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Reflection Artifacts in the ASI6200

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#251 GeneralT001

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Posted 19 September 2020 - 11:56 PM

I thought it was suggested the reflective side of the filters point to the telescope so that there's no refections between the filters and camera's sensor. 

 

Peter 

I was just reading another post on the Chroma issue and the consensus...in the end...was that the reflective side needs to be pointing at the camera...even though Chromas initial response was..."it doesn't matter...but if you have a problem...flip them". Thanks for the clear, authoritative answer Chroma smile.gif

 

 

https://www.cloudyni...entation/page-2


Edited by GeneralT001, 19 September 2020 - 11:57 PM.

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#252 rockstarbill

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Posted 19 September 2020 - 11:57 PM

I thought it was suggested the reflective side of the filters point to the telescope so that there's no refections between the filters and camera's sensor. 

 

Peter 

The AR coated side is called out as needing to be pointed to the camera. 



#253 Peter in Reno

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Posted 20 September 2020 - 12:11 AM

The AR coated side is called out as needing to be pointed to the camera. 

Ok, that sounds correct. Whew, I thought I was going open the FW to flip the filters again.

 

https://www.cloudyni...7#entry10511987

 

Peter 


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#254 rockstarbill

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Posted 20 September 2020 - 12:22 AM

Glass on results:

  • Grid pattern still present. No change.
  • Halos on less bright stars were still present. No change.
  • The ((o)) effect was not present at all with the sensor glass on either. This was a test with a refractor, so their immunity to this problem remains true.

The net of this is that the window on the camera does not help, nor hinder the effects at all. If the design goal of the window is to be net neutral compared to no glass at all, then it meets those objectives in a test with a refractor. If the design goal of the window is to aid in reducing the effects of reflective surfaces or events, it fails those miserably. 

 

Quick interject: The FLI8300 situation I linked to earlier showed that windows can actually aid in the reduction of reflective events, and in premium cameras they DO serve that purpose. It is my own personal advice to ZWO that the window should be improved. There is no way that no window performance should be identical to with window performance. That is likely indicative of the AR coating being poor and/or the window glass substrate being poor as well. I would lean to the substrate.

 

This same test needs to be carried out on a larger aperture and preferably Cassegrain based design. RC or DK would be the best. This test shows that sentiment from refractor users should be great, while the sentiment from Cassegrain scope users would be unchanged (obviously as we did not test that). 

 

To sum this up in a TL;DR:

  • Grid = Filter caused. Mounting of filter makes no difference other than the size of the donuts in the grid.
  • ((o)) = still unknown. This test did not provide data either way, as the issue did not reproduce on the refractor at all. These are more apparent in larger aperture and even more prevalent in systems that use spiders. (which this was not, it was a TOA130 refractor).
  • Halo effect = Unknown, but not exacerbated (nor aided) by the window at all. These are also more apparent in larger aperture telescopes, so more testing is needed.

Edited by rockstarbill, 20 September 2020 - 01:15 AM.

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#255 Lead_Weight

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Posted 20 September 2020 - 01:14 AM

I just pulled and tested an Astrodon filter and they have the same differences that the Chroma filters do. They are NOT the same on both sides.


Woah... this is an interesting discovery. I had always assumed they were the same on both sides. This also explains why we’re seeing the issue with these filters too, like my example of the 6” RC. Since I just ordered the 294mm and expect it this week I’ll be trying it out on my Edge 11 with the astrodon filters. I’ll check their orientation and flip them if necessary.
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#256 sharkmelley

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Posted 20 September 2020 - 03:48 AM

To sum this up in a TL;DR:

  • Grid = Filter caused. Mounting of filter makes no difference other than the size of the donuts in the grid.
  • ((o)) = still unknown. This test did not provide data either way, as the issue did not reproduce on the refractor at all. These are more apparent in larger aperture and even more prevalent in systems that use spiders. (which this was not, it was a TOA130 refractor).
  • Halo effect = Unknown, but not exacerbated (nor aided) by the window at all. These are also more apparent in larger aperture telescopes, so more testing is needed.

 

Excellent!  To the best of my knowledge this is the first time it has been demonstrated that the size of the reflected halos depends on filter orientation - making it possible to determine which side of the filter causes it.

