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Strange diffraction pattern

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#1 descott12

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Posted 14 February 2019 - 08:09 PM

All,

I was seeing this odd diffraction pattern last night when my ASI 294 was attached at prime focus to my Evo 8. I am used to the typical 4-quadrant spikes but this is new to me and I have never seen it when using the same equipment with my HyperStar.

 

I know the colimation is off a bit as central bright spot is not completely centered but I am not too worried about that. This was only visible on very bright stars like Sirus or Betelguese.

 

As far as I know there really isn't any internal structure inside the OTA to cause this, or is there?

 

Thanks

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#2 Jon Rista

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Posted 15 February 2019 - 01:43 AM

This is microlens diffraction. This can occur with many different sensors, and it can present in a very wide range of forms. But it has all the telltale signs of microlens diffraction. Mark (sharkmelly) and Frank (freestar8n) could probably help you verify for sure that this is indeed the case, and could probably accurately model the phenomena artificially as well, to figure out the exact characteristics that give rise to this specific pattern.



#3 descott12

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Posted 15 February 2019 - 03:00 AM

Interesting. Thanks Jon



#4 Berny

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Posted 15 February 2019 - 11:56 AM

Is this a problem with all astro-cameras?  If so, I would rather have a camera without micro lenses and put up with less sensitivity (and more noise?).

Or, is there a way to minimize the problem.



#5 CygnusBob

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Posted 15 February 2019 - 12:15 PM

How about simply avoiding having very bright stars in the field of view?  You might consider doing a mosaic using a smaller FOV and several images.  Having to give up the advantage of a high quantum efficiency sensor to avoid a problem like this sounds like a poor compromise.  No matter what you do you will get some sort of halo light or optical scattering effect from very bright stars.  I guess if you wanted to you could do what professional astronomers would do; use an occulting mask.  Form an image onto a glass plate with a small mask that blocks the bright star and then re-image it to the camera.  However, that would require some fancy optics.

 

Bob


Edited by CygnusBob, 15 February 2019 - 12:35 PM.


#6 Jon Rista

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Posted 15 February 2019 - 01:50 PM

Is this a problem with all astro-cameras?  If so, I would rather have a camera without micro lenses and put up with less sensitivity (and more noise?).

Or, is there a way to minimize the problem.

It is more an issue of how anti-reflective the sensor cover glass is, not the microlenses themselves. The issues are usually more pronounced the smaller the pixels are as well. 

 

With most cameras, it takes a ludicrously bright star, like Sirius, to cause an issue. Loss of microlenses would be devastating to Q.E., and there are not many sensors these days that do not have them. You could go from 70% Q.E. to 30-40%, which would have a large impact on integration times. 

 

I've only seen this kind of effect on particular stars. I can count them all on one hand, so it is not a particularly common problem, and the majority of the issues occur with the Panasonic M mono sensor. In the few cases where I have seen it on other cameras, it is always EXCEPTIONALLY bright stars that cause it, and usually with darn fast systems. 


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#7 descott12

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Posted 16 February 2019 - 10:59 AM

 it is always EXCEPTIONALLY bright stars that cause it, and usually with darn fast systems. 

Yes, it only happened on Sirius and Betelguese. But oddly, I never saw a it happen at F2 with my HyperStar. Only at a much slower F10 which is  the opposite of what you had stated...???



#8 Jon Rista

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Posted 16 February 2019 - 03:30 PM

Yes, it only happened on Sirius and Betelguese. But oddly, I never saw a it happen at F2 with my HyperStar. Only at a much slower F10 which is  the opposite of what you had stated...???

Well, the aperture is still huge with that scope. And those stars are extremely bright. 




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