SharpCap and QHY600M: Optimal Gain Setting

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Geoff_OPELS
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SharpCap and QHY600M: Optimal Gain Setting

#1

Post by Geoff_OPELS »

My team is using SharpCap and a QHY600M to do imaging with a RASA telescope. We need large dynamic range, from a few e- to >20000 per pixel. Can someone suggest a good "Gain setting" and "Offset" in SharpCap?

Based on the QHY600M manual, the best mode and "Gain" for us are: “High Gain Mode” and a Gain = 57. But our tests show that the "Gain" in SharpCap is not the same as the Gain in QHY600. We see good dynamic range with SHarpCap has a "Gain" set to 100 or more.

Can anyone suggest a "Gain" setting in SharpCap for a QHY600M, to achieve good dynamic range?

Thanks,
Geoff
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Re: SharpCap and QHY600M: Optimal Gain Setting

#2

Post by admin »

Hi,

I would suggest running the SharpCap sensor analysis function on your camera, which will measure the read noise and e/ADU for various gain values on the camera - see https://docs.sharpcap.co.uk/4.0/#!2!Sensor%20Analysis.

It's best to run this in SC 4.0 (rather than 3.2), since SC 4.0 takes account of the different read modes of the camera when measuring and storing measurement data. You will notice that in the data table that you get at the end of the measurement procedure you get the dynamic range of each gain value calculated (in this case, DR is defined as maximum signal level divided by read noise level - ie the brightest thing you can measure without quite saturating divided by the minimum image noise level).

Hope this helps,

Robin
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Re: SharpCap and QHY600M: Optimal Gain Setting

#3

Post by Geoff_OPELS »

Thanks, Robin. Excellent advice.
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Re: SharpCap and QHY600M: Optimal Gain Setting

#4

Post by Geoff_OPELS »

Dear Robin,

Your “Sensor Analysis” is absolutely brilliant. Thank you for creating this automated treasure.

May I offer you some feedback about my experience with your Sensor Analysis? I have done this calculation myself many times (for different cameras), by assuming Poisson fluctuations in the arrival of photons to determine the e/ADU. I have some questions about the results from "Sensor Analysis”. I hope to help clarify SharpCap.

My camera is a QHY600M, and I’m running SharpCap v4.0.7957, setting it in “High Gain Mode 16BIT”, “MONO16”, and Binning 1x1.

Here is the graph and table from Sensor Analysis.
Image
The listed Gain Value goes from 0 to 100. The e/ADU goes from 0.78 to 0.01 . The Full Well goes from 51141 to 612.

1. The SharpCap text and the Table continue the proliferation of a different terms called “gain”. SharpCap contains:

SharpCap refers to:
- "physical gain" (in the text at left)
- “Gain Value” (in the Table)
- e/ADU (in the text and Table)
- Relative Gain (in the Table)
- “Gain” (in the Camera Control Panel)
- “gain setting” (in the SharpCap User Manual)


I know what e/ADU is. The other terms containing the word “ gain” are mysteries. We care about e/ADU to compute Poisson photon noise. The rest of the “gain” terms confuse me and are specific to the camera, SDK, and SharpCap.

2. The left column of the Table has “Gain Value” values from 0 to 100. This is puzzling, because the Camera Control Panel (i.e. slider) allows values of the “Gain” from 0 to 200. Why is the Table missing “Gain Value” from 100 to 200?

(I am highly suspicious that the “Gain Value” listed in the Table is, somehow, 50% of the “Gain” shown in the Camera Control Panel. That would explain the discrepancy between the range of 0-100 vs 0-200.)

3. Dynamic Range: Could you please quote that actual ratio: Full Well / Read Noise .
The days of “stops” are so long in the past.

Again, thank you so much for this absolutely valuable diagnostic of the performance of the camera as a function of different settings.

Geoff
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Re: SharpCap and QHY600M: Optimal Gain Setting

#5

Post by admin »

Hi Geoff,

glad to hear that you find the analysis feature useful :)

Gain... yes, fun :)

The 'gain value' is the number that you see in the gain control. This may or may not have some physical meaning (depends on the camera manufacturer). In the case of your camera it doesn't by the look of things :(. This is also the 'gain setting' in the manual .

e/ADU you are obviously familiar with, but for a lot of users this is not a natural measure - 'relative gain' is much more obvious a measure - that is 'how many times brighter is this gain setting than the minimum gain'.

Now let me explain why SharpCap has only measured from 0 to 100. During an early part of the analysis SharpCap attempts to measure the gain range of the camera - starting at low gain and longer exposure and gradually increasing gain and reducing exposure to work out roughly how much brighter each value makes the image. SharpCap stops when it gets a gain that is more than about 300 times brighter than minimum gain or if the exposure gets close to the minimum exposure that the camera can support. This ensures that the later stages of the analysis can complete sucessfully. Normally gains of over 300x (~50dB) are fairly unusable, so you don't miss much by not analyzing the top end of the gain range. You may be able to get a little more of the range to analyze if you are hitting the minimum exposure limit - reduce the light level so that you start with longer exposures to try that.

I will have a go at clarifying some of the language and also see if I can squeeze in a 'dynamic range ratio' column along with the stops.

cheers,

Robin
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Re: SharpCap and QHY600M: Optimal Gain Setting

#6

Post by Geoff_OPELS »

Dear Robin,

Thank you for all of those insightful thoughts about "gain" settings for SharpCap and the QHY600M. Here is a note of agreement. The QHY600 User Manual shows the Read Noise has a sharp drop from 3.7 to 1.6 e- at "Gain Setting" = 57, referring to QHY's "Gain Setting". In comparison, the SharpCap Sensor Analysis shows a sharp drop from 3.7 to 1.8 e- occurring between Gain Value of 50 and 60 (see the Sensor Analysis table in my previous post). This represents agreement between the QHY600 "Gain Setting" and the SharpCap "Gain Value". So, maybe the two "gain" values are the same.

I'll examine the FITS images to see if the full well is also in agreement between that reported by the Sensor Analysis and that claimed by QHY.

Thanks again,
Geoff
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Re: SharpCap and QHY600M: Optimal Gain Setting

#7

Post by Geoff_OPELS »

An update on using QHY600M and SharpCap for photometry:
I use a QHY600M camera (16-bit) to take images that enable photometry.

1. I check that box about “Save 10/12/14 bit images in FITS files without scaling . . ." (I'm unsure if this avoids ADU scaling, to permit photometry.)
2. I set "MONO16".
3. I set (gamma, brightness, contrast) to (1.0, 0.0, 0.0), because Robin states it avoids stretching.
4. I set Gain = 0 and Offset = 0.

Adopting Gain=0 gives Full Well = 51k e- and read noise = 3.7 e-, and gives 0.78 e- per ADU

(An alternative: Adopt Gain=57, yielding Full Well = 23k e- and a lower read noise of 1.7e-, useful for faint sky and faint objects.)
deneb
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Re: SharpCap and QHY600M: Optimal Gain Setting

#8

Post by deneb »

the best gain for qhy600M is
in full mode and full mode II it is 23
in photo mode it is 10
in hig mode it is 0

offset 12 in the all modes
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