5.2 FIFI-LS

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5.2.1 Overview of AOTs

FIFI-LS specific instructions and reminders of general issues are given in the following topics below. It is necessary to read the FIFI-LS chapter of the Observer's Handbook before preparing detailed FIFI-LS observations in USPOT. Astronomical Observation Requests (AORs) should be created as described in Chapter 3.

The USPOT Observation drop-down menu lists the two Astronomical Observing Templates (AOTs) available for FIFI-LS: FIFI-LS and FIFI-LS OTF Mapping. Refer to the Observer's Handbook for a complete description of available combinations of configurations and modes for FIFI-LS.

The USPOT FIFI-LS Main AOR Window is divided into three columns. Figure 5.2-1 shows an example of the Main AOR Window of the FIFI-LS AOT. The instrument-specific fields are discussed in detail in this chapter. Contact the Help-Desk with any questions.

Figure 5.2-1.

Main AOR Window of the FIFI-LS AOT

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5.2.2 AOR Fields

The FIFI-LS AOR editing window is divided into three columns. The AOR Fields for FIFI-LS are discussed in the sections below. Table 5.2-1 lists the required fields for Phase I and Phase II for the available FIFI-LS AOTs. Conditional fields (i.e., fields not editable unless certain parameters are specified) are denoted with a footnote, with a reference to the required field to activate the conditional field. Fields that are not listed in these tables fall under one of three categories: fields not directly editable in USPOT (but may be affected by updating other fields, which are required; for more information on how particular fields may be related, refer to the corresponding sections within the Observer's Handbook—denoted in Table 5.2-1 by OH followed by the appropriate section number), fields intended for use only by SOFIA Support Scientists only, or optional fields.

All Observer's Handbook (OH) Reference links in the table below point to the latest version of the Observer's Handbook—currently Cycle 9. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

Table 5.2-1: FIFI-LS AOT: Required Fields for Phase I
Field Location Field Reference
New Target Window Specify Target § 3.4
Main AOR Window, First Column Rest Wavelength Blue; Width of Spectrum Blue; Rest Wavelength Red; Width of Spectrum Red § 5.2.2.1
Source Velocity (cz) OH § 3.2.2
Main AOR Window, Second Column & Lower Panel On source time per cycle; Cycles; MapType; 1Number of Points Along X; 1Number of Points Along Y; 2Import Map Offsets § 5.2.2.2; OH § 3.2.4
Main AOR Window, Third Column Instrument Mode OH § 3.2.1
Observing Condition & Acquisition / Tracking Window Target Priority § 3.4
Is Time Critical § 5.2.2.4

All Observer's Handbook (OH) Reference links in the table below point to the latest version of the Observer's Handbook—currently Cycle 9. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

FIFI-LS OTF Mapping AOT: Required Fields for Phase I
Field Location Field Reference
New Target Window Specify Target § 3.4
Main AOR Window, First Column Rest Wavelength Blue; Rest Wavelength Red § 5.2.2.1
Source Velocity (cz) OH § 3.2.2
Main AOR Window, Second Column & Lower Panel MapType; 1Number of Scans in Y direction; 1Number of Points in X direction; Map Offset X; Map Offset Y; Map Angle; 1Scanning speed in X; 1Scanning speed in Y; 1Scan Direction; 2Scan Length; 2Import Map Offsets § 5.2.2.2; OH § 3.2.4
Main AOR Window, Third Column Instrument Mode OH § 3.2.1
Observing Condition & Acquisition / Tracking Window Target Priority § 3.4
Is Time Critical § 5.2.2.4

All Observer's Handbook (OH) Reference links in the table below point to the latest version of the Observer's Handbook—currently Cycle 9. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

FIFI-LS AOT: Required Fields for Phase II
Field Location Field Reference
Main AOR Window, First Column Observation Order § 3.4
Width of Spectral Feature Blue; Width of Spectral Feature Red; Dichroic; Pointing Array § 5.2.2.1; OH § 3.1.1.2
Main AOR Window, Second Column Using multiple short AORs; Min Contiguous Exp Time; 1Step Size Along X; 1Step Size Along Y; 1Map Offset X; 1Map Offset Y; Map Priority; FOV Angle § 5.2.2.2; OH § 3.2.4
Main AOR Window, Third Column 3Nod Pattern; 4Chop Type; 5Total Chop Throw; 5Chop Angle Coordinate; 5Chop Pos Angle; 5Set Chop Angle Ranges button § 5.2.2.3; OH § 3.2.1; OH § 3.2.4
Reference Position Frame 6Ref Type; 6Map Ref. Pos.; Reference Name; 7RA Offset; 7Dec Offset; 8RA; 8Dec; 8Choose Position button § 5.2.2.3; OH § 3.2

