Step-by-Step APT Guide for NIRCam Deep Field Imaging with MIRI Imaging Parallels

This walk-through of the JWST Astronomer's Proposal Tool (APT) demonstrates how to specify the observations determined in the ETC step-by-step example for example science program #22: NIRCam Deep Field Imaging with MIRI Imaging Parallels.

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APT file

Main articles: NIRCam Imaging APT TemplateNIRCam Imaging Recommended Strategies
See also: JWST APT HelpJWST APT Training Examples and Video Tutorials

A filled out APT file can be accessed via the menu options FileJWST Example Science Proposals → NIRCam 22 NIRCam Deep Field Imaging with MIRI Imaging Parallels in APT. The APT file was created with version 27.3. There may be inconsistencies, warnings, or errors with other versions of APT.

Fill Out Proposal Information 

Main article: JWST Astronomers Proposal Tool overview

After opening APT, we selected New JWST Proposal under the New Document pull-down menu. On the Proposal Information page, we entered Title, Abstract, and Category of proposal and kept Cycle number at its default value.

Specify the target

Main article: APT Targets

This program is targeting the GOODS-S field with a 2 × 2 NIRCam mosaic. There are a few options for entering a target in APT; we will specify an individual fixed target. To start, create a new fixed target at RA = 03h 32m 42.7s, Dec = -27d 47m 59.7s. By setting this specific position (instead of the center of the GOODS-S field), the NIRCam and MIRI observations will fall completely within the CANDELS coverage (ETC Figure 1), given a specific position angle (see Special Requirements). 


Main article: APT Observations

To get started, create a new observation inside an Observation Folder. From there, select NIRCam as the Prime Instrument and NIRCam Imaging as the Template. Check the Coordinated Parallel box and select NIRCam-MIRI Imaging. Finally, choose the target defined above from the pull-down menu. When editing the "Observation", the tabs on the lower half of the GUI screen contain the detector specifications.

Define imaging observations

Main articles: NIRCam Imaging APT Template
See also: NIRCam Imaging Recommended Strategies

To set up the NIRCam imaging, click to the NIRCam Imaging tab (leftmost tab on the lower half of the APT screen). Since this program will be imaging a large area, set the Module to ALL and Subarray to FULL, which together encompass all pixels in all 10 NIRCam detectors, providing the largest possible spatial coverage. See the NIRCam Field of View article for details.

Data volume

Main article: JWST Data Rate and Data Volume Limits

For deep imaging that uses all 10 NIRCam detectors plus parallels, data volume can add up quickly. For this particular program, the data volume is high enough that the observations must be split into an individual observation for each filter instead of including all filters within a single observation, in the Filters panel. This increases the overheads slightly, while allowing for more flexibility in defining exposure times for each filter.

The data volume in each visit should be limited to:

  • < 58 GB (APT error)
  • < 29 GB (APT warning)
  • < 0.654 MB/s (so as not to exceed 58 GB in 24 hours)

For the third limit, APT gives no warning or error, but exceeding this limit may make a program difficult or impossible to schedule, since a data rate this high may not be sustained for more than 12 hours.

This program abides by all 3 limits.

Mosaic parameters

Main articles: JWST Mosaic Overview, NIRCam Dithers and Mosaics

To cover the necessary spatial area, this program uses a mosaic with 2 rows and 2 columns. The Row Overlap % is set to 20% and the Column Overlap % is set to 78%. These overlaps are defined relative to the NIRCam field of view. The 78% column overlap ensures 2 things: (1) that the large gap between the NIRCam modules is covered, and (2) that the MIRI coverage is continuous. The resulting NIRCam mosaic has 2 wide stripes of increased (2×) depth where the modules overlap. Figure 1 shows the Aladin visualization. The NIRCam mosaic covers approximately 25 square arcmin, and the MIRI mosaic covers approximately 7 square arcmin.

