Re: [TASS] Flat fielding

[Andrew Bennett <andrew.bennett@NS.SYMPATICO.CA>]

On Thu, 9 Sep 1999 09:50:29 -0400, you wrote:

>The current version of the Flat program is not affected by changing sky
>brightness because it uses a slightly different algorithm.  This is how it
>works:
>
>1.      An ordinary image, preferably one with a high background level, is
>dark subtracted.
>2.      The median value of each row is calculated.
>3.      Each pixel in the row is replaced by the ratio of its value to the
>row's median to create a scaled image.
>4.      The median value of each column in the scaled image is calculated to
>create a flat vector.
>5.      The scaled flat vector is written to a FITS image file where the
>values are scaled to fit the 2 byte integer range.
>
>This algorithm avoids the problem with changes in the sky brightness and
>also reduces its sensitivity to bright stars.
Yes - steps 2. and 3. should do a really good job of producing
a smoother flat vector in the face of the E-W or time gradient
in dusk shots.
BUT unless I am seriously mistaken (which has been known
to happen) the procedure passes N-S gradients unchanged.

Has anybody looked at night-night variations in flat
vectors? Seasonal variations? One would expect the N
edge to be darker (closer to the zenith) in the winter
and possibly brighter (closer to the set sun) in the
summer. Or either of the above if there is a bright
cloud nearby.

Needed: a dome flat technique for the Mk III.

Andrew Bennett, Avondale Vineyard, Nova Scotia, Canada.