The ETC calculates counts (e–) and count rates (e–/s) for given source and background parameters and assumed characteristics of the detectors. Once these are entered, the ETC then outputs signal-to-noise (S/N) ratios achieved for a given exposure time, or the exposure times required to achieve a given S/N ratio. The ETC supports both direct-imaging and spectroscopic (grism) observations. Starting in 2016 (Cycle 24), the ETC supports spatial scanning for UVIS and IR imaging and IR spectroscopy (see WFC3 STAN Issue 22). A variety of circular and square extraction apertures are available in the ETC, allowing the user to select either a radius in arcseconds or a size in pixels. It is also possible to input a calibrated spectral-energy distribution (SED) of your source directly into the ETC. The ETC also outputs peak per-pixel count rates and total count rates, to aid in feasibility assessment. Warnings will appear if the source will saturate the detector, which would not only compromise CCD and IR observations, but might affect subsequent exposures with the IR channel. See Appendix D of the WFC3 Instrument Handbook (IHB) for more details.
The UVIS shutter induces a small amount of vibration into the WFC3 instrument optics (IHB Section 6.10.4). As a consequence, the PSF of exposures shorter than about 5 seconds is slightly degraded.
For the UVIS channel, the ETC uses a CCD full-well value of 63,000 e–, the minimum value for either CCD chip, to determine saturation; users wishing to strictly avoid this occurrence should allow a buffer of at least 10% below this. IHB Section 5.4.5 presents a discussion of saturation.
For the UVIS channel, to the extent possible, one should select Detector chip 2 for UV filter exposures and place the target on chip 2 (IHB Section 6.4.4) since chip 2 has better quantum efficiency than chip 1 (IHB Section 5.4.1). Note that the quad filters have quadrant-dependent passbands (WFC3 ISR 2008-17).
To mitigate CTE losses, observers can add background to UVIS exposures by using the post-flash option (IHB Section 6.9.2). See WFC3 IHB Section 9.6 for equations that show how the post-flash background affects the calculation of S/N and exposure time.
For the IR channel, when the number of frames is 1, the ETC assumes a complete timing sequence (NSAMP = 15). For smaller NSAMP, the read noise is greater (IHB Section 5.7.3). For most programs, NSAMP should be greater than 5, and ideally more if possible, to achieve the best readnoise and cosmic ray correction (IHB Section 7.10.3).
The He I airglow line at 10,830 Angstroms was added as a component of the IR sky background in Cycle 24. It only contributes to the background observed in the F105W and F110W filters (IHB Section 7.9.5) and the G102 and G141 grisms (IHB Section 8.5).