[AZ-Observing] Re: Flats and Bias

Jeff Hopkins wrote:

> You can indeed take flats during daylight. The problem is getting the 
> illumination down to a value you can use. The larger the telescope 
> the harder it is.

   Not necessarily larger, but faster.  Think of the twilight or daytime 
sky as an extended object.  Image intensity then depends upon only 
illumination level per unit area, sensitivity and focal ratio.  In other 
words, at f/8, you'll expose an evenly illuminated source, the sky in 
this case, to the same density with the same sensor whether the aperture 
is 20 mm or 240 mm.  You'll collect more light with the larger aperture, 
but from a proportionately smaller region of the sky.

> A light box offers the convenience of being able to take flats 
> anytime you want. Plus you can make it so you can adjust the 
> brightness.

   The primary challenge with light boxes is the "evenly illuminated" 
part.  This in practice is very difficult to achieve.  How precisely you 
need to define "even" depends upon your goals.  If you're just looking 
to remove the gross effects of vignetting and dust particles in the 
image plane for "pretty picture" imaging, a light box is probably 
sufficient.  You can improve the correction somewhat by calibrating your 
flats taken with the light box against sky flats, as the comparison will 
reveal gradients within the light box.  If you're goal is scientific 
imaging, e.g. widefield photometry, you'll probably want sky flats each 
observing session for each filter.

   If you're taking sky flats, it's a good idea to know the size of your 
field and have a list of pre-determined regions devoid of brighter stars 
on hand.  Or at least take a number of flats covering several fields, 
and average them to reduce the effect of stars which aren't evident but 
affect the quality of the flat.

                                -- Mike --
-- 
cal_donley@xxxxxxx
--
See message header for info on list archives or unsubscribing, and please 
send personal replies to the author, not the list.

Other related posts: