6.1 System Throughput

A decision on a suitable exposure time will require the combination of

• The overall spectral response of the system ( Figure 2.4).
• The spectral transmission of the filters ( Chapter 3 and Appendix A).
• The spectral energy distribution and spatial profile of the target.
• The point response function and pixel size of the instrument
  ( Chapter 5).
• Criteria for specifying desirable charge levels.

When the transmissions of filters T() are combined with the overall system response Q(), we obtain detector quantum efficiency (DQE) plots (electrons-per-photon as a function of ) for each filter. These DQE plots link the output of the CCD to the photon flux at the input to an unobscured 2.4 m telescope.

These calibrations exist in the STScI Calibration Data Base, and are accessible with the STSDAS SYNPHOT package or with the XCAL software. The XCAL and SYNPHOT Users Guides should be consulted for further details.

The throughput calibration presented here is accurate to at least 10%-which is sufficient for planning observations, but not for the analysis of many programs. Investigators wishing to do photometry on WFPC2 images should refer to the HST Data Handbook for an explanation of the conventions used in determining WFPC2 zeropoints and should use the zeropoints given in Table 5.1 of the WFPC2 Data Handbook (Version 4.0, January 2002). For the most accurate and up-to-date calibrations, users should examine the on-line version of the Data Handbook to verify that no numbers of interest have changed since the last paper publication. A recent study has examined the issue of WFPC2 zeropoints (Heyer, et al. 2004, WFPC2 ISR 04-01) and is recommending using the zeropoints of Dolphin (2002, private communication) on his web site at http://www.noao.edu/staff/dolphin/wfpc2_calib/

In Table 6.1 the dimensionless efficiency and the mean wavelength for each filter are tabulated together with the effective width, the equivalent Gaussian dimensionless width, the maximum transmission, the derivative of the mean wavelength with respect to spectral index, the pivot wavelength, average wavelength, and wavelength of maximum transmission. The parameters are defined as follows. The dimensionless efficiency is

The mean wavelength is defined in Schneider, Gunn, and Hoessel (1993, ApJ 264, 337).

This rather unconventional definition has the property that the correspondingly defined mean frequency is just . It is in some sense halfway between the conventional frequency mean and the wavelength mean.

The pivot wavelength is defined as

The average wavelength is that defined in the simplest sense

The effective dimensionless Gaussian width is defined implicitly by

The effective width of the bandpass is

We note that all of the above integrals have been evaluated over the range to so as to avoid unrealistic contributions from imperfect blocking far from the bandpass. Where necessary, the integration range was further constrained to the range 1000Å to 11000Å.

Parameters and are the respective parameters at the peak throughput.

The parameter is defined in section Count Rates for Power Law Sources.

The final two columns in Table 6.1 are defined as follows. In the next-to-last column me/sec is the zero-point magnitude for 1 e^- s^-1 (with AB=0). The final column gives twfsky, which is the exposure time (in seconds) needed to make the sky noise equal to 5 e^- RMS (i.e. ~read noise) in the WFC for a sky level of V=23.3 mag arcsec^-2.

Table 6.1: System Efficiencies and Zeropoints. See Section 6.1 for definitions. 1

