8.4 Sensitivities and Exposure-Time Estimation

Table 8.2 lists current estimates of the detection limits for slitless spectroscopy with the UVIS and IR modes. The numbers in the table are the V magnitudes of point sources for which a 1-hour exposure yields a spectrum with a S/N of 5 per resolution element, with extraction boxes of 1 × 5 and 1 × 3 pixels for the UVIS and IR, respectively. V magnitudes are given for three different assumed source spectral types.

Table 8.2: V-band detection limits for the WFC3 grism modes for 1-hour exposure and 5σ detection.

Mode

Spectral Type

Wavelength
(nm)

O3 V

A0 V (Vega)

G2 V

UVIS2/G280

25.0

21.9

18.2

250

IR/G102

21.3

22.0

22.9

1050

IR/G141

20.5

21.3

22.8

1550


For more accurate estimations of exposure times, observers should use the WFC3 Exposure Time Calculator (ETC). The ETC supports all three spectroscopic modes.

For more detailed simulations of WFC3 slitless data, a simulator is available that can generate 2D images (dispersed as well as direct) of synthetic target fields. The simulator package, aXeSIM, is available via a Web interface (reduced functionality) and as a stand-alone IRAF package (full functionality). For further information, see: http://www.stsci.edu/hst/instrumentation/wfc3/documentation/grism-resources