 

Mark


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#257 rockstarbill

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Posted 20 September 2020 - 05:14 AM

Excellent! To the best of my knowledge this is the first time it has been demonstrated that the size of the reflected halos depends on filter orientation - making it possible to determine which side of the filter causes it.

Mark

Agreed. It also shows why folks with refractors likely look at this thread with confusion. The vector that causes this grid would be stars, which larger apertures would have better light gathering capability on, and can reproduce the grid issue much easier. Broadband filters do not show the grid problem in either scenario.

The qhy600 + PW 12.5 data now becomes a bit more delicious in the sense that none of the observed conditions occurred on it.

Edited by rockstarbill, 20 September 2020 - 05:19 AM.


#258 AtmosFearIC

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Posted 20 September 2020 - 05:24 AM

Given that it’s occurring due to the filters, I do wonder why the QHY600 doesn’t seem to have the issue?

I might check our my Astrodons tonight to see what way they’re facing.

#259 rockstarbill

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Posted 20 September 2020 - 05:28 AM

Given that it’s occurring due to the filters, I do wonder why the QHY600 doesn’t seem to have the issue?

I might check our my Astrodons tonight to see what way they’re facing.


If you saw my note about the window in this camera, it did nothing to help nor caused reflective events at all. Key part is the help bit. It's possible that the window in the QHY600 is preventing the reflective event from getting to the sensor. This could also explain why the ((o)) and small scale halo were eliminated as well.

 

Also, data to come later today at some point. It was late, and Monkeybird747 had to get to sleep. Posting the data as a flat share would be confusing as people would not know what they were looking at, so some file naming and stuff needs to happen to help people understand what they are looking at. The previous posts were notes I was taking as we were going.


Edited by rockstarbill, 20 September 2020 - 06:27 AM.


#260 Eric Benson

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Posted 20 September 2020 - 08:35 AM

Hi,

 

I have commented on filter reflection threads before:

https://www.cloudyni...dpost&p=8094003

https://www.cloudyni...dpost&p=8263050

 

Saying it another way...

The flipping of filters is solving a grazing incidence / light leak problem, light that hits your sensor has interacted with both filter surfaces the same number of times, each interaction will have the same transmission coefficient (0.x% or 99.y% hopefully). Light that does not hit your sensor can only have been reflected by the "mirror" surface of the filter (i.e. the dielectric filter, duh), if reflected once it will be reflected again if it tries a second time ;)

 

To get reflection artefacts (out of focus stars / donut images in reflectors) recorded on the sensor there needs to be two surfaces at play (this should be obvious). That means two surfaces are less than ideal, or not perfect enough depending how hard you are looking. N.B. there can multiple pairs involved, making different sized donuts (and interacting - see below).

 

There will always be some level of halo from any kind of passband filter (LRGB included). The thing is designed to pass 100% in some spectral band, and pass 0% (reflect) everywhere else, the reflectivity has to go through the 50% mark for some wavelengths somewhere...ideally the transition is very very sharp, and very little flux is contained in the partial reflectivity zone, but it can't be zero. AR coatings are never perfect. I make lasers for a living and routinely ask suppliers, can you make that coating better? 0.1% AR (anti-reflection) is a very good coating, it's rare to get better than 0.05% over a significant spectral range, 0.5% is a more routine AR coating, ~4% reflection is uncoated glass.

 

My bet is the sensor cover slip + filter AR coating is causing the halos, the sensor microlens + filter AR causing the grid halos, and SWAG, some interference between the two is causing the ((o)).

 

The fact the nothing happens with/without the chamber window mostly absolves ZWO of culpability, the AR coatings on the window are good enough to not caused more/worse reflections that what you are seeing.

 

The type of telescope focusing the light should be quite irrelevant (the small sample size here is not enough to make any robust conclusions), except that reflectors are generally much larger, show out of focus donuts instead of disks, and collect a lot more light...

 

Cheers,

EB


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#261 rockstarbill

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Posted 20 September 2020 - 08:46 AM

My bet is the sensor cover slip + filter AR coating is causing the halos, the sensor microlens + filter AR causing the grid halos, and SWAG, some interference between the two is causing the ((o)).

 

The fact the nothing happens with/without the chamber window mostly absolves ZWO of culpability, the AR coatings on the window are good enough to not caused more/worse reflections that what you are seeing.

