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)

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 prescriptions by Giavalisco et al. WFC3 ISR 2002-012

The geo-coronal 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

Background	Contribution
	Low	Average	High	Very High
Zodiacal [a]	23.3	22.7	22.1	NA
Earth_shine [b]	0	50	100	200 [c]

[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

Line		Low		Average		High
	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
Lyman_Alpha	1215.7	6.1e-14	0.04	3.05e-13	0.04	6.1e-13	0.04
O_I	1302	3.8e-16	0.013	2.85e-14	0.013	5.7e-14	0.013
O_I	1356	3.0e-17	0.013	2.5e-15	0.013	5.0e-15	0.013
O_II	2471	1.5e-17	0.023	1.5e-15	0.023	3.0e-15	0.023

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.

Thermal Background and Noise

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 3: Dark Current and Read Noise Values

Instrument	Dark_Current	Read_Noise
	(counts sec^-1 pixel^-1)	(for gain = 1)
ACS/HRC	0.00372	4.7 (for gain=2.0)
ACS/SBC	0.000012	NA
ACS/WFC	0.00622	4.1
COS/FUV	5.35e-06 target acquisition	NA
COS/FUV	4.37e-06 spectroscopy	NA
COS/NUV	9.92e-4	NA
STIS/CCD	0.0165	5.6
STIS/FUV	1.5e-4	NA
STIS/NUV	1.8e-3	NA
WFC3/IR	0.0221	14.6 (for gain=2.5)
WFC3/UVIS	0.00097 (0.0015 e-/s @ 1.55 e/DN)	3.0 (for gain=1.5)