Re: dawn and twilight sky flats

[Tom Droege <tdroege@veriomail.com>]

I notice that the camera used by Everett and Howell takes longer to read 
out than the Mark IV.  If they get more than one twilight flat, then they 
must adjust the exposure length and for us that would mean shutter 
problems.  They must have used dome flats since they had 120 of them.

OK, experienced observers, how do you get more than one or two twilight 
flats an evening?

Note that I assume that you do not want to put anything around the lens 
that will change its reflectance properties.  That is why I have not tried 
to use the flat box.  I am afraid that there will be more problems from the 
light bouncing off the introduced surfaces of the flat box than there is 
benefit from it's use.

The unit I build is designed to slip over the end of the telescope 
tube.  The face of the white box is several inches from the lens face and 
it is 8 inches in diameter for the 4 inch lens.  The walls of the box 
between the white box and the lens are "black".  Well it is black paper.  I 
think it will take a lot of work to get it even remotely black.  So there 
will be light reflecting off the black walls and getting into the lens tube 
at odd angles.  Comments anyone?  I think it is hard to do it right.  One 
needs really large dimensions, and really black paint, and many baffles.

Newcomers may recognize that this has been a much discussed subject.

Tom Droege

At 02:26 PM 11/7/01 -0500, you wrote:

>   Tom points out the Mark IV takes so long to read out that
>one can't get very many flats while the sky is bright, but not
>too bright.  D'oh!  I had completely forgotten about that.
>It would help if one set the exposure time to, say, 2 or 3 seconds
>instead of 60 seconds, but that fixed 40-second readout period
>is still a problem.  That makes twilight flats less attractive,
>for sure.
>
>   Tom asks if one could combine flatfield images from dawn and
>from dusk.  Yup.  If you point south (as the Mark IV typically
>does), then the sky gradient will go one way at dusk and the
>other way at dawn, so combining them will tend to remove the gradient.
>Sort of ...
>
> > Note that a lot of stars can be seen in a dawn flat ...
>
>   If one changes the operating procedure, so that the exposure
>time is only 2 or 3 seconds instead of the usual 60, there will
>be many fewer stars visible in these exposures.  That's a lot
>easier at dusk (when the operator is awake) than at dawn (when
>he's asleep), of course :-/
>
> >  The levels will thus vary wildly, and one must do something to
> >  normalize the data before combining.
>
>   Absolutely true.  I normalize all the exposures so that the mean
>level is the same in each before I combine them.
>
>
>                                                Michael