CCD-world: Rockwell Hawaii question - stars abound !

[Barry Starr]

The following was posted to CCD-world:

Hi Peter,

We're currently building our second camera based on the Rockwell 1024 x
1024 HAWAII Array.  The first system used a modified SDSU I system and a
CFHT preamp I designed specifically for the HAWAII array.  Our second
system is using the SDSU II system again with the CFHT preamp.  In our
system the preamp provides the clock drivers (and FPGA pattern
generator) which in the case of the HAWAII array is very simple due to
the CMOS input levels. I insert series resistors with the clock driver
IC (this works with the FPA input capacitance) to rolloff the rise time
to an appropriate rate of change and limit the clock current spiking.   

We had an occurence similar to this initially that turned out to be a
digital data handling artifact in our system, I don't think that's what
your describing here. In order to test for this we have digital pattern
generation capability in all our SDSU Timing Board readout routines to
test communication links and data processing. This has been very
valuable I recommend it if your don't currently use it, can be very
helpful in debugging.  In our setup we have not (yet) noticed your
particular artifact. 

We have noticed some other interesting artifacts in the HAWAII array
specifically an exponential gradient on read following a reset that
according to Rockwell has been noted before in the HAWAII arrays.  I'm
curious to hear if others have experienced this anomaly.  It is a
fucntion of time and clocking.  Fortunately with the appropriate
clocking delay the CDS removes the gradient effect very well after the
first line. 

Back to the issue at hand,  Rockwell has told me of charge injection
issues regarding the clocking and in particular the reset lines of their
FPAs, this may be what's happening.
A few general questions, sorry if any appear obvious.  Have you looked
at the indiviual images and seen where the effect is occuring? First
read? Second read? You mentioned that multiple reads increase the
occurence?  Can you store multiple reads as individual images?  If you
can, does the artifact stay in the subsequent images once it occurs?  In
other words is it coupling to the photosite or is it coupling into your
readout? Is there any commonality among or between quadrants? or is
there any evident spatial pattern to the events?

Possible causes I'd look in three areas: 
1) clock patterns
2) clock rates (particularly rise/fall times) 
3) bias and grounds

Regarding clock patterns with your reset read int read pattern (CDS),
Rockwell recommends pulsing the reset line at the beginning of each line
on the array, cycling through the entire array reset cycle without
reading, then they read (with reset inactive) the array storing this as
image 1, then integrate, followed by the second read image 2, final
image = image2 - image 1.  Is this your clocking pattern?  

Regarding clock rates, What clock rates are you using and what rise/fall
times on your clocks?  Does the anomaly vary when you slow the clocks or
in particular roll off the rise times? I'd recommend rolling them off as
much as your clock rates will allow (within reason of course).  I'd
suspect the rise/fall times first.

And finally on the bias and ground lines, are your connections low
impedance, some times this is difficult in dewar using constantin or
maganin wire.  Can you monitor these lines when your camera is warm and
open and see if they are stable and glitch free?  


Hope some of this helps, good luck on the fix, please let us know what
you find.

Barry

Peter Moore wrote:
> 
> The following was posted to CCD-world:
> 
> Dear Colleagues,
> 
> I am currently characterising a Rockwell 1024x1024 MCT array for use in
> our NIR imager INGRID due to be commissioned soon (to soon :-).
> (see http://www.ing.iac.es/IR/INGRID/ingrid1_home.htm)
> 
> I am experiencing problems of high noise due to very many hot pixels that
> are spatially stable. I am using a basic cds (reset - read - integrate -
> read) scheme running on a sdsu controller for readout. The number of hot
> events depends on some threshold that seems to be linked with the number
> of reads performed (more reads, more stars). The amplitude is significant
> (6K - 20K ADU at Vrst 0.55v) and can be varied using the reset voltage. If
> I hold the reset on while reading there are no events, if I switch the
> reset clock off, the events reach 60K ADU. I'm using offboard FET source
> followers with the onboard biasgate FET set to 3.4v and the biaspower at
> 5.2v. Holding the reset gate on and moving Vrst to 0v also surpesses the
> events.
> 
> Peeking between the events the noise floor is good at around 12e- rms.
> Response and linearity are good to about 100K e- at 4e-/ADU gain.
> 
> I'm thinking it has something to do with the current flowing through the
> horizontal mux switch but what to do about it is driving me nuts :-)
> 
> Any help on this problem would be very much appreciated !
> 
> Best regards to all of you,
>         Peter.
> 
> =+===+===+===+===+===+===+===+===+ ING +===+===+===+===+===+===+===+=
>  =     Peter Moore, Isaac Newton Group, La Palma Observatory, Spain.     =
> =   E-mail pcm at ing.iac.es                  Voice Office +34 922 405566    =
> =   S-mail Apartado de Correo 321,           Fax Office +34 922 405646    =
> =          38780 Santa Cruz de La Palma,     Voice Home +34 922 435042    =
>  =         Canary Islands, Spain.                                        =
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> 
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-- 
_____________________________________________________________________________

Barry Michael Starr                                    
starr at cfht.hawaii.edu
Canada France Hawaii Telescope                          Ph: 808-885-3139
PO BOX 1793  Kamuela, HI 96743                          Fax:
808-885-7288
_____________________________________________________________________________
- -- CCD-world -- --
CCD-world is fully moderated. Send posts to CCD-world at astro.ku.dk
Standard replies will go to the list; address personal replies manually.
For more information, please go to:  http://www.not.iac.es/CCD-world/