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Knowledgebase: IRAC
Why should I use dithers rather than repeats? Repeats take so much less telescope time!
Posted by Seppo Laine on 21 November 2008 11:59 AM
In-place repeats are successive frames taken at the same position, and dithers move the telescope between pointings. When you move the telescope, it takes time both for the slew and the subsequent settle before astronomical observations can begin. (In-place repeats are specified in the AOT window under For Each Pointing/ Number of Frames; dithers are specified lower in the AOT window under Dither Pattern. )

Dithering is used to eliminate array-dependent or transient artifacts from the true celestial map. A well-dithered map will mitigate the effect of pixel-to-pixel gain differences, which will average down when a celestial-coordinate mosaic is generated. Cosmic ray rejection is greatly facilitated by taking highly-redundant observations. And, bad pixels on the array are filled in when dithered images are combined.

Scattered light from bright sources near the edge of the field of view sometimes cannot be avoided, but it can be prevented from contaminating multiple frames by using a dither pattern larger than the characteristic size of the regions that produce stray light, e.g., medium or large scale dithers.

We discourage using in-place repeats (successive frames taken at the same position), especially in observations that have less than ten different dither positions, both for the reasons mentioned above and also because the residual first-frame effect will lead to a bias pattern difference between the repeats and the first frames of a repeat set. Taking in-place repeats is recommended only in the case of time series measurements with stringent requirements for stability. In general, the better handling of pixel-to-pixel variations using dithers will more than compensate for the reduced amount of integration time; that is, the realized signal-to-noise level will be higher using N-1 dithered observations than N observations with repeats.