6.5 Detector and Sky Backgrounds

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.

6.5.1 Detector Backgrounds

Table 6.1 shows the read noise and dark current characteristics of the detectors, taken from Chapter 7. Note that the FUV MAMA detector background is highly variable and this should be taken into account when observing faint sources. See the discussion in sections 4.2.3 and 7.5.2 for additional details.

Table 6.1: Detector Backgrounds.

¹ To convert to counts/s/pix for CCDGAIN=4, divide by 4.016. 
² Varies widely with position on detector and time since high-voltage turn-on.

6.5.2 Sky Background

The sources of sky background which will affect STIS observations include:

• Earthshine (ES)
• Zodiacal light (ZL)
• Geocoronal emission (GC)

The continuum background in counts/s/pix for spectroscopic observations can be computed as:

$$//<![CDATA[ \begin{array}{l}\displaystyle B^{\lambda}_{sky}=\frac{I_\lambda \times S^d_\lambda}{G}~,\end{array} //]]>$$

where:

• I_λ is the surface brightness of the sky background, in ergs/s/cm²/Å/arcsec²;
• $//<![CDATA[ \begin{array}{l}S^d_\lambda\end{array} //]]>$ is the diffuse-source sensitivity for the grating mode.

The background in counts/s/pix for imaging observations can be computed as:

$$//<![CDATA[ \begin{array}{l}\displaystyle B^{\lambda}_{sky}=\frac{\int I_\lambda \times S^d_\lambda d_\lambda}{G}~,\end{array} //]]>$$

 where:

• I_λ is the surface brightness of the sky background, in ergs/s/cm²/Å/arcsec²;
• $//<![CDATA[ \begin{array}{l}S^d_\lambda\end{array} //]]>$ is the diffuse-source sensitivity for the imaging mode.
• The integral is over the bandpass.

In Figure 6.1 we plot the "high" zodiacal light and earthshine 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 m_V = 22.1 arcsec^–2 from Table 6.2, while the low and average zodiacal light are scaled to m_V = 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 “average” values of these are plotted in Figure 6.1.

Background Variations and LOW-SKY

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 m_v = 20.9 arcsec^–2, i.e., about 9 times the polar value of m_v =  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).

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, sourced from Leinert et al. (1998).  See the ETC documentation for finer sampling.

Note: A value of “SA” denotes positions in the solar avoidance zone.

 

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.

Table 6.3: Count Rates by Sky Background and STIS Mode.

¹ Zodiacal contribution is the same as in Figure 6.1 and Table 6.4 (m_V=22.1 ­arcsec^–2).
² Corresponds to HST pointing 24° from the limb of the sunlit Earth.
³ Corresponds to HST pointing around 38° from the limb of the sunlit Earth, where the earthshine is 50% of the "extremely high" value.
⁴ Corresponds to HST pointing around 50° from the limb of the sunlit Earth, where the earthshine is 25% of the "extremely high" value.
⁵ Earthshine for shadow is 0 in the continuum, while the UV geocoronal emission lines are reduced from the high to the low values in Table 6.5.

Geocoronal Emission and Shadow

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 ergs/s/cm²/arcsec² where 1 Rayleigh = 10⁶ photons/s/cm²/[4π 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 triplet O I 1302, 1304, 1306 Å, which rarely exceeds 10% of Lyman-α. The typical strength of the O I 1302 Å triplet is about 2 kilo-Rayleighs (which corresponds to about 5.7 × 10^–14 ergs/s/cm²/s/arcsec²) 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 Å lines. The widths of the lines also vary. The line widths given in Table 6.5 are representative values assuming a temperature of 2000 K.

The geocoronal emission lines are unresolved at the first-order resolutions of STIS but the emission fills the slit in the spatial dimension. A wider slit or slitless observing does not increase the background counts per pixel from geocoronal emission but does increase the area (range of wavelengths or pixels in the dispersion direction) over which that background is received. Observations with a slit which is n pixels wide in dispersion will be affected by geocoronal emission in a roughly n pixel region centered on the relevant geocoronal emission line wavelength. For slitless spectroscopy in the UV, the effects of geocoronal emission must be taken into account at all pixels, unless a longpass filter is employed to block off the short wavelength emission (see also Section 5.3.5 and Section 12.1).

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).

An alternate strategy for reducing the effects of geocoronal emissions is to use time resolved observations, so that any data badly affected by geocoronal emission can simply be excluded from the final co-addition. This can be done either by doing the observations in TIME-TAG mode or by just taking a series of short (~5 min) ACCUM mode exposures over the course of each orbit.

Observers should also remember that geocoronal lines will also produce weak absorption features in the spectra of extended targets.