Observatory Support
STScI Exposure Time Calculators Oversight

Specifying the Appropriate Background

There are several potential types of background and noise that can affect the observations; the main contributors being:

1. Earth-shine
2. Zodiacal light
3. Geo-coronal emission Lines (UV)
4. Thermal background (IR)
5. Dark current
6. Read noise (CCDs and NICMOS)
7. Scattered light (Coronagraphy)

External Background

Earthshine can vary strongly depending on the Earth-target angle and the fraction of the sun-lit earth. The contribution of zodiacal light does not vary dramatically with time, and is constant within a factor of about 3 throughout the sky available to HST. Thus, while the Earthshine can be kept low by a careful choice of the epoch of observations, the zodiacal light can be the most dominant contribution.

Earthshine and zodiacal light values are based on the precriptions by Giavalisco et al. (WFC3 ISR 2002-012)

The geocoronal emission is confined to mostly a very few lines such as Lyman-alpha, which must be taken into account for UV observations.

The table below lists the contributions of the zodiacal light and Earthshine corresponding to these levels.

Table 1: Sky Background Contributions

^a Vega magnitudes per square arc second in Johnson/V band.

^b As a percentage of the “High” value.

^c Corresponds to limb angle 24 degrees.

Table 2: Geo-Coronal Emission Line Properties

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 Instrument Handbook and the HST Phase II Proposal Instructions.

Thermal Background and Noise

The thermal background is negligible below about 8000 Å and increases slowly towards longer wavelengths. For NICMOS and WFC3/IR, count rate (per unbinned pixel) is calculated by the pysynphot task THERMBACK 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 dependant upon the detector design and temperature. It is measured in counts per unbinned pixel per second.

CCD and CCD-like detectors (such as the NICMOS IR detectors) 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 3: Dark Current and Read Noise Values

^a Calculated for an operational temperature of 77.1ºKelvin.

Coronagraphy Scattered Light

When using the ACS HRC or NICMOS detector 2 with an available coronagraphic occulting spot, the ETC will estimate the background counts that are contributed by the scattered light from the “central” or occulted source.

The count rate of the central source is multiplied by the “Coronagraphic Central Source Fraction” to get the background count rate per square arc second in the region of the “field” or “target” source. This fraction is a function of effective wavelength and separation and is calculated using tables. A sampling of the Coronagraphic Central Source Fraction tables is given in Appendix B.