Never get "OK" Histogram Status for flats with QHY183C

[MarMax]

I've noticed that when taking flats with my QHY183C that the Histogram Status always says "Too many pixels below 10% saturation" no matter where the histogram level is set. See the attached screen grab for a recent flat taken with a histogram level of 70%.

[admin]

Hi,

I can see from your screenshot that there is definitely a medium sized peak in the histogram near the left hand end, which is what is causing the message about 'too many pixels below 10%'. Can you see on your image what part is causing that nearly dark peak in the histogram? Is it concentrated near the corners, or is it perhaps a very dark shadow in one area (maybe dust on or near the sensor). If you can work out the cause then maybe you can clear up the problem from that point of view which would then give you an 'OK' status.

If the dark areas are unavoidable then just go ahead and take the flats - the hints shown in SharpCap are guidelines for making a good flat, not rules. What you will find is though that the flat correction may not be very accurate in the very dark areas, as in those regions a very large boost to brightness will be added by the flat correction.

cheers,

Robin

[MarMax]

Great explanation Robin. This is with a C11 Hyperstar, which I should have included. I'm always getting dark corners in the flats and over-correction in the corners as I stretch the histogram (exactly what you said).

My sensor is dust free and my light panel provides good even illumination. Other than increasing the histogram to reduce that left side peak, what else can I try to make this better?

[admin]

Hi,

not a great deal sadly if the corners are badly darkened. You could use a slightly smaller ROI to exclude the corners of the full sensor frame, but to be honest it's just as easy to crop after processing if the corners look bad.

cheers,

Robin