STScI Logo

Hubble Space Telescope
WFPC-2 ETC Help

This is an Exposure Time Calculator (ETC) for the Wide Field and Planetary Camera onboard the NASA/ESA Hubble Space Telescope. The WFPC2 camera is described in detail in the Wide Field and Planetary Camera Instrument Handbook.

In its current form the ETC can be used to estimate the exposure time for point sources and extended sources of a given signal-to-noise ratio. Conversely, if the exposure time is specified, the expected signal-to-noise ratio for a point source or an extended source may be estimated.

The object spectrum may be characterised either by specifying stellar spectral type and visual magnitude, by specifying flux and spectral index for a power-law source, or by specifying a flux and wavelength for an emission line source.

Usage:

Most browsers retain the specified parameter values when you go back to the exposure specification page.

Also, note that the ETC fill-out forms contain many hi-lighted items; selecting any of these will give detailed help messages.

Batch Processing:

There are batch procedures which can conveniently execute many calculations. Users who need to do exposure time calculations in batches should contact John Biretta (biretta@stsci.edu).

Algorithms:

The stellar (and galaxy) source calculations are performed using Equation 6.1 in the WFPC2 Instrument Handbook. The specified spectral type is used to derive AB(nu) from Table 6.2, and the integral of the filter response is taken from Table 6.1.

For power-law sources, the calculation is similar except that V+AB(nu) is computed from the equation just below 6.1 in the WFPC2 Instrument Handbook. A small correction to the filter effective wavelength is also made using d(lambda-bar)/d(alpha) in Table 6.1.

For emission line sources the count rate calculation is different. The program merely converts the specified flux to a photon impact rate on a 2.4m diameter aperture. The plots in Appendix A.1 of the WFPC2 Instrument Handbook are then used to correct for the system+CCD+filter response.

Once the object count rate is determined, the noises due to readout, dark current, and sky background are estimated using information in WFPC2 Handbook Chapter 4, and Tables 6.4 and 6.5.

In the case of a point source, a correction is made for the fact that the PSF covers several pixels, and therefore several pixels of readout, dark, and background noise must be included (sharpness correction). Handbook Equation 6.5 (in modified form) and Table 6.5 are used for this correction. Note that this sharpness correction assumes optimal weighting of the PSF pixels during data analysis (e.g. PSF fitting). In general, simple aperture photometry will give lower SNR than computed by the ETC program.

For extended sources the calculations are performed per detector pixel, and there is no such correction to make.

Given the object counts and noises, the signal-to-noise is computed in a straight-forward way from the exposure time. In the reverse case, a quadratic equation is used to compute the exposure from the desired signal-to-noise. General discussion of these calculations can be found in the on-line guides (see references below) and in the WFPC2 Handbook.

Accuracy:

For stellar sources and power laws the accuracy is the same as the tables in Chapter 6 of the WFPC2 Handbook, or about 5 to 10%. For narrow band filters, results can be less accurate (typically around 20% accuracy) if narrow stellar absorption features interact with the filter bandpasses.

For spectral lines, both narrow lines (monochromatic assumption) and broad filters should have good accuracy (5 to 10%). If broad lines are observed with narrow filters, such that the filter throughput varies across the line, then the accuracy is reduced. We suggest using SYNPHOT in STSDAS to get better accuracy in situations with complex spectra/filter interactions.

Some effects are currently ignored. Cosmic-ray splitting is not included in the "Final Result," though there is detailed discussion of cosmic-ray splitting farther down the output page. For exposures where the noise is dominated by read out, the exposure splitting will significantly increase the noise (e.g. increase by root(2) for 2-way splitting). Information in the "intermediate results" output section can be used to judge the importance of the read out noise. Also, chip-to-chip differences in the read noise are ignored (6% effect). For the sky backgound, only a mean zodiacal contribution is used; sky background variations (up to ~40%) can occur if bright Earth light scatters into the telescope.

Current Restrictions:

Acknowledgements:

The WFPC2 ETC was written by Hans Martin Adorf at the ECF, and John Biretta (biretta@stsci.edu), at STScI. Initial WFPC2 E.T.C. algorithms were specified by Anatoly Suchkov, HTML files modified for use at STScI by Michael S. Wiggs.

References:

See general discussions in:

Exposure Time Estimation for WFPC2 Images of Stellar Targets

Exposure Time Estimation for WFPC2 Images of Extended Targets

WFPC2 Instrument Handbook


Please send comments about this page to biretta@stsci.edu.