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RE: Flats


With Michael's latest discourse, I now understand about the pixel to pixel
vs. full frame flatfields.  Thanks for your patience.

Now, I have a really stupid question.  Aside from the potential for
looking/transiting across the sun (and potentially going up in flames), why
not use daytime exposures to create flat fields?

I tried to search the tass site for "flatfield mark IV", but am getting
errors from the search page.  May be a problem here at work.  So I'm going
to ask a question which might of been answered in the past.  How far away
from the Mark IV would an appropriate flatfield contraption need to be?
Andrew mentions something which fits inside the lens tube, but suffers from
circularly variable illumination.  If a box was built like
http://www.mtco.com/~jgunn, could an illumination source be constructed so
that the roof of the box (or something on the inside of the roof) would
provide a decent flatfield source?  Then, flatfield pictures could be taken
all day...

Later,
Rob

> -----Original Message-----
> From: Tom Droege [mailto:tdroege@veriomail.com]
> Sent: Wednesday, November 07, 2001 10:38 AM
> 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
> 
>