 

The type of telescope focusing the light should be quite irrelevant (the small sample size here is not enough to make any robust conclusions), except that reflectors are generally much larger, show out of focus donuts instead of disks, and collect a lot more light...

 

Cheers,

EB

I was with you until this part. I think it is important to share the manufacturers AR claims on the window itself. They are 98.5% or 1.5% whichever way you want to slice it -- which based on previous comments you have shared -- is not very good. Now the testing on the refractor showed a net zero effect of the window. The refractors also had the lowest probability to show any of the conditions in question at all. In fact I suspected the window would show little to no difference based on the surfacing of the issue on those lenses to be very small or close to zero.

 

The type of telescope has been very relevant in the surfacing of this issue. So again I disagree with your assumptions. The data we have on hand in this thread shows otherwise. The same battery of tests needs to be done with a larger telescope, more prone to produce these conditions in question.

 

In the abscence of that, here is a frame to look at:

https://drive.google...iew?usp=sharing

 

This is the worst case scenario for this problem we are working though. 12.5" Corrected DK Telescope (Cassegrain design), with a Sony IMX455, Chroma 5nm HA filter, and a 20 minute exposure on a Mag 4 star. None of the issues outlined here surfaced. No halo, no ((o)), and no grid. 

 

I do appreciate your contributions to the thread though and hope that you continue to engage with this.


Edited by rockstarbill, 20 September 2020 - 10:00 AM.


#262 xthestreams

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Posted 20 September 2020 - 11:04 AM

Thinking out loud, would 65m, filters help, being that they could be mounted far enough ahead of the sensor that interactions would be minimal to non-existent?


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#263 Der_Pit

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Posted 20 September 2020 - 02:52 PM

Quick question regarding the ((o)) issue:  My first thought had been 'diffraction rings', which would match the fact that you get them with obstructed optics that will put more power in the rings, making them easier visible than in a refractor.  Maybe something for Mark to confirm/reject?


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#264 buckeyestargazer

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Posted 20 September 2020 - 05:14 PM

I'm going to offer something here that I know isn't a direct comparison, and I'm not sure how relevant this is.  But, here goes...

 

I have a RASA 8 f2 and a CFF105 reduced to f4.5.  For other reasons than the subject of this thread I wanted to directly compare images with both scopes using the same ASI2600MC camera.  I wanted to see how both scopes performed in my normal skies.  The image below is a crop of both sets of images, which have the exact same total integration time and were processed in the exact same way.  The RASA/ASI2600 image is on the left, the CFF105/ASI2600 image is on the right.

 

The bright star is Beta Cass, mag 2.25.  You can clearly see the ringing in the RASA image but not in the CFF105 image.  Same camera, different scopes, different results.  So there is something about the RASA that causes the rings that is not present in the CFF.  

 

Here's a link to download the full images in case anyone is interested.  

 

Again, this may not mean anything to this discussion, IDK.  

 

 

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#265 rockstarbill

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Posted 20 September 2020 - 05:36 PM

Thinking out loud, would 65m, filters help, being that they could be mounted far enough ahead of the sensor that interactions would be minimal to non-existent?


Good question.

The distance to the filter with the QHY camera is 30mm vs 20mm for the ASI camera. That could explain why we're not seeing the same issue on the QHY600.

 

This could also explain why the other cameras have not shown the same problems. The sensors are all much further away than 20mm. In the case of the 16200, the camera backfocus itself is 20.6mm. Would be worthwhile to test moving the ASI6200 10 mm further away from the wheel.


Edited by rockstarbill, 20 September 2020 - 06:31 PM.


#266 Eric Benson

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Posted 20 September 2020 - 06:52 PM

I'm going to offer something here that I know isn't a direct comparison, and I'm not sure how relevant this is.  But, here goes...

 

I have a RASA 8 f2 and a CFF105 reduced to f4.5.  For other reasons than the subject of this thread I wanted to directly compare images with both scopes using the same ASI2600MC camera.  I wanted to see how both scopes performed in my normal skies.  The image below is a crop of both sets of images, which have the exact same total integration time and were processed in the exact same way.  The RASA/ASI2600 image is on the left, the CFF105/ASI2600 image is on the right.

