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Reflection Artifacts in the ASI6200

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#1 rockstarbill

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Posted 12 September 2020 - 01:44 AM

Hello all,

 

I wanted to share some experiences with the ASI6200 camera on larger aperture telescopes. Before I get into this, I want to state clearly that these problems did not manifest at all with smaller aperture scopes, such as the AP130 GTX that I use for most of my widefield imaging.

 

For the test I used a AG Optical 10" iDK, ZWO ASI6200 camera, 2" ZWO Wheel, and M68 ZWO OAG. 

 

First frame I would like you all to review:

 

https://1drv.ms/u/s!...aGbnHg?e=l6u6bO

 

15 minute exposure of the Sadr region in HA using a 3nm Chroma HA filter. What you should see if you evaluate this frame with attention, is that there are a host of reflections in the image. They look like stacked donuts. If you cannot see them well, use the boosted stretch in PI to make them more visible.

 

Now, I flipped the HA filter and ran the test again, and the result was no different. This was very concerning to me, but I proceeded to do more imaging anyhow.

 

The next image I would like you to look at is this OIII image, taken with a 3nm OIII filter last night:

 

https://1drv.ms/u/s!...gpTHkA?e=Nka1fr

 

What you need to look at here are the brighter stars that show a clear off-centered reflection artifact.

 

Like this:

 

6200_Reflect.JPG

 

These are not present with the same scope, same filters, and a different camera. These are artifact issues experienced with larger apertures (and NB only), caused -- to my belief by a poor window coating and substrate. This is not the slip cover on the sensor itself that is in question. 

 

Would love to hear some discourse on this. If you start by blaming my optics though, I am going to laugh at you. wink.gif


Edited by rockstarbill, 12 September 2020 - 08:48 PM.


#2 sharkmelley

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Posted 12 September 2020 - 04:16 AM

The Ha image definitely shows a rectangular grid of reflected donuts.  Each donut is a defocused star and clearly shows the central obstruction of the optics.  The reason the reflections form a grid is microlens diffraction.

 

We can now perform some calculations.  The diameter of each donut is 870 pixels.  Given the 3.76micron pixel pitch of the sensor this is a diameter of:

870 x 3.76micron = 3.27mm. 

The focal ratio of the scope is f/6.7 - is this correct?  

This means that the light forming the reflection has travelled an extra distance of:

3.27 x 6.7 = 21.9mm

So each reflection has been generated by a reflective surface approximately 21.9/2 = 10.9mm above the sensor.

 

Is this the position of the lower surface of the camera window?

 

Mark


Edited by sharkmelley, 12 September 2020 - 04:21 AM.

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#3 sharkmelley

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Posted 12 September 2020 - 05:06 AM

You are seeing some kind of artefact centred around your stars but I'm not convinced they are reflections. 

 

The Ha image of GamCyg shows a set of very closely spaced rings (crop below is scaled 300%):

 

asi6200_ha.jpg

 

I don't know the exact cause but it may be some kind of diffraction.  If it is diffraction, the ring spacing will be proportional to wavelength - in this case Ha at 656nm.

 

Looking at the OIII image I think I'm seeing the same ring pattern but maybe more closely spaced (crop below is scaled 300%):

 

asi6200_oiii.jpg

 

We would need a better OIII example to be sure - maybe an OIII image of GamCyg.

 

Mark


Edited by sharkmelley, 12 September 2020 - 05:10 AM.

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#4 elmiko

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Posted 12 September 2020 - 12:35 PM

I'm very impressed by your diagnosis Mark!


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#5 rockstarbill

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Posted 12 September 2020 - 12:46 PM

The Ha image definitely shows a rectangular grid of reflected donuts.  Each donut is a defocused star and clearly shows the central obstruction of the optics.  The reason the reflections form a grid is microlens diffraction.

 

We can now perform some calculations.  The diameter of each donut is 870 pixels.  Given the 3.76micron pixel pitch of the sensor this is a diameter of:

870 x 3.76micron = 3.27mm. 

The focal ratio of the scope is f/6.7 - is this correct?  

This means that the light forming the reflection has travelled an extra distance of:

3.27 x 6.7 = 21.9mm

So each reflection has been generated by a reflective surface approximately 21.9/2 = 10.9mm above the sensor.

 

Is this the position of the lower surface of the camera window?

 

Mark

That sounds feasible, Mark. Here is the ZWO drawing for the camera. The chip is shown as 17.5mm from the faceplate on the camera, which is 5mm thick. In my setup that faceplate is removed, leaving 12.5mm from the face to the chip, with the window recessed. That recess is likely about 2mm or so.

