Is this a correct flat ?

[JGMoreau]

Hi,

I am wondering if my flat is correct. It was made by exposing for a few seconds on the evening clear sky
through a white bed sheet. My imaging scope is a Newtonian reflector.
I did as instructed by Vincent.

Most of my images after color calibration are too strong in red and/or lack blue.
Camera is SBIG ST-2000XCM one shot color.

Here is a screen capture of debayered flat and histogram :

http://www3.cablevision.qc.ca/astro/Flat_Histogram.jpg

As you can see the blue channel is off to the right.
Also, the flat is stretched (?)

Did i do something wrong ?

Your comments appreciated.

J Guy Moreau

[Nocturnal]

Hi,

your flat looks fine even if it's a bit blue. Most likely you're not bias subtracting your flat or not dark subtracting your lights.

[Cleon_Wells]

Guy, here's what my MasterFlat looks like for my Canon T1i on my 10"RC.
http://www.astronomybycleon.com/files/page0-1004-full.html
I did not convert the MasterFlat to gray scale.
I took the subs at dusk and rotated the OTA 180 degrees for the last 5 subs with a total of 10 flat subs.
You should be able to see the vignetting of your optics when you stretch the MasterFlat.
Cleon

[JGMoreau]

Thanks Sander and Cleon for replying.

Sander, i think i do make the flat according to instructions,
and calibrate my lights as they should, but i will check again.
Maybe i have not made all the mistakes wet

Cleon, i see that your flat is a linear image and you applied STF to make the content visible.
Well this is a clear sign that i goofed somewhere, the image in the link is my flat as it came
out of the making process and looks stretched!
Did you make it with PI ?
I made one with DSS and it is exactly the same, an autostretch lurking in the background ?

Hmmm, more testing is in order...

J Guy

[Cleon_Wells]

Guy, goto this link, it's for DSLR raw files witch I assume will work for you SBIG one shoot files.
http://pixinsight.com/forum/index.php?topic=2570.msg17312#msg17312
Yes I did every thing in PI.
Look at these MasterFlat comparisons on my web site, it's interesting that the debayered images from the Gray MasterFlat have a lower pixel level compared to the DeBayered image( lower left ) form my RGB MasterFlat.
http://www.astronomybycleon.com/files/page0-1005-full.html
Cleon

[pfile]

i have noticed the same thing regarding signal levels in flats between files debayered with BatchDebayer and DSLR_RAW... i posted a question sometime back but got no answers.

[JGMoreau]

For me signal level is pretty much the same either with the script or with debayer module.

http://www3.cablevision.qc.ca/astro/Flat_Histogram2.jpg

My main concern is with the different distribution of pixels values as depicted on the histogram.
Does this introduce a color bias on my lights ?

Guess that my understanding of how flats works is inadequate

Thanks Cloen for the link, will study.

Cheers,

JG Moreau

p.s. my latest :  http://www3.cablevision.qc.ca/astro/NGC3227-6.jpg