[AZ-Observing] Re: Flats and Bias

Brian,
It appears from what you said that one obtains flats over a number of 
days which will make getting them more doable because of the short time 
to take them during twilight.

You said "The problem with twilight flats is that the spectrum of the 
twilight sky does not resemble the (dark) night sky; the former is a 
very blue continuum like that of the Sun (unsurprisingly), whereas the 
night sky is rather red and has emission lines, something like the 
spectrum of a planetary nebula (the 'nebula' being the glowing ions in 
Earth's atmosphere)." If twilight has a blue continuum like the sun, why 
is a flat taken during twilight not suitable during a significant moon 
night?
Also, if a twilight flat is not a good match for the night sky because 
the night sky is 'rather red', would placing a sheet of red cellophane 
over the objective end of the telescope work to make a twilight flat 
more like a dark sky flat? If the red cellophane is too red, maybe there 
are some available that are not so red.

Finally, since there is always the danger of still imaging stars during 
the twilight, what about slow slewing the telescope in random directions 
as it is integrating? Am I correct that your reference to dithering is 
probably doing the same thing?
Stan

Brian Skiff wrote:

>     Jeff and Mike have mentioned most of the various trade-offs
>with regard to flat-field calibration images.  The usual procedure
>at most observatories is to take twilight flats with the telescope
>either not tracking or commanded to dither some small amount between 
>each exposure, the former being lots easier. - etc. 
>  
>
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