As part of the MMA imaging group's investigations into the MMA imaging characteristics of a Millimeter Array consisting of a large number of 8M antennas, in which short spacing information is obtained either from the 8m array elements operating in single dish mode, or from a separate 20m dish. This study overlaps with some of the work of earlier stdies (ref (1), (2), (3) and other referneces therein) and gives an independent confirmation of some of that work. The current study examines in more detail the relative importance of different components of the pointing error model, and presents an algorithm (PHFIT) which may lead to a relaxation of pointing requirements of the individual antennas.
The conclusions are summarized as: 1. Where similar parameters of noise and pointing errors have been used there is good agreement with earlier work ( refs(1), (2) )
2. Independent investigation of different pointing error components ( global, differential systematic, random) shows that the dominant components regarding degradation of dynamic range after single dish and MMA interferometric data have been combined are: (a) For single dish data, using either the 8m elements in total power or a separate 20 m large element (LE), global map pointing offsets. (b) For interferometric measurements, the differential systematic pointing error, i.e. the mean pointing error in the relative offset of one field of the mosaic with respect to its neighbours, is the dominant source of error.
A given uncorrected pointing error in short spacing data, derived either from a 20 m or an 8m antenna element, is more harmful than the smae pointing affecting interferometric data, by a factor typically of 4 to 6, depending on circumstances. If not corrected, a given global pointing error on 20m single dish is much more harmful than the same angular pointing error on a 8m dish. Purely random errors, such as tracking errors, are less important.
3. The effect of the gloabl pointing error of a single dish observations either with the 8m dishes or a separate 20m dish providing the short-spacing data, can effectively be eliminated by making use of of the overlap in the UV plane of regions common to both single dish and interferometric data. This technique is very effective, and probably should be applied as a standard correction to most combined single dish and interferometric data sets. In the presence of a single-to-noise ratio, the technique degrades more rapidly with a smaller single dish and its smaller area of UV plane overlap with interferometric data, than with a large single dish element.
4. If the phase overlap correction technique is applied, the 20m single dish always gives greater dynamic range in the combined MMA image than a corresponding 8m dish in total power
View a pdf version of MMA Memo 62.
Last modified: 2002-01-23
alma-memos@nrao.edu