Many of the observing modes offered allow the observer to select certain parameters such as a chopper avoidance angle (see, for example, the section called “Observing mode parameters - Chopping/nodding”) that requests that the observation be carried out in such a way that we do not chop over a certain angle on the sky.

Over the year the apparent rotation of the sky makes the position angle of the chopper on the sky change (this is the roll angle of the spacecraft, measured from north through east, using the spacecraft z-axis as reference - the z-axis is perpendicular to the orientation of the long axis of the PACS and SPIRE arrays). In other words, by selecting a chopper angle constraint we are effectively placing a timing constraint on our observations, stating that it may not be made at certain times of year. However, the Position Angle calculated in Figure 10.2, “The position angle calculated by HSpot for the ecliptic source Ganymede, the Jovian satellite.” has a strong latitude dependence. For sources in the ecliptic the Position Angle will barely vary with time during a visibility window. For the two observing windows available each year two values differing by exactly 180 degrees will be found (Figure 10.3, “Position angle variation for sources on the ecliptic and at the ecliptic pole, in the zone of permanent sky visibility. For sources at intermediate ecliptic latitude the annual range of variation of PA will be between these two extremes. This plot has been generated for a launch in early 2008; it remains valid for any launch date.”). In these cases defining a chopper avoidance angle is, at best, irrelevant (as the PA will only vary in a range of a few degrees anyway) and, at worst, catastrophic because it is may make all observations totally impossible, with no part of the visibility window permitted.

Note

Understanding chopper avoidance angles The PA that HSpot reports the spacecraft roll angle for any particular date of observation. The chop angle will be perpendicular to this angle. If, when you visualise an AOR, you find a bright source in your reference position, you must ADD 90 degrees to the PA in HSpot to avoid a position in the chopper off position. If you have a source in the nod off position you must SUBTRACT 90 degrees to the PA reported in HSpot.

At high ecliptic latitude we have a zone of permanent sky visibility and the PA of the chopper rotates rapidly with time. Here, even a quite wide chopper avoidance angle range may equate to only a relatively small effective restriction on dates. Figure 10.3, “Position angle variation for sources on the ecliptic and at the ecliptic pole, in the zone of permanent sky visibility. For sources at intermediate ecliptic latitude the annual range of variation of PA will be between these two extremes. This plot has been generated for a launch in early 2008; it remains valid for any launch date.” shows how the PA changes for a source almost at the ecliptic pole, which is within the permanent sky visibility zone.

At high ecliptic latitude we have a zone of permanent sky visibility and the PA of the chopper rotates rapidly with time. Here, even a quite wide chopper avoidance angle range may equate to only a relatively small effective restriction on dates. Figure 10.3, “Position angle variation for sources on the ecliptic and at the ecliptic pole, in the zone of permanent sky visibility. For sources at intermediate ecliptic latitude the annual range of variation of PA will be between these two extremes. This plot has been generated for a launch in early 2008; it remains valid for any launch date.” shows how the PA changes for a source almost at the ecliptic pole, which is within the permanent sky visibility zone.

At intermediate ecliptic latitudes there will be a break in the visibility windows, although this may be small. When the instrument +Z-axis crosses celestial north there will be a discontinuity in the PA value. Observers should take care of this when defining chopper avoidance angles for sources that are close to +60 degrees ecliptic latitude. A practical example of this is shown for PACS in Figure 10.4, “The position angle variation for PACS for an object at an ecliptic latitude of 59.5 degrees, close to the point of permanent visibility. The horizontal position is PA=000 degrees. The plotted positions of the PACS imaging detectors are for 2008 March 31st (start of visibility window) PA=127.4 degrees, 2008 June 15th (mid-window) PA=054.6 degrees, 2008 September 10th (end of visibility window) PA=333.7 degrees.” for an object at an ecliptic latitude of 59.5 degrees, close to the point at which there is continuous visibility, but where there is are still two annual visibility windows with a short gap between them. PA=000 degrees is shown (the horizontal position), along with the plotted positions of the PACS imaging detectors for 2008 March 31st (start of visibility window) PA=127.4 degrees, 2008 June 15th (mid-window) PA=054.6 degrees, 2008 September 10th (end of visibility window) PA=333.7 degrees.

Warning

Close to the ecliptic even a small range of chopper avoidance angle may equate to a huge scheduling restriction, potentially making observations impossible to schedule. At high ecliptic latitude it is easier for telescope scheduling to take a chopper avoidance into account. However, at high ecliptic latitude the chopper PA will often rotate through 360 degrees giving a dephase that must be taken into account when defining a chopper avoidance angle. In all cases an observer should consider very carefully if defining a chopper avoidance angle is really, genuinely necessary. All constraints on observations imply an increased observing overhead and thus decreased observing efficiency. Badly or incorrectly defined constraints are the biggest single cause of scheduling problems for Herschel.

Figure 10.2. The position angle calculated by HSpot for the ecliptic source Ganymede, the Jovian satellite.

Figure 10.3. Position angle variation for sources on the ecliptic and at the ecliptic pole, in the zone of permanent sky visibility. For sources at intermediate ecliptic latitude the annual range of variation of PA will be between these two extremes. This plot has been generated for a launch in early 2008; it remains valid for any launch date.

Figure 10.4. The position angle variation for PACS for an object at an ecliptic latitude of 59.5 degrees, close to the point of permanent visibility. The horizontal position is PA=000 degrees. The plotted positions of the PACS imaging detectors are for 2008 March 31st (start of visibility window) PA=127.4 degrees, 2008 June 15th (mid-window) PA=054.6 degrees, 2008 September 10th (end of visibility window) PA=333.7 degrees.