NIRCam dithers

Main articles: NIRCam Primary DithersCoordinated Parallels Custom Dithers

This program uses 4 INTRAMODULEBOX primary dithers to cover detector gaps in the SW channel and yield coverage of rectangular region. The 9-POINT-WITH-MIRI-F770W coordinated parallel subpixel dither pattern is also used to improve the spatial resolution of both the NIRCam and MIRI images. Altogether, there are 36 dithers for each exposure specification.

NIRCam filters

Main article: NIRCam Filters

In the Filters panel, specify the required filters, readout patterns, and exposures. NIRCam has a short wavelength and a long wavelength channel that produce simultaneous imaging (via a dichroic) over the same field of view. Programs can therefore select one filter for each channel for each exposure sequence in the Filters box, and both will be observed with the identical readout patterns and total exposure time. Filters used in this program are summarized in Tables 1 and 2.

NIRCam readout patterns

Main article: NIRCam Detector Readout Patterns

As described in the ETC Guide, this program uses the DEEP8 Readout pattern, which is designed for deep imaging and is required here to reduce data volume. To limit each integration time to <1,000 s, 5 Groups/Int are used. To achieve the required depth, this program uses 7 Groups/Int for each of the 36 exposures (dithers). The Total Exposure Time displayed at the end of the row in the Filters box includes all integrations and dithers, and is set up to achieve about 34 ks in some filters and 68 ks in others. See the ETC Guide for explanations about the selected exposure times.

MIRI parallels

Main articles: MIRI ImagingMIRI Detector Readout Overview

The parallel MIRI observations are specified under the MIRI Imaging tab. The program uses the F770W filter. Readout pattern set to SLOW is used for all observations, and is required to reduce data volume. Each MIRI integration time is set to match (or be slightly less than) each NIRCam integration times.

Table 1. Exposure parameters for prime NIRCam imaging observations

Observation #NIRCam SW FilterNIRCam LW FilterNIRCam ReadoutNIRCam Groups/IntNIRCam Integrations/ExpPrimary DithersSecondary DithersNIRCam Exposure Time (s)NIRCam Exposure Time (hr)

Table 2. Exposure parameters for parallel MIRI imaging observations

Observation #MIRI FilterMIRI ReadoutMIRI Groups/IntMIRI Integrations/ExpPrimary DithersSecondary DithersMIRI Exposure Time (s)MIRI Exposure Time (hr)

Define special requirements

Main article: APT Special Requirements

To ensure that the NIRCam and MIRI maps fall within the CANDELS region and includes the ACS Ultra Deep Field, we must restrict the position angle of the observations. Under the Special Requirements tab, add a PA Range of 280°–300° for each of the observations. It is important to set the PA range for each observation instead of using the Same PA Link option because the visits are so long (see below), it is impossible for them all to be observed at the exact same position angle.

Figure 1. Aladin visualization

Resulting coverage for each observation consisting of a 2 × 2 mosaic of NIRCam imaging (upper left) and MIRI imaging in parallel (lower right). Darker blue indicates increased depth (exposure time).

Run Visit Planner

Main article: APT Visit Planner

To determine the schedulability window of our proposed observations, we ran the Visit Planner ToolThe position angle requirement restricts the observations to a 32 day window in September/October.

Run Smart Accounting

Main article: APT Smart Accounting

After running Smart Accounting to reduce overheads, the total charged time (in APT 27.3) is 351.89 hours, of which 264.60 hours is science time with NIRCam and MIRI in parallel. The efficiency is therefore 75%.


JWST Proposal Planning Workshop (May 2017)

Presentations from the May 2017 JWST Proposal Planning Workshop



Latest updates
    Integration times reduced < 1000 seconds. Reduced data volumes with MIRI SLOW. Improved dithering. Simplified and synced with ETC Guide.

    Updated to APT 27.3: some parallel MIRI exposure times are slightly shorter.

    Revisions include adding links and improving clarity. Final charged time and efficiency updated to results from APT 27.1.

    Fixed typo in Exposure Parameters Table (changed MIRI depth units from "s" to "hr")