Column COOLING

As the stability of zero-points and transformation coefficients depends partly on the detector temperature, information must be given on the detector cooling arrangements. This is stored in a character column called COOLING, which may contain the strings 'REGULATED' if an active closed-loop cooling system is used (either thermoelectric or some other servo-controlled cooling); 'UNREGULATED' if the tube is cooled in some way, but not regulated; 'ICE' if ordinary (water) ice is used as coolant; 'DRYICE' if dry ice (solid carbon dioxide) is used without a heat-transfer fluid; 'MEASURED' if the temperature is measured but not regulated; or 'NONE' if the PMT runs at ambient temperature.

One should be careful not to push temperature-regulated systems beyond their capabilities. A servo-controlled system that tries to cool all the time and never oscillates about its set point is, in fact, unregulated rather than regulated. As long as dark current is well below sky measurements, dark noise probably adds little to the photon noise from the sky, and additional cooling is unnecessary. If the dark current is kept a little above the minimum possible value, so that it remains in the strongly temperature-dependent regime, it serves as a useful indicator of tube temperature and health.

If dry ice is used with a low-viscosity heat-transfer liquid such as ethyl acetate or Freon-11, COOLING should be set to 'REGULATED'; but if dry ice is used with a viscous fluid such as alcohol, 'DRYICE' should be used. Note: Freon-11 is one of the CFCs most destructive to the ozone layer, and should be avoided.

If COOLING='REGULATED', a Real*4 column named DETTEMP should contain the estimated detector temperature in kelvins. With dry ice and ethyl acetate, the value in DETTEMP should be 195, corresponding to $-78^{\circ}$C.

If COOLING='MEASURED', detector temperatures should be part of the regular data stream. In this case, a Real*4 data column DETTEMP should be in the data files, rather than in the instrumental ``.tbl'' file. (Cf. the similar usage of FILTTEMP, and further details in the next section.)

 

Table: Detector columns of the instrument table file

Column Label	Contents	Variable Type	Format	When used
DET	detector type	C*8 string	A8	always
DETNAME	detector name	C*12 string	A12	optional
NDET	detector number	I integer	I4	always
DETCODE	detector code	C*8 string	A8	if dets. are coded
SNUMBER	PMT S-number	C*9 string	A9	DET= PMT
MODE	PMT mode (PC/DC/CI)	C*9 string	A9	DET= PMT
DEADTYPE	PMT deadtime type	C*12 string	A12	DET= PMT
DEADTIME	PMT deadtime (sec)	R*4 real	E8.3	DET= PMT
DEADTIMEERROR	PMT deadtime error	R*4 real	E13.2	DET= PMT
COOLING	detector cooling	C*12 string	A12	always
DETTEMP	detector temperature	R*4 real	F8.1	COOLING= REGULATED
SPECTRESPTBL	actual spectral response	C*(*) string	A	(see section I.5.5)
BLUERESP	CCD blue response class	C*8 string	A8	DET= SILICON