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Canon EOS Ra star problems

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#1 agavephoto

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Posted 11 June 2020 - 09:43 PM

I've been getting a specific chromatic problem with stars on my Ra consistently now with 4 optical trains (three telescopes) where a 5D2 and 5D4 do not show this problem ever. I have some ideas, but would like to hear from others that know more than I do what they think as I am not certain. First, a sample image of the issue (please ignore the colour balance):

 

Ra_starproblem.jpg

 

The stars in the center of the frame are all not bad, but starting around 60% from the image center, the star images start to show this type of artifact. This star is ~ 75% left of center, just above vertical center. The pattern follows a circular symmetry about the image center. This shows up in a single raw file, and shows up when a standard raw converter creates a non-linear image (Adobe RAW) ... so, it's there with and without any of the calibration frames being used (including any combination of them used or not). I think this is related to the micro-lens array on the sensor and adjustments made for the mirrorless design's shorter flange distance. (This seems wrong now, so I added a strike-through). I also have consistent issues getting good flat field calibration with the Ra (not the 5D2 or 5D4) that results in a colour shift around the same place as the stars in the flat-corrected image.

 

Does anyone else see this or similar artifacts that can help me understand what the fundamental cause is? If you have an Ra, do you have issues with flat fields or with stars towards the edge of the field (maybe where you did not with a DSLR)?

 

Thanks in advance.

[EDIT, clarified last question.]

[EDIT 2] added strike-through where my initial thoughts appear to be wrong.


Edited by agavephoto, 12 June 2020 - 11:27 AM.


#2 sharkmelley

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Posted 12 June 2020 - 01:27 AM

A cascade of different size halos next to a star is usually caused by internal reflections within a filter.  Are you using a filter?

 

If you are not using a filter, then maybe it originates in the camera's filter stack or the sensor itself.  Can you provide a full image jpg at 100% scale so the geometry of the reflections can be calculated.

 

Mark


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#3 agavephoto

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Posted 12 June 2020 - 09:07 AM

Mark,

 

No filter used. The image I posted is 1:1 pixel scale ... or, are you saying you would like the full image also at 100% scale? Good point about internal reflections.

 

[Edit: I can English]


Edited by agavephoto, 12 June 2020 - 09:12 AM.


#4 johnpane

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Posted 12 June 2020 - 09:33 AM

You say the issue is present in both a single subframe. Is that what your example shows, or is it an integrated/processed image?

 

Were your 5D cameras modded to increase IR sensitivity?

 

Are the three scopes you tested with all refractors?


Edited by johnpane, 12 June 2020 - 09:34 AM.

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#5 sharkmelley

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Posted 12 June 2020 - 09:47 AM

No filter used. The image I posted is 1:1 pixel scale ... or, are you saying you would like the full image also at 100% scale? Good point about internal reflections.

If the image you posted is already 1:1 pixel scale then the calculations can be done on that image - I'll take a look tonight.

 

I would be interested to see the colour shift you mentioned.  Is it anything like what I see with the Nikon Z6:

https://www.cloudyni...s-a-case-study/

 

Mark



#6 agavephoto

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Posted 12 June 2020 - 09:48 AM

My example here is a stack of 15 subframes with an auto STF applied in PixInsight. 

 

My other cameras are NOT modified, and the Ra has not been modified.

 

All telescopes are refractors.



#7 sharkmelley

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Posted 12 June 2020 - 10:16 AM

I forgot to ask - which telescope was used for the image you posted?  Did it have a focal reducer?  It's the f-ratio of the optics I need.

 

Mark



#8 johnpane

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Posted 12 June 2020 - 11:13 AM

This looks like CA to me. The broader spectrum of the Ra would tax the CA performance of the refractors more than the 5D cameras.

 

I think this is related to the micro-lens array on the sensor and adjustments made for the mirrorless design's shorter flange distance. 

I doubt that. Your CA spans many pixels. The design works fine with EF lenses analogous to your refractor, when properly spaced. Are you using an EF-EOS R adapter or other spacer? Might you need to tune the spacing?


