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.
Table 6.2 shows the read-noise and dark-current characteristics of the detectors, taken from Chapter 7.
| Detector |
CCD |
NUV-MAMA |
FUV-MAMA |
|---|---|---|---|
| Read noise (electrons pix-1) |
4.2 (for CCDGAIN=1)7.6 (for CCDGAIN=4)1 |
0 |
0 |
| Dark current (electrons sec-1 pix-1) |
2.5 x 10-3 |
~2.0 x 10-3 |
~3.0 x 10-5 |
| 1 To convert to counts sec-1 pix-1 for CCDGAIN=4, divide by 4.039. |
The sources of sky background which will affect STIS observations include:
The continuum background in counts sec-1 pixel-1 for spectroscopic observations can be computed as:
is the surface brightness of the sky background, in erg sec-1 cm-2Å-1 arcsec-2.
is the diffuse-source sensitivity for the grating mode.
The background in counts sec-1 pixel-1 for imaging observations can be computed as:
is the surface brightness of the sky background, in erg sec-1 cm-2 Å-1 arcsec-2.
is the diffuse-source sensitivity for the imaging mode.
In Figure 6.1 we plot high sky 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 6.6. In the Exposure Time Calculator (ETC) and in this Handbook, the choices for earthshine of ``shadow'' and ``average'' correspond to 0 and 50% of these ``high'' sky background values in Table 6.6. For the zodiacal sky background, the values in Table 6.6 correspond to the typical value of mV = 22.7 from Table 6.3, while the low and high zodiacal light are scaled to mV = 23.3 and 22.1, respectively. The strength of the geocoronal line emissions are as shown in Table 6.7.
Figure 6.1: High Sky Background Intensity as a Function of Wavelength. The zodiacal contribution corresponds to a helio-ecliptic latitude and longitude of 30 and 180 degrees, respectively, which corresponds to mv = 22.7 per square arcsec. The Earthshine is for a target which is 24 degrees from the limb of the sunlit Earth. The upper limit to the [OI] 2471 intensity is shown. Use Figure 6.2 to estimate background contributions at other angles. The geocoronal day glow line intensities are in erg cm-2 s-1 arcsec-2..
In the ultraviolet, the background contains important contributions from air glow 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 air glow lines, at wavelengths shortward of ~3000 A, 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 6.3 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 mv = 20.9, i.e. about 9 times the faintest values of mv = 23.3.
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. In Figure 6.2, limits on the zodiacal light contribution are also given. 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 Earthshine contribution given in Table 6.6 and Figure 6.1 corresponds to this position.
Figure 6.2: Background Contributions in V Magnitude per square arcsec due to the Moon and the Sunlit Earth as a Function of Angle Between the Target and the Limb of the Earth or Moon
For observations taken 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 98-21).
Table 6.4 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.
| Mode |
counts sec-1 pix-1 |
|||
|---|---|---|---|---|
| Zodiacal1 |
Earthshine |
|||
| High (day)2 |
Typical3 |
Dark (shadow)4 |
||
| CCD Clear4 |
0.06 |
0.16 |
0.08 |
0 |
| Longpass CCD 4 |
0.03 |
0.08 |
0.04 |
0. |
| NUV-MAMA Clear |
4 x 10-5 |
7 x 10-3 |
3.5 x 10-3 |
5.7 x 10-4 |
| FUV-MAMA Clear |
1.2 x 10-8 |
0.05 |
0.025 |
4.5 x 10-3 |
| SrF2 (NUV-MAMA) |
3.75 x 10-5 |
1.2 x 10-3 |
6 x 10-4 |
6.25 x 10-6 |
| SrF2 (FUV-MAMA) |
1 x 10-8 |
3.8 x 10-3 |
1.9 x 10-3 |
2.4 x 10-5 |
| Lyman-  |
0 |
3.7 x 10-3 |
1.85 x 10-3 |
3.6 x 10-4 |
| Qtz (NUV-MAMA) |
3.6 x 10-5 |
3 x 10-4 |
1.5 x 10-4 |
1.4 x 10-6 |
| 1 Zodiacal contribution is the same as in Figure 6.1 and Table 6.6 (mv=22.7 per square arcsec). 2 Corresponds to HST pointing 24° from the limb of the sunlit Earth. 3 Corresponds to HST pointing around 40° from the limb of the sunlit Earth, when the Earthshine is 50% of the "high" value. 4 Earthshine for shadow is 0 in the continuum, while the UV geocoronal emission lines are reduced from the day to the night values in Table 6.7. |
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 degrees 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 degrees 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 (low, average, high) and Earthshine (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 6.3 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 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 a very few lines. The brightest line is Lyman- at 1216 Å. The strength of the Lyman- line varies between about 2 and 20 kilo-Rayleighs (i.e., between 6.1 x 10-14 and 9.2 x 10-13 erg sec-1 cm-2 arcsec-2 where 1 Rayleigh = 106 photons sec-1 cm-2 per 4
steradians) depending on the time of the observation and the position of the target relative to the Sun. The next strongest line is the OI line at 1304 Å, which rarely exceeds 10% of Lyman-. The typical strength of the O I 1304 Å line is about 2 kilo-Rayleighs (which corresponds to about 5.7 x 10-14 erg sec-1 cm-2 sec-1 arcsec-2) at the daylight side and about 150 times fainter on the night side of the HST orbit. O I] 1356 Å and [O I] 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 1304 Å line. The widths of the lines also vary. The line widths given in Table 6.6 are representative values assuming a temperature of 2000 K.
The geocoronal emission lines are essentially unresolved at the resolution of STIS but the emission fills the slit in the spatial dimension. Using a wider slit or observing slitless 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 Longpass-Filtered MAMA Imaging- F25SRF2 and F25QTZ and Slitless First-Order Spectroscopy).
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).
|
Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |