Chapter 11. PACS AOTs

Table of Contents

11.1. PACS Line Spectroscopy AOT
11.1.1. Wavelength Settings
11.1.2. Observing Mode Settings
11.1.3. PACS Time Estimator messages
11.2. PACS Range Spectroscopy AOT
11.2.1. Wavelength Settings
11.2.2. Instrument Settings
11.2.3. Observing Mode Settings
11.2.4. PACS Time Estimator messages
11.3. PACS Photometry AOT
11.3.1. Instrument Settings
11.3.2. Observing Mode Settings
11.3.3. PACS Time Estimator messages

11.1. PACS Line Spectroscopy AOT

In this AOT the PACS spectrometer is operating for observations of individual spectral lines. Depending on the requested grating order, the short-wavelength array observes in the 51-73 micron, or 70-105 micron band, while the long wavelength red array observes in the 103-220 micron band. For improved flat-fielding, the grating will be scanned by a number of discrete steps around the spectral line central wavelength, such that drifts in detector responsivity between individual pixels are eliminated. These grating scans provide a short spectrum covering ~1500 km/h with a resolving power of ~1700 in its highest resolution for each line and for each of the five by five spatial pixels. Background subtraction can be achieved either through standard chopping/nodding (with faint or bright lines option), or through 'unchopped grating scan' techniques of the grating mechanism (especially for line measurements of bright extended sources or point sources in crowded fields). The observer can select both chopping/nodding and unchopped grating scan techniques in combination with one of the two observing modes: "pointed" or "mapping".

The Line Spectroscopy AOT main window

Figure 11.1.  The Line Spectroscopy AOT main window

11.1.1. Wavelength Settings

The "Wavelength Settings" part of the PACS Line Spectroscopy start-up screen provides the observer with the opportunity to specify a set of spectral lines to observe within a single AOR. The main component of this functionality is the "PACS Line Editor" table which is where spectral line information is held. Before the observer provides table entries, the applicable wavelength ranges have to be specified. Selection of wavelength ranges

The user selects the applicable wavelength ranges (defined by the grating diffraction orders) via the first pull-down menu. There are two possibilities:

  • Option 1 - "[70-220] microns (2nd + 1st orders) " - default option

  • Option 2 - "[51-73] and [103-220] microns (3rd + 1st orders)"

As a result of the selection, the (redshifted) wavelengths of line centres have to be specified in the "PACS Line Editor" table, either within the 70-220 microns band, or within the 51-73 and 103-220 microns band.

In the 2nd + 1st orders combination the 103-105 microns range is overlapping as defined in the nominal wavelength ranges of these orders (70-105 and 103-220 microns). In case a line centre wavelength is reqested in this small overlapping range than PACS is observing this line in the 1st grating order.
As it is indicated in the options above, irrespective the blue order selection spectral lines can be always requested in the 103-220 microns band (1st diffraction order) of the red spectrometer array.
The observer has a choice of creating an observation of spectral lines located along the entire PACS wavelength range by grouping AORs. In this case at least two AORs have to be filled with flipped 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 PACS Line Editor

When setting up the list of spectral lines to be observed the PACS Line Editor is used. The AOT allows a maximum of ten lines to be entered into the table, although the use of "Repetition factor" parameter could result a further restriction on the number of observable lines (see below).

If your observation requires more than the allowed number of lines consider to set 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. Setting Values in the PACS Line Editor

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

Window to add- and modify spectral line entries

Figure 11.2.  Window to add- and modify spectral line entries

  • Add line manually - pressing this button allows the addition of a new spectral line in the chosen wavelength range. A "Create a new line" window is opened and the user can enter values in the enabled fields. The "Line ID" and "Wavelength" fields cannot be edited if the line is taken from a database. Once the "OK" button is pressed the spectral line settings are stored in the PACS Line Editor.

  • Add line from database - The selection of a given line/transition can be made via the HSpot spectral line managing system. Clicking on this button will make the "Add Lines to the Observation Request" dialogue window pop-up (see below). This dialogue allows the addition of a spectral line that is within the PACS [55-210] microns wavelength range. Once the line/transition is selected and the "OK" button is clicked the line settings are stored and a synopsis is displayed in the PACS Line Editor. Line attributes will show default settings in the table, you have to click on "Modify Line" to reset parameters.

  • Modify line - This button allows spectral line settings to be modified after they have been created with the "Add line manually" or "Add line from database" functionalities noted above. The window popping-up is identical to the "Create a new line" dialogue in all functionalities (see below).

