Testing the PT CCD Temperature Control

Date: Thu, 18 Jul 2002 11:06:32 -0600
From: Jon Brinkmann
Subject: PT CCD temperature
To: sdss-engineering

There has been some discussion as to problems controlling the
temperature of the PT CCD.  The CCD temperature primarily affects the z
quantum efficiency.  The PT CCD temperature is currently set to -90 C
and runs at -94 C.  I performed three experiments yesterday to find out
roughly how much the heater servo is used in this process.  The thought
was expressed that perhaps the mechanical connection between the
Cryotiger head and the CCD is removing too much heat, forcing the
heater servo to run full on and thus out of closed loop.

1)	Turn the heater full on by setting the set point to 0 C.  The heater
	doesn't have enough power to accomplish this, we'll find the 
	temperature of thermal equilibrium for the system.

                               Head CCD
Date     Time     Seconds      (K)  (C)
----     ----     -------      ---- ---
07/17/02 08:20:07 1026915601   93  -94
07/17/02 08:35:07 1026916501   93  -92
07/17/02 08:50:07 1026917401   93  -80
07/17/02 09:05:07 1026918301   93  -72
07/17/02 09:20:07 1026919201   93  -67
07/17/02 09:35:07 1026920101   93  -64
07/17/02 09:50:07 1026921001   94  -63
07/17/02 10:05:07 1026921901   94  -61
07/17/02 10:20:07 1026922801   94  -60
07/17/02 10:35:08 1026923702   94  -60
07/17/02 10:50:07 1026924601   94  -60
07/17/02 11:05:07 1026925501   95  -60

2)	Turn the heater completely off by setting the set point to -201 C.
	The Cryotiger will not cool the CCD to that temperature, but again,
	we'll find the temperature of thermal equilibrium for the system.

                               Head CCD
Date     Time     Seconds      (K)  (C)
----     ----     -------      ---  ---
07/17/02 11:20:03 1026926401   94  -73
07/17/02 11:35:07 1026927301   93  -86
07/17/02 11:50:08 1026928201   93 -100
07/17/02 12:05:07 1026929101   93 -110
07/17/02 12:20:07 1026930001   92 -116
07/17/02 12:35:07 1026930901   92 -119
07/17/02 12:50:08 1026931802   92 -121
07/17/02 13:05:07 1026932701   92 -123
07/17/02 13:20:07 1026933601   91 -124
07/17/02 13:35:07 1026934501   92 -124
07/17/02 13:50:07 1026935401   92 -124

3)	Adjust the running temperature to -90 C by settint the set point to -86 C.

0 TCHECK 

CAMERA 0 

LN2 TEMP
NOMINAL 77 K   MEASURED 93 K

CCD TEMP
DAC SET AT 110 =  -0.72 V = -86 C   MEASURED -90 C
 ok

So it appears that the heater is running at about 50% duty, i.e., half
way between -124 C and -60 C.  It and the servo loop are not overloaded.
It also appears that the -4 degree C offset is a constant in the servo
loop, not part of the physical CCD cooling connections.

Jon

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