Flats

[Tom Droege <tdroege@veriomail.com>]

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