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 background in counts e− s
−1 pixel
−1 for imaging observations
can be computed as:
|
•
|
Iλ is the surface brightness of the sky background, in erg cm −2 s −1 Å −1 arcsec −2.
|
|
•
|
 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:
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 arcsec
−2, 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.
In the ultraviolet, the background contains a bright airglow line at 2471 Å, which varies in intensity from day to night and as a function of HST orbital position. The airglow line may be the dominant sky contribution in the UV both for imaging and spectroscopic observations. Away from the airglow line, at wavelengths shortward of ~3000 Å, 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, but does vary by about a factor of about three throughout most of the sky as a function of distance from the Sun and ecliptic.
Table 9.4 gives the variation of the zodiacal background as a function of ecliptic latitude and longitude relative to the Sun. For a target near ecliptic coordinates (50,0) or (
−50,0), the zodiacal light is relatively bright at 20.9 mag arcsec
−2, i.e., about 9 times the faintest value of 23.3 mag arcsec
−2. Thus if you are considering deep imaging applications, you must carefully consider expected sky values.
On the other hand, 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 9.2. 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.
For observations taken longward of 3500 Å, 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 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 earth-shine 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 adjust separately both the zodiacal (low, average, high) and earth-shine (shadow, average, 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 brightest zodiacal background for an ecliptic target is 21.3 V mag/arcsec
2,
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. In the far-UV spectral region, the strongest geocoronal emission lines are Lyman−α at 1216 Å, O I at 1304 Å, and O I] at 1356 Å, but WFC3 is of course not sensitive at these wavelengths. The only significant geocoronal emission line to which WFC3 is sensitive is [O II] 2471 Å, shown in
Figure 9.1. In sunlight this line can be as bright as ~1.5
×10−15 erg cm
−2 s
−1 arcsec
−2, while in Earth shadow it is much fainter, typically ~7.5
×10−18 erg cm
−2 s
−1 arcsec
−2.
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).
and
are the plate scales in arcsec pixel−1 along orthogonal axes.
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
In the ultraviolet, the background contains a bright airglow line at 2471 Å, which varies in intensity from day to night and as a function of HST orbital position. The airglow line may be the dominant sky contribution in the UV both for imaging and spectroscopic observations. Away from the airglow line, at wavelengths shortward of ~3000 Å, 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, but does vary by about a factor of about three throughout most of the sky as a function of distance from the Sun and ecliptic. Table 9.4 gives the variation of the zodiacal background as a function of ecliptic latitude and longitude relative to the Sun. For a target near ecliptic coordinates (50,0) or (−50,0), the zodiacal light is relatively bright at 20.9 mag arcsec−2, i.e., about 9 times the faintest value of 23.3 mag arcsec−2. Thus if you are considering deep imaging applications, you must carefully consider expected sky values.On the other hand, 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 9.2. 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 9.2: 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 Å, 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.Table 9.4: Approximate zodiacal sky background as a function of ecliptic latitude and ecliptic longitude (in V-mag per arcsec2). SA stands for Solar Avoidance zone, in which HST observations may not be made.
is the monochromatic intensity of a line at wavelength L in 
