Creating Master Flats

[pfile]

if you forgot to take flats and you want to try to correct vignetting, you can use DBE. setting the mode to "division" is akin to calibrating with flats.

unfortunately DBE can't really discriminate between sky gradients and vignetting, so i guess you have to be judicious about sample placement and perhaps crank the smoothing factor way up. probably no matter what you'll calculate some combination of the vignetting and sky gradient and of course in an integrated stack with dithering even the vignetting becomes more complex due to the displacement of each frame.

i guess if you really wanted to pursue this you could integrate a bunch of unregistered frames, perhaps with low rejection sigmas, to come up with something that's more reflective of the vignetting. then you can compute a "background" model with DBE. by default DBE creates a model which has 1/2 the dimensions of the modeled image, but you can change that in the DBE settings. i suppose if you can come up with something that looks proper as a flat, you could save it out and then use it as the master flat in your calibration flow.

anyway just thinking out loud here. it might be enough to simply do DBE on your integrated stack per usual and you can take care of most of the vignetting.

rob

[jerryyyyy]

Thanks, did not really help.  I need to get some good flats.  I had this all figured out for the SBIG but the Maxim SkyFlat Assistant does not seem to work with the DSLR.  When you think of it must be very tricky since the flats would be different for RG&B.  Well I have some time to figure this out until my CCD comes back. 

[jcinpv]

My question is similar, but my equipment is different.
I just got a Canon 60Da. It has a clip-in LPS filter (Astronomiks).
When I take Flats, I'm using an LED white light, aperture covered with white cloth, ISO 800, A-V mode. Taking images from the camera, not any external software. RAW images. Camera is in same orientation as Lights on the Stellarvue SV80ED. No change in focus.
With the LPS filter in, it blocks much of the red as seen from the output of Statistics on a single image.
The RGB Mean values are 2817.2, 6245.8, and 9054.7.
Since this is the first time using a 60Da and with an LPS filter installed and taking Flats, what should I expect?

[Phil Leigh]

The "white" LED Light probably has very little red in it  - most white LED's are anything but white - and a lot of blue (regardless of the LP filter) - put the two together and...

"The RGB Mean values are 2817.2, 6245.8, and 9054.7" - is this measured in Image Statistics in 14-bit mode or 16-bit mode? If it's 14-bit mode it's OK - if in 14-bit mode the red channel is very under-exposed.

You do not need to get all three channels the same - that is not important. But you do need to get the lowest channel over ~2000 when analysed in 14-bit mode - and to make sure that the highest channel does not exceed ~8000 (max value in 14-bit mode is 16384).