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Hubble Space Telescope
WFPC2 ETC Accuracy

In general, the accuracy is the same as the tables in Chapter 6 of the WFPC2 Handbook, or about 5 to 10%.

For narrow band filters and stellar sources, results can be less accurate (typically around 20% accuracy) if narrow stellar absorption features interact with the filter bandpasses.


IMPORTANT NOTE: The algorithm used for point source calculations assumes optimal weighting of pixels in the PSF during point source photometry (e.g. PSF fitting). In cases where the SNR is limited by background noise (i.e. readnoise, dark current noise, or noise from sky emission), simple aperture photometry can give much lower SNR values than predicted by ETC, especially if large apertures are used (i.e. with areas much larger than 10 WF pixels, or 20 PC pixels). For large apertures in the background noise limited case, aperture photometry will give an SNR of approximately:

SNR(aperture) = SNR(ETC optimal) * 0.17 / R   

for WFC, or 

SNR(aperture) = SNR(ETC optimal) * 0.11 / R   

for the PC, where SNR(ETC optimal) is the ETC result, and
R is the aperture radius in arcseconds.
