The first step to create a PACS Photometry AOR is the filter selection in the blue channel. The two mutually exclusive options are [60-85] or [85-130] microns bands. Irrespective the blue filter selection, parallel data are always obtained in the red channel.

Clicking on the "Source Flux Estimates" action button a pop-up window will offer four entry fields:

Figure 11.19.  The Photometry AOT main window

These are optional AOR parameters, source estimated data used only for S/N calculation except if very a bright source is specified. In case "Point source photometry" observing mode is selected the extended source entry fields get disabled.

	Warning
	Specifying a very bright source will induce a change in gain settings. Check the applicable flux limits in the HSpot Observers Manual.

On the right hand side of the observing mode settings area a text box labelled "Repetition factor" can be found. The absolute sensitivity of the observation can be controlled by entering an integer number between 1 and 120. However, there are further rules to apply depending on the observing mode selection. The repetition factor has to fit within the range:

Figure 11.21.  Point Source Photometry tab

This mode is reserved for observers who want to target a source that is completely isolated and point-like, or smaller than one blue matrix (approx. 85 arcseconds). It uses chopping and nodding, both with amplitude of 1 blue matrix, and dithering with an amplitude of a single pixel.

The point-source photometry mode is using a classic 4-position chopping, with dithering along the chopper axis, combined with nodding in the perpendicular direction. This setup compensates for the different optical paths in the most efficient way since the source is always kept on-array.

The observer specifies a repetition factor that is used to adjust the number of nod cycles to reach the requested integration time. The minimum time per nod position is 64 seconds. This is to block very short observations that have a poor observing efficiency, but is done at the expense of a stepwise behaviour in the AOR total time.

Depending on the user specified source flux level, the photometer gain is adjusted by the backend logic to provide a comprehensible dynamic range.

Chopper avoidance angle

The chop direction is determined by the date of observation; the observer has no direct influence on this parameter. If an emission source would fall in within the chopper throw radius around the target the observer may consider setting up a chopper avoidance angle constraint. The angle is specified in Equatorial coordinates anticlockwise with respect the celestial North (East of North). The avoidance angle range can be specified up to 345 degrees.

Warning

Specifying the chopper avoidance angle place restrictions on when the observation can be scheduled. This constraint might reduce the chance that your observation will be carried out, especially targets at low ecliptic latitudes could be inaccessible for certain chopper angles. You should use chopper avoidance angle only for observations where it is absolutely necessary. (You may check your observation's footprint orientations by clicking in the HSpot menu: Overlays -> AORs on current image...)

Dithering

A predefined set of dithering positions can be observed by clicking on this parameter. In the current AOT logic three dithering positions are used (see the PACS Observers' Manual (http://herschel.esac.esa.int/Docs/PACS/html/pacs_om.html) for further details).

	Note
	In the Photometry AOT dithering is performed only along the spacecraft Y-axis using the PACS focal plane chopper for beam modulation. More complicated two-axis dithering patterns are not allowed in the current design.

Figure 11.22. Small Source Photometry tab

Here the term "small source" refers to sources that are smaller than the array size, yet larger than a single matrix. To be orientation invariant, sources must fit within approximately 2 by 2 arcminutes. Classic three-position nodding is performed in a 2 by 2 raster minimap (with fixed map parameters), in this configuration the source cannot be kept on the array at all times. The chopper is moving along the long size of the field of view with largest possible 4 arcminute throw. Again, as in the point-source photometry mode, the unit is one nod cycle, and the total duration is computed as multiples of this cycle. The only option of this observing mode is the "Chopper avoidance angle". For detailed description of this parameter see "Point Source Photometry" above.

If the source is significantly larger than the array, or the objective is to cover a large contiguous area of the sky, there are two possible observing modes: chopped raster and unchopped scan mapping.

Figure 11.23. Chopped Raster tab

The raster map option, combined with on-array chopping, preserves the PACS highest spatial resolution without degrading the point spread function. Maps can be specified in instrument coordinates, the chopper axis is perpendicular to the raster lines. The observing unit is the chopper cycle; the total duration is computed by multiples of this cycle.

Setting the map size

The centre of the map is at the coordinates given by the target position. The map size along a raster line can be expressed as the number of raster points per line times the raster point step. In perpendicular direction, the map size is given by the number of raster lines times the raster line step. These four parameters have to be given in the following ranges to define a raster map:

• Number of raster points per line in range [2-100]

• Number of raster lines in range [1-100]

• Raster point step (arcseconds) in range [2-210]

• Raster line step (arcseconds) in range [2-105]

	Note
	The maximum step sizes are limited by the angular size of the PACS photometer footprint. As a consequence, sparsely sampled maps are not possible in this mode. The maximum raster map size is limited to 350 times 175 arcminutes in this mode.

	Warning
	Raster maps are time efficient only for areas smaller than 10 by 10 arcminutes respectively. The observatory overheads make this mode inefficient for large fields (see the PACS Observer's Manual for further information).

Setting the map orientation parameters

In raster mapping mode mode the map (raster line) orientation is defined by the chopping direction, the observer has no direct access to the map orientation parameter. However, there are two choices to influence the map orientation.

