Flat frames - not flat! Asi294mc pro

[Robstaffs]

With following set up I can’t get flat frames flat all the way to the edge. Normal? 
 

zenithstar 61 + FF

asi 294mc pro

using asi air pro to calculate exposure, ADU value looks right, but I get steep gradients at the edge as shown in attached image. I’ve tried the T shirt method, plus a light pad with and without the shirts, but the result is the same. Any ideas? 
 

 

[david_taurus83]

Is that image stretched? Does your light frame look the same? If so it's vignetting and the flat frame should remove it from the lights! That's the idea of flats.

[Robstaffs]

Hi, no it isn’t stretched. I do see some of the same gradient on lights sometimes. But I think you answer my question in that this is what I should be seeing when taking flats. If we ever get a clear night I’ll get some data to process with them and see what happens! 

[alacant]

Hi

We should be able help with -links to- a light frame, flat frame and, if you've made them yet, a dark-flat frame.

Cheers

[ollypenrice]

Rather than eyeball your flat you should just measure it. Open it in a program which lets you mouse over it to see the ADU values recorded in the middle and the corners to get an idea of the illumination you're getting in those places. It's possible to obtain a perfectly good result with as much as a 25% drop-off in brightness between corner and centre. One of my rigs generally gives me a reading of about 23,000 ADU in the middle and 19,000 in the corners. The flats correct this to produce a good clean final image, however.

Olly

Edited April 3, 2021 by ollypenrice
Typo

[Robstaffs]

My error, this is of course a stretched image, the AAP has stretched it rather than me hence my confusion. The corresponding fits file DOES look even and the ADU is about 25000