Flat scaling

[gvanhau]

Hello
I'm having some trouble with my flats in PI.

When I use my SCT with a f6.3 reductor and my QHY8L cam I get slightly vigneted images.
When I then apply the flats to my lights, the efect is that the four corners become more iluminated than the center of my images.
I tried to solve this adjusting exposure timing of my flats (getting flats ranging fom 1000 to 40000 adu's), with no results.

To solve this ultil now, i have to go trough te tedious work consisting in adjusting a pixelmath poportional substraction of the master flat to each of the lights until I get an even iluminated image.

When I use the same set of images in DeepSkyStacker the result is much better in that aspect.

Can some one give me a tip how to correct this?

Regards
Geert

[Nocturnal]

Hello Geert,

this is happening because of the following reasons:

- your flats aren't linear. This is not to be confused with linear vs. stretched images in PI. Flats indicate brightness relationships of all the pixels in your image assuming that the entire scope was exposed to the same light flux. You see it's important to stay in the linear range of your sensor so that there is a linear relationship between light flux (intensity) and ADU count. 40K Is probably too high.

- your flats aren't calibrated properly. For the above described linear relationship to work flats need to be calibrated by subtracting a flat dark which is generally just a bias frame when flats are short enough not to accumulate dark current.

Using the explanation above you can investigate why things aren't working the way they should. If you can't then I suggest describing in detail how you calibrate your subs in PI.

[gvanhau]

Thanks Nocturnal for your fast answer.

- your flats aren't linear. This is not to be confused with linear vs. stretched images in PI. Flats indicate brightness relationships of all the pixels in your image assuming that the entire scope was exposed to the same light flux. You see it's important to stay in the linear range of your sensor so that there is a linear relationship between light flux (intensity) and ADU count. 40K Is probably too high.

As I said, I tried with diferent flat exposure times, 40000 adu where the highest ones, I've been trying with 1000 ADU, with 2000 ADU, with ..16000.. 32000 ADU, etc.

- your flats aren't calibrated properly. For the above described linear relationship to work flats need to be calibrated by subtracting a flat dark which is generally just a bias frame when flats are short enough not to accumulate dark current.

I've been calibrating my flats with bias only since flat exposure times are always less than 1/2 sec.

Could there be another factor? 

Regards
Geert

[Nocturnal]

I forgot one more thing. Your lights need to be calibrated properly but presumably they are. Flattening is essentially (light - dark) / (flat - flat dark).

Finally it's possible your data is all correct but PI isn't doing the right thing, either due to a bug or incorrect usage. That's why I suggested you describe exactly what you're doing. I don't calibrate in PI so we'll have to wait for someone else to pick up and hopefully provide some assistance.

[zvrastil]

Hi Geert,

just a stupid idea - do you calibrate your flats beforehand or using "Calibrate" checkbox on the ImageIntegration process? Doing both would result in bias being subtracted twice from the flat frame and it could explain overcorrection you see.

regards, Zbynek

[gvanhau]

Hello

the settings I use for calibrating darks, flats and bias are more or less those sugested in this tutorial:
http://pixinsight.com/tutorials/master-frames/en.html

Hi Geert,

just a stupid idea - do you calibrate your flats beforehand or using "Calibrate" checkbox on the ImageIntegration process? Doing both would result in bias being subtracted twice from the flat frame and it could explain overcorrection you see.

regards, Zbynek

I dont think this is stupid idea, I´m not shure about that, I will check if this evenenig (now I'm at work).

Regards
Geert

[gvanhau]

Hello
I redid  the whole process from bias, dark and flat ensuring "calibrate" check was always off.

The result was much better, but I still feel that the results obtained with DDS are flatter than those I get with PI.

Regards
Geert