The Surf package supports the following SCUBA observing modes:
This is the mode used for setting the X and Y alignment of the secondary mirror. This mode usually consists of 5 measurements, one for each secondary mirror position. Currently the standard observing mode uses a single pixel to calculate the best secondary mirror position since it is the most efficient method for determining the alignment – this is similar to the heterodyne X and Y focus observations – and this mode is not supported by Surf. Alternatively it is possible to ALIGN using the entire array and, since these data are simply 5 JIGGLE/MAPS, this mode can be reduced with Surf. Care must be taken to make sure that each measurement is rebinned independently (switch off the measurements that are not required by using change_quality) otherwise you will end up with the average of all the measurements at all secondary mirror positions (the unfocussed images will dominate). Note that no special processing is perfomed on these data and Surf does not provide a way of calculating the secondary mirror offset.
This is the mode used for focussing the Z axis of telescope. This mode is similar to ALIGN in that five measurements are taken and that the single pixel mode is not supported. The same care must be taken when reducing the unfocussed images since each measurement will be from a different secondary mirror position.
This is the main imaging mode for sources which are smaller than the array (i.e. less than about 2 arcmin). All JIGGLE/MAP observations (including ALIGN, FOCUS and POINTING) are reduced in the same way using rebin to make the final image. [info on how a JIGGLE/MAP is taken - reference exposures and switches]
This is the mode used to measure the noise behaviour of the array. Data are reduced using reduce_noise. Noise data can be inspected with scunoise.
This mode is used to measure the flux of a point source. In its simplest guise the observation involves pointing a single bolometer at the source, measuring the signal, chopping and nodding to reduce the effect of sky emission, and integrating to build up the signal-to-noise. SCUBA also allows for 2 or 3 bolometer photometry (chopping on the array), simultaneous photometry using the long and short wave arrays, and jiggling on source to reduce the effects of seeing. The scuphot task is used to reduce photometry observations to a simpler form (one data point per integration) for further analysis.
This mode is used to check the pointing of the telescope by observing a bright source with a known position. A single-pixel observing mode is available and is not supported by Surf. The JIGGLE/MAP implementation can be processed as a standard JIGGLE/MAP. The pointing offset can be checked by using, say, the Kappa centroid task.
These are PHOTOM or MAP observations observed in polarimetry mode. For these observations a measurement is taken (usually consisting of one integration) for different positions of a half-wave plate. Surf can be used to reduce these measurements so they can be processed by a specialized polarimetry data reduction package such as Polpack.
In SCAN/MAP mode (also known as ‘on-the-fly’ mapping) the telescope is continuously scanned across the sky (usually at a Nasmyth position angle of 18 degrees so that the image is fully-sampled on a single pass) whilst using the secondary mirror to chop so that atmospheric contributions can be minimized. Scan rates of up to 24 arcseconds per second are possible in this mode and it is the most efficient SCUBA mapping mode. One problem with this mode is that the data is taken in dual-beam mode; each data point is the difference between the left and the right chop positions. This means that sources appear twice in data, separated by the chop throw: a positive beam and a negative beam – this dual beam response must be taken out in software. Two forms of data taking are implemented: one involves chopping along the scan direction [11] whilst the other involves chopping in a fixed direction on the sky but combining data from many chop configurations [12, 13].
This mode measures the sky brightness temperature at a range of elevations and uses that data to calculate the zenith sky opacity. In most cases the values found in the RO file should be sufficient and skydip data should not need to be reanalysed. The skydip task can be used to re-reduce the data if necessary (see also sdip). A summary of the skydip observations in a given directory can be produced with the skysum command.