System Throughput and SNR / Exposure Time Estimation
The sky background can contribute significant Poisson noise in broad and medium band filters, and must be taken into account during noise calculations. The actual sky brightness depends on the heliocentric ecliptic coordinates (latitude and longitude) in a manner summarized in Table 6.3. The appropriate ABn can be taken from Table 6.2. To convert mag arcsec-^2 to mag pixel-1 one needs to add 5 magnitudes (WFC) or 6.7 magnitudes (PC1). These values are actually lower limits on the effective sky-brightness that will be seen, because light from the bright Earth limb can scatter into the aperture.
If your observations are sky background limited, and signal-to-noise is a driver, consider the use of the special requirement LOW-SKY as described in the Call for Proposals or the Phase II Proposal Instructions. LOW-SKY has two effects:
For many targets LOW-SKY will have minimal impact on the observing efficiency. Note, however, that targets in the Continuous Viewing Zone (CVZ) cannot be observed if LOW-SKY is specified. See "Observing Faint Targets" on page 143 for more information.
- It causes the observation to be scheduled at the time of year when the zodiacal background light is within 30% of the minimum possible background value for the target, and
- It requires that the observation be made when the bright Earth limb is more than 40 degrees from the OTA axis, which greatly reduces scattered light.
Table 6.3: Sky Brightness (V mag arcsec^-2) as a Function
of Heliocentric Ecliptic Latitude and Longitude.
Another option for reducing the sky brightness, is the special requirement SHADOW, which forces the observation to be made when HST is in the Earth's shadow. This usually has a large negative impact on the observing efficiency, and is recommended only when attempting to avoid geocoronal lines when observing far-UV emission lines (e.g. Ly a and OI 1304Å). Moreover, it does not attempt to minimize zodiacal emission, which dominates at visible wavelengths.
Table 6.4 shows approximate sky count rates for the WFC and PC1 for filters with significant sky count rates. An average sky brightness of V=22.9 mag arcsec-2 is assumed. Filters not listed in the table have sky count rates below that of the dark current, so the sky contribution will generally be unimportant. Values for other filters or sky brightnesses can be computed from Table 6.2, Table 6.1, Table 6.3, and Equation 6.2.
Table 6.4: Sky Count Rate per Pixel (P_sky).