Detector Linearity and Noise

Both the response and the noise of the detectors have been characterized as function of signal strength. Tab. 5.1 summarizes the overall properties of the detectors.

For sensitivity calculations, the noise  per reset interval may be summarized as

with S the signal in V/s and dimensionless (happens to be equal to the reset time in seconds). The empirically determined noise parameters (read noise, in V/s) (dark current noise, in V/s), and (signal shot noise, V /s ) have been obtained by forcing a fit through this function, and are summarized in Tab. 5.2. For the SW grating and the FP detectors the function works well up to a sec. For the LW grating detectors the function does not work well in describing the dependency, and the resulting values are not necessarily directly to be associated with read noise and dark noise. Parameter values are chosen in such a way that they give the best description for the short reset intervals.

For Band 4, test measurements show consistently a very poor increase of SNR for sec. The parameters listed apply to = 1 sec and = 2 sec. For longer no improvement in noise should be expected, but the function still applies.

 

Table 5.1: Summary of SWS-FM detector properties for a reset time sec

detector	T	bias	minimum		int.	average response	min. system
			system noise
	(K)	(V)	( V/s)	(electrons)	( V/s)	( V/s/Jy)	(W/ )
			gain=16
Gr-InSb	3	-	0.7	35	0.32	3.0
Gr-Si:Ga	3	12	1.5	75	0.32	1.6
Gr-Si:As	4.5	3.0	3.0	150	0.40	1.5
Gr-Ge:Be	2	0.6	2.0	100	1.00	1.3
F-P-Si:Sb	10	1.1	1.1	70	0.40	0.007
F-P-Ge:Be	2	0.6	3.0	200	0.40	0.0037

Notes to Tab. 5.1:
All minimum noise and NEP's are conservative values for sec and gain=16 . 's include the transmissions of the instrument (i.e., directly relate to 's).
1) values listed for element 2; noise = 0.7 V/s for element 1
2) excess photon noise at V/s
3) measured at a signal power at which photon noise is negligible; except FP-Ge:Be where still some photon-noise is suspected; estimated photon-noise free = W/
4) int. is intercept: photon noise at signal = 10 V/s
5) 1 Jy = WHz , ISO telescope area taken as 0.28 m