RE: Flats

["Gutzwiller, Michael" <mgutzwiller@lanvision.com>]

For my system I hope to use dome flats.  I intend mount a couple of
photographic gray cards, one for each camera, on the wall of the
observatory.  I haven't worked out the lighting completely but I should be
able to illuminate them fairly evenly, at least as evenly as a 4 degree
twilight flat would be.  I would also have the advantage of being able to do
flats at any time and repeat as many as I needed to get the noise to an
acceptable level.

Mike G.


-----Original Message-----
From: Tom Droege [mailto:tdroege@veriomail.com]
Sent: Wednesday, November 07, 2001 12:38 PM
To: tass@listserv.wwa.com
Subject: Flats


Michael Richmond writes,

>Indeed, since the Mark IV has a large (4x4 degree) field of view,
>it will see a portion of the twilight sky which is NOT of uniform
>brightness: the side of the field closer to the setting sun will be a
>bit brighter than the other side of the field. These variations
>(of size roughly 1% per degree) will lead to errors in the relative
>measurements of stars on one side vs. stars on the other side of
>the field -- it's true. On the other hand, if one concentrates on
>a small section of the entire image (as Andrew noted in his messages),
>then this large-scale variation becomes very small. AND, if one
>is interested only in the CHANGE in brightness of a given star
>relative to its neighbors, then such large-scale gradients can
>be ignored completely.

Hmmmm!  How about combining an equal number of twilight and dawn fields?

Tom Droege

.