IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
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-pixe l dithering is ma inta ined,
independent of scale.
Sub-pixel dithering, combined with the drizzle technique (Fruchter & Hook 2002, [11]) to reconstruct the
images, can improve the sampling of the mosaics that are obtained from IRAC (or any other)
observations. Such strategies have been used for the WFPC2 and NICMOS instruments on the HST for
some time (for details see the HST Drizzle Handbook). Dithering is also needed to calibrate intra-pixe l
sensitivity variations, and needed for programs requiring accurate photometry and astrometry (Anderson
& King 2000, [2]). To be effective, however, accurate pointing and low image distortion are required.
The offsetting accuracy of Spitzer is in the range 0.1”−0.4”. This, combined with the image distortion in
the IRAC arrays, places a limit of about ¼ pixel on the sub-sampling that is likely to prove useful in
practice. For example, the distortion of the IRAC camera is < 1% (see Figure 2.5). Thus for the largest
dither patterns, which typically offset up to ±64 pixels from the starting point, the offsets will be up to
±0.6 pixels from the nominal values. Thus only in the small scale patterns, where the offsets are less than
±16 pixels, will the sub-pixel sampling work well, though even on the larger scales some improvement of
the images will probably be noticeable.
Operating Modes 29 Dithering Patterns