All Observer's Handbook (OH) Reference links in the table below point to the latest version of the Observer's Handbook—currently Cycle 9. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

FIFI-LS OTF Mapping AOT: Required Fields for Phase II
Field Location Field Reference
Main AOR Window, First Column Observation Order § 3.4
Dichroic; Pointing Array § 5.2.2.1; OH § 3.1.1.2
Main AOR Window, Second Column Min Contiguous Exp Time § 5.2.2.2; OH § 3.2.4
Main AOR Window, Third Column [None]  
Reference Position Frame 6Ref Type; 6Map Ref. Pos.; Reference Name; 7RA Offset; 7Dec Offset; 8RA; 8Dec; 8Choose Position button § 5.2.2.3; OH § 3.2

1For MapType = Grid (FIFI-LS AOT) or Tile (FIFI-LS OTF Mapping AOT)
2For MapType = Custom; these observations also require maps to be imported via the Import Map Offsets button
3For Instrument Mode = Asymmetric Chop or Spectral Scan
4For Instrument Mode = Spectral Scan
5For Instrument Mode = Symmetric Chop, Asymmetric Chop or Spectral Scan
6For Chop Type = Asym
7For Ref Type = By Offset
8For Ref Type = By Position

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5.2.2.1 First Column: Selecting the Grating Parameters

The Observer's Handbook links below point to the latest version of the Observer's Handbook—currently Cycle 9. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

The first column is used to set the grating configuration. The parameters that can be set for the FIFI-LS OTF Mapping AOT are a subset of those for the FIFI-LS AOT.

The rest wavelengths for the blue (Rest Wavelength Blue field) and red (Rest Wavelength Red field) transition must be entered to be observed with this AOR. The rest wavelengths need to be accurate to 0.001 μm. Line lists are available at the MPE Garching. For both transitions also enter the width of the spectral feature of interest and the total width of the spectrum to be observed in km/s except for the Spectral Scan mode where the unit is microns.

The Width of Spectral Feature is only used by the instrument scientist together with the information in the proposal to judge whether there is enough baseline on both sides of the feature, when the observation returns a spectrum of the requested width. The value used in the execution of the AOR is the width of spectrum parameter. Both width parameters for each channel can be left at 0 km/s for unresolved lines and a minimal spectral dither pattern will be executed, which will include slightly more than the bandwidth (see Section 3.1.2.2 of the Observer's Handbook). If a wider spectrum than the bandwidth is requested, the grating scan will be adjusted to include the specified spectrum width (the observing wavelength will be in the center of the spectrum). In the second column, adjust the observing time accordingly (factor ℓ in Section 3.1.2.3 of the Observer's Handbook).

Enter the radial velocity of the source in km/s in the optical (cz) velocity convention in the Source Velocity field. The radial velocity can be rounded to 100 km/s, since the spectrum will be at least 1000 km/s wide wide and the velocity resolution is at best ~200 km/s (note that this means differences between the topocentric, LSR and barycentric frames can be disregarded).

One of the two dichroics needs to be selected so that an observation of both lines is possible. Typically, the 105_micron Dichroic is used unless a wavelength between 100 and 115 μm or shorter than 52.5 μm is observed (see Section 3.1.1.2 of the Observer's Handbook).

The choice in the Pointing Array field only affects the telescope pointing. It does not indicate a scientific priority. For most applications, the Pointing Array can be left at Blue. This choice will place the target coordinates (plus any mapping offsets, if applicable) on the center of the blue array. Ideally this would also be the center of the red array, but actually the red array is offset about 10 arcsec. This offset is reflected in the USPOT visualizations starting with version 3.4.2. Choosing the red array as the Pointing Array will put the target in the center of the red array but relatively close to the edge of the blue array. For the OTF Mapping AOT, the Pointing Array also controls the Step Size Along X and Step Size Along Y parameters (reported in the second column) as these are fixed by the size of the array.

Spectral 1 (FIF_BLUE) and Spectral 2 (FIF_RED) are fixed values.

For the OTF Mapping AOT, the maximum integration time per spaxel calculated from the inputs in column two (for MapType = Tile) is reported at the foot of column one.

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5.2.2.2 Second Column and Lower Panel: Setting the Integration Time and Map Type

The links below point to the latest version of the Observer's Handbook—currently Cycle 9. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

This section differs substantially between the FIFI-LS and FIFI-LS OTF Mapping AOTs.

