An important innovation enabling Spitzer to accomplish ambitious scientific goals at a modest cost is its Earth-trailing heliocentric orbit (shown in Figure 3.5), in which the observatory drifts away from Earth at the rate of ~ 0.12 AU/year. This separation from the Earth as a heat source substantially helped prolong the coolant lifetime by supporting an operating regime in which most of the cryogen is used to take up the power dissipated by the detector arrays, rather than lost to parasitic heat loads. In addition, this orbit has less-constrained visibility, compared to what would have been the case in a near-Earth orbit, allowing all parts of the sky to be visible for at least two extended periods each year and some zones to be visible continuously.
Figure 3.5: Spitzer’s solar orbit projected onto the ecliptic plane and viewed from ecliptic North. In the rotating frame, the Earth is at the origin and the Earth-Sun line is defined as the X-axis. “Loops” and “kinks” in the trajectory occur at approximately 1-year intervals when Spitzer is at perihelion. Spitzer’s orbit is also slightly inclined with respect to the ecliptic.
3.5.2 Pointing Constraints
Spitzer’s view of the sky was limited by two hard pointing constraints, illustrated in Figure 3.6 and (in a different fashion) in Figure 3.7:
· The angle between the boresight and the direction of the Sun may never be less than 82.5º. (NB: this was updated in early 2004.)
· The angle between the boresight and the direction of the Sun may never exceed 120º.
The area defined by these hard constraints is called the Operational Pointing Zone (OPZ). In addition, some bright objects (such as the Earth) are normally avoided, because they would degrade the quality of the observation, due to direct exposure or stray light, but this is not a strict constraint (see section 3.5.6 and Appendix A). Note that the definition of the OPZ precludes observing Mercury or Venus with Spitzer. A second-order effect on the OPZ is provided by the limited roll angle of Spitzer around the Y-axis (see Figure 3.4), which is just ±2º.
Figure 3.7 shows the actual pointings of the observatory for one day in the life of Spitzer. Note the locations of the OPZ and how it constrains where the telescope actually pointed on that day.
Figure 3.6: The main geometric observing constraints form an area called the Operational Pointing Zone (OPZ).
As the mission progressed, the pitch angle of the boresight relative to the sun when it was communicating with Earth gradually increased as Spitzer’s distance to the Earth increased (see Figure 2.5). In December of 2013 this angle exceeded the edge of the OPZ (in the anti-sun direction, fortunately!) in order for the high gain antenna to be pointed at Earth. In order to continue to operate the spacecraft the onboard OPZ checks were disabled before each downlink to Earth.
Once the spacecraft was pointed outside the OPZ, it was no longer power-positive when downlinking to Earth and the onboard battery was used to power the spacecraft until it could be returned to a pitch angle within the OPZ. In October of 2015 the pitch angle of the science for two hours following each of these downlinks was restricted to 82.5 to 100 degrees so that the solar panels were approximately normal to the sun and recharged the battery faster. At the time of decommissioning the boresight/sun angle was 144.6 degrees, which was 24.6 degrees outside of the OPZ. Downlinks had a maximum duration of 140 minutes so as not to deplete the battery too far (the maximum battery depletion during a downlink measured was 53% (47% state of charge). Since the round trip light time to at this point Spitzer was 29 minutes and 32 seconds the downlinks also had a minimum duration of 120 minutes so that there was enough time to send and receive confirmation of sequences uplinked to the telescope.
