CMOS sensor microlens diffraction patterns

[MarkS]

I seem to have spent most of my free time recently investigating weird stuff such as the Nikon star eater problem that no-one believed still existed and also microlens diffraction patterns that seem to become very obvious under some circumstance when imaging with particular cameras using H-alpha filters.

The explanation of the microlens diffraction patterns is something I'm particularly proud of. 
So here's the link for those who might be remotely interested:
https://www.cloudynights.com/topic/635937-microlens-diffraction-effect-explained/?p=8967693

I have now written pages and pages of maths at home to explain what's going on but in the end and quite surprisingly it all boils down to a simple coordinate transformation which I implemented in a PixInsight script.

The weird thing is, I don't actually own one of the affected cameras but it's such an interesting phenomenon that I became really intrigued by it, especially since there was no good explanation for its occurrence.

Now, finally, I might be able to get back to my huge backlog of images waiting to be processed 

Mark

[Carole]

Flippin heck Mark, you do get involved in some very technical bits of analysis.

I must admit much of it went over my head.  But I have seen images presented on SGL with this effect and had no idea what the cause was.

So in simple English, is this something that CMOS chip owners are stuck with on bright stars?  I am wondering whether it has anything to do with Duncan's coloured blotches he had trouble with recently.

Well done anyway.

Carole

[RobertM]

Great explanation Mark and a good read.  That's an excellent piece of detective, work well done.

So in simple English, is this something that CMOS chip owners are stuck with one bright stars?

Carole it's got nothing to do with being a CMOS sensor.  You will see this in CCD chips also such as the KAI8300 that a lot of people use.  The effect is much reduced with an AR cover glass over the sensor.  I had an ML8300 which produced an incredible diffraction pattern and had to go back to get the cover glass changed to one with a higher anti reflection coating.  For some reason Gamma Cass is the one everyone seems to notice the problem on and it was with me also.

Robert

[ApophisAstros]

For some reason Gamma Cass is the one everyone seems to notice the problem on and it was with me also.

Robert

In the spec of mine (zwo asi183mm), it says "AR protection window built-in - full transmission even in infrared", so should i be ok?
Heres my 300s Luminance of Gamma Cass, so i think my cam is not affected by this.



Roger

[Carole]

Hmmm, well I have to say I have never come across or seen this before the last couple of weeks when I saw a post on SGL.  It was a CMOS sensor in this case. 

Hopefully my cameras already have the relevant cover glass on them.  I used to own a KAF8300 (if that's the same thing) and never got it on there either.  I have only imaged Gamma Cass a couple of times and again did not get this affect.

Anyway, excellent bit of work Mark, you are obviously held in great respect on Cloudy Nights. 

Carole

[MarkS]

In the spec of mine (zwo asi183mm), it says "AR protection window built-in - full transmission even in infrared", so should i be ok?
Roger

ZWO is talking about the optical window that protects the humidity controlled chamber in which the sensor sits.  But the diffraction pattern involves a reflection off the cover slip on the sensor itself.  ZWO has no control over this unless the sensor manufacturer produces a version with anti-reflective coatings.

It's not just astro-cameras that suffer.  There was a great example on a DPReview forum recently where someone had an array of green dots around the sun in their photo of a lighthouse:
https://www.dpreview.com/forums/post/61926940

It's exactly the same diffraction effect but probably involves a reflection off an interface elsewhere in the filter stack.

Mark

[RobertM]

In the spec of mine (zwo asi183mm), it says "AR protection window built-in - full transmission even in infrared", so should i be ok?
Roger

This is the reason I bought a 183 based camera, the sensor cover glass is AR coated but on the Panasonic sensor does not.  As Mark has pointed out, camera manufacturers only have control over the camera optical window so it's an important point to note.

In my case the sensor cover slip was replaced and that made a light and day difference.

Robert

[The Thing]

Nasty. Nice write up Mark. Not had this problem as far as I can tell.