Scan map data can be taken using two techniques (both based on chopping). The first technique is to chop in the direction of the scan and deconvolve each scan independently (the EKH method [11]). This technique must be used for single pixel mapping although it can also be used for array scan mapping. Problems with this technique are that it is very sensitive to spikes, every scan must be completely off-source at both ends and correlations with adjacent scans/pixels are ignored.
For array scan maps we scan the array across the source whilst chopping in a fixed direction on the sky. Following the work of Emerson[12] we take data using a number of different chop configurations in order to sample as many spatial frequencies as possible (we are not sensitive to structure that is larger than the chop throw). Multiple chop throws in 2 orthogonal directions are used with chop amplitudes chosen so that, except at the origin, the zeroes in the Fourier transform of one do not coincide with the zeroes in the FT of the other up to the spatial frequency limit of the telescope beam. For SCUBA, it is recommended that 6 different chop configurations should be used: Chop throws of 20, 30 and 65 arcsec each with chop position angles of 0 (Dec chopping) and 90 degrees (RA chopping) in a coordinate frame fixed on the sky. This will give the best coverage of spatial frequencies but reasonable maps can also be obtained by combining four of the chop configurations. This mode also has the advantage that the deconvolution occurs after the images have been regridded; this means that data can be salvaged even if a scan did not go completely off source (by combining with data that does) and small spikes will be averaged out.
During commissioning it has been shown that the new method can result in a substantial improvement in signal-to-noise over the EKH method[13].
SURF -- SCUBA User Reduction Facility