For the FIFI-LS AOT:

Apart from the map parameters, the integration time is set in the second column. First, specify tnod (On-source exp. time per cycle) per nod cycle. Since a nod cycle must not take too long, the maximum values are 30 s and 15 s for the symmetric and asymmetric chopping, respectively. These values yield the best observing efficiency. To achieve longer exposure times, increase the number of nod Cycles to reach the desired ton, the on-source integration time per map position, as ton = cycles x tnod. The on-source exposure time ton is derived by the FIFI-LS exposure time calculator on the SOFIA Instrument Time Estimator (SITE) (see also Section 3.1.2. of the Observer's Handbook).

If shorter integration times are sufficient, the maximum values might still be the best option as the observing efficiency goes down with smaller values for tnod and the grating scan will get coarser (less spectral redundancy) as less time is available for it. The smallest tnod in the Symmetric Chop Instrument Mode (Sym) is 20 s and 10 s in the Asymmetric Instrument Mode. For bright objects, where one chop cycle is already sufficient, i.e. ton is 10 s or less, the Asymmetric Instrument Mode can be used with the ABA or AABAA Nod Pattern. These are more efficient because two or four map positions respectively are observed per reference position. The Total Power Instrument Mode always uses the ABA Nod Pattern and does not chop the secondary.

The overhead estimate for the Spectral Scan Instrument Mode is a rough estimate for this non-standard mode and will depend on the exact nature of the observation. This is only offered as an engineering mode, not for standard observing. Contact the Help-Desk for details.

The field On-source time per map position reports tnod times the number of cycles. Multiplying this by the number of map positions gives the total on-source exposure time for the whole observation. More detail on the time estimate can be found by selecting the Observation Est... button to get the total on-source time, the overhead, and total observing time. The total duration includes a 300 s overhead to setup the observation. For observations that are composed of multiple short AORs (e.g. multiple spectral settings on a single target or mapping of a source using separate AORs for each map pointing) this can be reduced to 60 s by setting the field Using multiple short AORs to true.

The Min Contiguous Exp Time field can be left at >0 s unless a long observation ( >1 h) is requested that must not be split and scheduled on separate observing legs or flights. Set this value to the minimum duration required for an observing leg. See also Sect 4.1 of the Flight Planning White Paper.

The position angle of the FOV of FIFI-LS is specified via the FOV angle parameter (see also Section 3.1.1.3 of the Observer's Handbook). If the angle is 0, the FOV is aligned so that North is up on the array. This angle rotates the FOV and any map offsets counterclockwise.

Two types of maps are supported via the MapType parameter: Grid and Custom. If Grid is selected, a rectangular grid of map positions can be specified, including an offset of the center of such a grid from the target position. If Custom is selected, the map offsets for a custom map optimized for the source geometry can be read in from a two-column csv file containing the map offsets in arcseconds. Make sure that there are no empty lines in the file. For both types, the offsets are specified along the FOV axes.

The map position either from Grid or Custom maps can be exported as csv files either as offsets from the source coordinate or as absolute coordinates with the respective buttons. It can be useful to create a Grid map first, export the offsets, and trim it and/or create shifted extra coverage in an editor for csv files. After importing the edited csv file, the result can be checked in the overlay.

The parameter Map Priority informs the instrument scientists how to prioritize the map observation. If Map Order is selected, the order of the map positions is strictly followed as listed in USPOT. If for unforeseen circumstances the observing time is cut short during a flight, the last map positions might be missing but most of the map positions will have been observed as long as planned. If Coverage is selected, the map is observed by looping through the map positions a few times, which ensures that the whole map is observed if the observing time is cut short but it will be (partly) less deep than planned.

For the OTF Mapping AOT:

The Min Contiguous Exp Time field can be left at >0 s unless a long observation ( >1 h) is requested that must not be split and scheduled on separate observing legs or flights. Set this value to the minimum duration required for an observing leg. See also Sect 4.1 of the Flight Planning White Paper.

The MapType parameter is used to select whether the OTF maps are created using the Tile or Custom method. If Tile is selected, the number of scans in X and Y directions and the scanning speed in X and Y are set in this column and the start points of the scans are calculated by USPOT and displayed in the lower panel. In the Custom method, the start points, scan speeds and scan directions are read in directly from a csv file. In both modes the current scan parameters can be exported to a csv file, so it is possible to create a template using the Tile method, export it and edit it, then read it in as a Custom map. The Custom map also supports changing the Scan Length in seconds, which is fixed at 30s for the Tile map, although this can only be changed for the whole AOR rather than for each scan.

