[billdan (Bill)]

Gooday Neil,

In theory you should subtract a bias or a flat dark to get them calibrated correctly. However you can subtract an offset using pixel maths to get the same result.

Flat darks must be the same exposure length, in my case I always use 5sec flats and flat darks. Bias subs are at 0 sec exposure time.

Flats do not need to be taken at the same temperature as the lights and definitely not the same exposure time.

Yes you can take the flats at any time as long as nothing has changed ( focus, orientation etc). You can use these flats over and over as long as nothing has changed.

The master flat I am using is now a month old and as long as I keep the optics clean and orientation the same it will always work.

I wouldn't use a flash light to create flats, better off pointing the scope at a white wall or white cardboard with even illumination.

I use Astroart V5 software and I can create a Pseudo Flat from the stacked lights and then divide the pseudo flat into the stacked lights, this works OK as well.
So this is another option to consider, in fact after stretching the stack, if a gradient appears I can create another Pseudo flat to remove the gradient.

Hope this helps

Cheers
Bill