For all targets the main components of background are the zodiacal light (at short wavelengths, with only slow angular variations and little granularity) and the Interstellar Medium (ISM) at longer wavelengths (with much greater granularity). For a fixed target the ISM will have a fixed value at any wavelength, being highest for targets in the Galactic Plane and the zodiacal light will vary with ecliptic latitude and solar elongation. For a moving target the ISM background will, logically, vary with time, although these variations will be a function of the object's heliocentric and geocentric distance - for distant planets the time variations will be slower but, as a corrolary is that an object will take longer to escape from a region of bad background. Note that Infrared Cirrus is highly structured and this structure will affect observations of faint targets. For very faint solar system targets, or where high signal-to-noise is essential, a careful examination of the cirrus may be necessary to look for a hole that will allow deeper observations; once a suitable hole is identified, you can put a time constraint on your observations to ensure that they are made against it - HSC Mission Planners will make great efforts to satisfy such requests, when properly justified.

As an example, the following shows how the PA (Figure 6.3, “PA variation for a typical solar system object: Neptune's satellite Triton. Note how the PA variations over the course of a full observing window amount to less than 2 degrees. This makes it effectively impossible to accomodate map orientation or chopper angle avoidance constraints. Although this example was calculated originally for a Herschel launch in 2007, the amplitude and timescale of variation remains the same for the actual launch date.”) and the estimated background at 80 microns (Figure 6.4, “The background variation for Triton at 80 microns. The background is dominated at this wavelength by the Zodiacal Light contribution. As the elongation changes over the course of the observing window the background effectively doubles with time. At longer wavelength the ISM component will also change as the target moves across areas of different background. For objects relatively close to the Sun the ISM component may vary enormously in a comparatively short space of time. Although this example was calculated originally for a Herschel launch in 2007, the amplitude and timescale of variation remains the same for the actual launch date.”) vary through a visibility window for the satellite Triton of Neptune (NAIF ID 801). At this wavelength the zodiacal light dominates and increases as the solar elongation decreases. Note too how the PA barely changes over the duration of an observing window, meaning that the chopper throw is almost fixed in direction with time; this has strong implications for any potentially orientation-constrained observations.

Figure 6.3. PA variation for a typical solar system object: Neptune's satellite Triton. Note how the PA variations over the course of a full observing window amount to less than 2 degrees. This makes it effectively impossible to accomodate map orientation or chopper angle avoidance constraints. Although this example was calculated originally for a Herschel launch in 2007, the amplitude and timescale of variation remains the same for the actual launch date.

Figure 6.4. The background variation for Triton at 80 microns. The background is dominated at this wavelength by the Zodiacal Light contribution. As the elongation changes over the course of the observing window the background effectively doubles with time. At longer wavelength the ISM component will also change as the target moves across areas of different background. For objects relatively close to the Sun the ISM component may vary enormously in a comparatively short space of time. Although this example was calculated originally for a Herschel launch in 2007, the amplitude and timescale of variation remains the same for the actual launch date.

Note that for satellites of solar system objects HSpot only calculates the visibility window with a solar elongation criterion. It does not take into account if the object is genuinely observable by Herschel. It is the astronomer's responsibility to make the necessary checks. Many solar system satellites experience transits and occultations by their parent planet. Similarly, a stellite may not be resolved at the wavelength of observation, or instrument safety constraints may make it impossible to observe a satellite when at less than a certain elongation from the parent planet, or only on one side of the planet (please contact Helpdesk (http://herschel.esac.esa.int/esupport/) for specific, detailed enquiries about this topic).

As an example, the following plots show how the elongation of Io, Jupiter's innermost Galilean satellite (NAIF ID 501), varies from the centre of the disk of Jupiter. In the first plot (Figure 6.5, “The variation of the elongation of Io from the centre of Jupiter with time. The area in grey is the region when Io is either superimposed on the disk of Jupiter (in transit) or behind the disk of Jupiter (occulted). HSpot does not warn the user if visibility of a planetary satellite is limited in this way.”) we see how the elongation varies with time over part of a visibility window. In the area marked in grey the satellite is either in transit, or occulted and thus, by definition unobservable. The second plot (Figure 6.6, “The variation in the offset of Io from the centre of Jupiter through an entire visibility window. The grey ellipse represents the approximate mean size of the disk of Jupiter. Note that the entire area of this plot is smaller than the field of view of either PACS or SPIRE. If requesting observations of a planetary satellite the observer should check the visibility of the satellite using the JPL Horizons program at the url: http://ssd.jpl.nasa.gov/horizons.cgi.”) shows the offsets in R.A. and Dec. (in arcseconds) over a full observing window. The ellipse marks the approximate size of the disk of Jupiter which suffers a variation of about 10% with time. Note that the entire area of the plot is smaller than the PACS or SPIRE instrument array (see Table 3.1, “The main imaging capabilities of PACS and SPIRE. Please note that the wavelength range of detector sensitivity is approximate and the instrument sensitivities depend on the observing mode, so the values given are only orientative: please consult the relevant observing manual for more detailed values.”).

Figure 6.5. The variation of the elongation of Io from the centre of Jupiter with time. The area in grey is the region when Io is either superimposed on the disk of Jupiter (in transit) or behind the disk of Jupiter (occulted). HSpot does not warn the user if visibility of a planetary satellite is limited in this way.

Figure 6.6. The variation in the offset of Io from the centre of Jupiter through an entire visibility window. The grey ellipse represents the approximate mean size of the disk of Jupiter. Note that the entire area of this plot is smaller than the field of view of either PACS or SPIRE. If requesting observations of a planetary satellite the observer should check the visibility of the satellite using the JPL Horizons program at the url: http://ssd.jpl.nasa.gov/horizons.cgi.

Warning

If requesting observations of a planetary satellite the observer should check the visibility of the satellite using the JPL Horizons program at the url: http://ssd.jpl.nasa.gov/horizons.cgi. The ephemeris should be requested specificially for the "Herschel Space Observatory" (site code "500@-486). The observations will almost certainly have to be entered in HSpot with a time constraint save for small, distant satellites.