Image Histogram doesn't match the L value on a flat frame?

Sequence Generator
1

Observe that the image histogram spike is very close to the left, while the
L value showed very large number, indicating that the pixel is almost white.

Why is this?? Did I do something wrong?

SGP.jpg1482×883 87 KB

I was relying on the histogram to give me proper exposure for flats, I think I have overexposed the flat?

Camera setting used: QHY163M Gain 20, Offset 100, 5 second exposure
Filter used with the flat frame: 8.5nm OIII filter

3

The ‘Image Statistics’ module is a better way to get the right exposure for flats. Just adjust the exposure length until the mean value is equal to your target value. For a lot of CCDs this is somewhere between 20,000 and 30,000.
From the look of the screen shot there is a very strong screen stretch applied. Right-Click the image and select ‘Auto Stretch - None’ to see what it really looks like.

Clear Skies

Mick

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