MIRI MRS Dedicated Sky Observations

Even with the MIRI cryocooler, the telescope thermal background is significant for MIRI MRS spectroscopic observations longer than 20 μm. Depending on the science target, it may be necessary to get a reference sky observation with the medium-resolution spectrometer (MRS) in a location further away from the science target than possible with standard MRS dithering.

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For example, if an extended object or group of objects filled the entire MRS field of view, an offset sky observation would be necessary in order to reliably measure and subtract the background signal.  Similarly, if an extended source dither pattern was specified it would be necessary to observe a dedicated sky pointing since the total dither offset will be insufficient for pairwise background subtraction.

Obtaining a sky background

There are 2 options for obtaining a sky background: (1) use the offset background option in the Astronomer's Proposal Tool (APT) or (2) create a hand-built mosaic.

Use the dedicated background option

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

The checkbox on the APT MRS template within APT (in the Fixed Targets form) is used to specify that the object requires a companion background observation and allows the user to select a sky target from the target list. This sky target may be created in the usual manner as if it were an astronomical object, and should have an observation of its own (note that Acq Target should be set to NONE in the observation form for dedicated background observations).

In the Special Requirements section of the APT science target observation form, the science target and background target observations may then be linked together as a non-interruptible sequence by grouping the observation timings so that the background is obtained as soon as possible after the last exposure on the target.  A to-be-determined upper limit will be imposed on the maximum allowable offset between the science and background targets.

It is recommended to dither dedicated background observations in order to mitigate the effects of bad pixels and cosmic rays; updated guidance on the required depth and minimal number of dithers for dedicated background observations will be provided based on in-flight data obtained during commissioning.

Hand-built mosaic

The template allows an N × N mosaic to be specified. A 2-position mosaic, one position on the target and one on the background sky, can also satisfy the background requirement.  However, using the offset background option is preferable since it more clearly indicates which of the mosaic tiles is the science target and which is the background (which allows for automatic processing by the JWST pipeline).

Latest updates
    Updated guidance on necessity of dedicated backgrounds
Originally published