Scan maps

Scan maps, i.e. mapping while continuously scanning the array over the source, require a few additional steps in the reduction procedure. The reduction process is also different depending on whether we made conventional scan maps or used the ``Emerson2'' technique. Conventional scan maps refer to maps done while chopping in the scan direction and restoring the resulting dual beam map with the EKH algorithm (the Emerson-Klein-Haslam algorithm - known to all who have ever used NOD2 or JCMTDR [7]) before transforming the map into equatorial coordinates. The ``Emerson2'' technique is essentially a basket weaving technique, where one can scan in an arbitrary angle but chop in two orthogonal directions and restore the dual beam map in the Fourier plane after converting the dual beam maps to equatorial coordinates. This method therefore requires a minimum of two maps, one where we chop in RA and one where we chop in Dec. The standard setup for SCUBA is to use six maps, three of which are done while chopping in RA with chop throws of 30, 44 and 68'', and three while chopping in Dec with the same three chop throws. The chop throws are chosen so that we should be sensitive to most spatial frequencies in the map. If possible one should try to choose the map size so that it covers the whole source and provides an additional baseline region off source, but as we all know this is not always possible.

For all scan maps we can do the first three reduction steps: reduce_switch, flatfield, and extinction, the same way as we would do for any jiggle map. We also blank out noisy bolometers, but from here onwards we need to apply slightly different methods. Scan maps are also affected by sky noise, especially when we use the ``Emerson2'' technique. This is because the time difference between when the positive and the negative beam passes the same position on the sky can be significant, and sometimes even longer than in jiggle-maps. This is especially noticeable for large maps and large chop throws. We can crudely remove sky noise in scan maps, but not as well as in jiggle maps, calcsky is our main tool and needs several repeats of the same maps to work efficiently.