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#9 agavephoto

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Posted 12 June 2020 - 11:14 AM

Thanks, Mark.

 

That particular image is with the WO SpaceCat51, f/4.9, no additional optics. I just re-checked some other data with this camera, and I forgot I used it with a fourth telescope as well, still showing the issue. The severity decreases as the focal ratio increases. That is, of the telescopes I have, it's worst to least as one moves from f/4.9, f/5.3, f/5.5, f/7.  At f/7 it appears as though the side facing away from the center of the telescope has a shift in colour, more subtle but clearly there. 

 

To answer your other question, we spoke about the flat issues early in the year and I do think it's similar to what you see in the linked thread.



#10 agavephoto

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Posted 12 June 2020 - 11:19 AM

I agree this specific issue does not appear to be related to the microlens array at this point, I had not considered the internal reflection issue. I am using the EF-EOS R adapter for the proper back focus spacing. I have not had the time (or weather) to test this camera with just a camera lens, though that was not my intended use for it, anyway. I'm glad to hear you are getting good results with your EF lenses.

 

This looks like CA to me. The broader spectrum of the Ra would tax the CA performance of the refractors more than the 5D cameras.

 

I doubt that. Your CA spans many pixels. The design works fine with EF lenses analogous to your refractor, when properly spaced. Are you using an EF-EOS R adapter or other spacer? Might you need to tune the spacing?



#11 sharkmelley

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Posted 12 June 2020 - 03:09 PM

That particular image is with the WO SpaceCat51, f/4.9, no additional optics. I just re-checked some other data with this camera, and I forgot I used it with a fourth telescope as well, still showing the issue. The severity decreases as the focal ratio increases. That is, of the telescopes I have, it's worst to least as one moves from f/4.9, f/5.3, f/5.5, f/7.  At f/7 it appears as though the side facing away from the center of the telescope has a shift in colour, more subtle but clearly there. 

 

To answer your other question, we spoke about the flat issues early in the year and I do think it's similar to what you see in the linked thread.

What follows is an approximate calculation.

 

The diameter of each successive circular reflection increases by 25 pixels which is 134microns, given a pixel pitch of 5.36microns.

So the extra distance travelled by the light rays for each successive reflection is 134x4.9 = 650microns, given an f-ratio of f/4.9.

Assuming this is an internal reflection inside a layer of glass this means the thickness of the glass (in air equivalent optical terms) is approximately half this, i.e. 325microns, although this needs adjusting by the refractive index of glass.  To obtain the actual thickness of glass we multiply by the refractive index: 325*1.5 is approx. 500 microns or 0.5mm

 

So I reckon the cause is successive internal reflections within the sensor coverslip or within a very thin filter layer in the sensor's filter stack.

 

Mark


Edited by sharkmelley, 12 June 2020 - 03:10 PM.

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#12 agavephoto

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Posted 12 June 2020 - 03:26 PM

Thanks for the analysis, Mark!


Edited by agavephoto, 12 June 2020 - 03:43 PM.


#13 asanmax

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Posted 12 June 2020 - 03:27 PM

I've seen this before in optical systems, in your case this may be caused by the reflection in the internal camera filter, as sharkmelley pointed out already.

The distance to the next half-circle increases gradually and every half-circle is fading.

I would take some shots using an artificial star and see what the results are.

If this is true that would be such a shame for a camera like the Ra to have this kind of flaw.


Edited by asanmax, 12 June 2020 - 03:48 PM.

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#14 sharkmelley

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Posted 12 June 2020 - 06:14 PM

The distance to the next half-circle increases gradually 

That's because of the angle the primary ray hits the sensor for this star, 75% from the centre.  So each successive internal reflection is further from the centre of the image. For a bright star in the centre of the field-of-view I would expect to see concentric rings, that's if there are any rings at all.  In this particular example in the original post I estimate the primary ray to be hitting the sensor at 6 degrees off the orthogonal and the outer rays of the f/4.9 light cone forming the star to be hitting the sensor at up to 12 degrees off the orthogonal.

