This morning i refined my technique for taking flats.
I aimed the scope into the pre dawn sky.
Then (using sharpcap) watched the histogram and the live image.
Varying the exposure time I was able to find the optimum exposure to reveal a lens problem.
Tried cleaning the primary lens and fault is still there.
So I have a field flattener. Maybe the problem is there, or maybe on the camera lens?
Back to basics: best to eliminate problems at the source.
As to the formula I really don't understand it.
Does it assume the dust is a certain size?
I was being a little tongue in cheek, Peter. I agree it's better to remove the dust but sometimes you can't. I use remote/unattended systems and flats deal with dust donuts just fine in between infrequent cleaning and maintenance.
I have never thought deeply about the formula but it does seem to work. If you have a bunch of dust specks at the same distance the shadows all look pretty much the same size even though the specks presumably vary in size. They are a shadow of the aperture (solid for a refractor and donuts for scopes with a central obstruction.) Something to ponder when I run out of data to process
where D is the image diameter of the dust bunny in mm.
This is an approximation of the geometrical intersection of lines between aperture, dust grain and focus plane.
Here, the dust size is assumed to be zero, a good approximation as long as it is much smaller than it's image. Bigger dust grains will primarily darken the bunny.
(I'll send ya the fits files...)
