Flats Calibration Problem

[oldwexi]

Geoff ... I'm with you.  I always thought that the entire purpose of the flat was to correct these problems, but the logic here seems to be missing.

Hi Hytham!
Thats correct as long as the flat was produced correctly. The logic that your flat does not work is very simple - it is overexposed in addition to the too small
image circle.
Check the histogram of your flat, check the statistic of it. Try to use the simplified calibration rule from my first post.
this helps to understand the logic that with "black Corners" in the flat there is no way for proper calibration.
Again simplified what happens in the corners:
(light / flat)
(0.1 / 0.01 results in clipped 1 = White)

Gerald

[topboxman]

I've always thought that part of the reason to use flats is to remove vignetting. Vignetting results in dark corners when taking flats. I've had vignetting many times with my C-8 with F/6.3 focal reducer and large CCD camera and always have dark corners when taking flats and never had issues calibrating dark corners flats with lights. Am I missing something? 

Peter

[pfile]

i assume what gerald is saying is that the drop off is so great that there's no way to properly expose the corners while simultaneously properly exposing the center.

in this case maybe the only thing to do is to expose as much as possible so that the center is just at the end of the linear range of the sensor, and then use many, many flats to get the SNR in the dark areas up.

rob

[Hytham]

Gerald,

My apologies and I have skewed your results!  One thing I should have mentioned is that I have regenerated my flats for a lower exposure (because as you state they were very much over exposed), but the result with the Heart and Soul nebula is still the same over brightening of the corners, but when using the very same set of regenerated flats to calibrate the Elephant Trunk, the result is very much ideal.

As soon as I am home, I can show you a images of both nebulae with the same flats  with very different results.

Sorry for the confusion - absolutely my fault.

-Hytham

[bhwolf]

I have nothing to add that is useful, except, I am finding this thread EXTREMELY useful -- thank you!  I image frequently with a hyperstar, which suffers from some pretty severe vignetting.  My flats have worked previously just fine, but in an image I just started processing, I'm seeing this exact problem.  Many thanks to all the contributors...

[Hytham]

Here we go:

Both images have been calibrated with the exact same bias (100), darks (20), and flats (20).  The difference is integration time; 3.6h for Elephant, and 9h for Heart and Soul.  The ADU count for the flats was approximately 37,000.

[oldwexi]

Hi Hytham!
Find attached a link to a screenshot which shows different diagonal profiles of the  used
images and flats.
https://dl.dropboxusercontent.com/u/57910417/Image23.jpg

I have normalized them with PixelMath for better comparison.
In the top row you see the Image, in  the bottom row below you see the diagonal Profile Curve.
On the left the extreme Profile of your first flat, right of it the Profile Curve of my corrected flat.
This right curve fits relatively good to the curve of the heart and soul image.(Far left in the screenshot))

On the right side below the lights you see in the Profile a preview rectangle which represents the clear image circle.
Look at the curves inside the previews, they are the same in both previews. Outside the preview in the obstructed area
depending of the light collected the curve is steeper or more flat.
Less light collected gets a more flat curve, more light collected
gets a steeper curve. Best visible in the overexposed flat at the leftmost image.

So, because of the obstruction:  the flat  work properly on all images parts inside the unobstructed area,
but not outside, here the curve needs adaption by making the flat steeper or more flat.
Depending of the light collected in your light.

I do not see a possiblity to use only 1 flat for all your Images in this Kind of Image train.
The new flat you made sounds like it worked for the darker Image but not for the brighter Image.
This is what i wanted to explain with the screenshot.

Gerald