MIRI WFSS APT Template

Instructions for filling out the Astronomer's Proposal Tool (APT) MIRI WFSS template are provided in this article.

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An observation in the MIRI WFSS mode proceeds as follows:

  1. A direct image filter is selected in the imager filter wheel
  2. A single (i.e., non-dithered) image of the scene is taken, called the "pre-image."
  3. The filter wheel is set to the double prism, P750L
  4. Dithered prism images are taken. 
  5. At the last dither position, another image is taken of the scene with the same filter as the pre-image; this is called the "post-image." This "post-image" can optionally be executed with 2 extra dithers to image sources that have partial traces in the dispersed image. 
  6. Optional: select a different filter and repeat steps 1–5.
  7. Optional and recommended to mitigate overlapping spectral traces: duplicate the observation with a PA Offset Link to assist in disentangling overlapping spectra.

Further information and guidance on executing these steps are in the documentation below.

 

Generic parameters

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

The following parameters are generic to all templates, and are not discussed in this article: observation Number, observation Label, observations Comments, Target name, Optional ETC ID (in the Filters dialog box), Mosaic Properties, and Special Requirements.


Target ACQ  

 Target acquisition is not available for MIRI WFSS observations. 

Direct Images

A typical WFSS observation will obtain direct images of the field both prior to and after the dispersed exposure sequence, the pre- and post-images. At the last dither position of the dispersed exposure sequence, an image is taken of the scene with the same filter as the pre-image; this is called the "post-image." This "post-image" can optionally be executed with 2 extra dithers to image sources that have partial traces in the dispersed image. The goals of these images are:

  • Enable proper identification of object(s) seen in the dispersed image(s); and 
  • Determine the origin of the absolute wavelength scale for the prism spectra for each object in the field.  

The direct images can be taken with any of MIRI's imaging filters that provide a (near-)overlap with the throughput range of the prism:

  • F560W
  • F770W
  • F1000W
  • F1280W
  • F1500W

The direct pre- and post-WFSS  imaging must use the same filter.  

If images of the field are required in multiple imaging filters, the WFSS sequence can be repeated multiple times to gain the additional filter coverage. If images in other MIRI filters are required (i.e., those at wavelengths beyond the F1500W coverage), these should be added as regular MIRI imaging observations. 

Subarray Parameters

Subarray 

WFSS spectroscopy uses only the FULL frame readout.

Dither

By default direct pre- and post-WFSS images are taken as a single exposure, not dithered. The post-WFSS image can, however,  be taken with a 3-position dither pattern to image any out-of-field sources that may be partially dispersed into the science field. 

The dispersed exposures must use a dither pattern. STScI offers the default imager CYCLING pattern for maximum flexibility in the number of dither points. A minimum of 4 dither points are recommended to provide enough redundancy. It offers 3 different pattern sizes, with CYCLING-LARGE considered the default. 

The CYCLING pattern consists of 311 points. This pattern is a random Gaussian pattern designed to be flexible. Observers will be able to choose (1) the starting position in the dither table and (2) the number of dither positions to maximize observational flexibility. For observers who request more than 311 dither positions, the CYCLING pattern will wrap so that the 312th dither position is the same as the 1st position. Each set of 4 consecutive dithers provides complete ½ pixel sampling.

Users are strongly encouraged to check their observation by visualizing it with the Aladin Viewer.

Exposure Time 

Users should use the Exposure Time Calculator (ETC) to determine the best exposure configuration to optimize the signal-to-noise ratio.

Readout Pattern

See also: MIRI Detector Readout Overview

Number of groups and integrations

See also: Understanding JWST Exposure Times

The MIRI timing pattern per exposure is defined by 3 MULTIACCUM parameters: 

  • Groups/Int: the number of groups during an integration;
  • Integration/Exp: the number of integrations during an exposure, where integration is defined as the time between resets;
  • Exposures/Dith: the number of exposures per dither position. This default to 1, and requires prior approval by the MIRI Team through the JWST Help Desk to be > 1.

Total Dithers (not user-editable) returns the total number of dither steps for the observations.

Total Integrations (not user-editable) shows the total number of integrations for the observation, taking into account the dither and exposure settings. The formula used is Total Integrations = Total Dithers × Integrations/Exp × Exposures/Dith.

Total Exposure Time (not user-editable) returns the total exposure time calculated from the dither and exposure settings. The formula used is Total Exposure Time = Groups/Int × Total Integrations × "group read time". 


Other tabs

Mosaic Properties

See also: JWST Mosaic Overview

See also:  Specifying Mosaics in APT 

The MIRI WFSS may be used to obtain data for a region larger than the slit size by creating a mosaic pattern under the Mosaic Properties tab. Use of the mosaic pattern is not supported when using the LRS in slitless mode.

Special Requirements

A variety of observatory level Special Requirements may be chosen under the Special Requirements tab. 

Comments

The Comments field (under the Comments tab) should be used for observing notes.



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