 

My guess is the anti-reflective coatings on this piece of glass (i.e. coverslip or filter) are unable to cope with long wavelength (i.e. red) light hitting the AR coating at such angles.  This is why the reflected rings are coloured red and it would also explain why we see only half circles instead of full rings.

 

By the way, if you do some experiments with an artificial star, make sure it is a full spectrum light source including H-alpha and beyond.

 

Mark


Edited by sharkmelley, 12 June 2020 - 06:53 PM.

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#15 asanmax

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Posted 12 June 2020 - 07:58 PM

That's because of the angle the primary ray hits the sensor for this star, 75% from the centre. 

Exactly to the point.

 

 

By the way, if you do some experiments with an artificial star, make sure it is a full spectrum light source including H-alpha and beyond.

 

Mark

100% with you on that.



#16 mmalik

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Posted 13 June 2020 - 07:54 AM

So I reckon the cause is successive internal reflections within the sensor coverslip or within a very thin filter layer in the sensor's filter stack.

If we are implying (Ra) stock camera sensor's (optical) layers alone are responsible for such reflections, that may be highly unlikely. I don't think Cannon would have missed such a stock flaw. My take is it is primarily the optics and/or some combination of the two (Ra and the optics OP is using). I would like more testing of this on more samples, I mean other than the OP's (Ra as well as the optics). Regards


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#17 sharkmelley

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Posted 13 June 2020 - 09:02 AM

I would like more testing of this on more samples, I mean other than the OP's (Ra as well as the optics). Regards

There are already plenty of images out there on the web showing the problem with a wide variety of optics.

 

For instance, take a look at two excellent examples shot with a Canon EOS Ra on two different (Canon!) lenses, in a post on this German forum:

https://forum.astron...-eos-ra.283700/

 

Make sure you click on  "Größere Version" and  "Größere Auflösung" to see the full size versions.

 

Mark


Edited by sharkmelley, 13 June 2020 - 09:16 AM.

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#18 agavephoto

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Posted 13 June 2020 - 10:41 AM

If we are implying (Ra) stock camera sensor's (optical) layers alone are responsible for such reflections, that may be highly unlikely. I don't think Cannon would have missed such a stock flaw. My take is it is primarily the optics and/or some combination of the two (Ra and the optics OP is using). I would like more testing of this on more samples, I mean other than the OP's (Ra as well as the optics). Regards

I also would like to see more samples from other SNs of the Ra, taken on other optics (or the same optics) ... which is exactly why I started this topic as I wanted to see what other people were seeing with the camera. I have no interest in gear shaming anything or anyone. I am trying to learn if I have a defective camera, if there was some range of SNs affected, if this is a more fundamental problem with this model, or if it has nothing to do with the camera at all, because I did not expect this type of artifact. The image below is a 1:1 crop taken near the bottom of the frame from my first light with the Ra. This field didn't have many brighter stars in the frame, but the one very near the lower edge does show these concentric rings. The telescope used is the new Astro-Physics Stowaway at f/5.3 using the AP 0.8X reducer/flattener and the Ra at the proper back focus distance. This image is from the stacked and processed result. There are other examples online showing the Stowaway with reducer gives this star shape on full frame cameras, so please ignore that. It shows the same concentric ring portions as my initial image here, which are also shown in the first image from the German forum linked to above by Mark.

 

Ra_starproblem2.jpg

 

[Edit: just looks better with a space between the words and image.]


Edited by agavephoto, 13 June 2020 - 10:42 AM.


#19 agavephoto

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Posted 13 June 2020 - 10:46 AM

There are already plenty of images out there on the web showing the problem with a wide variety of optics.

 

For instance, take a look at two excellent examples shot with a Canon EOS Ra on two different (Canon!) lenses, in a post on this German forum:

https://forum.astron...-eos-ra.283700/

 

Make sure you click on  "Größere Version" and  "Größere Auflösung" to see the full size versions.

 

Mark

Thanks for finding this, Mark. The first image clearly has the concentric partial red rings.



