December 7, 2015: The latest version of SSpot (v2.9.2) includes a separate Astronomical Observation Template (AOT) for on-the-fly mapping using the upGREAT Low Frequency Array. Instructions are available in the Guide to Planning On-The-Fly Maps with the upGREAT Array in SSpot. The previous version of SSpot may be used for all other GREAT modes.

Guide to Planning Observations with SOFIA/GREAT provides an overview of the instrument, the offered observing modes, SSpot specific information, and includes example sensitivity calculations.  

GREAT-specific instructions and reminders of general issues are given in the following topics below. Note that all menu selections and selectable options are boldfaced, field names appear as in SSpot, and text to enter into SSpot is displayed in Courier New.


SSpot has AOTs for the GREAT Single Point, Raster Map, and On-The-Fly (OTF) Map modes. For observations using the upGREAT Low Frequency Array (LFA) single point and raster map, select the relevant GREAT observing template and then select GRE_LFA for Mixer 2. The GREAT OTF Array Mapping AOT should be used for upGREAT OTF maps.

Please take note of the following information when preparing Astronomical Observation Requests (AORs):

1) Enter the rest frequencies of the line to be observed with Mixer 1 and of the line to be observed with Mixer 2. The specific mixers to use must be selected. In the Velocity field, enter the source velocity in the local standard of rest frame (V_LSR). With this information, the flight team will decide the optimal local oscillator (LO) frequencies for observing these lines (upper side band (USB) versus lower side band (LSB), position of the line in the intermediate frequency (IF)), taking into account frequency-dependent system performance and atmospheric transmission. If you have specific requests, such as observing other lines in the same setting, please note those in the comments section of the AOR.

2) If you are searching for a line or if you are uncertain about its strength or velocity, do at least one frequency shift. The easiest way to do this is to change the V_LSR by something like 10 to 20 km/s. The mixer will have to be retuned and a new calibration done, which involves a small cost in time (~2 minutes).

3) You do not have to choose a backend. Data gets automatically recorded on all backends.

4) For all observing modes, the integration time per point on the sky must be entered. This generally should not be more than 60 seconds (< ~40 seconds in OTF mode). If longer exposures are needed, then increase the number of cycles. For maps, Cycles indicates the number of times the maps are repeated.

5) Plan each AOR so that it does not take longer than about half an hour to execute. If you are making a big OTF map, make it into a set of smaller maps. With the entry Min Contiguous Exposure Time you can specify the minimum acceptable observing time in case the observation has to be split over multiple parts for, e.g., efficient flight planning.

6) For all AORs, the Instrument Mode Total Power refers to Position Switch.

7) In the Dual Beam Switch Instrument Mode (for point and raster map AORs), a Chop Angle can be specified, defined in the usual north through east direction.

8) For Total Power observations, choose your Reference Position as close to your source as possible. It should be no farther away than half a degree and preferably within 10-15 arcminutes of your source. A Reference Position more than half a degree away from your target is not likely to work and may result in really poor baselines. The same is true if you observe in beam switched Instrument Mode. For most observations a chop throw of 100” or less should be sufficient. Large chop throws affect the pointing of the telescope, introduces coma into the beam, and degrades the data quality.

9) For beam-switched observations, a Chop Throw of 100” or less should be sufficient. Large chop throws affect the pointing of the telescope, introduce coma into the beam, and degrade the data quality.

10) In OTF mode, the Single Beam Switch Instrument Mode is rarely used. Position Switch is preferred (Total Power).

11) In OTF mode, SSPOT refers to scan rows as ‘along latitude’ and column as ‘along longitude’. If the map rotation angle is not zero (the default), however, the map is rotated with respect to the longitude-latitude frame. If in doubt about the map orientation, use the Overlays to Current AOR option to plot the OTF coverage on a sky map.

If you have any questions, please contact the SOFIA help-desk.