bin 2 flats form 1x1 flats: is it possible?

[lucchett]

Hello,
I wonder if it is possible to save time and build the binning 2x2 flats form the 1x1 series, using some resampling and scaling.
That would save me a lot of time when I am going on site.
Many thanks,
Andrea

[bitli]

The problem is that usually you make 2x2 flats for 2x2 images that were taken with different filters (like RGB) than the 1x1 images (usually L).  The dusts and other filter related effects will not be corrected correctly if you use flats not made with the same filter and same focus.

So in theory you should make the proper flats with each image configuration (including each filter), on the same night of the image. If you have a flat box and a reasonable control program, this is not very long.

Naturally this depends highly on the quality of image you want to achieve, the time you have and other pragmatic considerations.  You may always try to resample the 1x1 master flat made using a geometry/resample or rescale (I do not remember the name) and see if it is very different from the master flats made at 2x2 and if this is acceptable to you. But it is definitely not the same quality as proper flats.

-- bitli

[lucchett]

Thanks Bitli.
I do work in RGB (and Ha), all binned 1x1.

During winter and for current comets I need to shot bin 2x2
filter's dust should be downsampled well.
I am not sure how to scale (if needed) to the right adus values.

Also interested in doing the opposite: from bin2 to bin 1: that would save me a lot of time

Many thanks,
Andrea

[pfile]

hopefully you don't need to scale the 'binned' flat - just try Resample to make it's size 50%. you might want to change the interpolation method to something simple though, like bilinear.

rob

[Geoff]

Note that when you do a resample from bin 1 to bin 2 and save the image, the fits header file gives the correct new pixel dimensions but still reports binning 1, so you have to manually change this otherwise the BPP will give an error.