 

The bright star is Beta Cass, mag 2.25.  You can clearly see the ringing in the RASA image but not in the CFF105 image.  Same camera, different scopes, different results.  So there is something about the RASA that causes the rings that is not present in the CFF.  

 

Here's a link to download the full images in case anyone is interested.  

 

Again, this may not mean anything to this discussion, IDK.  

The difference I see in the images is the radial streaks, the broad smooth underlying halo looks the same to me. What are the streaks from? I think this is obvious, the RASA has a cable (or two?) traversing the objective, the cable(s) causes diffraction. The amount of energy in each spike is proportional to the thickness of the cable. Any straight edge in the entrance pupil will cause a spike, a perfectly curved cable will spread the diffraction over an area (not producing a spike), however cables used here (USB, power twin conductor?), can not be held in a perfectly smooth arc, there are tons of little 'kinks' along the way providing the RASA image with many radial spikes in all directions. If you could mount the cable along a straight edge you would get one brighter spike.

 

EB


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#267 Eric Benson

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Posted 20 September 2020 - 06:57 PM

Good question.

The distance to the filter with the QHY camera is 30mm vs 20mm for the ASI camera. That could explain why we're not seeing the same issue on the QHY600.

 

This could also explain why the other cameras have not shown the same problems. The sensors are all much further away than 20mm. In the case of the 16200, the camera backfocus itself is 20.6mm. Would be worthwhile to test moving the ASI6200 10 mm further away from the wheel.

Bigger filters further away do help by making the artefact bigger, and hence fainter per unit area. Of course bigger filters cost that much more, and perversely might be worse because it's harder to make a good coating on a bigger optic keeping the thickness the same (i.e. induced potato shape is a bigger problem, and fancier coating have more layers which can cause more stress). So you pay more but you might get less AR quality.

 

EB


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#268 sharkmelley

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Posted 20 September 2020 - 06:57 PM

Quick question regarding the ((o)) issue:  My first thought had been 'diffraction rings', which would match the fact that you get them with obstructed optics that will put more power in the rings, making them easier visible than in a refractor.  Maybe something for Mark to confirm/reject?

I've been re-thinking this. 

 

I was quite adamant that the rings were not Airy rings because Airy rings are too closely spaced to be resolvable by the sensor.  However, I have done some further background reading on this.  It would appear that something very interesting happens when there is a central obstruction.  The central obstruction sets up a regular pattern of bright and dark rings and this pattern is large enough to be resolvable even though individual rings are not resolvable.

 

I don't know if anyone has information on these ring patterns, otherwise I can always generate them from first principles using Bessel's Functions.  Ideally I would also like to see another Gamma Cygni image from rockstarbill but without binning.

 

I can then start comparing this potential theory to actual examples.

 

Mark


Edited by sharkmelley, 20 September 2020 - 07:02 PM.

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#269 rockstarbill

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Posted 20 September 2020 - 07:03 PM

It will be a bit. The smoke is gone now (yay) but the clouds are going to stick around for a bit based on the weather forecast. When things clear up I can get more test data.


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#270 xthestreams

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Posted 20 September 2020 - 07:08 PM

Ah Wisdom - I hate wisdom.... ;-)

 

Great point and reminds me why I still feel fondly about my weeny 1.25” fittings - everything was that much less $ back then....

 

Will be interesting to see if the problem/symptoms vary as a function of the back spacing between optic and sensor, it doesn’t sound like a lot in gross terms but a 50% variation in distance might be enough to swing it either way - i cant say with any certainty not owning the product in question (yet, hence the lurking).

 

Bigger filters further away do help by making the artefact bigger, and hence fainter per unit area. Of course bigger filters cost that much more, and perversely might be worse because it's harder to make a good coating on a bigger optic keeping the thickness the same (i.e. induced potato shape is a bigger problem, and fancier coating have more layers which can cause more stress). So you pay more but you might get less AR quality.

 

EB



#271 Eric Benson

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Posted 20 September 2020 - 07:34 PM

I was with you until this part. I think it is important to share the manufacturers AR claims on the window itself. They are 98.5% or 1.5% whichever way you want to slice it -- which based on previous comments you have shared -- is not very good. Now the testing on the refractor showed a net zero effect of the window. The refractors also had the lowest probability to show any of the conditions in question at all. In fact I suspected the window would show little to no difference based on the surfacing of the issue on those lenses to be very small or close to zero.

