QHY174mGPS 12 or 16bit?

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TimHaymes
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Joined: Sun May 28, 2017 8:11 am

QHY174mGPS 12 or 16bit?

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Post by TimHaymes »

Hi:
I was in a recent ZOOM meeting that was discussing fast frame rates for IMX174 cameras. I have the QHY174mGPS. I use it for deep sky imaging as well as occultations. Typical settings are:
Occultations: 2x2bin, Gain 380, exposure 100ms
Deep Sky : 1x1, Gain 180, exposure 60 sec

I use gain 180 because it was unit gain (1e/ADU) with a read noise of 3.9e and full well about 40000. It seems a good "point" in the calibration.
I would like to know if there is a transition from 12 to 16 bit near this gain setting, and should i be using gain 100 (10,500 full well) to achieve the best results ?
The speaker suggested there was a transition point. If so, how would i recognise this is the images ?

Thanks for any insight

Cheers - Tim
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Re: QHY174mGPS 12 or 16bit?

#2

Post by admin »

Hi Tim,

the IMX174 sensor used in the QHY174 and various similar models from other manufacturers is a 12 bit sensor with a maximum FWD of 32000 to 33000e per pixel. If you increase the gain from minimum to 180 (QHY values), you do indeed get close to unity gain. At that point, the FWD is only about 4096e (1 electron per ADU, 12 bit ADC so 4096 total possible ADU values).

For this particular model, there is no 16 bit mode at all, and even binning does not help you get additional bits of information, since sadly QHY implement binning by adding 4 pixel values together. A better approach for a 12 bit camera is to average the 4 pixel values, since the averaged values can use a 14 bit range.

I should point out that some cameras do change mode at particular gain values, switching from 'Low Conversion Gain' (LCG) to 'High Conversion Gain' (HCG). This is usually accompanied by a sharp drop in read noise, but does not affect the bit depth. The 174 sensor does not do that. There are also some QHY cameras that have multiple read modes that are more complex, again the 174 does not have those.

cheers,

Robin
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