The CCD that wouldn't

[Tim Abbott]

Hello all,

	At CFHT, we're currently seeing a range of bizarre behaviour from
our newest CCD and I thought I might pass it around a bit to see if
anyone has any insight on what is going on.

	This is a 2048^2 Orbit CCD, thinned by Mike Lesser.  There was
a problem with the initial AR coat and it was stripped and recoated before
being shipped.  Mike was apparently able to pin it successfully.

	The CCD was initially run at CFHT in February this year and
a successful, but uncalibrated, pin appeared to degrade over the course of
about a week.  The flat-field had the general appearance of horizontal
and vertical "brushstrokes".  The device was then shelved for a few 
months while we brought a new (Leach) controller on-line.

	We are now trying to bring the CCD on-line again to become 
our workhorse device.  Unfortunately, we are having a hard time 
acheiving a uniform pin.  We are using a Cadmium lamp as UV source.

These are the observed phenomena:

1) After a 45 min UV flood in medical O2 atmosphere, there is a large amount
of structure in the flat field, typical of an unpinned or poorly pinned
thin CCD, but not like the data from February.  This flat field appeared 
stable over the course of a week.  (a sample flat-field can be seen in 
http://www.cfht.hawaii.edu/~tmca/orbit1.html, click on "image 1").

2) We were unable to remove this partial pinning - even after the
CCD was kept warm (but in vacuum) for a week and after soaking in
H2 gas for 1.5 hrs, it was still only slightly degraded.

3) Since the flat field had the appearance that one might expect from
a contaminant deposited by a condensation event on loss of vacuum, we
washed the CCD in Isopropyl and rinsed with ethanol.  This had no significant
effect.  (The same patterns are visible, but only very faintly, in the
data from February).  The dewar was thoroughly cleaned at the same time.

4) After a 3 hour UV flood, the flat-field was significantly smoother with
only about 1/3 of the imaging area compromised.  However, there was significant
(1-0.1 electrons/pixel/sec) dark current which took several hours
to dissipate.

4) The same dark current reappeared after the control system was power cycled.
The initial form of the dark current was donut-shaped - ie. a bright ring
centered on the CCD with a dark center and corners (possibly representing
the temperature profile of the CCD?).  As it decayed, over the course of 5 
hours, it did so non-uniformly, with a bright blob appearing at upper right
after about 2 hours, which had disappeared within an hour.

5) A small dark current appeared when the system was turned off for ~6 minutes,
prompting the hypothesis that it is dependent on the system being powered
down.

6) Therefore, we attempted a further 3-hr UV flood, this time with the control
system turned on and idling such that the CCD was continually flushed.  This
appeared to severely degrade the pinning, but not to remove it entirely.

7) On the theory that cycling the CCD clocks during the warmup and UV
flood caused the loss of pin, we tried another UV flood, for 4 hours 
with the system turned off.  This appeared to make little or no difference. 
(a sample flat-field can be seen in 
http://www.cfht.hawaii.edu/~tmca/orbit1.html, click on "image 2").
Could we have "overpinned" the CCD...?

So our questions are:

Does any of this sound familiar to others with experience of pinning CCDS?

Does anyone have any suggestions of how we might pin the CCD?

The dark current that appears on cycling system power is completely
novel to us.  Might anyone be able to suggest a mechanism?

Finally, is "overpinning" a realistic concept and how might it fit in
with the current theories of the pinning mechanism?

Many thanks for any replies,

	Tim Abbott
	On behalf of the CFHT detector group.

-- 
      Timothy M. C. Abbott, Resident Astronomer, tmca at cfht.hawaii.edu
                http://www.cfht.hawaii.edu/~tmca/tmca.html
      Canada-France-Hawaii Telescope, P.O. Box 1597, Kamuela, HI 96743