Re: flatfields

[aah@nofs.navy.mil]

Tom wrote:
>Siemens      930 - 980
>             840 - 925
>             645 - 680
>             615 - 665
>             590 - 630
>             570 - 620  
>             555 - 580
>             545 - 570
>             440 - 520
  It sounds like the V bandpass is reasonably well covered, though you will
have to combine several LEDs and adjust their light intensities to match
a black body curve.  Someone should offer an LED that fills in the
520-545nm region.  Note also that you will probably have to use several
sets of these LEDs to give even illumination.

>Why am I so pig headed about building a light box with LEDs?  I guess I 
>just like the way I can drive them.  By varying the number of pulses one
>can do a very nice linearity test.  I know this is not the prime purpose,
  The mark IV has a shutter, so why can't you do a good linearity test
with it?  Granted, the short exposure end is dominated by shutter
effects, but you can use longer exposures for the flatfield to get out
of that problem.  Likewise, with the mark III, you can adjust the VCO rate
with a fixed intensity illumination system to give you the linearity
measurement.
  The pulsed LED system might be a problem for the mark III
cameras because of their drift scan readout method.  If you: don't clock
the CCD; give it a number of LED pulses; then read it out, you
will have differing dark current from start to end of the readout, which
is ok as long as the flat exposure level is much larger than the dark
current.  Likewise, I'm not quite sure how one would turn off the VCO
for the exposure, and then start it up again for the readout, without
some initialization errors.  Should be a fun exercise for someone!
  The main thing you gain by pulsing an incadescent is the higher color
temperature.  I still think leaving the incadescent on, filtering the light
to get a bluer response, and using the shutter for controlling the exposure,
is the simplest method for the mark IV.
  However, I'm always open to new ideas (really!).  If you can find LEDs
that fill in the missing spectral regions (and can afford them!), then
give your pulsed LED system a try.  Like I said earlier, though, I would
compare _any_ new technique to the tried-and-tested incadescent method
to make sure that no funny structure is present with the new technique.
Arne
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Arne Henden                               Instruments/software/CCDs
US Naval Observatory Flagstaff Station    Cepheids/photometry/IR
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