Subject: DIS red operational and improved


Submitted: Tue, 12 Dec 2006 13:13:10 -0700

Message number: 1002 (previous: 1001, next: 1003 up: Index)

We are pleased to announce that the DIS red camera is operational  
again, having been fitted with a new deep-depletion CCD.  Thanks go  
to Jim Gunn and Dale Sandford for coming to APO a second time to  
complete this upgrade.  Thanks for earlier efforts also go to Jeff  
Morgan, and to George Wallerstein for providing the funds to purchase  
the detector.

A complete characterization of this new detector is underway, but  
users are encouraged to begin science data taking right away.  There  
is one half-blocked column, but otherwise the cosmetics are very  
good.  The read noise is estimated at 4.2 electrons, comparable to
the old red chip. There is some indication that nonlinearity sets
in at around 30000 DN, a bit lower than with the previous detector.

Spectral fringing in the red has been reduced by a factor of 3 - 4  
(which is the main reason we changed to a deep-depletion device), and  
the reduced fringing kicks in now at a much redder wavelength than  
did the old chip.  The new chip is advertised to have better QE in  
the far red, and there are indications that we are seeing improved

As expected, the new device is more prone to cosmic-ray  
blemishes, and their removal in the reductions may be challenging-- 
the increased thickness of the substrate results in more multiple-pixel
cosmic-ray hits. We do believe, however, that the streaks can be dealt with  
effectively, and that exposure times in excess of 30 minutes are  
still practical. You may wish to consider splitting long exposures for
CR-removal; as long as the individual exposures are each background (not
readout noise) limited, there will not be a noise penalty.

As always, we are interested in hearing your comments on how this new  
detector works for your science programs.  We'd also like to hear any  
feedback regarding any elements of the DIS upgrades done this  
past year ( 3 new gratings, a new prism assembly in the blue channel for
increased UV response, new red CCD and field flattener), which are now 
essentially completed.  

Jon Holtzman
Suzanne Hawley
Bruce Gillespie

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