Both methods support offsetting the map center from the defined target position using the Map Offset X and Y parameters. These add with the offsets defined in a Custom map, with the result being reported in the lower panel – note that the offsets are reset to zero when a csv file is read in so must be re-applied after this. Both modes also support altering the angle of the map using the Map Angle parameter – this defines the angle of the map scans, the angle of the array is fixed relative to these scans to always give optimal coverage. If the angle is 0, the map is aligned so that North is up (Y direction); this angle rotates the map and any map offsets (both for the map center and for the individual scans) counterclockwise.

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5.2.2.3 Third Column and Reference Position Frame: Selecting the Chop and Nod Parameters

In the third column and the reference position frame, the instrument mode and the chop and nod parameters are set. For the FIFI-LS OTF Mapping AOT only the instrument mode (which is always set to OTF Mapping) and the nod parameters in the reference position frame are available.

For the FIFI-LS AOT, the user chooses between the following Instrument Mode selections: Symmetric Chop, Asymmetric Chop, and Total Power (and, in engineering mode only, Spectral Scan). When either the Asymmetric Chop or Spectral Scan mode is chosen, the Nod Pattern can be chosen to be ABA (the default), ABBA, or AABAA; for Symmetric Chop mode this is fixed as ABBA and for Total Power it is fixed as ABA. When the Spectral Scan mode is selected, the Chop Type can be chosen to be either symmetrical (Sym) or asymmetrical (Asym). Otherwise, the Chop Type is fixed by the instrument mode selection.

Specify the Total Chop Throw in arcseconds and the position angle for the chop (Chop Pos Angle). The position angle of the chop is specified as Chop Pos Angle relative to the chosen Chop Angle Coordinate system: J2000 or HORIZON (the default). In J2000, the Chop Pos Angle runs counterclockwise from north. The choice of HORIZON selects to chop relative to the quasi-horizontal telescope coordinate system and the Chop Angle is fixed to 0 to achieve that. As Total Power is an unchopped mode, the chop parameters are not available in this mode.

If the Chop Type is asymmetric (Asym) or none (including Total Power and OTF Mapping modes), a reference (nod) position is required. This can be done by specifying an offset from the target position or by specifying an absolute position in the Ref Type box. If specifying an absolute position (Ref Type → By Position), the Choose Position button allows the user to enter sexagesimal numbers or to resolve object names. The map offsets in the second column will not be applied to the reference position unless Map Ref. Pos. (located in the third column, Reference Position frame) is set to true. A warning will be given for reference positions more than 1800” from the target position and an error for reference positions more than 2700” from the target position.

Use the visualization in USPOT to check the chop and nod parameters. It is important to ensure there is no chopping/nodding into emission. The best estimate for continuum emission may be Herschel/PACS-photometer maps which may be available at 70 μm, 100 μm, or 160 μm, which can be loaded into USPOT from a FITS-file. Figure 5.2-2 is a screen shot from USPOT visualizing the asymmetric chop with only one map position on a PACS 100 μm map. If the Chop Angle Coordinate system is selected to be HORIZON, the visualization in USPOT cannot know the corresponding position angle on the sky and plots the chop aligned with the FOV. Different position angles can be simulated by changing the FOV Angle in the third column.

Figure 5.2-2

Screen shot from USPOT visualizing the asymmetric chop with only one map position

Figure 5.2-2. The concentric red and blue squares are the FIFI-LS FOVs.The star in it denotes the source coordinates, which is in the center of the blue array because in this example the pointing array was set to blue. The green squares are the off-source chop separated from the target by 300 arcsec at a position angle of 220˚ (north is up here, but that depends on the orientation of the loaded background image). The turquoise squares are the reference position specified as a relative offset. A magenta square shows the off-chop for the reference position, but is outside of the image here. A chop angle range is specified and indicated by the yellow lines with triangles.

When chopping asymmetrically, the maximum chop amplitude varies with the position angle. The maximum chop throw varies between 250 and 600 arcsec. The range of position angles (PAs) where the maximum chop throw is below 600 arcsec is fixed with respect to the telescope. That means that the range of PAs with a limited chop throw is limited with respect to equatorial coordinates (J2000) depends on the rotation of field during the observation or in other words, when the observations is carried out. Since that is not known while the AORs have to be prepared, a range of possible chop angles must be specified if the chop throw is larger than 250 arcsec. Then the button Set Chop Angle Ranges is activated. Use it to open a dialog box to enter range(s) of possible chop angles. The visualization above shows a possible range from 190˚ to 250˚.

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5.2.2.4 Observing Condition & Acquisition / Tracking Window

Observations that are time critical, i.e. have to be performed during a certain date range (other than due to ordinary visibility constraints that are already included in the flight planning), should be marked as such in the Observing Condition & Acquisition/Tracking Window and the appropriate date constraint should also be entered in the same window (these are grayed out until Is Time Critical is set to Yes).

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