This pull-down menu in the "Range scan or SED mode" frame gives the choice to select between two scan modes the flexible "Range scan" and the fast "SED" (Spectral Energy Distribution) mode. Depending on the applicable wavelength ranges (defined by the grating diffraction orders) these options are further divided all together into five categories:

As a result of the selection, the spectral ranges to observe have to be specified in the PACS Range Editor as being within either the 71-98 and 102-210 micron band, or within the 55-72 and 105-210 micron bands. However, PACS always observes in two parallel bands, beside the 'nominal' range, data are obtained in the 'parallel' channel as well.

	Note
	As is indicated in the options above, irrespective of the blue order selection, PACS always observes a part of the [102-210] microns range (1st order) in the red spectrometer array. Details about the covered ranges and grating step sizes in 'nominal' and 'parallel' channels can be found in the PACS Observers' Manual (http://herschel.esac.esa.int/Docs/PACS/html/pacs_om.html).

	Note
	The PACS spectrometer has a sensitivity gap from 98 to 102 microns.

Tip

The observer has a choice of creating an observation comprising of spectral ranges spread along the entire PACS wavelength range (55-210 micron) by grouping AORs. In this case, at least two AORs have to be requested, one with each of the grating order options. See Section 6.5.3, “Group/Follow-on Constraints” and Section 16.2, “Timing Constraints ” (grouping and timing constraints) for further information.

The use of range scan options in combination with high sampling density (see "Instrument settings") is recommended to observe broad spectral lines of FWHM typically larger than few hundred km/s. The poorer sampled SED modes or the range scan options in combination with Nyquist sampling are more suitable for continuum studies.

Tip

Very long integrations over the full spectrum in range scan mode with high sampling density is possible with the PACS spectrometer but not recommended. Indeed, this is a very time efficient way to explore the continuum and detect spectral lines at the highest possible resolution but data could be barely calibratable. Two alternatives should be considered: (i) split up the full wavelength range into parts defining several AORs if the detection of faint lines is the goal, or (ii) increase the repetition factor in SED mode (or in range scan options in combination with Nyquist sampling) in order to increase the continuum sensitivity.

In SED mode parameters stored in the PACS Range Editor are ignored by the AOT backend except the reference wavelength and continuum flux density parameters if given. The observation will cover the blue or the red SED domain as defined by the selected grating order combination. In this mode, the observer has no control over the grating sampling density; the "Instrument settings" field is disabled for this purpose.

	Note
	In case one of the "Range scan" mode is selected in full wavelength range combined with "Nyquist sampling" then the observation is identical to the corresponding SED mode option.

The PACS Range Editor is used to set up the list of spectral ranges to be observed. The AOT allows a maximum of ten ranges to be entered into the table, although the use of the "Repetition factor" parameter could result in a further restriction of the number of observed ranges (see below).

	Tip
	If your observation requires more than the allowed number of ranges consider setting up a group of AORs. See Section 6.5.3, “Group/Follow-on Constraints” and Section 16.2, “Timing Constraints ” (grouping and timing constraints) for further information.

The table gives a summary of spectral range dependent attributes of your AOR. You cannot enter, modify, or remove values directly in the table. The three buttons at the bottom of the table allow the following:

Figure 11.11.  Window to add- and modify spectral range entries

On the right hand side of the observing mode settings area there is a button labelled "Nodding or map repetition cycles". The absolute sensitivity of the observation can be controlled by entering an integer between 1 and 100. This parameter has the effect of increasing the on-source time by repeating the nodding pattern the number of times that you specify. For each of the nod positions the sequence of range scans is repeated with the relative depth specified in the PACS Range Editor (see "Range Repetition" above). In SED mode this parameter has a very similar effect, the entire spectral range is repeated the number of times that is specified. In case crowded-field observation with mapping with off-position is used the on-source time is increased by repeating the map the number of times that is entered. No nodding is applyied in this latter configuration.

This mode is for observing point sources, i.e. sources that are not extended at the wavelength of observation. The integral-field concept allows simultaneous spectral and spatial multiplexing for the most efficient detection of weak individual spectral lines, with sufficient baseline coverage and high tolerance to pointing errors, without compromising spatial resolution. The PACS spectrometer arrays have 5 by 5 spatial pixels covering a 49 x 49 arcsecond field of view. Both channels view identical positions on the sky. The line flux from a point source object will always be collected with the filled detector array. Therefore, for a simple detection of a line source, one pointing is sufficient.

Figure 11.13.  Options for the Pointed observing mode

Background subtraction can be achieved through standard chopping/nodding technique.

