Re: Everett and Howell Paper

[Andrew Bennett <andrew.bennett@ns.sympatico.ca>]

On Wed, 7 Nov 2001 11:02:31 -0500, Stupendous Man
<richmond@a188-l009.rit.edu> wrote:

> ...
>
>  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.
>
>  So, IF your goal is to measure changes in brightness very precisely,
>then a high-signal flatfield image with some large-scale variations
>is just fine -- and much better than a low-signal flatfield image
>made from a combination of night-sky images.
>
>  On the other hand, if your goal is to measure accurately the 
>magnitudes of stars all the way across the frame, in order to make
>a photometric catalog, then you should NOT use a twilight sky flat,
>because it WILL introduce systematic errors of a few percent.
>Instead, you might choose the low-signal night-sky flatfield image:
>it will cause a larger (random) scatter, but a smaller (systematic)
>error within each frame.

Is this true?
Around here, the sky gets brighter nearer the horizon
all night long and the lights of towns extend an awful
long way towards the zenith. 1% per degree at around 45
degrees zenith angle wouldn't surprise me in the least.
I have always felt that this was a problem with the
MK III (putting in unnecessary gradients for Arne to take
out again) and it is far worse for the MK IV because it
changes azimuth and elevation.

Dome flats.

Andrew Bennett, Avondale Vineyard