5.1 EXES

v8.1.0

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5.1.1 Overview of AOTs

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

The USPOT Observation drop-down menu lists the four Astronomical Observation Templates (AOTs) available for EXES: HIGH_MED, Medium, HIGH_LOW, and Low. Refer to the Observer's Handbook for a complete description of available combinations of configurations and modes for EXES.

The USPOT EXES Main AOR Window contains several frames: EXES and Nod & Map. Figure 5.1-1 shows an example of the Main AOR Window of an EXES AOT. The instrument-specific fields are discussed in detail in this chapter. For instrument and calibration questions, contact the instrument PI. For USPOT related questions and any other questions, contact the Help-Desk.

Figure 5.1-1.
Main AOR Window of an EXES AOT

Figure 5.1-1. An example of an EXES AOT Main AOR Window, using the EXES HIGH_MED AOT.

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5.1.2 AOR Fields

Table 5.1-1 lists the required fields for Phase I and Phase II for all available EXES 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 5.1-1 by OH followed by the appropriate section number), fields inteded for use only by SOFIA Support Scientists only, or optional fields.
 
Table 5.1-1.

Phase I Required Fields
All Observer's Handbook (OH) Reference links in the table point to the latest version of the Observer's Handbook—currently Cycle 8. 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.

Field Location Field Reference
New Target Window Specify Target Section 3.4
EXES Frame EXES clock time SITE ; 5.1.2.1
Desired S/N per resolution element EXES Central Wavelength OH §2.2
Slit OH § 2.1.2.1
Nod & Map Frame Nod Style OH § 2.1.2.1
Nod & Map Frame, Map Tab 1Step Size X 1Step Size Y 1Num Steps OH §2.2
Observing Condition & Acquisition/Tracking Window Is Calibrator § 3.4
Is Time Critical § 5.1.2.1

Phase II Required Fields

Field Location Field Reference
Main AOR Window Observation Order § 3.4
Echelle Order Min Contiguous Exp Time § 5.1.2.1
No Peak-Up No Wavelength Setup § 5.1.2.1; OH §2.2.3
Detector Shift § 5.1.2.1
Reference Position Frame, Reference Position Tab 1Dedicated Refere... 2RA Offset 2Dec Offset OH §2.2
Reference Position Frame, Map Tab 1X Position 1Y Position 1Rotation Angle Frame OH §2.2
1For Nod Style = Map
2For Nod Style = Map and Dedicated Refere... = Yes
Return to: Table 5.1-1
 

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5.1.2.1 EXES Frame

The latest EXES sensitivities are available using the exposure time calculator accessed through the SOFIA Instrument Time Estimator (SITE). It is the main source of information for determining instrument settings, most of which depend strongly on wavelength, including:

  • available echelle orders
  • available slit widths
  • available single setting instantaneous wavelength coverage
  • ability to nod on or off slit (at many wavelengths in the high resolution modes, the slit is too short to be able to nod on slit)
  • EXES clock time

It is important to check in SITE whether an observational setup is supported, as the validation button in USPOT may not catch some unsupported settings combinations. The EXES clock time field (note that this is not the on-source time) includes overheads due to nodding and instrument inefficiencies. For each AOR, USPOT will add to the time request 15–23 minutes of peak-up, wavelength optimization, flux calibration, and flat field overheads to the clock time. If no peak-up is necessary (e.g., after a wavelength change on the same target, or if the source is extended, or if the continuum emission is too weak), the overheads can be reduced by setting the No Peak-Up field to True. Overheads can also be reduced if multiple sky positions are observed in the same wavelength setting. In this case, set the No Wavelength Setup field to True. Note however that the time on a given target on a single flight is limited to 90–180 minutes, so full overheads may be needed again once the sum of AOR times exceeds 90 minutes. Conversely, if a single observation takes more than 90 minutes, it may need to be split into multiple AORs, each with full overheads. Please consult the EXES and SOFIA staff in these cases.

The Min Contiguous Exp Time field is the minimum amount of clock time that must be observed in a single flight. Values smaller than the total clock time will improve the probability that the AOR will be scheduled. At Phase II, this field must not be left at 0 seconds. If the entry is larger than 5400 seconds, it will be very difficult to get the AOR scheduled.

The EXES Central Wavelength field of the line of interest should be corrected for heliocentric motions. If applicable, use the Time Critical fields under the Observing Condition & Acquisition / Tracking button to indicate the range of dates at which the emission/absorption line of interest is sufficiently well separated from any atmospheric lines (such as for corrections for the Earth’s motion with respect to the Sun which are generally not important for the EXES Central Wavelength setting.)

