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ACS Instrument Handbook
ACS Insturment Handbook for Cycle 24
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Advanced Camera for Surveys Instrument Handbook for Cycle 24 > Chapter 9: Exposure-Time Calculations > 9.4 Detector and Sky Backgrounds

9.4
When calculating expected signal-to-noise ratios or exposure times, the background from the sky, and the background from the detector must be taken into account.
9.4.1 Detector Backgrounds
See Table 3.1 for read noise and dark current characteristics of the detectors, including variations by amplifier and GAIN for the CCDs.
9.4.2 Sky Background
The sources of sky background which will affect ACS observations include:
The background in counts/second/pixel for imaging observations can be computed as:
Where:
Iλ is the surface brightness of the sky background, in
erg/second/cm2/е/arcseconds2.
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:
where
The total sky background is:
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 mv = 22.1 from Table 9.4, while the “low” and “average” zodiacal light is scaled to mv = 23.3 and 22.7, respectively.
Figure 9.1: High sky background intensity as a function of wavelength.
The zodiacal contribution (ZL) is at ecliptic latitude and longitude of 30°,180°, and corresponds to mv = 22.1 per square arcseconds. The Earthshine (ES) is for a target which is 38° from the limb of the sunlit Earth. Use Figure 9.2 to estimate background contributions at other angles. The daytime geocoronal line intensities are in erg/cm2/ second/arcseconds2 (see Table 9.5).
Background Variations and LOW-SKY
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= 20.9, i.e. about 9 times the faintest values of m= 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.
Figure 9.2: Background contributions in V magnitude per arcseconds2
Background due to the zodiacal light, Moon, and the sunlit Earth, as a function of angle between the target and the limb of the Earth or Moon. The two zodiacal light lines show the extremes of possible values.
Table 9.4: Approximate zodiacal sky background as a function of ecliptic latitude and ecliptic longitude. (In V magnitudes per square arcseconds.)
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 mv = 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 signific