  • Delete line - Allows the deletion of a spectral line. Click on the table entry holding the spectral line to highlight it then hit the "Delete" button to remove it. Deleting a line has no effect on other lines in the table. Default line IDs will not change for already configured lines. Add Lines to the Observation Request dialogue

The "Add Lines to the Observation Request" dialogue is a shopping basket which filters PACS detectable transitions from the HSpot "Manage Lines" facility (see Section 18.1, “Managing Lines”).

"Add Lines to the Observation Request" dialogue

Figure 11.3.  "Add Lines to the Observation Request" dialogue

The "Add Lines to the Observation Request" dialogue has some rules applied to it:

  • Only one line may be selected in a single dialogue. To retrieve additional spectral lines, click on again the "From Database" check box and repeat the procedure for the next line in the table.

  • If your "Manage Lines" window does not contain any user defined lines that fall in the specified PACS wavelength range the "Add Lines to the Observation Request" dialogue will open containing the default lines only. You should click on "Cancel" and fill your line database with appropriate transitions through the "Manage Lines" dialogue (see Section 18.1, “Managing Lines”).

Default lines included in the "Add Lines to the Observation Request" dialogue provides a handful set of most often requested strong lines. Some of these transitions (mostly atomic lines) are not included in the online available catalogues linked to HSpot.
An extended list of lines can be added to the "Add Lines to the Observation Request" window from online catalogues and/or individual line information provided by the user (see Section 18.1, “Managing Lines” and the functionalities of the "Manage Lines" dialogue). Create a new line / Update a line dialogues

Pressing the "Add line manually" or "Modify line" button will pop-up either the "Create a new line" or the "Update a line" window. The content of these two windows are identical and allow the user to enter/modify values in the PACS Line Editor.
The Create a new line / Update a line dialogues include optional parameters for signal-to-noise calculation in case the observer can supply source estimates. Optional parameters are highlighted in green text in the HSpot windows.
Line ID

Mandatory parameter. A unique line identification label. It has to be at least one character different for the subsequent lines, default values are "Line 1, Line 2, ... Line 10". User defined Line Ids could contain any character string including spaces up to 40 characters. If the user removes a line from the table (see below) the Line Id will not change for the remaining set of lines.

If a spectral line is taken from the "Manage Lines" database (see Section 6.5.3, “Group/Follow-on Constraints” and Section 16.2, “Timing Constraints ”) then the Line Id will be copied from the database. This string can be overwritten as can default line Ids. If a line is taken from a database the user is not allowed to reset its Line Id and wavelength in the Line Editor Table. Attributes of database transitions can be changed using the "Manage Lines" functionality (see Section 18.1, “Managing Lines”).
Wavelength (microns)

Mandatory parameter. The rest wavelength of the line centre. If no redshift is specified, the PACS grating will perform an up- and down scan, centred around the line's central wavelength. For about two thirds of the on-source time PACS measures the (unresolved) spectral line and about one third of the time is spend on measuring the baseline. See the PACS Observers' Manual ( for more information.

  • Lines/transitions taken from a database always refer to the rest wavelength.

  • The wavelength order of spectral lines as entered in the Line Editor Table has no effect on the duration of the observation. The AOT backend logic sorts the observing sequence of spectral lines to minimise instrument (grating) overheads.

Redshifted wavelength (microns)

Parameter calculated by HSpot, it appears only in the PACS Line Editor and indicates the redshifted wavelength of the line centre. The bottom pull-down menu allows the reference frame for the redshift to be input.

  • The redshifted wavelength has to fit within the selected grating order combination.

  • Each redshifted wavelength in the PACS Line Editor have to be at least 0.001 microns apart.

Line flux unit

This menu gives the flexibility to switch between physical input units supported by the PACS AOT logic. The pull-down menu allows the unit of line flux to be defined:

  • Option 1 - "Flux (10^-18 Watt/m^2)", default option

  • Option 2 - "Flux (10^-15 erg/s/cm^2)"

The "Line Flux" parameter of the Line Editor Table is interpreted in the selected units.

In cgs units the flux is measured in erg s^-1 which can be converted to watts, 1 watt = 1 x 10^7 erg s^-1.
Line flux

Optional parameter. User supplied line flux estimate in units specified by the "Line Flux Units" option. Line flux input is used for signal-to-noise estimation as well as for the optimization of the dynamic range. Leaving the parameter as the default 0.0 value means the PACS Time Estimator will not perform signal-to-noise estimation (sensitivity estimates are still provided) and default integration capacitor may be used with the smallest dynamic range.

