SharpCap Forums

Hi Robin,

Thanks for your response. Can you check my screenshots above and see if I did the Bias test properly? They show that at a gain of 200 and brightness of 80 (the max), I'm still clipped on the left side.

Mark

Hi,

So I can see in your post from 2 May that there is a very small peak at the left-hand side of the histogram shown in fits Liberator. The question is, how many pixels does that represent out of the total of 15 million or so in the image. The vertical scale of that histogram is going to be a logarithmic scale so that the number represented by that small peak is going to be less than you perhaps think. Fortunately fits Liberator has a feature that lets you highlight the black clipped pixels in a dark blue colour. The checkbox to turn this on is on the right-hand side of the fits Liberator window. I'd suggest turning this option on and exploring the image to see how many pixels are clipped at the black point. I think the number will be small enough that it's really not worth worrying about.

Cheers, Robin

Ok, thanks. I'll do some additional tests to see how much is being clipped on the left. Here's the big question. I did these bias tests for my ASI294MC Pro camera and the graph is below. As you can see, I needed very little Brightness at low gains and ran out of Brightness range at a gain around 450. Yet for my ASI071MC Pro, I'm needing a very HIGH Brightness value at low gains, and quickly run out of brightness range at a gain around 150.

Mark

As an experiment, take a bias frame using each of the presets in the ASCOM driver. I am curious to see what the histograms look like .

Dave

Hi Dave and Robin,

Here are the screenshots when using the three different ASCOM camera settings that are offerred:

Highest Dynamic Range (Gain was set to 0, Offset defaulted to 65) - Histogram is not clipped
Unity Gain (Gain was set to 90, Offset defaulted to 65) - Histogram is not clipped
Lowest Read Noise (Gain was set to 240, Offset defaulted to 65) - Histogram is clipped on the left, what I expected.

So now we know that ZWO is defaulting the offset value to 65 but it can be edited in the ASCOM driver window. The good news is that the clipping at a gain of 240 matches what I found independently using the SharpCap camera selection of "ZWO ASI071MC Pro". There is not enough Brightness/Offset prevent clipping at the higher gain values.

Robin, as you suggested, I check the "Black Clipping" box in FITS Liberator but saw no blue anywhere in the screen for the "Lowest Read Noise" test where the histogram obviously is clipped. Am I missing something?

Thanks,

Mark

Mark

UG looks promising. This fits in with the ZWO forum post about offset. I thought this might happen for one of the settings.

Dave

Mark

Have a read of https://www.cloudynights.com/topic/5420 ... o-cameras/. A hard read but should be relevant to your question and post #30 on page 2 sums it up nicely I think.

Dave

Thanks Dave!