Dawn Flats

[Tom Droege <tdroege@veriomail.com>]

Michael Richmond writes:

>However, one alternative which WILL improve the flatfielding
>is to use twilight sky flats. If one takes pictures of the sky
>as the sun goes down, one will begin with saturated images.
>As the sky darkens, the images will stop being saturated, yet
>have very high signal levels in each pixel (in the case of the
>Mark IV, just below saturation, about 60,000 electrons per pixel).

Twilight flats are hard to get as I am usually turning on and am quit 
busy.  However, I always run through dawn.  I have a lot of dawn 
data.  Looks like I get about 1 dawn flat a night.  On November 3, for the 
three 100 second exposures around dawn the means were 1243, 12,391, and 
saturated.  Note that the 12391 exposure was actually 37,000 counts above 
the pedestal and the 1243 exposure was26,000above pedestal.  This is about 
80%  and 60% of the full scale (before saturation) range of 45000 counts 
for the Mark IV.

These exposures look lopsided to me.  However, I took a near dawn image, 
dark subtracted and divided by a flat made from many object frames, and the 
result came out looking pretty flat.

Note that a lot of stars can be seen in a dawn flat, so one will have to 
use a lot of them.  I have a number of runs days now, 20 or so, and for 
most I kept data through dawn.  I could make up a disk of this data if 
anyone wants to work with it.  Because of the way I am running - following 
a field - there will be only one useful dawn flat an evening.

>If one continues to take pictures, stars will begin to appear
>as the sky level decreases. Eventually, one ends up with
>ordinary night-sky images: a very low background sky level and
>prominent stars.

I think Michael is probably used to looking at big telescope data where 
there is little chance of a bright star in the twilight flat.  Even with an 
exposure near saturation there are lots of stars visible.  In fact, one can 
tell a dawn image just by looking at it (without measuring) because so many 
of the stars have saturation tails.  I remember seeing this in Mark III 
data.  I used to wonder why there seemed to be so many bright stars near dawn.

Note also, that when it takes 46 seconds to read out an image, one does not 
get very many twilight flats.  One or at most two will be in a range of 
interest.  The levels will thus vary wildly, and one must do something to 
normalize the data before combining.

I assume that there is no difference between a dawn and a twilight image??

Tom Droege