#20 SandyHouTex

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Posted 13 June 2020 - 12:45 PM

I wonder if someone could take a pic with an all reflecting system using the Ra.  It’s well known that camera lenses and refractors focus Ir at a different place than visual.  That’s why, if using a camera lens manually, there are different lines on the lens for regular focus and Ir.

 

Another interesting experiment would be to see if other DSLR astronomy cameras exhibit the same behavior.  So take two pics, one with the Ra and one with the 60Da, or D810a, and compare.



#21 Uggbits

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Posted 13 June 2020 - 01:04 PM

I had dismissed it as an internal flare from my lens, but I appear to have captured a similar issue on Rigel in this stacked shot: 

 

Orion_CN.jpg

 

Sigma 105mm at f1.4


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#22 johnpane

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Posted 13 June 2020 - 01:49 PM

There are already plenty of images out there on the web showing the problem with a wide variety of optics.

 

For instance, take a look at two excellent examples shot with a Canon EOS Ra on two different (Canon!) lenses, in a post on this German forum:

https://forum.astron...-eos-ra.283700/

 

 

Wow, the implication is that Canon didn't even critically test the Ra before releasing it.


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#23 SandyHouTex

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Posted 13 June 2020 - 05:30 PM

Wow, the implication is that Canon didn't even critically test the Ra before releasing it.

I seriously doubt that, but then again all cameras, DSLR, mirrorless, dedicated astro camera, have issues.

 

Plus the Ra has been out for quite awhile and many people rave about it.  It seems odd that this issue has all of a sudden popped up.


Edited by SandyHouTex, 13 June 2020 - 05:31 PM.


#24 agavephoto

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Posted 13 June 2020 - 05:55 PM

I seriously doubt that, but then again all cameras, DSLR, mirrorless, dedicated astro camera, have issues.

 

Plus the Ra has been out for quite awhile and many people rave about it.  It seems odd that this issue has all of a sudden popped up.

I don't think the issue is new, but I think people are now looking for it since they've been made aware. I have seen this since I was able to get first light in February ... the last image I posted is from that first set of data in February. The image in the first post was from the this June. I pre-ordered my Ra, and received it in early December ... but weather was pretty terrible for me until February. My first light happened to also be with a new telescope as well. Since I am familiar with how Canon DSLRs performed for many years now, I thought it might be the new telescope, but needed to test more before any conclusions were made because a sample size of one isn't really useful. So, next was that same telescope, but with a flattener only. Same result,  not as bad, but still there. My sample size is now two. Weather and my own time prevented further testing with other cameras on the new scope, but I did keep using the Canon Ra on other telescopes as weather and time allowed, resulting in the last example from June in my first post, which is when I was also able to shoot with the 5D4 on the first light telescope, without any star issues like this where the Ra did. That was when the data suggested I needed to post here and ask for input from others and for the experience of others with the Ra. To have four telescopes now show this result over five optical trains where the 5D2/5D4 do not strongly suggests the issue is with this camera, and now others are able to see this if they look critically. I am concerned that there is a significant design issue that was overlooked, but still the sample size is low, so that is why it's important that others look at their data. Having seen results from the Ra that don't appear to show this (at the low resolutions posted that I saw), is why I thought it would be useful to start the conversation here.

 

Sandy, I thought I read somewhere here that you have an Ra ... is that correct? have you had a chance to use it for a session where you would integrate many images that would also happen to have brighter stars towards the edge of the sensor?


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#25 2ghouls

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Posted 14 June 2020 - 12:39 PM

HI Drew, 

 

I can confirm that the red rings on bright stars affects my Canon EOS Ra as well. A bit disappointing since I have the exact same issue on my LifePixel modified Nikon D800, so I still haven't been able to get lucky with a full frame camera sensitive to Ha that doesn't have this. I have only noticed it on bright stars, but none of my pictures with the camera have been deep yet. With your sample what it is the total integration? Here are some samples using the bright stars framing the Jellyfish Nebula (IC443).

 

Center:

center.jpg

 

Edge:

edge.jpg

 

Tech specs: 

Stellarvue SVQ86 (Quad), no filter, 16x3min, 48 min. total, Bortle 6

 

Cheers, Nico


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