 

The type of telescope has been very relevant in the surfacing of this issue. So again I disagree with your assumptions. The data we have on hand in this thread shows otherwise. The same battery of tests needs to be done with a larger telescope, more prone to produce these conditions in question.

 

In the abscence of that, here is a frame to look at:

https://drive.google...iew?usp=sharing

 

This is the worst case scenario for this problem we are working though. 12.5" Corrected DK Telescope (Cassegrain design), with a Sony IMX455, Chroma 5nm HA filter, and a 20 minute exposure on a Mag 4 star. None of the issues outlined here surfaced. No halo, no ((o)), and no grid. 

 

I do appreciate your contributions to the thread though and hope that you continue to engage with this.

In the image you shared I indeed can see a grid, or better called mottling in this case since the spots are almost overlapping, just very closely spaced near the star in the surrounding (normal) halo. The grid here is possibly from the coverslip to microlens which might be less than 1 mm distance and at f/8 (?) the spots remain small.

 

In any case what are the differences the OTA can impart on the light cone received by the filter wheel/camera?

-Angle of cone, determined by f/ratio.

-Spectral content: mostly determined by the source star, with refactors losing more in the extreme violet due to absorption in the glass. (BTW a corrected reflector has much the same properties here as a refractor since the lenses are made of glass ;)

-Polarization: not measurable by our sensors, and nearly always perfectly random from the source anyways.

-Transverse intensity distribution: here a small difference is the hole in obstructed telescopes pupil pattern. Can we think of any reasons why this would matter?

-Type and amount of diffraction: Refractors have much less of this for sure, and no spikes due to spider vanes or cables, but how does this affect haloes which are reflections?

-Intensity or amount of photons: aperture and exposure time determine this.

 

IMHO the problem you are trying to address is multiple reflection out of focus artefact, the type of OTA does not have great leverage here.

 

BTW you can estimate the transmission product of the non-perfect contributing surfaces. Image a star, but do not saturate, record total integrated ADU in the seeing disk (e.g. aperture tool in Maxim). Now record long exposure to recover the unwanted halo. Find average adu level per pixel from the halo (above background) and halo diameter in pixels. Multiply avg adu by halo area, this is the halo power. Divide halo power by the star power times the integration time ratio. This number is R1*R2, where R1 and R2 are the reflection coefficients of the surfaces involved.

 

A formula might be clearer:

R1R2 = (avg halo count * halo area) / (star signal * LongTime/ShortTime)

 

Typical 'good' R would be 0.5% which is 0.005, so 0.005 squared is 25E-6, not much! So for a 1,000,000 adu star (way saturated!), 25 adu's of signal will be spread over the entire halo (1E6 * 25E-6).

 

For the halos to register I feel the star must be saturated in the order of 40E6 adu, those anti-blooming gates are just too good!

 

Regards,

EB



#272 AtmosFearIC

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Posted 21 September 2020 - 02:55 AM

I'm planning on checking out my filters tonight but here is something that you all might find interesting. This is my RH200 with QHY183M HaRGB filters.

 

There are a couple of things to note, this is towards the edge of the frame, Ha is on the other side of meridian to RGB. And this does show the extent of trefoil astigmatism.

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#273 sharkmelley

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Posted 21 September 2020 - 04:17 AM

This is my RH200 with QHY183M HaRGB filters.

Excellent - I will try to explain my hypothesis using this example.

 

Mark


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#274 wenjha

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Posted 21 September 2020 - 04:40 AM

I draw a photo to show the halo issue between optics

narrow band filter is the key point

it will become better if you use a better narrow band filter absorb some light instead of reflecting lights

 

Halo.jpg


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#275 AtmosFearIC

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Posted 21 September 2020 - 05:43 AM

Excellent - I will try to explain my hypothesis using this example.

Mark


If it helps, I’m using the QHY bolt on system which has a 10.5mm back focus from the filter wheel. It’s the US version which chews up 15mm of optical back focus so the filter is likely to be about 19mm. That’s just an approx.

From memory, when I was using a QHY163M I didn’t get this kind of ringing artefact but I did get a single larger halo that I estimated being about 350 microns distance traveled which put it within the sensor cover glass above the microlens’. I don’t see that kind of reflection with the QHY183M.


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