Chopping/Nodding

This mode allows a classical three position nodding observation to eliminate inhomogeneities between the telescope and sky background. The PACS focal plane chopper is moved between the on-target and the off-positions during a grating scan. The whole sequence of spectral line scans is then repeated in the nod position. In total, one half of the integration time is spent on-source. This mode is always enabled.

Observing mode parameters - Chopping/Nodding

The chopper throw and chopper avoidance angle can be selected. The choice of throws "Small", "Medium" and "Large" refer to 1.0, 3.0 and 6.0 arcminute chopper throws on the sky respectively. The chopping direction is determined by the date of observation, the observer cannot directly influence this parameter. If a source of emission falls within the chopper throw radius around the target, the observer should consider setting up a chopper avoidance angle constraint. The angle can be 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

Setting up a chopper avoidance angle requires an additional constraint on mission planning, therefore this parameter should have to be used only for observations where it is absolutely necessary. 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 if a constraint is essential for your observation (see Section 10.5, “Position angle and chopper avoidance”). You may check your observation's footprint orientations by clicking in the HSpot's "Overlays" menu (see Chapter 19, Fixed Target and AOR Visualisation and Chapter 20, Moving Target Visualisation ).

This mode is offered for taking data on a point-like object in a similar way to the "Pointed" observing mode (see above). The underlying pointing mode is slightly different, as dithering will be carried out by small spacecraft movements perpendicular to the chopper direction. In this configuration the observing mode can compensate for image slicer effects, especially important for faint targets (see further explanation in the PACS Observer's Manual). The user parameters are identical to the "Pointed" mode.

Figure 11.14.  Options for the Pointed with Dither observing mode

Setting up the instrument mode parameters on the Mapping screen is identical to that for the "Pointed" or "Pointed with dither" modes. The difference can be seen at the bottom of the tab under "Raster map parameters" and the "Off-position" settings.

Figure 11.15.  Options for the Mapping observing mode

This mode allows the observer to set up a raster map observation in combination with chopping/nodding or off-position. If the field of view would not be placed in an emission-free area by chopping 6 arcminutes away, a selected off-position on the sky can be given as a reference position. Note that the reference position must be within 2 degrees of the target. The frequency of calibration blocks at the off-position is optimised by the AOT backend; the user has no control over it.

The figure above shows the options for raster mapping, however, parameter validation and ranges are slightly different depending on whether chopping/nodding, or an off-position is selected.

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. Mapping parameter ranges are defined as:

• Raster point step in [2, 480] arcseconds

• Raster line step in [2, 480] arcseconds

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

• Number of raster lines in [1, 100]

	Note
	The maximum map size is restricted to 6 arcminutes in chop/nod mode to allow for clean off-positions with the chopper for each raster position. Only the large chopper throw is allowed; sparsely sampled maps are possible. The maximum map size is 2 degrees in off-position mode; sparsely sampled maps are possible.

The breakdown of the observation is provided in the "PACS Time Estimation" screen (Figure 11.16, “ PACS Time Estimator main window”). It indicates the on-source time, calibration time, instrument and observatory overheads and the total observing time. Further details on calculation of the timing components can be obtained in the PACS Time Estimator messages window (see below).

	Note
	The 180 seconds observatory overhead is always charged to the observer as a compensation for the initial slewing time. In case the AOR is constrained (see Section 10.5, “Position angle and chopper avoidance” and Chapter 16, Timing Constraints Editing ) a total of 10 minutes observatory overhead is charged.

The spectrometer sensitivities over the requested ranges can be obtained by clicking on the "Range Sensitivity Plots" button of the time estimator main window. The pop-up window has two tabs, one for RMS sensitivities of a point source in 10^-18 W/m^2 units, the other is for point source continuum sensitivities in mJy units.

The individual curves represent system level sensitivities as a function of wavelength for each range specified in the PACS Range Editor plus sensitivities in the corresponding parallel ranges. The sensitivity estimation takes into account the relative line repetition factors, the number of observation cycles and relevant properties of the selected observing mode.

Figure 11.17.  Range sensitivity plots for spectral lines

Figure 11.18.  Range sensitivity plots for continuum

	Note
	Depending on the grating order/wavelength range combination a user selected range could have 0, 1 or 2 parallel ranges. These are shown as "free" ranges on the legend of the plot.

	Tip
	The observer should always check the range sensitivity plots before completing an AOR. This would help to avoid unnecessary duplications of data.

	Tip
	Using the left mouse button you can zoom in/out in the image. Hitting the right mouse button some further options can be obtained (Properties, Save as, Print..., Auto Range).

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

Figure 11.19.  PACS Time Estimator messages window

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