QHY294C - Sensor Analysis

gwsnyder
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QHY294C - Sensor Analysis

Post by gwsnyder » Mon Dec 02, 2019 7:06 pm

I seem to be having an issue running the Sensor Analysis tool (SharpCap 3.2) for my QHY294C astrocam. For the most part, the 'e/ADU' measurements are happening correctly although I sometimes see the little green '+' all over the place and not in a straight line. No issues with measuring the Darks. However, when it comes to measuring the Relative Gain, the tool always errors out with "Measurement Failed - Minimum exposure reached" regardless of how low I set the light level (I'm using a lightbox app on an iPad).

Any ideas?

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Re: QHY294C - Sensor Analysis

Post by admin » Mon Dec 02, 2019 7:24 pm

Hi,

Can you tell me what the minimum and maximum values that are allowed in your gain slider control for your camera?

Additionally, if at all possible can you work out how much brighter the maximum gain is than the minimum gain. The way to do this is to set the camera to maximum gain and then adjust the exposure to get about 50% peak level in the histogram. Note down the exposure time required for this, then set the camera to minimum gain and again adjust the exposure (which will be much longer) to get the histogram peak of 50% and again note the exposure. The ratio between the two exposure times will be the amount of gain that the camera can apply by going from minimum to maximum gain values.

The reason I have to ask this is that while most brands of camera have a consistent way of handling the gain control (and Qhy used to), QHY seem not to have maintained this recently, meaning that different cameras have different scaling's for their gain controls. Sometimes the gain control give such a wide range of adjustment that it makes the sensor analysis difficult to complete. In those cases SharpCap tries to exclude the highest gain settings, but if it doesn't understand the gain values it can't do that.

Cheers, Robin

gwsnyder
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Re: QHY294C - Sensor Analysis

Post by gwsnyder » Tue Dec 03, 2019 7:52 am

Hi,

The minimum gain value is 0 and the maximum is 3624. The exposure range is 1/64000 sec (0.016ms) and 15 min (900 sec).

As far as trying to measure the exposure times between minimum gain and maximum gain @ 50% in the histogram is 2.2ms and 2 sec when I had a moderately light lightbox app setup in front of the telescope. I'm not sure if this is correct or not as I think the light level will impact the readings.

Thanks,

Greg

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Re: QHY294C - Sensor Analysis

Post by admin » Tue Dec 03, 2019 6:57 pm

Hi Greg,

Thanks for that. I think that means that the gain range is about 1000, which isn't excessive. Another thing that could be tripping the procedure up is that for some cameras the brightness of the image seems to increase for a dark frame image as you turn up the gain, particularly if you set the offset to a nonzero value. That may trip up the procedure into continually thinking it needs to shorten the exposure. If you could post the log file after you have attempted to run the sensor analysis then that would give me some more indications of what might be going on and how to fix it. Note that after the sensor analysis has been running the log file might be quite long! You can find the log information by going to the help menu, choosing 'show log' then missing the copy button to copy all of the text.

Cheers, Robin

gwsnyder
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Re: QHY294C - Sensor Analysis

Post by gwsnyder » Tue Dec 03, 2019 8:49 pm

Hi Robin,

I've attached the actual log file to this reply. You'll find where the Sensor Analysis gives up (literally) towards the end of the file.

Thanks,

Greg
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Log_2019-12-03T12_09_06-7940.log
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Re: QHY294C - Sensor Analysis

Post by admin » Wed Dec 04, 2019 8:06 pm

Hi,

Thanks for the log file, it wasn't quite what I expected – the analysis seems to be getting stuck because the brightness of the frame isn't responding to the exposure changes in the way the program will expect. SharpCap is trying to get a histogram peak at about the 65% level and its finding that when it sets the exposure to 376 ms or more it gets about 68% and when it sets it to 372 ms or less it gets about 58% – obviously you wouldn't expect such a big change for a 1% difference in exposure value!

I can think of two possible causes for this. One is that the camera is not changing exposure lengths in the way that it's asked to properly for some reason. The other is the the possibility of light from the monitor forming part of the illumination on the camera – that might explain why setting a shorter exposure would have a bigger effect than expected on the camera output (because it would darken the screen and therefore reduce the light level falling on the camera). Is it at all possible that the monitor light is having this effect in your setup?

Cheers, Robin

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Re: QHY294C - Sensor Analysis

Post by gwsnyder » Wed Dec 04, 2019 8:47 pm

Hi,

What are your thoughts on the former (camera not changing exposure lengths)? Is that something I could test somehow or could it be driver related? Do you know of any QHY diagnostic routines? As for the possibility of light from the monitor, the setup I'm using is a white t-shirt over the telescope (no wrinkles) with a lightbox app on an iPad providing illumination. The iPad is set directly next to the lens/t-shirt. The light illumination is constant (I can adjust the light level in the app).

Thanks,

Greg

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Re: QHY294C - Sensor Analysis

Post by admin » Thu Dec 05, 2019 8:30 pm

Hi Greg,

Yes, you can test the exposure response of the camera without too much trouble – set up your illumination again and then dragged the exposure slider gradually backwards and forwards through the range that we discussed (350 to 400 ms). If everything is well then the camera brightness should vary steadily as you move the slider backwards and forwards. If there are any jumps in the brightness at any point then it would seem likely that the camera exposure is not responding directly to the command sent by SharpCap.

If you make the control panel area on the right-hand side wider then you will get better control of the exposure because the slide will be longer.

Cheers, Robin

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Re: QHY294C - Sensor Analysis

Post by gwsnyder » Fri Dec 06, 2019 6:09 am

Hi Robin,

There definitely seem to be jumps in brightness as I drag the Exposure slider to the right of 60ms. Up until about 60ms, the increase in brightness is pretty steady and the histogram follows along. The brightness spike is very brief jump and the histogram moves about pretty wildly for a moment and then settles back pretty quickly. Where does that leave us?

Thanks again,

Greg

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Re: QHY294C - Sensor Analysis

Post by admin » Fri Dec 06, 2019 8:03 pm

Hi Greg,

I've certainly seen some cases where some pretty weird image frames come through shortly after changing camera settings on the QHY cameras. To try to deal with this SharpCap waits for five frames after making a change before it starts measuring the brightness (in theory to allow the brightness to settle).

Did you notice any weirdness about the 360 to 380 ms mark where SharpCap was having problems?

Cheers, Robin

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