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Wide Field Camera 3 Instrument Handbookfor Cycle 22 > Chapter 9: WFC3 Exposure-Time Calculation > 9.7 Sky Background

9.7
As noted in the previous section, the backgrounds from the sky and from the detector must be taken into account when calculating expected S/N ratios or exposure times.
Detector backgrounds (as well as read-out noise) for the CCDs and IR detector are discussed in Chapter 5. Background added to CCD exposures using the post-flash option is discussed in Section 6.9.2. This section deals with the sky backgrounds that can affect WFC3 observations.
The most important sources of sky background are:
The background in counts e s−1 pixel−1 for imaging observations can be computed as:
where:
Iλ is the surface brightness of the sky background, in
erg cm2 s−1 1 arcsec2.
is the point-source sensitivity for the imaging mode.
In the case of slitless spectroscopy, the image of the sky through a disperser is not uniform, because some wavelengths fall off the detector for regions of sky near the edge of the field of view. The regions of lower sky background will be strips at the long- and short-wavelength edges of the field of view; a UVIS grism spectrum is roughly 270 pixels long, while an IR grism spectrum is roughly 170 pixels long. The maximum width of the strips from where the signal starts to decline to the edge, where the signal is down by roughly a factor of 2, will be about half the total length of a spectrum of a point source, i.e., roughly 135 pixels (UVIS) or 85 pixels (IR), in the case of a sky background with a continuum of wavelengths. These small strips of lower sky background are ignored in the following formulae. Furthermore, 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 [O II] 2471, which will dominate the background through the UVIS/G280 grism:
where
The total sky background is:
Figure 9.1 and Table 9.3 show “high” sky background intensity as a function of wavelength, identifying the separate components which contribute to the background. The “shadow” and “average” values of the earth-shine contribution in the WFC3 Exposure Time Calculator (ETC) correspond, respectively, to 0% and 50% of the “high” values in Figure 9.1 and Table 9.3. For the zodiacal sky background, the values in Figure 9.1 and Table 9.3 correspond to the high value of V-band surface brightness of 22.1 mag arcsec−2 from Table 9.4, while the “low” and “average” zodiacal light is scaled to V surface brightnesses of 23.3 and 22.7 mag arcsec2, respectively.
In Table 9.3 we present the “high” sky-background numbers, which are plotted in Figure 9.1. See the text and the caption of Figure 9.1 for more details. These high sky values are defined as the earth-shine at 38 from the limb and the high zodiacal light of V = 22.1 mag arcsec−2.
Table 9.3: High Sky Backgrounds.
(erg cm2 s−1 −1 arcsec2)
(erg cm2 s−1 −1 arcsec2)
(erg cm2 s−1 −1 arcsec2)
Figure 9.1: Sky background intensity as a function of wavelength.The total sky spectrum (black) for the “high-background” case adopted in the WFC3 ETC, along with the individual contributions from zodiacal light and earth-shine. These data correspond to a V-band surface brightness of 22.1 mag arcsec2. The only significant geocoronal emission line in the WFC3 spectral range is [O II] 2471 , shown as a line; it has a flux of 1.51015 erg cm2 s−1 arcsec2, extending beyond the upper limit of the plot.
9.7.1 Zodiacal Light, Earth Shine, and LOW-SKY
Zodiacal light varies as a function of angular distance of the target from the Sun and from the ecliptic. Table 9.4 gives the variation of the zodiacal background as a function of heliocentric ecliptic longitude and ecliptic latitude in V-mag arcsec-2. As shown by this table, over most of the sky the zodiacal background is within 1 magnitude of the minimum value of 23.4 V-mag arcsec-2. It is greater than that at low ecliptic latitudes and at angles from the sun approaching the minimum permitted observing angle of 50 deg. At the lower ecliptic latitudes, it rises rapidly as this 50 deg limit is approached. For a target near heliocentric ecliptic coordinates (50,0) or (50,0), not shown in the table, the zodiacal light level is 20.9 V-mag arcsec2, i.e., about 10 times the faintest value.
Table 9.4: Approximate zodiacal sky background as a function of heliocentric ecliptic longitude and ecliptic latitude (in V-mag per arcsec2). SA stands for Solar Avoidance zone (HST pointing within 50 deg of the sun), where observations may not be made.
The zodiacal background level observed in WFC3/IR exposures is being analyzed as a function of ecliptic latitude and angular distance of the target from the sun (Pirzkal et al. in prep). The dependence derived for filter F160W is shown in Figure 9.2.
Figure 9.2: Zodiacal background level for filter F160W as a function of the target’s ecliptic latitude and angular distance from the sun.
