Re: [TASS] Flat Fielding

[Chuck Shaw <cshaw@GHG.NET>]

> Date:    Fri, 10 Sep 1999 10:24:48 -0700
> From:    "Borcher, David (NG-EX)" <DBorcher@GI.COM>
> Subject: Re: Flat fielding
> - - - - -
> I once saw someone use a sheet of very thick (about 1") white
> "semi translucent" plastic, illuminated by incandescent bulbs
> along each side.  I think the bulbs were a quartz halogen type.


I built a "Light box" to do flat fields with for my camera (CB245) since my
imaging is mostly done from surburban skies.  A brief article is on my
website in the "Stars" section.  The results are excellent (IMHO).  The acid
test was to rotate the light box to different orientations and shoot flats
in each orientation, then subtract them.  If the light source really is
uniform, you will not see any values remaining in the "subtracted" results
from the two images.  I found I had "virtually" no differences in mine
("virtually" = less than 2 pv (out of 0-4095) in the resulting subtracted
image).  It was quite easy to build, and very inexpensive.

Since I use mine for color work, I am planning on  making a couple of
upgrades.  The first is to add a variable control, to be able to adjust the
light level so I can shoot longer or shorter, depending on the optical
system's f-ratio I am using.  The second is sinceI am using a red bulb, and
when I shoot flats for my blue or cyan filters, the exposures get too long
for my patience.  So I will change to a "white" bulb.

Hope this helps........

Chuck

Chuck Shaw
cshaw@ghg.net  (home e-mail)
cshaw@ems.jsc.nasa.gov  (Work e-mail)
http://www.ghg.net/cshaw (S4 : Stars, Sailing, Spaceflight & Stuff)