Re: shutter

[aah@nofs.navy.mil]

Tom's 'sliding pocket door' shutter should work.  One concern is the speed,
since you are talking 200milliseconds to open/close the shutter.  This
means, center to edge, you have a variation of 400ms in exposure time.
Either this has to be calibrated out (software again!), or else you need
to use quite long exposures not to have shutter vignetting be a major
contribution to the photometry.  The software solution is not difficult,
but is just another calibration to make.  You can do a linearity test
with such shutters, either by using lower light intensity and longer
exposures, or by including the vignetting function.  Note that linearity
should not be a function of wavelength, so you could use an incadescent
for flatfielding, and then include a single pulsed LED as an add-on for
doing a linearity test.
  An alternative is to use the open-hole shutter.  We are using a square
hole (curtain shutter?) on our 1.3m mosaic camera, and using solenoids to
move it back/forth for high speed.  The direction of motion, from closed_a to
open to closed_b is the same, so that each pixel on the CCD sees the same
duration of exposure.  Then the shutter is reversed in direction for the
next exposure, so that it goes from closed_b to open to closed_a.  This
action eliminates shutter vignetting, but does require a bigger space than
Tom was envisioning for the Schmidt system.
  I would definitely include both 'bulb' and 'timed' exposure commands
in the stamp.  For dark/bias frames, you would want to expose with the
shutter closed, and for many tests, you would want the shutter left open
indefinitely.  At the same time, you may want the stamp to time the
exposure to remove that overhead and possible latency from the PC.  Having
both command sets gives you the desired flexibility.  However, isn't there
just one stamp in the system, and can it properly time three separate
cameras, that may be exposing/reading out at different times?
  One of the experiments I want to do on first mark IV light here is to
get a week's worth of data on Polaris.  This cepheid had 0.2mag variations
in the 1960's, but has decreased to its current 0.02mag variation.  I'd
like to see if the mark IV can resolve the light curve.  This means that
the mark IV either has to be able to reach the pole on its Dec drive, or
else I run the mount rotated 180 degrees for a week or so.  Therefore, my
vote is for a modified mount that can reach the pole, and probably down
to the horizon to the south.  Note that the galactic center is at -30 degrees,
so it would be nice to be able to reach at least that from North American sites.
Many events, such as novae, are galactic in nature, so reaching as much of
the galactic plane/bulge as possible improves your chances of finding them.
Atmospheric effects can be calibrated out, so that shouldn't be a major
consideration, but hardware may limit the possible range of motion.
Arne
--------------------------------------------------------------------
Arne Henden                               Instruments/software/CCDs
US Naval Observatory Flagstaff Station    Cepheids/photometry/IR
P.O. Box 1149                             ftp: 192.68.148.67  
Flagstaff, AZ 86002-1149                  Voice: (520)779-5132
aah@nofs.navy.mil                         FAX:   (520)774-3626