JWST Slit Spectroscopy Roadmap

A roadmap to guide users, step-by-step, through the process of designing a JWST slit spectroscopy observing program using NIRSpec or MIRI.

Each step listed below is followed by a list of articles with additional details.

1. Choose which instrument you wish to use, based on the wavelength range of interest. Roughly, 5 um is the boundary between NIRSpec and MIRI.

2. Choose your method of target acquistion.
NIRSpec Target Acquisition - NIRSpec has three options.  In brief, None is not recommended, and WATA will generally suffice.
MIRI LRS Slit Target Acquisition - TA is recommended for the LRS slit.

3. Decide if a mosaic or a single slit pointing is desired.  See the detailed mosaic guides for more information
NIRSpec FS and IFU Mosaic APT Guide
MIRI LRS Mosaics

4. Decide on the slit(s), subarray(s), grating(s), and filter(s). 
NIRSpec Dispersers and Filters- NIRSpec provides many options.
For MIRI, you should choose FULL subarray in the APT to ensure that you observe in the slit.  If a spectral resolving power greater than 200 is desired, then the MRS may be a better choice.

5.  Choose a dither pattern. 
NIRSpec FS Dither and Nod Patterns - NIRSpec offers many patterns. 
MIRI LRS Dithering - If observing a compact or point source in the LRS slit, use the standard two-nod dither.  If observing a more extended source, place it in the center of the slit (with the MAPPING dither) and use a background observation.  See LRS Recommended Strategies and APT Targets - backgrounds for more information.

6. Calculate exposure times, both for TA and for science observation, using the Exposure Time Calculator (ETC).  Both NIRSpec and MIRI provide a great deal more information on readout modes:
NIRSpec Detector Readout Modes and Patterns
MIRI Detector Readout Overview

7. Create an APT file. Step-by-step instructions are available for both NIRSpec and the MIRI LRS:
NIRSpec Fixed Slit Spectroscopy APT Template
For questions about the APT not covered by the two links above, see the JWST APT Overview.

Go to the Getting Started Guide to complete the steps for proposal submission.


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