The first figure for each imaging mode gives the integrated system throughput. This is the combination of the efficiencies of the detector and of the optical elements in the light path. The bottom section of the throughput figures includes wavelengths beyond the nominal passband of that mode, showing any red or blue “leak” on a log scale (dotted sections are manufacturer’s lab measurements). The throughput is defined as the number of detected counts per second per cm2 of telescope area relative to the incident flux in photons per cm
2 per second. For the CCD “counts” is the number of electrons detected. For the MAMA, “counts” is the number of valid events processed by the detector electronics after passing through the various pulse-shape and anti-coincidence filters. In both cases the detected counts obey Poisson statistics. The throughput includes all obscuration effects in the optical train (e.g., due to the HST secondary and due to the STIS CCD Lyot stops). The “effective area” of the mode can be computed from the throughput by multiplying by the physical area of the HST primary mirror (45238.93416 cm
2). This is shown on the ordinate label on the right side of each plot.
The table for each mode gives the throughput and the point source sensitivity as a function of wavelength. Throughput has the meaning described above. The imaging point source sensitivity Spλ has units of counts/s/Å per incident erg/cm
2/s/Å for the MAMAs, and e-/s/Å per incident erg/cm
2/s/Å for the CCDs. Counts and electrons refer to the total counts from the point source integrated over the PSF.
counts/s/pix per incident erg/cm2/s/Å/arcsec
2 for the MAMAs and e-/s/pix per incident erg/cm
2/s/Å/arcsec
2 for the CCDs.
Thus Spλ and
Sdλ are related through the relation:
where m is the plate-scale in arcsec per pixel. Here, we have assumed that the diffuse source has a uniform brightness over the area of interest.
The point source S/N figures are shown for average sky levels (i.e., the ‘average zodiacal+average earthshine’ background level used in the STIS ETC), and for sky levels during orbital night (i.e., average zodiacal + low earth). For the CCD the read noise has been computed assuming a number of readouts
NREAD = integer (
t / 1000 s), where
t is the exposure time, with a minimum
NREAD=2. That is, each exposure has a minimum
CR-SPLIT=2. Different line styles in the figures are used to indicate which source of noise dominates.
For the CCD, the dominant sources of sky background are zodiacal light and scattered earthshine. The LOW-SKY requirement can be used to ensure that these backgrounds are kept as low or lower than the rates assumed for these plots. If your source falls within the sky-dominated portion of the figures, you may want to consider imposing the
LOW-SKY requirement.
For the NUV-MAMA the sky background has different contributions which can dominate depending on the filter used and on whether the observation takes place on the day or night sides of the orbit. The FUV geocoronal lines dominate on the day side if
CLEAR,
F25SRF2, or a neutral-density filter are used due to the strength of those lines (see
Table 6.6) and to the significant sensitivity of the detector at FUV wavelengths (see
Figure 14.23 and
Figure 14.41). The NUV [O II] 2471 Å geocoronal line is the second most important contribution and dominates on the day side of the orbit if
F25QTZ is used. Zodiacal light provides the largest contribution for the rest of the filters on the day side of the orbit and for most of them on the night side, where geocoronal emission is greatly reduced. The dark current is larger than the sky background in all cases except for CLEAR observations in the day side of the orbit.
For the FUV-MAMA, the dominant source of background is geocoronal emission (see
Table 6.4 and
Table 6.5). The lines vary strongly from the day to night side of the orbit. For broad-band the contribution from the geocoronal lines can be minimized by using the
F25QTZ filter, or observing with the
DARKTIME special requirement.
We now give a sample S/N calculation using these plots. Consider a V=27 star of spectral class G2V, for which we want to obtain S/N = 20 with F28X50LP observing with the CCD. From
Table 14.1 we find a correction AB
ν= −0.21 to go from V magnitude to AB magnitude near the center of the
F28X50LP bandpass. We thus have V+AB
ν=26.79. We look at
Figure 14.8 and find this value on the horizontal axis and read up to find the curve that intersects the desired S/N. We find ~10,000 seconds are needed to reach S/N = 20 in conditions of low sky background.
(see 