Table 6.1 shows the read noise and dark current characteristics of the detectors, taken from
Chapter 7.
In Figure 6.1 we plot the “high” sky background intensity as a function of wavelength, identifying the separate components which contribute to the background. The information in this figure is presented in tabular form in
Table 6.4. In the
ETCs and in this Handbook, the choices for earthshine of “shadow”, “average”, and “extremely high” correspond to 0, 50% of, and twice the “high” values in
Table 6.4. For the zodiacal sky background, the values in
Table 6.4 correspond to a high value of
mV = 22.1 arcsec
-2 from
Table 6.2, while the low and average zodiacal light are scaled to
mV = 23.3 arcsec
-2 and 22.7 arcsec
-2, respectively. The strength of the geocoronal (airglow) line emissions are as shown in
Table 6.5.
Earthshine, on the other hand, varies strongly depending on the angle between the target and the bright Earth limb. The variation of the earthshine as a function of limb angle from the sunlit Earth is shown in Figure 6.2. The figure also shows the contribution of the Moon which is typically much smaller, and the full range of the zodiacal contribution. For reference, the limb angle is approximately 24° when the HST is aligned toward its orbit pole (i.e., the center of the CVZ).
Table 6.3 contains the expected count rates from different sky backgrounds in various STIS modes, which you can use to determine whether your observations would be background limited.
Observations of the faintest objects may need the special requirement LOW-SKY in the Phase II observing program.
LOW-SKY observations are scheduled during the part of the year when the zodiacal background is no more than 30% greater than the minimum possible value for the given sky position.
LOW-SKY also invokes the restriction that exposures will be obtained at angles greater than 40° from the bright Earth limb to minimize earthshine and the UV airglow lines. The
LOW-SKY special requirement limits the times at which targets within 60
° of the ecliptic plane will schedule and limits visibility to about 48 minutes per orbit.
The ETC provides the user with the flexibility to separately adjust both the zodiacal (none, low, average, high) and earthshine (none, average, high, extremely high) sky background components in order to determine if
LOW-SKY is advisable for a given program. However, the absolute sky levels that can be specified in the
ETC may not be achievable for a given target. As shown in
Table 6.2, the minimum zodiacal background level for an ecliptic target is m
v = 22.4, which is brighter than both the low and average options with the
ETC. By contrast, a target near the ecliptic pole would always have a
zodiacal=low background in the
ETC. The user is cautioned to carefully consider sky levels as the backgrounds obtained in HST observations can cover significant ranges.
Background due to geocoronal emission originates mainly from hydrogen and oxygen atoms in the exosphere of the Earth. The emission is concentrated in a very few lines. The brightest line is Lyman-α at 1216 Å. The strength of the Lyman-
α line varies between about 2 and 20 kilo-Rayleighs (i.e., between 6.1
× 10
–14 and 6.0
× 10
–13 erg/s/cm
2/arcsec
2 where 1 Rayleigh = 10
6 photons/s/cm
2/[4
π steradians]) depending on the time of the observation and the position of the target relative to the Sun. The next strongest contribution is from the doublet [O I] 1302 + 1306 Å, which rarely exceeds 10% of Lyman-
α. The typical strength of the [O I] 1302 + 1306 Å doublet is about 2 kilo-Rayleighs (which corresponds to about 5.7
× 10
–14 erg/s/cm
2/s/arcsec
2) at the daylight side and about 150 times fainter on the night side of the HST orbit. [O I] 1356 Å and [O II] 2471 Å lines may appear in observations on the daylight side of the orbit, but these lines are at least 10 times weaker than the [O I] 1302 + 1306 Å line. The widths of the lines also vary. The line widths given in
Table 6.5 are representative values assuming a temperature of 2000 K.
It is possible to request that exposures be taken when HST is in the umbral shadow of the earth to minimize geocoronal emission (e.g., if you are observing weak lines at ~1216 or ~1304 Å) using the special requirement SHADOW. Exposures using this special requirement are limited to roughly 25 minutes per orbit, exclusive of the guide-star acquisition (or reacquisition) and can be scheduled only during a small percentage of the year.
SHADOW reduces the contribution from the geocoronal emission lines by roughly a factor of ten, while the continuum earthshine is set to 0. If you require
SHADOW, you should request it in your Phase I proposal (see the
Call for Proposals).
is the diffuse-source sensitivity for the grating mode.
is the diffuse-source sensitivity for the imaging mode;
In Figure 6.1 we plot the “high” sky background intensity as a function of wavelength, identifying the separate components which contribute to the background. The information in this figure is presented in tabular form in Table 6.4. In the ETCs and in this Handbook, the choices for earthshine of “shadow”, “average”, and “extremely high” correspond to 0, 50% of, and twice the “high” values in Table 6.4. For the zodiacal sky background, the values in Table 6.4 correspond to a high value of mV = 22.1 arcsec-2 from Table 6.2, while the low and average zodiacal light are scaled to mV = 23.3 arcsec-2 and 22.7 arcsec-2, respectively. The strength of the geocoronal (airglow) line emissions are as shown in Table 6.5.The zodiacal contribution corresponds to mv = 22.1 arcsec-2. The earthshine is for a target which is 38° from the limb of the sunlit Earth. The upper limit to the [OII] 2471 Å intensity is shown. Use Figure 6.2 to estimate background contributions at other angles. The geocoronal airglow line intensities are in erg/cm2/s/arcsec2.In the UV the background contains important contributions from airglow lines. These vary from day to night and as a function of HST orbital position. The airglow lines are an important consideration for imaging mode observations and can be for spectroscopic observations as well. Away from the airglow lines, at wavelengths shortward of ~3000 Å, the background is dominated by zodiacal light and is generally much lower than the intrinsic detector background. The contribution of zodiacal light does not vary dramatically with time and varies by only a factor of about three throughout most of the sky. Table 6.2 gives the variation of the zodiacal background as a function of helio ecliptic latitude and longitude. For a target near ecliptic coordinates of (50,0) or (-50,0), the zodiacal light is relatively bright at mv = 20.9 arcsec-2, i.e. about 9 times the polar value of mv = 23.3 arcsec-2.Earthshine, on the other hand, varies strongly depending on the angle between the target and the bright Earth limb. The variation of the earthshine as a function of limb angle from the sunlit Earth is shown in Figure 6.2. The figure also shows the contribution of the Moon which is typically much smaller, and the full range of the zodiacal contribution. For reference, the limb angle is approximately 24° when the HST is aligned toward its orbit pole (i.e., the center of the CVZ).The values are V magnitude per square arcsecond due to the moon and the sunlit Earth as a function of angle between the target and the limb of the bright Earth or moon. Zodiacal light levels range between mv=22.1 and 23.3 mag arcsec-2.For observations longward of 3500 Å, the earthshine always dominates the background at small (<22°) limb angles. In fact, the background increases exponentially for limb angles <22°. The background near the bright Earth limb can also vary by a factor of ~2 on time scales as short as two minutes, which suggests that the background from earthshine also depends upon the reflectivity of the terrain over which HST passes during the course of an exposure. The total background at limb angles greater than the bright-Earth avoidance angle of 20° appears to show no significant dependence on position within the small HST fields of view. Details of the sky background as it affects STIS are discussed by Shaw et al. (STIS ISR 1998-21) and Giavalisco et al. (WFC3 ISR 2002-12).Table 6.2: Approximate Zodiacal Sky Background in V magnitude arcsec-2 as a Function of Helioecliptic Coordinates
