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SSpot Details for GREAT Observations
December 7, 2015: The latest version of SSpot (v2.9.2) includes a separate 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.
INSTRUCTIONS FOR PHASE II PLANNING OF GREAT OBSERVATIONS WITH SSpot
SSpot has Astronomical Observation Templates (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 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. Also enter the source velocity (LSR). With this information, the flight team will decide the optimal LO frequencies for observing these lines (USB versus LSB, position of the line in the 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 source velocity (Vlsr) 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 “Minimum 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 Dual Beam Switch Mode (for point and raster map AORs), a chopping 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 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.
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 in SSPOT to plot the OTF coverage on a sky map.
If you have any questions, please contact the SOFIA help-desk