The observing mode described below is useful for observing compact
sources, such as faint point sources, which are small compared to the
array and chopper throw.
To begin chopping the secondary mirror, type
UIP> SEC 39 1.388888888 4 4
at the UIP prompt. (Wait a minute or two for the auto-tuning to
complete before observing.) The throw, specified as 39 arcsec = 8
pixels X
4.85 arcsec/pixel, can be enlarged for moderately extended
sources. The frequency 1.388888888 Hz corresponds to a period
of 720 msec, which is 20 times the detector sampling period of 36
msec. To stop the secondary mirror at the end of chopped
observing, execute "UIP> SEC /STOP".
A scan pattern is still needed with chopped observing, and it should be
slow enough that the motion in one chopper period is less than ~3
arcsec. A suitable scan pattern for compact sources is
UIP> SWEEP 30 70.7 /YAMP=20 /YPER=50 /ALT"
The quick-look reduction software for analyzing pointing scans is
called "sharcchop" and is currently available on champinux. Run
champinux% sharcchop #####
to analyze scan #####. "sharcchop" currently assumes that the
chop throw is 39 arseconds.
Collecting analyzable chopped data requires that a cable be connected from the
chopper monitor to the SHARC II electronics.