|
|
IRAC: Dither Patterns |
|
Jump directly to: Dither Pattern | Dither Strategies | Dither Scales | Dither Characteristics | Dither Tables
Dither PatternsDithering performs a number of functions. It allows the identification and removal of small-scale detector defects such as bad pixels, it reduces noise from the effects of pixel-to-pixel errors in the flat field (in fact for large numbers of dithers a flat field can be constructed from the data themselves), and sub-pixel dithering can be used to recover some level of information which would otherwise be lost through undersampling of the array. These considerations drive the optimization of the dither patterns in different ways; there is thus no "ideal" dither pattern. For the standard IRAC dither patterns, we have chosen designs that we think should please most observers most of the time. However, the option always remains for observers to define their own dither patterns using array offsets in cluster mode.For the full-array mode there are two types of dither patterns available. Five such patterns are fixed patterns, which are 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 is performed at each map grid position. Different patterns are available in subarray mode, as the angular scales covered by the arrays are quite different. Two fixed patterns are available for this mode. See this page for copies of the actual dither tables themselves.
Dithering StrategiesThe dithering strategy adopted for a particular observation will depend on the type of object and the required depth and resolution of the final image. The Reuleaux Triangle patterns were designed with the idea of optimizing the Figure of Merit of Arendt, Fixsen, & Moseley (2000, Astrophysical Journal, 536, 500). They thus sample a wide range of spatial frequencies in a fairly uniform manner, and are 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 1/4 sub-pixel 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 are designed for AORs having many mapping/dithering observations, but may also be of more general use. The large and medium patterns are Gaussian distributions (with dithers >128 pixels removed). The small pattern is specifically designed for mapping, where only a few dithers are taken at each map position. It is also based on a Gaussian distribution, but the center is downweighted to decrease the fraction of small dithers in the pattern, and it is truncated at a maximum dither of 11 pixels to ensure that maps with up to 280 arcsec spacing have no holes, even if there is only one dither per map point. All the patterns are constrained to have no pair of dithers closer than three pixels in any run of four consecutive points. To use these patterns, you specify the number of dithers to perform at each map grid position and the starting point in the cycling dither table. You may specify a single dither at each map grid position. This will result in a single observation at each map grid position, but the location of that observation will be slightly shifted from the rectangular grid. Such a "fuzzy" grid should allow a better flat field determination and reduce jail-bar or other pattern noise in the resulting mosaic. The cycling dither table will wrap around once the final (311th) element is reached. For example, selecting a starting point of 310 and three dithers per map, the first grid position will use points 310, 311 and 1 from the table, the second 2, 3 and 4 and so on. This pattern has a 1/2 sub-pixel sampling pattern superposed on it, starting with point 1 and repeating continuously every four points (at point 311, the final cycle is simply truncated early, thus patterns which wrap around the table will miss a sub-pixel dither point). The five-point Gaussian pattern is a general use pattern suitable for shallow observations where the exact sub-pixel sampling is unimportant. It has a 1/2 sub- pixel pattern, with the 5th point at sub-pixel (1/4,1/4). The Figures below show the dither patterns at the default (large) scale and the distribution of both the dithers and of the separation between dithers for each scale.
 Example of one
IRAC dither pattern, at the large scale factor. Click for
full-size imageIRAC Dither patterns for the 'large' scale factor.
 Example of one
IRAC dither pattern. Click for
full-size imageCharacteristics of the cycling dither patterns, in pixels.
Dither ScalesEach of the IRAC dither patterns is available in three sizes, large (default), medium, and small. For most of the patterns, the scaling of the large, medium, and small patterns is approximately in the ratio 4:2:1. Exceptions are the small cycling pattern, which is about 1/5 of the size of the large cycling pattern and has a lower-weighted inner region to reduce the numbers of small separation dithers, and the 4-point subarray pattern where the scaling is 4:3:1.5. For all the patterns, the sub-pixel dithering is maintained, independent of scale.The dialog box in Spot for the dither patterns is shown below, which illustrates how the pattern and scale are selected.
 IRAC Dither patterns dialog box.
Dither Characteristics
Dither TablesSee this page for copies of the actual dither tables.
Go back to the AOT Description, or
|
help@spitzer.caltech.edu http://ssc.spitzer.caltech.edu/irac/dither.html