For WFC3/IR programs where a low background level is critical to the science goals, observers with a target at low ecliptic latitude may want to increase the solar avoidance angle from the required minimum of 50 deg to 60 deg or greater by excluding the range of dates when the target would be at the undesired angles.
Earth-shine varies strongly depending on the angle between the target and the bright Earth limb. The variation of the earth-shine as a function of limb angle from the sunlit Earth is shown in Figure 9.3. The 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). The earth-shine contribution shown in Figure 9.1 and Table 9.3 corresponds to this position.
Figure 9.3: Background contributions in V magnitude per arcsec2 due to the zodiacal light, Moon, and 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.
For observations taken longward of 3500 , earth-shine 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 earth-shine also depends upon the reflectivity of the terrain and the amount of cloud cover over which HST passes during the course of an exposure.
Observations of the faintest objects may benefit from the use of the Phase II Special Requirement LOW-SKY, which should be requested and justified in the Phase I proposal. 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 target position. LOW-SKY also invokes the restriction that exposures will be taken only at angles greater than 40 from the bright Earth limb to minimize earth-shine and the UV airglow lines.
The disadvantages of using the LOW-SKY requirement are that it greatly reduces scheduling opportunities and limits visibility to about 48 minutes per orbit. The restriction on zodiacal light may be stricter than scientifically necessary, especially for targets at high ecliptic latitudes where the zodiacal background is relatively low, and for filters where other background components dominate the noise. For targets and filters where high zodiacal background could be a problem, observers may want to limit that background by placing their own (less stringent) restrictions on scheduling dates, as discussed above. Most WFC3/IR exposures with higher than average earth-shine experience the high background in only one or two frames of the timing sequence taken at one end of the orbit, so that selective data reduction can be a better option than accepting the shorter visibility period required by LOW-SKY. WFC3/UVIS observers should remember that CTE losses are greater for very low background levels, so that reducing the background below the recommended level (12 electrons/pixel for transfers the height of the CCD chip) can be counter-productive for the detection of faint sources (Section 6.9).
The ETC provides the user with the flexibility to adjust separately both the zodiacal and earth-shine sky background components in order to determine if planning for use of LOW-SKY is advisable for a given program. The RA, Dec and date option can be used to compute the zodiacal light for specific targets, which is especially useful for targets at low ecliptic latitudes where the zodiacal background increases rapidly as the angle between the target and the sun approaches the HST observable limit of 50 degrees. Note that the standard normalizations for zodiacal light and earth-shine (from none to high or extremely high) provide approximations that may not accurately predict the levels in individual exposures.
9.7.2 Geocoronal Emission, Airglow, and SHADOW
Background due to geocoronal emission originates mainly from hydrogen and oxygen atoms in the exosphere of the Earth. In the far-UV spectral region, the strongest geocoronal emission lines are Lyman−α at 1216 , O I at 1304 , and O I] at 1356 , but WFC3 is of course not sensitive at these wavelengths. The only significant UV geocoronal emission line to which WFC3 is sensitive is [O II] 2471 , shown in Figure 9.1. This line varies in intensity from day to night and as a function of HST orbital position. It can be the dominant sky contribution in the UV both for imaging and spectroscopic observations. In sunlight it can be as bright as ~1.510−15 erg cm2 s−1 arcsec2, while in Earth shadow it is much fainter, typically ~7.510−18 erg cm2 s−1 arcsec2.
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 re-acquisition), 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 earth-shine is set to zero. SHADOW requirements must be included and justified in your Phase I proposal (see the Call for Proposals).
Recent analysis of archival WFC3/IR grism and broad-band filter observations suggests that the IR channel is susceptible to a strong airglow line of He I at 10830 , perhaps related to auroral activity and seen even at high angles above the bright earth limb (see Section 7.9.5). Its relationship to geocoronal emission and the possible utility of SHADOW are currently under investigation. An alternative for imaging exposures is to consider using filters that exclude it.

Wide Field Camera 3 Instrument Handbookfor Cycle 22 > Chapter 9: WFC3 Exposure-Time Calculation > 9.7 Sky Background

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