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Question about Back Illuminated Sensors

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#1 Agreegator

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Posted 23 September 2021 - 04:29 AM

I have a confusion about QE of back illuminated CMOS sensors.

 

I was checking some ZWO cameras:

- ASI 120 MM Mini - Regular Sensor, QE Peak is 80%

- ASI 183 MM - Regular Sensor, QE Peak is 84%

- ASI 290 MM - Back Illuminated Sensor, QE Peak is 80% 

- ASI 183 MM Pro - Back Illuminated Sensor, QE Peak is 84%

 

Shouldn't a back illuminated sensor provide markedly higher sensitivity and collect more photons? Then why are they in the same range, or in case of 290 MM, lower at QE than Regular Sensor cameras ? Especially 120MM mini, which is as basic as it gets.

 

Is there something I'm missing here, there seems to be almost no noticeable difference in QE and in many cases, a downgrade with Back Illuminated Sensors - can someone please explain ? 

 

Thanks.


Edited by Agreegator, 23 September 2021 - 04:33 AM.


#2 jkmccarthy

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Posted 23 September 2021 - 08:45 PM

Are you certain the 183MM uses a front-side illuminated sensor?   I google'd ASI 180MM, and learned the sensor is Sony IMX183 (see:  https://astronomy-im...t/asi183mm-mono... I don't think this is the Pro model?) which is actually a backside-illuminated CMOS per the Sony description:

https://www.sony-sem...CQJ-J_Flyer.pdf

 

 

Meanwhile the 120MM reportedly employs the ON Semiconductor AR0130CS sensor, whose datasheet is here:

https://www.onsemi.c.../ar0130cs-d.pdf

... where on page 26 the QE curve shown peaks at 78% in the 600nm (red) wavelength region, decreasing to 50% at 400nm.

 

CMOS detectors are not like CCDs (whose polysilicon gate structure that defines the pixel rows covers the entire pixel area, accounting for the much lower QE of frontside illuminated CCDs compared to backside illuminated CCDs).  In CMOS, however, manufacturers have been able to increase the QE of frontside illuminated devices by concentrating the active pixel circuitry in a small portion of the pixel area, and using a thin gate layer over the photosensitive region to enhance response.  Skimming the AR0130CS datasheet, I did not find a discussion of how this particular sensor is designed to provide good QE, but certainly in the blue its QE does not appear to be competitive with AR-coated backside-illuminated devices.

 

Hope this helps,

 

        -- Jim


Edited by jkmccarthy, 23 September 2021 - 08:47 PM.



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