Filter	QT d/				QTmax	d/d	p	<>	max	me/sec	twfsky
F122M	0.00010	1305.6	239.1	0.0778	0.00107	7.90	1321.6	1326.2	1260	18.48	1.1E+07
F130LP	0.10175	4285.9	4755.4	0.4712	0.13936	951.53	5814.6	6137.5	6398	26.01	1.9E+02
F160BW	0.00024	1473.0	449.1	0.1295	0.00074	24.69	1521.9	1534.9	1400	19.46	9.2E+05
F165LP	0.10091	4494.4	4528.9	0.4279	0.14066	823.00	5852.9	6155.6	6400	26.00	1.9E+02
F170W	0.00057	1707.7	545.1	0.1355	0.00169	31.37	1769.7	1786.3	1857	20.38	2.3E+06
F185W	0.00038	1941.6	334.3	0.0731	0.00196	10.38	1962.3	1967.7	1940	19.93	7.3E+06
F218W	0.00059	2177.4	395.0	0.0770	0.00286	12.92	2203.1	2209.6	2242	20.42	5.4E+06
F255W	0.00080	2577.7	395.1	0.0651	0.00462	10.92	2599.4	2604.9	2536	20.76	2.6E+06
F300W	0.00571	2919.8	740.2	0.1077	0.01974	33.84	2986.8	3004.3	2804	22.89	6.7E+04
F336W	0.00497	3329.3	374.3	0.0477	0.03558	7.59	3344.4	3348.2	3454	22.74	3.6E+04
F343N	0.00003	3426.9	23.5	0.0029	0.00397	0.03	3426.9	3427.0	3432	17.27	4.7E+06
F375N	0.00008	3732.2	24.4	0.0028	0.00983	0.03	3732.3	3732.3	3736	18.24	1.1E+06
F380W	0.00779	3940.5	681.8	0.0735	0.03752	21.27	3982.7	3993.1	3999	23.22	7.1E+03
F390N	0.00031	3888.0	45.0	0.0049	0.01999	0.09	3888.2	3888.2	3889	19.72	2.1E+05
F410M	0.00183	4085.7	146.8	0.0153	0.04027	0.95	4087.6	4088.1	4097	21.65	2.7E+04
F437N	0.00022	4369.1	25.2	0.0025	0.03065	0.03	4369.2	4369.2	4368	19.37	1.7E+05
F439W	0.00576	4292.6	473.2	0.0468	0.03903	9.41	4311.3	4316.0	4318	22.90	7.1E+03
F450W	0.01678	4483.6	950.8	0.0901	0.08671	36.36	4555.4	4573.0	5069	24.06	2.0E+03
F467M	0.00250	4667.7	166.5	0.0151	0.05582	1.07	4669.8	4670.4	4731	21.99	1.2E+04
F469N	0.00027	4694.4	25.0	0.0023	0.03784	0.02	4694.4	4694.4	4698	19.56	1.1E+05
F487N	0.00034	4865.1	25.9	0.0023	0.04811	0.02	4865.2	4865.2	4864	19.81	8.1E+04
F502N	0.00041	5012.4	26.9	0.0023	0.05800	0.03	5012.4	5012.4	5009	20.04	5.9E+04
F547M	0.01342	5467.8	483.2	0.0375	0.11515	7.70	5483.3	5487.1	5558	23.81	1.6E+03
F555W	0.03012	5336.8	1228.4	0.0977	0.11194	50.99	5439.0	5464.6	5550	24.69	7.3E+02
F569W	0.02343	5582.3	965.7	0.0735	0.11518	30.13	5642.0	5657.4	5549	24.42	8.9E+02
F588N	0.00145	5893.2	49.0	0.0035	0.13078	0.07	5893.5	5893.5	5896	21.40	1.3E+04
F606W	0.04513	5860.1	1502.4	0.1089	0.14220	69.46	5996.8	6030.8	6185	25.13	4.2E+02
F622W	0.02882	6137.4	917.1	0.0635	0.14096	24.71	6186.2	6198.6	6405	24.64	6.3E+02
F631N	0.00084	6306.4	30.9	0.0021	0.12632	0.03	6306.4	6306.4	6301	20.81	2.1E+04
F656N	0.00049	6563.8	21.5	0.0014	0.11273	0.01	6563.8	6563.8	6562	20.21	3.5E+04
F658N	0.00068	6590.8	28.5	0.0018	0.11443	0.02	6590.8	6590.8	6591	20.58	2.5E+04
F673N	0.00113	6732.2	47.2	0.0030	0.11978	0.06	6732.3	6732.3	6730	21.12	1.4E+04
F675W	0.02344	6677.4	866.8	0.0551	0.13604	20.29	6717.4	6727.6	6624	24.42	7.0E+02
F702W	0.03429	6818.0	1384.7	0.0862	0.14185	50.71	6918.5	6944.3	6513	24.83	4.6E+02
F785LP	0.00900	8627.9	1381.2	0.0680	0.04831	39.88	8707.0	8727.5	8226	23.38	1.3E+03
F791W	0.01694	7811.2	1230.7	0.0669	0.09530	34.97	7880.6	7898.4	7397	24.07	7.6E+02
F814W	0.01949	7904.8	1539.4	0.0827	0.10343	54.06	8012.2	8040.3	7255	24.22	6.5E+02
F850LP	0.00473	9086.1	1037.5	0.0485	0.03939	21.37	9128.8	9139.8	8810	22.68	2.4E+03
F953N	0.00016	9544.7	52.5	0.0023	0.02213	0.05	9544.9	9545.0	9525	19.00	6.9E+04
F1042M	0.00017	10220.5	448.9	0.0187	0.00481	3.56	10227.6	10229.4	10110	19.10	6.0E+04
FQUVN-A	0.00033	3764.4	73.2	0.0083	0.01326	0.26	3764.5	3764.6	3801	19.78	2.5E+05
FQUVN-B	0.00030	3829.3	57.3	0.0064	0.01557	0.15	3829.5	3829.6	3828	19.68	2.4E+05
FQUVN-C	0.00037	3912.6	59.5	0.0065	0.01900	0.16	3912.9	3913.0	3909	19.92	1.7E+05
FQUVN-D	0.00047	3991.8	63.6	0.0068	0.02329	0.18	3992.2	3992.3	3989	20.17	1.2E+05
FQCH4N-A	0.00076	5435.3	34.4	0.0027	0.09537	0.04	5435.4	5435.4	5442	20.70	2.9E+04
FQCH4N15-B	0.00088	6199.2	33.8	0.0023	0.12242	0.03	6199.4	6199.4	6202	20.85	2.0E+04
FQCH4N33-B	0.00087	6199.3	33.8	0.0023	0.12165	0.03	6199.4	6199.4	6202	20.85	2.0E+04
FQCH4N-C	0.00070	7278.5	38.1	0.0022	0.10275	0.04	7278.5	7278.5	7278	20.60	2.1E+04
FQCH4N-D	0.00021	8930.2	54.9	0.0026	0.02917	0.06	8930.2	8930.2	8930	19.31	5.0E+04
POLQ_par	0.06695	4978.4	4226.0	0.3605	0.09998	646.91	6099.9	6355.5	6493	25.56	-
POLQ_per	0.01494	6257.6	5233.7	0.3552	0.04268	789.39	7613.6	7843.4	8001	23.93	-

1All values have been computed using the WF3 chip, except for the Quad filters.