IRAC Instrument Handbook - IRSA - California Institute of Technology

3.2 Map Grid Definition
IRAC Instrument Handbook
If “No mapping” was selected in the AOT, the map grid consisted of a single position at the coordinates
specified in the Target section of the AOT. With “No mapping” selected, and selecting both fields of
view, first the 4.8/8.0 µm field of view was pointed at the target, then the telescope repositioned so that
the 3.6/5.8 µm field of view pointed at the target. In both cases, data from all 4 arrays were collected,
whether they were pointed at the target or not.
If the mapping mode was used, a rectangular map grid needed to be specified in either array or celestial
coordinates. In array coordinates, the map grid is aligned with the edges of the array, such that the map
rows and columns correspond to rows and columns of the array. Specifically, a column is along a line of
constant solar elongation, and a row is along an ecliptic parallel (line of constant ecliptic latitude). It is
worth noting that the two IRAC fields of view are at approximately constant solar elongation, so that a
map with 1 column and several rows made a strip along the direction of the separation between the two
fields of view and yielded 4-array coverage along part of the strip (if it was long enough). In celestial
coordinates the rows and columns correspond to J2000 right ascension and declination. A position angle,
degrees E of N, could be specified to orient the raster in equatorial coordinates. Specifically, if the
position angle is zero, a column is along a line of constant right ascension, and a row is along a parallel
(line of constant declination). The map can be offset from the specified coordinates by giving a map
center offset.
3.3 Dithering Patterns
For the full-array mode there were two types of dither patterns available. Five such patterns were fixed
patterns, which were performed identically at each mapping position. The cycling pattern is a set of dither
positions (also referred to as points), a different subset of which was performed at each map grid position.
Different patterns were available in subarray mode, as the angular scales covered by the arrays were quite
different. Two fixed patterns were available for this mode.
The characteristics of the available dither patterns are given in Table 3.1. The Reuleaux Triangle patterns
were designed with the idea of optimizing the Figure of Merit of Arendt, Fixsen, & Moseley (2000, [3]).
They thus sample a wide range of spatial frequencies in a fairly uniform manner, and were well suited to
the Fixsen least-squares flat fielding technique. The 9-point and 16-point patterns were designed to be the
optimum size for 1/3 and ¼ subpixel dithering, respectively. The random 9 pattern is based on a uniform
random distribution. The spiral 16 pattern was designed by R. Arendt to provide a pattern which is both
compact and has a good figure of merit for self-calibration. The cycling patterns were designed for
observations (“AORs”) having many mapping/dithering observations. The large and medium patterns
were Gaussian distributions (with dithers >128 pixels removed). The small pattern was specifically
designed for mapping, where only a few dithers were taken at each map position. It was also based on a
Gaussian distribution, but the center was downweighted to decrease the fraction of small dithers in the
pattern, and it was truncated at a maximum dither of 11 pixels to ensure that maps with up to 280”
Operating Modes 27 Map Grid Definition