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- 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 Astronomical Observing Template (AOT) available for FIFI-LS. Refer to the Cycle 6 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 dividied into thre 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.
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 list the required fields for Phase I and Phase II for the available FIFI-LS AOT. 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 for Cycle 6—denoted in Table 5.1-1 by OH6 followed by the appropriate section number), fields inteded for use only by SOFIA Support Scientists only, or optional fields.
Required Fields for Phase I
|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||§ 18.104.22.168|
|Source Velocity||OH6 § 3.2.2|
|Main AOR Window, Second Column||On source exp. time per cycle Cycles MapType 1Number of Points Along Lat 1Number of Points Along Lon 2Import Map Offsets||§ 22.214.171.124; OH6 § 3.2.4|
|Main AOR Window, Third Column||Instrument Mode||OH6 § 3.2.1|
|Observing Condition & Acquisition / Tracking Window||Target Priority||§ 3.4|
|Is Time Critical||§ 126.96.36.199|
Required Fields for Phase II
|Main AOR Window, First Column||Observation Order||§ 3.4|
|Width of Spectral Feature Blue Width of Spectral Feature Red Dichroic Pointing Array||§ 188.8.131.52; OH6 § 184.108.40.206|
|Main AOR Window, Second Column||Min Contiguous Exp Time 1Step Size Along Lat 1Step Size Along Lon 1Map Offset RA 1Map Offset Dec Map Priority FOV Angle||§ 220.127.116.11; OH6 § 3.2.4|
|Main AOR Window, Third Column||3Chop Type Total Chop Throw Chop Angle Coordinate Chop Pos Angle Set Chop Angle Ranges button||§ 18.104.22.168; OH6 § 3.2.1; OH6 § 3.2.4|
|Reference Position Frame||4Ref Type Map Ref. Pos. Reference Name 5RA Offset 5Dec Offset 6RA 6Dec 6Choose Position button||§ 22.214.171.124; OH6 § 3.2|
2For MapType = Custom; these observations also require maps to be importved via the Import Map Offsets button
Return to: Table 5.2-1
The first column is used to set the grating configuration.
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, if there is enough baseline on both sides of the feature, when the observation returns a spectrum of the requested width. Thus, the value used in the execution of the AOR is the second width 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 126.96.36.199 of the Observer's Handbook). If a wider spectrum is requested the grating scan will be adjusted to include the specified 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 188.8.131.52 of the Observer's Handbook).
Enter the radial velocity of the source in km/s 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.
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 is observed (see Section 184.108.40.206 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.
Spectral 1 (FIF_BLUE) and Spectral 2 (FIF_RED) are fixed values.
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 (see also Section 220.127.116.11 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 Bright Object Instrument Mode can be used. It is more efficient because two map positions are observed per reference position.
The overhead estimate for the Spectral Scan Instrument Mode is a rough estimate for this shared risk mode and will depend on the exact nature of the observation. Contact the Help-Desk for details.
The field On-source exp. time field reports tnod times the number of map positions. Multiply this duration with the number of cycles to get the total on-source exposure time for the whole observation or select the Observation Est... button to get the total on-source time, the overhead, and total observing time. The total duration includes a 60 s overhead to setup the observation.
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 18.104.22.168 of the Observer's Handbook). If the angle is 0, the FOV is aligned so that North is up on the array. This angel rotates the FOV and any map offsets counterclockwise.
Two types of maps are supported: 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 might 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.
In the third column, the chopping mode and parameters are set. The user chooses between the following Instrument Mode selections: Symmetric Chop, Asymmetric Chop, Bright Object, and Spectral Scan. When the Bright Object or Spectral Scan is selected, the Chop Type can be chosen to be either symmetrical (Sym) or asymmetrical (Asym). Otherwise, the Chop Type is fixed.
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. 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.
If the Chop Type is asymmetric (Asym), a reference 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.
Use the visualization in USPOT to check the chop 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.
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˚.
If an altitude higher than 38,000 ft was entered in the Exposure Time Estimator, select Low or VeryLow for Requested WV Overburden in the window accessed via the Observing Condition & Acquisition/Tracking button.