannot say for sure.

 

Kaos

[SandyHouTex]

I don’t have the issue but again, I have Orion narrowband filters that are mounted.  When I look through them and turn them sideways I don’t see any brightening at the edges.