 

6200-Mechanical-Diagram-1024x744.png



#6 sn2006gy

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Posted 12 September 2020 - 12:52 PM

It must be your optics. I don't see this on other 6200s :)



#7 rockstarbill

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Posted 12 September 2020 - 01:08 PM

Whenever the smoke lets up here I can get a test shot of GamCyg with the OIII filter as well. I do have a 15 minute OIII sub taken of the Wizard Nebula which has HD 215588 in its FOV which shows this reflection a bit better. I have uploaded that frame here:

 

https://1drv.ms/u/s!...HtmVhw?e=hzR02W


Edited by rockstarbill, 12 September 2020 - 01:27 PM.


#8 mrszymon

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Posted 12 September 2020 - 01:08 PM

Chroma filters are known to sometimes produce artefacts similar to this; the solution from Chroma is to turn the filter around in the filter wheel. Not saying that’s definitely the cause here, but probably worth a try (this kind of thing doesn’t happen with Astrodons).

#9 rockstarbill

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Posted 12 September 2020 - 01:12 PM

Chroma filters are known to sometimes produce artefacts similar to this; the solution from Chroma is to turn the filter around in the filter wheel. Not saying that’s definitely the cause here, but probably worth a try (this kind of thing doesn’t happen with Astrodons).

I flipped the HA filter and that had no effect. I wish I still had Astrodon filters to test and see if the same behavior occurs. I will note that this is the only camera I have that produces this effect with the Chroma filters, and that the effect is not present in my smaller aperture scope, 130mm refractor in this case, using the same camera. 



#10 rockstarbill

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Posted 12 September 2020 - 01:13 PM

It must be your optics. I don't see this on other 6200s smile.gif

The same effect occurs on a 12.5" RCOS. I will see if I can get some data from Monkeybird747, who has this exact same problem on a larger aperture scope and the 6200MM Pro camera.


Edited by rockstarbill, 12 September 2020 - 01:13 PM.


#11 sn2006gy

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Posted 12 September 2020 - 01:17 PM

I flipped the HA filter and that had no effect. I wish I still had Astrodon filters to test and see if the same behavior occurs. I will note that this is the only camera I have that produces this effect with the Chroma filters, and that the effect is not present in my smaller aperture scope, 130mm refractor in this case, using the same camera. 

Ya, we know someone who had to flip his filters and that worked.

 

A quick search on astrobin and looking for images with diffraction spikes, i'm not seeing concentric rings there but they could be post processed out and/or masked by compression



#12 rockstarbill

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Posted 12 September 2020 - 01:26 PM

Ya, we know someone who had to flip his filters and that worked.

 

A quick search on astrobin and looking for images with diffraction spikes, i'm not seeing concentric rings there but they could be post processed out and/or masked by compression

I uploaded frames in the original post. The HA GamCyg frame was the most concerning. Go actually look at it. No way someone is processing that out as a viable workaround. 



#13 sn2006gy

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Posted 12 September 2020 - 01:32 PM

I've discovered one thing with ASI cameras that is annoying - that could be causing weirdness. Some config in ASI seems to be stored in NVRAM, so for example, if you polar align with sharpcap and it sets your camera to a low bit mode or different white balance those settings are preserved in non volatile ram.

 

If you switch to an imaging/sequence program that doesn't then set those parameters back to what it expects, you end up using whatever was last written to nvram.

 

If you use ascom drivers, it seems, the defaults are sane and get reset between uses. However, if you use native drivers, those settings can be preserved and not reset between uses.

 

Hard to explain... but i found out the white balance was messed when using native drivers because native drivers don't set sane defaults, whereas the ascom drivers seem to default to 50/50 

 

Do you happen to use sharpcap for alignment or focusing or anything and switch to another sequencer with the 6200?



#14 rockstarbill

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Posted 12 September 2020 - 01:35 PM

I've discovered one thing with ASI cameras that is annoying - that could be causing weirdness. Some config in ASI seems to be stored in NVRAM, so for example, if you polar align with sharpcap and it sets your camera to a low bit mode or different white balance those settings are preserved in non volatile ram.

 

If you switch to an imaging/sequence program that doesn't then set those parameters back to what it expects, you end up using whatever was last written to nvram.

 

If you use ascom drivers, it seems, the defaults are sane and get reset between uses. However, if you use native drivers, those settings can be preserved and not reset between uses.

 

Hard to explain... but i found out the white balance was messed when using native drivers because native drivers don't set sane defaults, whereas the ascom drivers seem to default to 50/50 

 

Do you happen to use sharpcap for alignment or focusing or anything and switch to another sequencer with the 6200?

Not sure what white balance has to do with reflections, but yes I use SharpCap to align and Voyager to run sequences. Both programs leverage native mode drivers. 



#15 sn2006gy

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Posted 12 September 2020 - 01:36 PM

I uploaded frames in the original post. The HA GamCyg frame was the most concerning. Go actually look at it. No way someone is processing that out as a viable workaround. 

I'd imagine it would get worse with stacking and i'm just not seeing terrible circles on astrobin but its an integration/post processed site, not raw subs so hard to tell.