For the Echelle Order field, select from the options listed in the exposure time calculator on SITE at the wavelength of interest. Note that in the exposure time calculator, the Echelle Order is referred to as the observing order—not to be confused with the Observation Order field in USPOT, which is the order in which this observation should be executed relative to others in the program (e.g., if you want certain wavelength settings done before others). This sets the wavelength coverage and slit length and thus, depending on the echelle order specified, determines whether on-slit nodding is possible.

In Observing Condition & Acquisition / Tracking Window (accessed via the Observing Condition & Acquisiton / Tracking button in the AOR editing window), give all AORs for the same target the same priority in the Target Priority field. For each target specify the order in which the AORs should be scheduled (Observation Order in the main EXES frame). Enter any finer scheduling priorities in the AOR comments section. For the Medium and Low AOTs, partial detector read-outs may be required to avoid detector saturation, which will reduce the effective length of the slit to 30-60", depending on the echelle order and background. Please consult with the PI and Instrument Support Scientist if a long slit length is important to the observation strategy.

The Detector Shift field value in pixels can be determined by using exposure time calculator on SITE. High resolution configurations (above 19 μm) contain wavelength gaps between the echelle orders; these gaps can be covered by shifting the detector array.

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5.1.2.2 Nod & Map Frame

USPOT does not determine whether on-slit nodding (Nod Style = NOD_ON_SLIT) is possible given the slit length. For this, use the exposure time calculator on SITE and modify the Nod Style entry in USPOT accordingly.

If nodding off slit is needed (Nod Style = NOD_OFF_SLIT), check that the nod position is free from emission at the wavelength of interest. The AOR image overlay function may be useful. Adapt the nod throw and/or angle accordingly. The angle is relative to the slit in the SIRF coordinate system, and east of north in the SKY/ERF system.

In mapping mode (Nod Style = MAP), EXES observes a single stripe of slit positions. The Num Steps field is the number of slit positions on the sky. Note that multiple stripes must be defined in separate AORs and the stripe orientation on the sky cannot be controlled.

The flux calibration and correction for telluric absorption lines will be done using sky and blackbody measurements, which are accounted for in the standard overhead. Lines overlapping with sky lines may need a telluric calibrator. These calibrator observations must be defined in separate AORs and they will count against the allocated time. Because of the difficulty of scheduling a given telluric calibrator with the science target in a given flight, the specific calibrator will need to be chosen at the time of flight planning in consultation between the proposer, instrument PI, and SMO support scientist. For wavelengths below 8–10 μm this will most likely be a hot, bright star (e.g., Vega or Sirius) and at longer wavelengths an asteroid. Galilean moons will also be considered, provided they are well separated from Jupiter. In USPOT, a separate observation entry should be entered by selecting New Target with Target Name Cal_target, where target is the name of the associate science target (e.g. Cal_IRC+10216), and given the coordinates RA:00:00:00, Dec: +00:00:00. One must use the EXES exposure time calculator on SITE to estimate the clock time needed, assuming a continuum brightness of 100 Jy below 10 μm and 150 Jy above 10 μm for the HIGH_MED and HIGH_LOW AOTs. For the Medium AOT, a brightness of 50 Jy should be assumed, and for Low, 25 Jy at all wavelengths. One is urged to limit the EXES clock times on the telluric standard at a given wavelength and instrument configuration to less than about 30 minutes. Further improvement of the removal of telluric absorption features may be achieved by employing models of the Earth's atmospheric transmission.

The Example Rotation Angle field is meant only for visualization of the AOR on the sky. It will not set the EXES slit orientation during the observations. Additionally, the EXES slit orientation on the sky cannot be controlled, except when putting time constraints on the observation. Note that time constraints will reduce the probability that the AOR will be scheduled. SOFIA’s Target Visibility Tool, now also included within USPOT, is intended to determine the EXES slit orientation on the sky at a given date and position on Earth.

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5.1.2.3 Comments Window

Enter the following information in the Comments section (accessed via the Comments... button at the bottom of the AOR editing window):

  • The wavelengths of other important lines expected in the same setting.
  • If a telluric or photometric calibrator must be observed, mention explicitly which AOR this corresponds to. Do the same in the notes of the calibrator AOR.
  • Any information that will help with the target acquisition. Telescope acquisition and guiding is done in the optical (up to z-band). Mention if no guide stars brighter than 16th magnitude within a 9 arcmin radius are available. For accurate positioning of the slit, indicate if the target is extended or multiple at the wavelength of interest.

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