The observer can select the map orientation reference coordinate system and map orientation angle constraint through the following parameters:

• Position angle reference frame, options: "Array" or "Array with sky constraint". If "Array with sky constraint" is selected then the map orientation can be limited within a given range of angles.

• Constraint angle from (degrees). Angle calculated East of North (counterclockwise in equatorial coordinate system).

• Constraint angle to (degrees). Angle calculated East of North (counterclockwise in equatorial coordinate system).

The other way to influence the map orientation is setting up a timing constraint on the AOR.

Warning

Specifying a map orientation constraint place restrictions on when the observation can be scheduled. These constraints might reduce the chance that your observation will be carried out, especially targets at low ecliptic latitudes could be inaccessible for certain chopper or map angles. You should use constraint angle only if it is essential for your observation. (You may check your observation's footprint orientations by clicking in the HSpot menu: Overlays -> AORs on current image...)'

Line scan maps are the suggested default to map large areas of the sky, for Galactic as well as extragalactic surveys. In this mode the signal modulation is provided by the spacecraft motion. The extended emission is filtered out by fitting the temporal signal, but point sources can be detected without losing sensitivity. Line scan maps can be performed in spacecraft coordinates, or in sky coordinates, although there is no direct control of the homogeneity of the map coverage when performed in sky coordinates. The observer has the choice of selecting the scan speed (low, medium and high), which determines the on-source time and the depth of the observation.

Figure 11.24. Scan map tab

Setting the map size

The centre of the map is at the coordinates given by the target position. The map size along a scan line is given by the "Scan leg length" parameter. In perpendicular direction, the map size is given by the number of scan lines times the cross-scan step. These parameters have to be given in the following ranges to define a scan map:

• Scan leg lengths (arcminutes) in range [1-1200]

• Number of scan legs in range [1-1500]

• Cross-scan step (arcseconds) in range [2-210]

The map parameter panel allows to set up two further parameters what might help to optimize the reconstructed map quality and the observing time. These parameters are:

• Homogeneous coverage. Setting the pull-down menu to "Yes" the AOT logic will calculate automatically the cross-scan step size for a given map angle. This functionality is enabled only if the position angle reference frame is either "Array" or "Array with sky constraint". The calculated step size ensures that all positions in the map are seen the same amount of time, therefore the signal-to-noise ratio can be kept almost constant ovet the mapped area.

• Square map. If "Homogeneous coverage" option is selected then the number of scan legs can be calculated automatically by the AOT logic to form a nearly squared shape map on the sky.

	Note
	The maximum cross-scan step size is limited by the size of the PACS photometer footprint. As a consequence, sparsely sampled maps are not possible in this mode. The maximum scan map size is limited to 20 times 4 degrees in this mode.

	Warning
	Scan maps are time efficient only for areas larger than 10 by 10 arcminutes respectively. The observatory overheads make this mode inefficient for small fields (see the PACS Observer's Manual for further information).

Map orientation parameters

The observer can select the map orientation reference coordinate system and map orientation angle through the following parameters:

• Position angle reference frame, options: "Sky", "Array", Sky with array constraint" and "Array with sky constraint"

• Position angle (degrees). If Position angle reference frame is given in "Sky" coordinates then this angle is defined East of North (counterclockwise in equatorial coordinate system) to the scan direction. If Position angle reference frame is given in "Array" coordinates then this angle is defined between the instrument +Z-axis and the first scan leg to the scan direction.

• Constraint angle from (degrees). Angle calculated in the selected reference system.

• Constraint angle to (degrees). Angle calculated in the reference system.

Warning

Specifying a map orientation constraint place restrictions on when the observation can be scheduled. These constraints might reduce the chance that your observation will be carried out, especially targets at low ecliptic latitudes could be inaccessible for certain chopper or map angles. You should use constraint angle only if it is essential for your observation. (You may check your observation's footprint orientations by clicking in the HSpot menu: Overlays -> AORs on current image...)'

The breakdown of the observation is provided in the "PACS Time Estimation" screen. It indicates the on-source time, calibration time, overheads and the total observing time.

More detailed information can be obtained by clicking the "Messages" button on the "PACS Time Estimation" screen.

Figure 11.26.  PACS Time Estimator messages window

The message window presents information of two kinds: (1) a summary of the data entered in HSpot (instrument settings and Ïs); and (2) several relevant pieces of timing information, as well as the RMS noise and signal-to-noise for the resulting times.

The breakdown of calculated confusion noise in the "PACS Time Estimation" screen. It indicates the confusion noise specific for the AOR configuration (see the "Herschel Confusion Noise Estimator Science Implementation Document for details). On startup this table is kept empty, in order to receive a confusion noise estimation the "Update confusion noise estimates" button has to be pushed.

	Note
	In case the estimated confusion noise is higher than the instrument sensitivity then HSpot provides a warning message. You may consider to reduce the number of repetition factors unless there is a well established reason to observe deeper than the local confusion noise limit.

	Note
	Confusion noise estimates are by their nature preliminary at any planning phase before Herschel's routine phase starts. Please consult the HSC website to learn plans about future updates of the estimator.