 

I do see some great images, but if i pixel peep on images with diffraction spikes i can see slight halos from filters but i haven't gone deep enough to see what filters they used and i'm not sure there is enough 6200 data on there yet to distill anything from that.



#16 sn2006gy

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Posted 12 September 2020 - 01:38 PM

Not sure what white balance has to do with reflections, but yes I use SharpCap to align and Voyager to run sequences. Both programs leverage native mode drivers. 

just an example, but if its setting your camera to 12 or 14 bit mode and that is left in NVRAM perhaps you're seeing artifacts of lower dynamic range

 

edit: I noticed several different quirks because of this nvram issue - the dynamic range was one i experienced, but i have seen others.  You may want to try taking similar subs with the ascom vs native drivers just to make sure this isn't an issue. 


Edited by sn2006gy, 12 September 2020 - 01:40 PM.


#17 andysea

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Posted 12 September 2020 - 01:41 PM

Chroma filters are known to sometimes produce artefacts similar to this; the solution from Chroma is to turn the filter around in the filter wheel. Not saying that’s definitely the cause here, but probably worth a try (this kind of thing doesn’t happen with Astrodons).

Wait wait. This is a 900s sub with the QSI 583wsg, Astro Tech 6" RC and Astrodon 3nm H-alpha filter. This image is from 6 years ago. I don't know if AD filters have changed since then but I can see the same rings. I don't think it's the filter.

Attached Thumbnails

  • rings.jpg

Edited by andysea, 12 September 2020 - 01:42 PM.

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#18 rockstarbill

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Posted 12 September 2020 - 01:44 PM

just an example, but if its setting your camera to 12 or 14 bit mode and that is left in NVRAM perhaps you're seeing artifacts of lower dynamic range

 

edit: I noticed several different quirks because of this nvram issue - the dynamic range was one i experienced, but i have seen others.  You may want to try taking similar subs with the ascom vs native drivers just to make sure this isn't an issue. 

SharpCap in my case is not set to 12 or 14 bit mode. It is taking images in 16 bit mode. Bin 2x2 for Polar Alignment. None of the other values are changed. It does use a higher gain setting, but Voyager forces the gain and offset values specifically for each frame it takes. 



#19 sn2006gy

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Posted 12 September 2020 - 01:45 PM

Heh, I was looking at some of your images for comparison Andy.  I see those concentric circles on the raw subs but when i look at your stacked data it looked like the familiar soft halos you see on filters



#20 sn2006gy

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Posted 12 September 2020 - 01:47 PM

SharpCap in my case is not set to 12 or 14 bit mode. It is taking images in 16 bit mode. Bin 2x2 for Polar Alignment. None of the other values are changed. It does use a higher gain setting, but Voyager forces the gain and offset values specifically for each frame it takes. 

ok.. i've seen a lot of odd things with the native drivers they don't document. I think that part of zwo's drivers does suck and it has bitten people in many different ways.  Without access to voyagers code I just have to take the word for it that they're not susceptible while everyone else was (sgp, nina,  et all)



#21 rockstarbill

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Posted 12 September 2020 - 01:50 PM

Wait wait. This is a 900s sub with the QSI 583wsg, Astro Tech 6" RC and Astrodon 3nm H-alpha filter. This image is from 6 years ago. I don't know if AD filters have changed since then but I can see the same rings. I don't think it's the filter.

Thanks for chiming in Andy. I saw similar problems with the FLI 8300 camera I have, which is an older version of the camera, similar to the 583, mine was built in 2009. That camera had to be sent back to FLI to have the original window removed and a new window installed that used better substrate and coatings. Once that was done, the reflections were completely gone.

 

That experience is what led me to examine and identify the problem. The HA GamCy frame shows behavior I have never seen before. This suggested to me that the coating and/or substrate used in the ASI6200 window was suspect. 



#22 andysea

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Posted 12 September 2020 - 01:56 PM

I need to do some forensic search but I have about 10 years of data with several CCD and Cmos cameras and I have seen the same airy rings across the board. I will see if I can dig out some examples.

#23 rockstarbill

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Posted 12 September 2020 - 01:58 PM

I need to do some forensic search but I have about 10 years of data with several CCD and Cmos cameras and I have seen the same airy rings across the board. I will see if I can dig out some examples.

Thanks!

 

What I am seeing is more than just the ring pattern though:

 

Star_Artifact.JPG

 

There is an off centered halo effect as well. 



#24 andysea

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Posted 12 September 2020 - 02:08 PM

Oh yes I was referring to the ring pattern, sorry for not clarifying.

#25 rockstarbill

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Posted 12 September 2020 - 02:26 PM

Richard Crisp has a good writeup here:

 

http://www.narrowban..._tests_49mm.pdf

 

This shows the 8300 window, but you can see similar patterns in this data to some of the windows that were tested. 




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