The uplink logic automatically selects the integrating capacitance based on estimated continuum and line fluxes. If an observation contains lines that fall in different flux regimes, the largest applicable capacitance will be chosen for the entire observation. If lines in the same observation fall in different flux regimes, it is recommended to split the observation into separate observations per flux regime. The HSpot Time Estimator Message (click on "Observation Estimation / PACS Time Estimator Messages") indicates if other than the default capacitance has been selected for a given combination of lines.

If continuum and expected line fluxes are higher than the saturation limits for the default capacitance, it is mandatory to enter the expected continuum and line flux for every line in HSpot. Observations that are saturated because no HSpot flux estimates were entered by the observer will not be considered as failed for technical reasons. Saturation limits are presented in PACS Observers' Manual (
Continuum flux density (mJy)

Optional parameter. Continuum flux density estimate at the line (redshifted) wavelength. The value of this parameter is interpreted by the PACS Time Estimator as flux density for a spectrometer resolution element. Leaving the parameter as the default 0.0 value means the PACS Time Estimator will not perform signal-to-noise estimation (sensitivity estimates are still provided) and default integration capacitor may be used with the smallest dynamic range.

The PACS spectrometer spectral resolution as a function of wavelength and gration order can be found in the PACS Observers' Manual (
Line width unit

This menu gives the flexibility to switch between physical input units supported by the PACS AOT logic. The pull-down menu allows the unit of line width to be defined:

  • Option 1 - "1 km/s", default option

  • Option 2 - "1 micron"

The "Line Width" parameter in the PACS Line Editor is interpreted in the selected unit.

Line width (FWHM)

Optional parameter. The spectral line full width at half maximum value in units specified by the "Line Width Units" pull-down menu. Line width input is used only for checking purposes. It helps the observer to ensure the specified line width fits within the predefined wavelength range hard coded in the PACS Line Spectroscopy AOT logic (see the PACS Observers' Manual ( for further details). None of the instrument or observing mode settings can be influenced by this parameter. The PACS Time Estimator will perform signal-to-noise estimation for the specified line width.

The PACS Line Spectroscopy AOT is designed to measure unresolved or narrow spectral lines. HSpot raise a warning message if the specified line is too broad for a complete profile detection in PACS Line Spectroscopy AOT. In such a case the observer should consider to measure this spectral line in PACS Range Spectroscopy AOT high sampling density range scan mode.
Line Repetition

Mandatory parameter. The relative line strength (fraction of on-source time per line) is taken into account by specifying the grating scan repetition factor for each line. A maximum of 10 repetitions in total can be specified in the table. For instance, in the case that 10 lines are selected, the "Line repetition" factor has to be 1 for each line; if 3 lines are selected then the total of the 3 repetition factors has to be less or equal to 10 (e.g. 4+5+1 or 2+3+3 ...). If the sum of repetitions exceeds 10 then you must either remove spectral line(s), or reduce the scan repetition factor(s).

The following considerations have to be made when specifying the repetition factor:

  • Each grating scan has equal duration irrespective of the observation wavelength.

  • The grating scan pattern is repeated for every on-source and nod position (except in wavelength switching mode).

  • Higher sensitivity can be achieved either by scanning one line more often within a nod position, or by repeating the nod pattern of grating repetitions more often (see "Observing mode settings"). The latter would then affect all lines in the PACS Line Editor. If you are observing only one or two lines you should consider increasing the "Line repetition" factor in the table. This is more efficient as it minimises the nod slewing overhead.

  • In wavelength switching mode the "Line repetition" factor has precisely the same effect as the "Nodding or wavelength switching cycles". For instance, 3 repetition of a line with 2 cycles gives exactly the same on-source time and same measurement sequence if 2 line repetition would be combined with 3 cycles.

You can specify an observation with more than ten grating scans per pointing position by grouping AORs. Redshift Selection

The redshift selection menu allows the observer to make adjustments to the observing wavelength. The input can be specified either in known radial velocities or redshifts. Once any area in the PACS Line Editor is clicked on the "Redshifted Wavelength" field turns to show the wavelength to be observed.

If the line rest wavelength is shorter than 55 microns then HSpot will reject the line even if the redshifted wavelength fits to the valid PACS range. For such a line you need to enter directly the redshifted wavelength into the PACS Line Editor and set the redshift to default zero.

11.1.2. Observing Mode Settings

The "Observing Mode Settings" screen provides the observer with the opportunity to specify the combination of instrument modes with spacecraft pointing modes. To start the procedure just click on the "Set the Observing Modes" action button in the main AOT window. This returns a screen with the choice of observing modes, simple click on a tab to select the mode required. The default observing mode is "None selected". You must then choose another tab to validate the selected observation.

