There are several potential types of background and noise that can affect the observations; the main contributors being:
The sum of the External Background components is presented on the Results page. Note that each of the components of External Background can be switched off (set to None or No airglow) so that one of the components can be selected for display on the Results page. All components should be included when simulating observations, as artificially turning off the sky does not represent actual observing conditions.
Earthshine can vary strongly depending on the limb angle, on the fraction of the earth that is sun-lit, and on the fraction of cloud cover. At low limb angles, it can dominate the zodiacal light, which varies by only a factor of ~3 throughout the sky available to HST (See Giavalisco et al. WFC3 ISR 2002-012).
The table below lists the relative levels of Earthshine selectable in the ETC.
Table 1: Earthshine Background Contributions Relative to “High”
|Limb angle||N/A||50 deg||38 deg||24 deg|
The phase II special requirement LOW-SKY ensures that the limb angle will be greater than 40 degrees, i.e. that it will avoid the limb angle of the High Earthshine case by at least 2 deg. This requirement greatly reduces scheduling opportunities, so should be requested only when critical to the science and should be well justified in the phase I proposal.
The Zodiacal light values given in Table 2 are in ecliptic co-ordinates (ecliptic latitude) and (helio-ecliptic longitude or elongation). See Figure 1 below.
When a user request to calculate the contribution of the Zodiacal light with position, first the Equatorial coordinates are converted to Ecliptic coordinates. The relation between these set of coordinates is:
Since we know we can then use (i) and (ii) to calculate and .
To solve for the ambiguity of the sign you should calculate the equations (iii) and (v) and
The ETC then calculates the helio-ecliptic logitude or elongation (sun angle between sun-earth and earth particle) to determine the brightness along the line of sight:
is derived as follows:
Once the Helioeclipic coordinates are calculated, the ETC performs a bilinear interpolation using the values in Table 2 in order to derive the Johnson V magnitude for the Zodiacal contribution. The result is then fed to the pysynphot task renorm to renormalize the Zodiacal light spectrum model (currently zodiacal_model_001.fits) to the derived V magnitude.
Table 2 provides the approximate values of the Zodiacal light background () in ecliptic latitude () and helio-ecliptic longitude (). The SE values correspond to the solar exclusion zone. This table has been updated for cycle 24 using a finer grid to provide better estimates of the zodiacal light as a target approaches the 50 degree HST-to-target limiting angle.
Table 2: Zodiacal Light Background Contributions
This is used to normalize the Zodiacal Light spectra used by the ETC and shown in figure 2 below.
Table 3: Geo-Coronal Emission Line Properties
|Wavelength Angstrom||Flux(erg cm^-2 s^-1)||FWHM Angstrom||Flux(erg cm^-2 s^-1)||FWHM Angstrom||Flux(erg cm^-2 s^-1)||FWHM Angstrom|
The strength of the geo-coronal Lyman alpha varies between about 2 and 20 kilo Rayleighs, depending on the time of observations and the position of the target relative to the Sun, and can be kept low by the special requirement “SHADOW”. For more details, see the corresponding Instrument Handbook.
Short-wavelength WFC3/IR imaging filters (F105W and F110W) and WFC3/IR grisms (G102 and G141) are subject to excess background emission caused by metastable Helium atoms in the solar-illuminated upper atmosphere. The background is due to He I emission at 10830 Angstrom. Its contribution is negligible in Earth’s shadow but increases sharply when the telescope is outside of shadow (up to 5 e-/s/pix in extreme cases). The He background also increases with decreasing limb angle, but can still be significant as high as 40 degrees above the Earth limb. For more information on this background component, see WFC3/ISR 2014-03 (Brammer et al.)
The contribution of atmospheric helium to the WFC3 background is negligible in Earth’s shadow, corresponding to the “None” default ETC setting. The “Average”, “High” and “Very High” values implemented in the ETC correspond to 0.1, 0.5, and 1.5 e-/sec/pix in F105W and are the 50, 75, and 95 percentile fluxes of the excess flux, per exposure, from the Helium line determined from archival observations. The ETC model of this emission is a gaussian source at 10830 Angstrom with FWHM = 2 Angstrom.
The thermal background is negligible below about 8000 Angstrom and increases slowly towards longer wavelengths. For WFC3/IR, the thermal count rate (per unbinned pixel) is calculated by an algorithm that is described in detail in “Thermal Background Limitations for IR Instrumentation Onboard HST”, Sosey, M., Wheeler, T., Sivaramakrishnan, A., 2003, NICMOS ISR 2003-007
Detector dark current is an intrinsic source of background counts. The dark current rate is dependent upon the detector design and temperature. It is measured in counts per unbinned pixel per second.
CCD and CCD-like detectors (such as the WFC3 IR detector) are subject to noise caused by the process of “reading out” the charge accumulated by the pixels. The amount of read noise varies by detector and as a function of gain. Read noise is measured per binned pixel, per read.
Table 4: Dark Current and Read Noise Values
|Instrument||Dark_Current (counts sec^-1 pixel^-1)||Read_Noise (for gain = 1)|
|ACS/HRC||0.058||4.7 (for gain=2.0)|
|COS/FUVB||4.40e-06 target acquisition||NA|
|COS/FUVA||4.21e-06 target acquisition||NA|
|STIS/CCD||0.018 (middle of detector)||6.2|
|WFC3/IR||0.0192||14.6 (for gain=2.5)|
|WFC3/UVIS1||0.0020||3.0 (for gain=1.5)|
|WFC3/UVIS2||0.0022||3.0 (for gain=1.5)|
|||Assuming a STIS FUV-MAMA glow region value of counts/pixel/s contributing to the total dark rate.|