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SSpot Details for EXES Observations
EXES observations are described by AORs, which are defined using the SSPOT tool. The instrument team will take the AOR parameters to construct scripts that are executed during the flights.
Please read the overall Cycle 4, Phase II instructions.
EXES-specific instructions and reminders of general issues are given below.
1) If you don’t have a SOFIA DCS account yet, create one on the Data Cycle System (DCS) website. An account is required to retrieve and save AORs (and later access your data).
2) SSPOT 2.9.0 (or higher) must be used for Cycle 4.
3) AORs are created automatically with the function File → Download AORs from DCS from the “Observing Plan” generated by SPT at the time of proposal submission. They can subsequently be modified and saved. The latter includes an option to upload the modified AORs to the DCS system.
4) The latest EXES sensitivities are available using the EXES Exposure Time Calculator (ETC). Please use it. 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
5) SSPOT entry “EXES Clock Time”: Enter the clock time, not the on-source time. The clock time includes overheads due to nodding and instrument inefficiencies. For each AOR, SSPOT will add 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" option to "True". Overheads can also be reduced if multiple sky positions are observed in the same wavelength setting. In this case click the "No Wavelength Setup" button. 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.
6) SSPOT entry “Central Wavelength”: The central wavelength of the line of interest should be corrected for heliocentric motions. Corrections for the Earth’s motion w.r.t. the Sun are generally not important for the wavelength settings (but see the next point).
7) If applicable, use the “Time Critical” fields under the “Observing Condition” button to indicate the range of dates at which the emission/absorption line of interest is sufficiently well separated from any atmospheric lines.
8) SSPOT entry “Echelle Order”: Select from the options listed in the ETC at the wavelength of interest. This sets the wavelength coverage and slit length. Thus, depending on the order, on-slit nodding may be possible or not. Note that the entry under Observing Condition → Order is not the echelle order.
9) SSPOT entry “Minimum Contiguous Exp Time”: This 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. Do not leave it at 0 seconds. If the entry is larger than 5400 seconds, it will be very difficult to get the AOR scheduled.
10) SSPOT entry “Nod Style”: SSPOT does not determine whether on-slit nodding is possible, given the slit length. For this, use the ETC and modify the “Nod Style” entry in SSPOT accordingly.
11) If nodding off slit is needed, 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.
12) In mapping mode (Nodstyle=Map), EXES observes a single “stripe” of slit positions. Multiple stripes must be defined in separate AORs. The stripe orientation on the sky cannot be controlled (see below).
13) The flux calibration and correction for telluric absorption lines will be done using sky and blackbody measurements. These are accounted for in the standard overhead mentioned above. 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 program GI, the instrument PI and the 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 SSPOT, a separate observation entry should be entered with object name "Cal_target", where "target" is the name of the associate science target (i.e. "IRC+10216" and "Cal_ICR+10216"), and given the coordinates RA:12:00:00, Dec:+90:00:00. One must use the EXES ETC 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_Medium and High_Low configurations. For the Medium configuration, 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.
14) Please enter the following information in the comments section:
- 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 arcminute radius are available. For accurate positioning of the slit: is the target extended or multiple at the wavelength of interest?
15) Give all AORs for the same target the same priority (Observing Condition → Target Priority). For each target specify the order in which the AORs should be scheduled (Observing Condition → Order). Enter any finer scheduling priorities in the AOR comments section.
16) Be aware that the AOR image overlay function does generally not display the correct slit length. For the HIGH MEDIUM and HIGH LOW configurations, the slit lengths are strong functions of the wavelength and the echelle order. Please check the ETC for the correct slit length. For the MEDIUM mode, the slit length is correct, but for the LOW mode partial read-outs may be needed to avoid saturation, which will reduce the slit length.
17) SSPOT entry “Example Rotation Angle”: This entry is meant only for visualization of the AOR on the sky. It will not set the EXES slit orientation during the observations (see also the next point).
18) The EXES slit orientation on the sky cannot be controlled, except when putting time constraints on the observation. This is strongly discouraged as it limits the probability that the AOR will be scheduled. SOFIA’s Target Visibility Tool is intended to determine the EXES slit orientation on the sky at a given date but it currently gives incorrect values. Therefore, do not use it. It will be fixed after the analysis of commissioning observations.
19. Contact information: for SSPOT related questions, contact the EXES support scientist Adwin Boogert (firstname.lastname@example.org; 1-650-604-1095). For instrument and calibration questions, please include the instrument PI, Matt Richter (email@example.com) on the emails.
If you have any questions, please contact the SOFIA help-desk