STUDY SUMMARY - IPMU

SUMMARY REPORT
WIDE FIELD FIBER-FED OPTICAL
MULTI-OBJECT SPECTROMETER (WFMOS)
has a 4k × 4k CCD detector with imaging optics and will look at a quarter of the positioner focal
plane. Calibration of the camera distortion and mapping to the focal plane will be achieved by
imaging backlit fixed fiducial fibers of known position located on the prime focus positioner
plane.
Figure 3.6-1: Schematic of Single Camera on the Prime Focus Support Strut
To reposition the science fibers onto a new object field, each metrology camera takes an image
of a quarter of the focal plane. This is achieved by pulsing the LED light sources of the backlit
science and fiducial fibers.
Since the patrol fields overlap, multiple images of the focal plane with different illuminated
sets must be taken in order to identify the fibers uniquely. One-third of the science fibers are
backlit at a time using an LED source in the spectrograph, and metrology imaging is performed
three times. The science fibers are then moved to new positions based on predicted positions of
the chosen targets. The metrology camera then takes a new set of images, and the positions of the
science fibers are measured in the same way as before. The error from the required position is
then calculated and the fibers moved again. This process is repeated six times.
The stringent subpixel centroiding requirement means back-illuminated CCDs are preferred
to front illuminated ones, as they offer better subpixel uniformity. The baseline detector is the
1100S camera from Spectral Instruments, Inc with a E2V230-84. The camera read-out time is 2
s, and the centroid computation time is 1 s.
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