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Knowledgebase: ALMA Observing Tool (OT)
Should I use a mosaic or individual pointings to observe my target(s)?
Posted by Maria Diaz Trigo, Last modified by Fumi Egusa on 16 March 2017 04:30 AM
Individual targets that are larger than 1/3 of the Field of View (FOV) of the 12-m Array will require a Nyquist sampled mosaic to obtain uniform sensitivity across the field due to the sensitivity fall-off towards the edges of the primary beam. This can be set up either as a Rectangular Field mosaic or a custom mosaic containing overlapping individual pointings in the OT (see the Knowledgebase article on setting up mosaics for details). A list of representative sizes of the FOV for each ALMA band is available in document "Observing With ALMA - A Primer", available from the Science Portal under 'Documentation'. The FOV and the inner 1/3 of the FOV where maximum sensitivity is reached can also be visualised in the Spatial Field Editor in the OT: the FOV of a pointing is displayed as a red circle, the inner 1/3 is shown by a green dashed circle.

A set of unresolved (point) sources that are close in the sky, i.e. separated by less than the FOV, are also observed most efficiently using a mosaic (either custom or rectangular). In contrast, a set of targets that are widely separated (by more than the FOV) are best observed as offset pointings (deselecting the custom mosaic option) if they are smaller than 1/3 of the FOV and share the velocity information and expected source properties. If they are extended and/or have different properties they should be defined as separate sources each containing a mosaic if necessary.

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