3.5.3 Viewing Periods
The amount of time during the year any particular target is visible depends primarily on the absolute value of its ecliptic latitude (Figure 3.8). As seen in Figure 3.6, Spitzer’s instantaneous window of visibility on the celestial sky forms an annulus (the OPZ), perpendicular to the ecliptic plane, of ~40º width and symmetrical with respect to the Sun. This annulus rotates with the Sun over the period of a year; the edges of the OPZ move along the ecliptic at about 1º/day as Spitzer orbits the Sun. For an object near the ecliptic plane, the length of the visibility period is ~40 days twice a year (modulo periods when undesired bright moving objects are also present and bright object avoidance has been selected; see section 3.5.6). The visibility periods increase to ~60 days twice/yr at an ecliptic latitude of ~45º, ~100 days twice/yr at latitudes ~ 60º, then becoming a single long window ~250 days long for latitudes near 60-70º, finally reaching constant viewing near the ecliptic poles. About a third of the sky is visible to Spitzer at any time. Figure 3.9 illustrates how the total number of days of visibility varies over the sky in three coordinate systems.
Figure 3.7: OPZ boundaries for 24 Nov 03 (0h UTC), with dots representing the actual locations of the telescope boresight for the subsequent 24 hours.
Figure 3.8: Variation of length of visibility period as a function of ecliptic latitude for all of the targets in the April 2003 ROC. (This figure is provided as indicative of the general concepts, despite the fact that the ROC has changed substantially since April 2003.)
Figure 3.9: Total days of visibility per year in equatorial, ecliptic and galactic projections.
3.5.4 How Visibility Evolves with Time
Figure 3.10 illustrates the zone of visibility in equatorial coordinates for four specific dates during the first year of operations. Note how the sky passes into and out of visibility as the calendar date advances. Each set of contours shows the available and forbidden zones on a particular date (in this plot, 1 Sep 2003, 1 Dec 2003, 1 Mar 2004, and 1 Jun 2004). On each date, there is a forbidden zone in the anti-Sun direction, as well as a second, more extended-looking forbidden region on the side of the spacecraft towards the Sun; the Sun-ward forbidden region is in the center of the plot on 1 Sep and on the edges for the 1 Mar plot. Note that even targets at high declinations may fall in the forbidden zones part of the time, although objects at extreme declinations are generally visible for most of the year.
Figure 3.10: Example of time evolution of visibility zones over a year; see text.
3.5.5 Orientation of Focal Plane and Slits against the Sky
Spitzer had very limited ability to rotate the focal plane of the telescope. At any given time, the center of the sunshade (X-Z plane; see Figure 3.4) was always kept within ±2º of the Sun. This limited freedom in the “roll” angle was used to maintain a consistent orientation for long observations (AORs) and was not selectable by the observer. For each target, the orientation of the focal plane on the sky is a function of the position of the spacecraft along its solar orbit (i.e., the date of observation) and of the ecliptic latitude of the target. Targets in the ecliptic plane have only two possible focal plane orientation angles which are at 180º with respect to each other. Elsewhere in the sky, the focal plane has two ranges of orientation angles.
Science instrument aperture orientation within the focal plane is physically fixed. Because the spectrometer slits have quite different orientations within the focal plane (see Figure 3.1), this could be particularly limiting for IRS observations, but it also affected mapping with MIPS and IRAC.
3.5.6 Bright Object Avoidance
No object outside the solar constraint zone posed a threat to instrument safety. However, an observer may have wished to avoid observing the Earth and other bright moving objects to avoid compromising observations of faint targets. Therefore, the visibility windows calculated by Spot (the observation planning software: see section 4.5.1) avoided certain bright moving targets by default, although the observer could choose to override the default.
With bright object avoidance turned on (the default), the visibility windows calculated for both inertial and moving targets excluded regions of time when the positions of (a) the Earth and Moon, and (b) a fixed list of bright moving objects (e.g., Jupiter, Saturn, bright asteroids) coincide with the target position. The visibility windows was trimmed to delete any time periods when the Spitzer target is within 7º of the Earth or Moon and when the target is within 30 arcminutes of the other bright objects. Earth-Moon avoidance was only an issue early in the mission. As Spitzer moved farther away from the Earth, the Earth fell into the solar avoidance zone. The complete list of the bright moving objects is in Appendix A. The observer may have chosen to override the default (a) Earth/Moon or (b) other bright object avoidance. For example, to observe Jovian satellites, one would turn off (b) and leave (a) in effect.