Rick,

I usually take flats for every night of imaging. Still I sometimes have
problems with flats that either over or undercorrect the dust donuts, don't
know why. On other occasions I find that I can use flats from two weeks ago
if I have not changed the setup.
I don't have problems with vignetting with my small chip, though this might
change once I get the QHY8.
I have not seen small changes in focus (e.g. when I have to refocus because
of dropping temperatures) to affect the flats, at least I never made the
connection.
I usually do an artificial flat in addition to a "real" one as I almost
always have some gradients left. They are small enough not to be a problem
in b/w images, but they can get nasty in colour images, especially with
larger objects where an artificial flat is more difficult.

Stefan


"Rick Johnson" schrieb im Newsbeitrag
ster.com...


Stefan Lilge wrote:
I got myself a Skywatcher ED120 as a wide field scope recently. This is
"second" light ("first light" was such an incredibly cold night that
mount, scope and operator were not functioning correctly) in a very
bright night with city lights, bright moon and a snow cover that
reflected both. Sky background in a 10 minute exposure at f/7.5 was over
5000 counts though my Astronomik 13nm OIII filter. In a "normal" city
night this would be below 1000.
The ED120 gives good starshapes in Halpha, OIII and also broadband. This
image is not sharpened in any way.

Taken from the middle of Berlin with a Skywatcher ED120 at f/7.5 on a G11
mount, SXV-H9 camera, 14x10 minutes Halpha, 6x10 minutes OIII.

The picture can also be found at
http://ccd-astronomy.de/temp2/M97colourgut.jpg

Stefan

Looks like you have a winner with that combination!

My sky is really bright of late here too but that's in LRGB. Still I
normally run about 350 in 10 minutes lum binned 2x2 but am now running
1200 to 2000 with no moon, just scattering of starlight off a zillion ice
crystals in the air. Most years it is rather dry so they tend to sublime.
My counts can hit 600 to 800 but with high humidity this year that isn't
happening. I am finding flats very sensitive to temperature at this
background level. My focus changes with temp and that changes vignetting
considerably so I have to take a zillion flats at all focus positions to
find a set that matches. Then a new dust speck shows up and I do it all
over again. Spending all day taking flats it seems.

Does your ACF have that problem? I'm using a 3" Crayford with mirror
locked. Move the mirror and the flats really change, more than just
changing the Crayford's position. With the wide temp changes this
winter, -35C one night and only -10C the next, I need new flats daily.

Your smaller chip might not see that vignetting problem, I'm working right
to the edge of the usable FOV where the internal baffle is doing the
vignetting. Still a change in focus changes the size of the dust mots.
So they don't flat out right.

Rick

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