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A New Chip for GoldCam (1Sep93)
(from KPNO, NOAO Newsletter No. 35, 1 September 1993)

We have recently replaced the mediocre Ford 3K x 1K CCD in GoldCam with
one of much better performance. The old chip suffered from a large
number of traps in the imaging area which compromised regions of the
spectrum and necessitated a preflash, increasing the effective noise.
The new chip solves these two problems. The only traps are below pixel
600, where the focus starts to deteriorate seriously. The read noise is
8.5 electrons, and no preflash is needed. The sensitivity of the new
chip is quite similar to the CCD it replaced. So far, measurements have
only been made with grating 240 which gives spectral coverage from 3500
to 7000 A.  The total telescope/instrument/CCD throughput peaks with
this grating at 5000 A where the system has 11.5% total efficiency.

The two remaining problems with this system are the fringing, which
remains significant in the red, and the resolution, which remains no
better than 2.8 pixels FWHM. The fringing is due to the poor
anti-reflection properties of the coating in the red.  While we had
hoped to decrease the amplitude of the fringing by depositing a thicker
coating, an error in the calibration of the coating process left us
with one quite similar to the coating on the previous chip.  This can
be remedied by removing the chip and depositing an additional layer of
the coating material.  We plan to do this in the fall. This will
somewhat reduce the UV sensitivity of the chip. The second remaining
problem is the resolution. Although we have been very careful with the
positioning of all the optics and the detector, the best focus images
have gone from around 1.8 pixels FWHM with the old TI CCD to 2.8 pixels
FWHM with each of the two Ford chips. We are still trying to understand
this better, but we now believe this is a result of mis-spacing the
optics in the camera. We will work on this further in the fall also.

Todd Boroson, Rich Reed, Di Harmer

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