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etendue calculator

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#51 james7ca



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Posted 16 September 2019 - 02:55 AM

...Imaging Etendue equals 424.36 * QE * pixel size^2  / focal ratio^2


...Edit:  Going back through the thread in detail, I see where jhayes tucson had already expressed it this way, albeit in longer form: https://www.cloudyni...ator/?p=9349503


All the best,



Or, you could reference the post I made back in 2018 where I showed the relationship between plate scale and aperture versus pixel size and f-ratio. Noted just above in post #47 of this thread or by direct link here:



#52 Coconuts


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Posted 16 September 2019 - 07:06 AM

james7ca:  Good point (and post).  That earlier thread was a bit thick and contentious, but the takeaway is now clear.  I'm delighted to have a simple, accurate way to assess imaging speed/etendue among different scope/lens and camera combos.  I took the liberty (and have notified the OP) of adding a simplified section to his original spreadsheet, link here:



I have had my eyes on Sony's new line of BSI CMOS sensors since they first appeared on the Sony roadmap a few years ago.  They have pixel counts of 61M, 100M, and 150M (IMX455, IMX461, and IMX411, respectively).  They appear nearly perfect: as large as your wallet will support (42, 55, and 67 mm diagonals), very low read noise, high QE, decent full well (51Ke-), 16 bit ADC, color and mono versions, etc.  Astro cameras based on the IMX455 are shipping now (QHY600M/C), and there will be an ASI6200 available towards the end of the year.  And minor variants (autofocus pixels added) will soon become available in consumer MILCs (the Sony a7r IV any day now, and the Fuji GFX100 in November).  But their pixels are all a bit on the small side: 3.76 um square.  Since imaging speed scales as inverse pixel size squared, this seemed at first glance a disadvantage.  On reflection, though, I realized that binning, either in camera or in post processing, allows the pixel size to be doubled (or even tripled) in cases where the focal length will support this from a sampling perspective.  Since these are CMOS sensors, and their "binning", unlike CCDs, requires multiple reads, this capability has been dissed.  While it provides no advantage from a read noise perspective, though, I'm not sure that this matters much, as the read noise is already 10x+ lower than CCDs.  Where this does have real value is in increasing imaging etendue and hence speed by a dramatic 4-9X.  This gear shift from 3.76 um (ideal for camera lenses) to 7.52 or 11.28 um pixel sizes provides really nice flexibility.  And you are still left with plenty of pixels!  Interesting times, indeed...


All the best,



Edited by Coconuts, 16 September 2019 - 07:12 AM.

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