HSpot allows the observer to define many different kinds of constraints on observations. This may be to observe an object at a certain time, to carry out observations in a certain sequence, or with a certain detector orientation, or to repeat observations at a certain interval. However, observers should be wary of overconstraining their observations and of defining constraints that are not strictly necessary, as each constraint that is added makes an observation more difficult to schedule.
![[Warning]](../../admonitions/warning.gif) | Warning |
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| Overconstrained observations may be impossible to schedule. |
![[Tip]](../../admonitions/tip.gif) | Tip |
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| When you add a constraint, you should use the "AOR Visibility" button (double click on the AOR to bring up the pop-up with the button) to check that the AOR visibility with the constraint is as you expect. This button looks at the AOR that you have defined and includes all the factors that may limit its visibility (map size, orientation constraints, avoidance angles, etc.) and gives you the effective visibility of the observation. |
In all chopped observations there is a certain danger that a nearby bright source could lie in the chop position, which is at 90 degrees to the position angle reported by HSpot. HSpot allows chopper avoidance angles to be defined. If, even when the chopper throw is changed, it is impossible to avoid a nearby bright object then defining a chopper avoidance angle should be considered. A chopper avoidance angle tells the observation planning system that the observation should be scheduled in such a way that the chopper will not chop at this range of angles. This however should be done with great caution as a star that looks bright in a DSS or 2MASS image is unlikely to be bright, even at the shortest Herschel wavelengths. A chopper avoidance angle is only necessary when there is a strong far-IR source present in the reference position.
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 has a strong ecliptic 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 6.1, “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. These plots were made originally for a Herschel launch in 2007, but the range and timescale of variation remains unaltered for the actual 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.
Figure 6.1. 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. These plots were made originally for a Herschel launch in 2007, but the range and timescale of variation remains unaltered for the actual launch date.
![[Note]](../../admonitions/note.gif) | Note |
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Understanding chopper avoidance angles 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 6.1, “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. These plots were made originally for a Herschel launch in 2007, but the range and timescale of variation remains unaltered for the actual 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 6.2, “An illustrative example. 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 a hypothetical case with 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. The situation is effectively identical for other dates.” 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 are for a hypothetical case of a 2007 launch of Herschel, with 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. The timescale and amplitude of variations does not change for the actual launch date.
Figure 6.2. An illustrative example. 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 a hypothetical case with 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. The situation is effectively identical for other dates.
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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. However, given the very small range of Position Angle change close to the ecliptic, any chopper avoidance angle will either be irrelevant (the PA will never be within the defined avoidance), or catastrophic (the avoidance angle range makes the observation impossible by definition by covering the entire range of PA change). 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 de-phase 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. |
PACS and SPIRE offer the possibility to define a map orientation constraint. In other words, the telescope should scan in a certain direction only, or within a certain range of directions. Further details of such orientation constraints and their limitations can be found in the relevant instrument manual.
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| An map orientation constraint equates to a telescope scheduling restriction and implies that an observation may only be made at a certain, limited range of dates, thus making their execution more problematic. Over-restricting observations may mean that for operational reasons it becomes impossible to carry them out. |
In certain cases there may be a strong scientific reason for requesting that an observation be carried out at a fixed time. A flag can be put in the AOR defining that the observation be carried out at a set time defined by the astronomer. This obliges the observation planning system to block the observation at this date and time, usually to within a few seconds, although at the cost of putting severe constraints on telescope scheduling, particularly as instruments have to be blocked by days.
A less constraining way of fixing the time is to define a timing window during which the observation should be carried out. A range of dates may be defined during which the observation must be made. This gives the observation planning system more liberty to work around the constraint.
Concatenation or chaining of observations may be defined to oblige the observation planning system to carry out observations together. Concatenation improves planning efficiency by avoiding the need for unnecessary slews, so the observer benefits because no slew overhead is applied to the observation). (
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| The saving may not always be exactly 180 seconds because some set-up is done while the telescope is slewing and so, if a set-up needs to be done for the second, or later observation in a concatenation - for example, an internal calibration - this time will still be charged against the observation. |
Four methods of chaining of observations are permitted:
Concatenation of observations
Two or more AORs for the same target are linked together (concatenated). These must use the same instrument and the same observation type (i.e. you cannot combine PACS and HIFI spectroscopy in a single chain, nor can you combine SPIRE photometry and spectroscopy in a single chain, nor SPIRE PACS Parallel Mode with any other PACS, SPIRE or HIFI mode). At present HSpot does not permit observations in different HIFI bands to be chained either. You can mix a SPIRE photometry map and point source photometry, or a PACS Line Spectrum and a Range Spectrum. The mission planning system will treat these observations as a single pointing. If it is important for observations to be carried out together, they should be concatenated.
Targets must be separated by no more than 1 degree to be chained. Fixed and moving targets can be chained, although it is the observer's responsability to ensure that they will be less than 1 degree apart at some point during the missiona and thus that the observation is schedulable.
As many chains as are required may be defined and as many observations as are required may be put in each chain, but the total observing time requested in each chain must be less than 18 hours.
The great advantage for the observer, apart from ensuring that observations are carried out together, is to avoid the need for a slew between integrations, thus saving a 180 or 600s slew overhead.
Follow-up observations
This mode is for repeat observations, for example of a variable source. A time between repeat observations can be defined. Chained observations can be sequenced so that the entire chain is repeated after a number of hours or days. The chain or sequence can be repeated several times if monitoring is required over a period of time.
Warning The observer can request that a sequence be carried out with a very exact interval, or within a band of time (e.g. each observation should be within 8 and 12 days of the previous one). The stricter the constraint, the more difficult it will be to accommodate the observations in the observing schedule, to the point that highly constrained observations may be impossible to carry out. There is a regular planning cycle of instruments over each two week period, with instruments available on sets days in each period: your constraints should be compatible with this cycle. Sequencing
This mode is to carry out observations in a particular order, although not necessarily the same day. This may be necessary when two or more measurements are required and it is essential that one be carried out first to allow the other observations to be reduced when carried out.
Group within
In this mode observations must be carried out in a certain time frame, but with no constraint as to when. An observer can specify that all the observations in the group should be carried out within a maximum of, for example, one month; in this case the observatory planning system will complete all the AORs within a month of carrying out the first one. The observations may be carried out in any order within this time interval.