The Exposure Time Calculators (ETCs) are online tools that predict the count rates and S/N in various observing modes given specified source characteristics.
The UVIS near UV (200-260 nm) throughput measurements during obtained System Level Thermal Vacuum testing has a significant calibration uncertainty (about 30%). It is possible, but not certain, that this uncertainty will be reduced by recalibration of the test equipment during July. The ETC reflects a more conservative (lower) estimate of the UV throughput. Observers with bright sources that would exceed 60% of the detector full well are requested to indicate such cases via comment.
Observers are reminded that the IR detector has significant image persistence (Handbook Sections 7.9.4 and D.2). While no restrictions are being placed on the observation of bright sources with the IR channel, observers should (1) be aware that subsequent images may be impacted by residual images from saturated sources, (2) indicate via comments to STScI those exposures which will badly (>10^7 electrons/pixel) saturate, understand that some risk is being accepted in proceeding without restrictions.
The throughput of the F775W filter was incorrectly measured at the component level. The ETC has been corrected to reflect its measured performance during system level testing. This increases the throughput of this filter by ~15% and places it in family with the other broad band filters of similar construction. See ISR WFC3 2008-08.
The UVIS shutter induces a small amount of vibration into the WFC3 instrument optics. This occurs typically for about 200 msec following the opening of the shutter. As a consequence, the PSF of exposures shorter than about 10 seconds is slightly degraded. Total flux is well conserved but the energy in the peak pixel is decreased by 5 to 15 percent in 1 second exposures. Observers obtaining PSF reference stars are strongly encouraged to select sources permitting exposure times greater than 10 seconds. It is difficult to predict with absolute certainly the amplitude of this effect in zero-G so programs dependent upon the exact structure and repeatibility of the PSF in short (<10 second) exposure should so indicate via comments. An ISR will be forthcoming.
As of May 30, 2008, we have updated the ETCs to reflect our preliminary analysis of recent ground tests. Those analyses are still underway, and thus there may be future changes. The changes are documented in WFC3 ISR 2008-11.
The UVIS filter transmissions were measured at a temperature of 20C but will be operated in flight at 0C. On Feb. 20, we updated the transmission curves of 18 narrow-band filters to account for this temperature shift, moving their edges approximately 0.1 nm to the blue.
The spectroscopic ETCs for WFC3 were built upon the ACS spectroscopic ETC, which does not convolve the input spectrum with a point spread function. In contrast, the NICMOS spectroscopic ETC does employ this convolution. There are two pitfalls here. First, a user can obtain an overly optimistic estimate of the signal-to-noise ratio in an emission line if an extremely narrow line width is specified in the input spectrum. Second, comparison of the results from the ETCs for these instruments will not necessarily be realistic. Emission lines can be simulated via a template, a user-supplied spectrum, or the emission lines section of the ETC input page. It is possible via these methods for a user to specify a line that is more narrow than a pixel. For the ACS and WFC3 ETCs, the result will be a simulation with most or all of the flux confined to a single pixel, whereas in reality the emission line would span multiple pixels. The WFC3 spectroscopic ETCs will incorporate a PSF convolution in a future release, but for now, this problem can be mitigated by specifying an appropriately broad emission line. The approximate instrumental resolution in each of the grisms is 1 pixel (25 Angstroms) for the IR/G102, 1.2 pixels (56 Angstroms) for the IR/G141, and 1.9 pixels (27 Angstroms) for the UVIS/G280. These are the widths that should be specified for an infinitesimally narrow line; for lines with a significant intrinsic width, the width specified in the ETC should be the quadrature sum of the intrinsic and instrumental widths. E.g., if a line has an intrinsic width of 20 Angstroms and the observer is using IR/G102, the line width should be specified as sqrt(400+625)=32 Angstroms.
The "extended source" option in the ETCs is intended for objects at least 2 pixels across.
We see platform-dependent changes in the ETC output due to changes in the way floating-point calculations are handled. These changes are generally on the order of a few percent but have been as high as 10% in our testing, with the large changes occurring in calculations involving narrow-band filters.