WORKING WITH TRIFFID
This Galway build photometer goes on several telescopes
around the world, including WHT/CASS. An array of dichroics
splits up the incident beam in broad band regions, each region
projected onto the detector. The
photon-counting tube detects each photon arrival by knocking
of an electron (photo cathode) which is accelerated through
an electric field until it hits a micro channel plate which produces
a shower of electrons. This shower is detected on a fine grid of wires
and, using a techique called coincidence logic, the photons xy position
is recorded together with the arrival time. The two-dimensional image
builds up on the 'MAMA' display. Furthermore, 3 fibres acts like single
channel photometers (ADC). Detector size is ~ 1000 x 200 'resolution
elements'. each element 24 microns corresponding to 0.1" on sky.
FOV ~ 100" x 20"
Use ISIS setup.
The TV camera can be used in direct mode to view the field,
i.e. ICL>agcomp. Remember there is a focus offset between
direct view and cass focal plane of ~0.4mm
The observing system runs on a linux machine
supplied by the TRIFFID team. Data acquisition & storing is likewise
controlled by their own computers. However, the ICL is still needed
with aux/isis setup to control autoguiding & tv acquisition (like for
The nominal telescope focus is determined on the TRIFFID
realtime display. As the image builds up, the observers measure the
FWHM. This is repeated for several focus values.
Standard cass procedure: direct tv view, find rotator center,
calibrate on rc.
Determine aperture offset to instrument center:
Rotator center is close to the center of the triffid array
(MAMA) -> no ap offsets needed.
Object is typically centered on one of the fibres (APDs) seen
on the MAMA display. Autoguiding starts, and observing goes on for hours
No worries; Observers writes their triffid data on Magneto-Optical disks.
If no other instrument has been used during the night, the log will
only contain the header!