6.4 Sky Background

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 AB 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:

• It causes the observation to be scheduled at the time of year when the zodiacal background light is no more than 30% greater than 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° from the OTA axis, which greatly reduces scattered light.

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 for more information.

Table 6.3: Sky Brightness (V mag arcsec-2) as a Function of Heliocentric Ecliptic Latitude and Longitude. "SA" denotes that the target is unobservable due to solar avoidance.

Heliocentric Ecliptic Longitude	Ecliptic Latitude
	0°	15°	30°	45°	60°	75°	90°
180°	22.1	22.4	22.7	23.0	23.2	23.4	23.3
165°	22.3	22.5	22.8	23.0	23.2	23.4	23.3
150°	22.4	22.6	22.9	23.1	23.3	23.4	23.3
135°	22.4	22.6	22.9	23.2	23.3	23.4	23.3
120°	22.4	22.6	22.9	23.2	23.3	23.3	23.3
105°	22.2	22.5	22.9	23.1	23.3	23.3	23.3
90°	22.0	22.3	22.7	23.0	23.2	23.3	23.3
75°	21.7	22.2	22.6	22.9	23.1	23.2	23.3
60°	21.3	21.9	22.4	22.7	23.0	23.2	23.3
45°	SA	SA	22.1	22.5	22.9	23.1	23.3
30°	SA	SA	SA	22.3	22.7	23.1	23.3
15°	SA	SA	SA	SA	22.6	23.0	23.3
0°	SA	SA	SA	SA	22.6	23.0	23.3

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 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 (Psky). An average sky brightness of V = 22.9 mag arcsec-2 is assumed. Filters not listed have sky rate significantly below the dark current.

Filter	Sky Count Rate (Psky) (e- s-1 pixel-1)
	WFC	PC1
F336W	0.0009	0.0002
F380W	0.005	0.001
F439W	0.005	0.0011
F450W	0.018	0.004
F467M	0.003	0.0006
F547M	0.021	0.0045
F555W	0.052	0.010
F588N	0.002	0.0006
F569W	0.040	0.0081
F606W	0.090	0.020
F622W	0.060	0.012
F673N	0.002	0.0006
F675W	0.056	0.012
F702W	0.082	0.0016
F785LP	0.024	0.0050
F791W	0.048	0.010
F814W	0.054	0.011
F850LP	0.012	0.0024