The Observing Mode Settings window in its start-up status

Figure 11.4.  The Observing Mode Settings window in its start-up status

The main driver of observing modes is the pointing mode in which you wish to observe. The tab labels refer to "Pointed" and "Mapping" observing modes, each of them can be combined either with chopping/nodding or unchopped grating scan mode. Nodding, unchopped grating scan or mapping cycles

On the left hand side of the observing mode settings area a text box labelled "Nodding, grating scan or mapping cycles"can be found. The absolute sensitivity of the observation can be controlled by entering an integer number between 1 and 100. In chop mode, the on-source time is increased by repeating the nodding pattern the number of times that is entered. For each of the nod positions the sequence of line scans is repeated with the relative depth specified in the PACS Line Editor (see Section, “The PACS Line Editor”). In case of unchopped grating scan mode, this cycle is used to repeat the ON-OFF blocks the number of times that is entered. No nodding is applied in this latter case. Pointed mode

This mode is offered for taking data for a point-like object. 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 square field of view of 47 by 47 arcseconds. 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.

Options for the Pointed observing mode

Figure 11.5.  Options for the Pointed observing mode

Background subtraction can be achieved through a standard chopping/nodding beam modulation technique or applying the unchopped grating scan mode.


This mode allows a classic three-position nodding observation to eliminate inhomogeneities in to the telescope and sky background. The PACS focal plane chopper is moved between the on-target and the off-positions during a grating scan and the whole sequence of spectral line scans is repeated in the nod position. One half of the total integration time is spent on-source. This mode can be selected by clicking on "Chopping/nodding" in the "Observing mode selection" field.

The chopper throw and chopper avoidance angle can be selected. The choice of "Small", "Medium" and "Large" refer to 1.5, 3.0 and 6.0 arcminutes chopper throws respectively on the sky. 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.

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 (see Section 10.5, “Position angle and chopper avoidance”). You may check your observation's footprint orientations by clicking in the HSpot "Overlays" menu (see Chapter 19, Fixed Target and AOR Visualisation and Chapter 20, Moving Target Visualisation ).
Chopping/nodding (bright lines)

This option is similar to the normal chopping/nodding option described above but optimized for spectral line measurements relaxed in sensitivity. By the grating up- and down scan only 10 positions are visited to cover the central part of the faint-line spectral range. Grating step size is identical to the default faint-line mode. This mode is 3-4 times faster than normal chopping/nodding depending on the order.

Unchopped grating scan
This is a 2nd generation observing mode, replacing wavelength swtiching in HSpot v5.0 and later versions.

The unchopped grating scan is an alternative to the chopping/nodding mode if by chopping to a maximum of 6' the off position field-of-view cannot be on an emission free area, for instance in crowded-fields or for spectral line mapping of extended objects with diameter larger than 5' respectively. This mode is not recommended for faint lines, target lines needs to be above typically ~1 Jy peak-to-continuum and the continuum level can be recovered in a reliable way only for bright sources, i.e. at a minimum continuum level of ~ 20-30 Jy.The continuum level can be determined by off-position subtraction which could efficiently eliminate the telescope background for bright objects.

Unchopped grating scan (bright lines)

In HSpot versions v5.3 and later the unchopped grating scan mode has been extended with a sub-mode option for bright lines, this is called "Unchopped grating scan (bright lines)". This option is similar to the normal unchopped grating scan described above but optimized for spectral line measurements relaxed in sensitivity.

This sub-mode should be used with single line repetition only, for bright lines where the signal-to-noise is at least 20-25 in this configuration.

In the bright line mode the grating scans over a shorter wavelength range (30% shorter) than in the standard unchopped mode suitable for faint lines. This is a compromise, the continuum on both sides of an unresolved spectral line is less sampled, but still all the 16 spectral pixels of the PACS detectors scan through the full line profile. The standard unchopped calibration scheme remains valid for this new sub-mode, however, the shorter continuum coverage might cause a slight degradation in data quality because transient effects in system response may not be elimiated at high accuracy over the wavelength range of the line profile. Please consult the PACS Observer's Manual for more informarion on this mode.

Unchopped grating scan off-position panel

The off-position can be specified by relative offset in arcminutes with respect the target coordinate, or alternatively, you can define an absolute position within a two degrees radius. In case you prefer to use the absolute position option then it is highly recommended to specify RA/Dec via the standard HSpot target definition window. You can have access to this window by clicking on the "Choose Position" button. This panel is enabled only for the unchopped grating scan mode. Pointed with Dither mode

This mode has been deprecated. HSpot v5.0 and later versions allow the reading of AORs in 'Pointed with dither' mode but time estimation has been disabled and submission to HSC is not possible.

