[Aavso-photometry] Lightbox flat question

[Richard Huziak]

Rob,

You need equal illumination across the front of the light box, and the 
box screen has to be big enough for your lens angle to 'see' that equal 
illumination and not go beyond it.  The viewing angle of your 8" scope 
is about 2 degrees (ie - approx 'straight ahead').  However, your 28mm 
lens may have a viewing angle of 75 degrees and the 130mm lens about 18 
degrees (guessing a bit on the exactness of the angles).  So the wider 
the lens, the closer it has to be to the screen.  With the 28mm, if 
angle is 75 degrees, and if your screen is 1 foot wide, you'd have to be 
no further than 14 inches from it, provided your screen is truly equally 
illiminated at the extremes.  Seems to me that the closer the better 
applies here for a light box, except I can envision issues with changes 
in illumination from the front screen resulting from the 'diffraction 
angle' from the screen into the lens (ie - attenuation at the edges on 
very close angles).  That then contradicts my initial impression on 'the 
closer the better'.

It may be better to do sky flats with a diffuser over the lens instead, 
but I can see some real problems with very wide angle lenses, since the 
sky is not equally illuminated at all angles at dusk.

 From your '0.6 to 2x lens f/l' statement, it seems you know the above 
issue already.  Others, are, however, doing telephoto photometry - what 
are you guys doing?

Rick

Robert J. Modic wrote:

>I've taken lightbox flats with my 8" f/5 Newtonian for some time
>now with mostly good results.  The lightbox is placed on the front end
>of the tube (about 1x the scope's f.l. from the primary mirror). 
>Recently, I've tried to use several short f.l. lenses (28 to 130 mm
>f.l.) for wide-field CCD photometry.  When I try to use a lightbox to
>take flats, the results are sometimes uneven.  Depending on the
>distance of the lightbox from the lens, the resulting flats can look a
>bit mottled and they don't flat field my science frames as well as I
>would like.  I've experimented with distances ranging from 0.6x to 2x
>the lens f.l.  The shorter distances seem to give the best results.
>
>Is there an optimal distance to have a lightbox from a lens?
>
>Bob Modic
>
>
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Richard Huziak
Manufacturing Engineering
SED Systems, Saskatoon
tel. (306) 933-1676
<huziak at SEDSystems.ca>
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