See Table 3.1 for read noise and dark current characteristics of the detectors, including variations by amplifier and GAIN for the CCDs.
The background in counts/second/pixel for imaging observations can be computed as:
|
•
|
Iλ is the surface brightness of the sky background, in erg/second/cm 2/е /arcseconds 2.
|
|
•
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 is the point source sensitivity for the imaging mode. |
The image of the sky through a disperser is not uniform, since some wavelengths fall off the detector for regions of sky near the edge of the field of view (FOV). Since the ACS grism spectra are of order 200 pixels long, the regions of lower sky will be strips at the long and short wavelength edges of the FOV. The maximum width of the strips from where the signal starts to decline to the edge, where the signal is down by roughly 2x, is about half the total length of a spectrum of a point source, i.e., roughly 100 pixels in the case of a sky background with a continuum of wavelengths. In the case of the HRC, the sky for the dispersed mode will not have the low background strips, since the FOV is not masked to the detector size. These small strips of lower sky background in the SBC and the WFC are ignored in the following formulae. Furthermore in the SBC and the WFC, since the spectra do not lie along the direction of the anamorphic distortion, the plate scales of
and
above must be replaced by the plate scales
and
in the orthogonal spatial and dispersion directions, respectively. Interior to the strips, a point on the detector sees a region of sky over the full wavelength coverage of the disperser. Thus, for
spectroscopic observations:
For a monochromatic sky emission line at
like Lyman-α, which will dominate the background through the LiF prism:
Figure 9.1 and
Table 9.6 show “high” sky background intensity as a function of wavelength, identifying the separate components which contribute to the background. The “None” and “Average” values of the Earthshine contribution in the
ETC correspond, respectively, to 0 and 50% of the “High” values in
Figure 9.1 and
Table 9.6.
“Extremely high” Earthshine corresponds to twice the “High” value. For the zodiacal sky background, the values in Figure 9.1 and
Table 9.6 and the ETCs correspond to the high value of m
v = 22.1 from
Table 9.4, while the “low” and “average”
zodiacal light is scaled to m
v = 23.3 and 22.7, respectively.
In the ultraviolet, the background contains bright airglow lines, which vary from day to night and as a function of HST orbital position. The airglow lines may be the dominant sky contributions in the UV both for imaging-mode and spectroscopic observations. Away from the airglow lines, at wavelengths shortward of ~3000 е, the background is dominated by
zodiacal light, where the small area of sky that corresponds to a pixel of the high resolution
HST instrumentation usually produces a signal that is 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 9.4 gives the variation of the
zodiacal background as a function of ecliptic latitude and longitude. For a target near ecliptic coordinates of (50,0) or (-50,0), the
zodiacal light is relatively bright at m
v = 20.9, i.e. about 9 times the faintest values of m
v = 23.3. Deep imaging applications must carefully consider expected sky values!
On the other hand, Earthshine 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 9.2. This figure also shows the contribution of the Moon, which is typically much smaller than the
zodiacal contribution, for which the upper and lower limits are shown. For reference, the limb angle is approximately 24
° when the
HST is aligned toward its orbit pole (i.e., the center of the CVZ), this limb angle corresponds to “Extremely High” Earthshine. The Earthshine contribution shown in
Figure 9.1 and
Table 9.6 corresponds to a limb angle of 30
°, High Earthshine.
For observations taken longward of 3500 е, the Earthshine dominates the background at small (< 22°) limb angles. In fact, the background increases exponentially for limb angles < 22
°. The background near the bright limb can also vary by a factor of ~2 on timescales 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. Details of the sky background as it affects ACS, as well as STIS, are discussed by Shaw, et al. (
STIS ISR 98-21). The impact of Earthshine on ACS observations is discussed in
ACS ISR 03-05.
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
light is no more than 30% greater than the minimum possible
zodiacal light for the given sky position.
LOW-SKY in the Phase II scheduling also invokes the restriction that exposures will be taken only 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 use of
LOW-SKY must be requested and justified in the Phase I Proposal.
The ETC provides the user with the flexibility to separately adjust both the
zodiacal (Low, Average, High) and Earthshine (None, Average, High, Extremely High) sky background components in order to determine if planning for use of
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; e.g., as shown in
Table 9.4 the
zodiacal background minimum for an ecliptic target is m
v = 22.4 which is still 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 the four lines listed in Table 9.5. The brightest line is Lyman
−α at 1216 е. The strength of the Lyman-
α line varies between about 2 and ~30 kilo-Rayleighs (i.e., between 6.1x10
−14 and 6.1x10
−13 erg/second/cm
2/arcseconds
2 where 1 Rayleigh = 10
6 photons/second/cm
per 4
π steradian) depending on the position of
HST with respect to the day-night terminator and the position of the target relative to the Earth limb. The next strongest line is the OI line at 1304 е, which rarely exceeds 10% of Lyman-
α. The typical strength of the OI 1304 е line is about 1 kilo-Rayleighs (which corresponds to about 2.85x10
–14 erg/second/cm
2/arcseconds
2) on the daylight side and about 75 times fainter on the night side of the
HST orbit. OI 1356 е and OI 2471 е lines may appear in observations on the daylight side of the orbit, but these lines are ~10 times weaker than the OI 1304 е line. The width of the lines also vary with temperature, the line widths given in
Table 9.5 are representative values assuming a temperature of 2000
°K.
Except for the brightest objects (e.g., planets), a filter or prism mode which does not transmit at Lyman-α should be employed. To minimize geocoronal emission the special requirement
SHADOW can be requested. 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 zero.
SHADOW requirements must be included and justified in your Phase I proposal (see the
Call for Proposals).
and 
are the plate scales along orthogonal axes.
is the monochromatic intensity of a line at wavelength L in 