Originally, this mode has been offered to take data for a point-like object in a very similar way to "Pointed" observing mode (see Section, “Pointed mode”). Dithering was done by small spacecraft movements (one third of a spatial pixel size respectively) perpendicular to the chopper direction and perpendicular to the image slicer orientation. In such a configuration this observing mode was aimed to compensate for image slicer effects, especially important for faint targets.

It has been proven during the Performance Verification Phase that flux reconstruction from a single pointed observation is as good as in dithering mode, therefore dithering option is not recommended anymore. For sources with a well confined photocenter (point- or compact sources), the pointing mode can be changed from 'Pointed with dither' to 'Pointed'. To maintain the observation integration time, nod repetition and/or scan repetitions should be increased until the original observing time is reached. Nod or range repetition x 3 should be the appropriate change for most observations. Observations requiring spatial oversampling should use a minimum 2x2 size raster with recommended step sizes. Mapping mode

Setting up the instrument mode parameters on the "Mapping" screen is identical to that for the "Pointed" or "Pointed with dither" modes. The one difference can be seen at the bottom left of the tab within the "Raster map" frame.

Options for the Mapping observing mode

Figure 11.6.  Options for the Mapping observing mode

This mode allows the observer to set up a raster map observation in combination with chopping/nodding or unchopped grating scan techniques. The chopping/nodding layout is identical to the logic applied for "Pointed" mode, however, nodding cycles are repeated for every raster positions.

The figure above (Figure 11.6, “ Options for the Mapping observing mode”) shows the user options for raster mapping, however, parameter validation and ranges are slightly different depending on whether chopping/nodding, or unchopped grating scan 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]

  • 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 unchopped range scan mode; sparsely sampled maps are possible.

Setting the map orientation angle

Selecting the chop/nod mode the map (raster line) orientation is defined by the chopping direction, the observer has no direct access to the map orientation parameter (disabled field). However, the observer has two choices to influence the map orientation:

The sky reference frame can be selected only in unchopped grating scan mode. Please note, in unchopped grating scan mode the HSpot default option is 'sky' reference, but we highly advise to switch to 'instrument' mode if suitable for the science case.

In unchopped grating scan mode, if an AOR raster covers an elongated area (e.g. a nearby edge-on galaxy) then the observer might have no other option then using sky reference frame and turn the raster to the right direction. If the target is at higher ecliptic latitudes then you may select instrument reference frame and put a time constraint on the AOR. The appropriate time window can be identified in HSpot "Overlays/AORs on images..." option by changing the tentative epoch of observation. This way the array can be rotated to the desired angle by the time dependent array position angle.

Wavelength Switching
This mode has been deprecated. For crowded-field spectroscopy, and in more general, for science cases where chopping is not possible it is advised to use the unchopped range scan mode instead. Observers with AORs in wavelength switching mode could still read these observations in HSpot but time estimation is not possible any further in HSpot v5.0 and later versions.

11.1.3. PACS Time Estimator messages

To get a time estimate for the observing mode configuration chosen and its associated noise level, click the "Observation Estimation" button on the bottom of the AOT main screen. This runs the PACS Time Estimator and once the calculation is done brings up a window "PACS Time Estimation".

PACS Time Estimator main window

Figure 11.7.  PACS Time Estimator main window

Running the estimator invokes a sequence of optimisation processes through possible combinations stored in instrument configuration tables. This process selects the most efficient way of observing on-target, reference and calibration measurements and also minimises the instrument overheads.

The exact time for the observing sequence and the associated expected noise are presented to the observer. A set of messages and information regarding the sequence chosen are also given. PACS Instrument Performance Summary

The breakdown of the observation is provided in the "PACS Time Estimation" screen (Figure 11.7, “ 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).

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. PACS Time Estimator messages window

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

PACS Time Estimator messages window

Figure 11.8.  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:

  • HSpot decoding logic; number of repetitions per spectral line and observing time per line per pointing position

  • Observing time required; minimum observing time and actual requested time

  • AOT, pointing mode and nodding / wavelength switching info

  • Global AOT durations

  • Setup and calibration summary

  • Spectral line summary; observing wavelengths, continuum sensitivity for a point source, spectral line sensitivity for a point source, observing time spent per line, on-source time plus observing time on reference position (chop-off field)