1.2.2 Comparison of WFPC2 and NICMOS

Both WFPC2 and NICMOS are capable of imaging at wavelengths between ~8000Å and ~11,000Å. At longer wavelengths NICMOS must be used; at shorter wavelengths WFPC2, STIS, or ACS must be used. Table 1.3 compares the detective efficiency of WFPC2 and NICMOS in the wavelength region where both instruments overlap in capabilities. Count rates for a V=20 star of spectral class A0 are given for all filters at common wavelengths; the signal-to-noise (S/N) is also given for a 1 hour exposure of this same star. For bright continuum sources WFPC2 and NICMOS offer similar efficiency over the spectral range from 8800Å to 10,500Å; the choice of instrument will likely depend on other factors such as field size and details of the passband shape. However, for very faint sources, the lower read noise of WFPC2 (5e^- for WFPC2 vs. 30e^- for NICMOS) should prove advantageous.

Both instruments have polarimetry capability, but the WFPC2 polarizers are not viable above 8000Å; above this wavelength NICMOS must be used for polarimetry. We note that the ACS WFC is optimized for the far red and has polarimetric capability.

Table 1.3: Comparison of WFPC2 and NICMOS Count Rates for a V=20 A0 Star.

Instrument	Filter	Mean Wavelength (Å)	Effective Width (Å)	Count Rate (e- s-1)	SNR in 1 hour1
WFPC2	F785LP	9366	2095	14	215
	F791W	8006	1304	30	314
	F814W	8269	1758	33	333
	F850LP	9703	1670	7.1	150
	FQCH4N (Quad D)	8929	64	0.47	34, 292
	F953N	9546	52	0.21	19, 15 2
	F1042M	10,443	611	0.20	18, 15 2
	LRF3	8000 9000 9762	105 113 126	1.5 0.64 0.23	66 40 20
NICMOSe	F090M4	8970	1885	17.4	89
	F095N 4	9536	88	0.883	9.2
	F097N 4	9715	94	1.19	12
	F108N 4	10,816	94	1.17	9.9
	F110W (Camera 1)	11,022	5920	73	170
	F110W (Camera 2)	11,035	5915	83.7	290
	F110W (Camera 3)	11,035	5915	75.9	390

1WFPC2 SNR assuming two 1800s exposures for cosmic ray removal. NICMOS SNR for central pixel of PSF. 2Values given for WFC (0.10" pixels) and PC (0.046" pixels). 3LRF filter is continuously tunable from 3710Å to 9762Å. LRF field-of-view is 10"x10". 4These NICMOS filters are available only on Camera 1 which has 11"x11" field-of-view. 4e. The NICMOS ETC performs S/N calculations for the brightest pixel with the detector temperature at 77.1oK.