Wide Field and Planetary Camera 2 Instrument Handbook for Cycle 14
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
- 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
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.
Filter QT d/ QTmax d/d p <> max me/sec twfsky
1All values have been computed using the WF3 chip, except for the Quad filters.
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