Implementation

The background model described in this memo requires the following user inputs:

• coordinates
• wavelength
• date of observation.

Using these inputs, the background estimator can

• evaluate the zodiacal light model
• extract nearest value from the interstellar medium map
• scale interstellar medium map by average spectrum, and
• add all these together with the cosmic background.

The first task involves a numerical integration of a parameterized function. The second task involves a table look-up through two $4096\times 4096$ pixel FITS files (north and south galactic hemispheres). Figure 4 shows the spectra of the background at two different positions and dates.

  

Figure: Background spectrum on April 10 for two different sky positions, at galactic coordinates $(30^\circ ,30^\circ )$ for panel (a) and $(30^\circ ,8^\circ )$ for panel (b). In each panel, the total brightness is shown as a solid curve, the contribution from zodiacal light as a dashed curve, the contribution from the interstellar medium as a dotted curve, and the extragalactic background as a dash-dotted curve.

When the date of observation is not known, the background estimator will determine the minimum and maximum zodiacal light brightness expected for the given target. To do this, the program finds the dates when the solar elongation of the observation would be at the edges of the 80-120$^\circ$ viewing swath. There are two swaths, corresponding to observations leading and trailing the Earth in its orbit, so there are 4 dates. The zodiacal light model is evaluated for each date, and the minimum and maximum brightness are returned. For ecliptic latitudes above $80^\circ$, the field is always visible, so we use the amplitude of a sinusoidal fit to the sky brightness as a function of date. Thus a typical use of the background estimator in Spot, for a typical observer (who will not be fixing the observation date), is to find the range of background brightnesses possible for the target coordinates and wavelength of observation. It is up to the judgment of the observer to decide whether to use the average background or the maximum background (to be conservative), and the choice should be specified when explaining how backgrounds were used in a proposal.