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 Guide for Example Science Program #22: NIRCam Deep Field Imaging with MIRI Imaging Parallels.
The APT file for this Example Science Program is available for download: Final_DeepField.aptx (created with APT 27.1).
Fill Out Proposal Information
Main article: JWST Astronomers Proposal Tool overview
All information must be filled out before submitting the proposal, but is not necessary to fill out all "Proposal Information" parameters while setting up the detector specifications. Note that red x's will appear in the "Tree Editor" on the left side of the GUI until all required information is filled out.
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 (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.
Defining NIRCam imaging observations
To set up the NIRCam imaging, proceed to the "NIRCam Imaging" tab (leftmost tab on the lower half of the APT screen). Since this program will be imaging a large area, choose ALL from the "Module" menu and the FULL from the "Subarray" menu, which together encompass all pixels in all 10 NIRCam detectors, providing the largest possible spatial coverage. See the NIRCam field of view page for details. For smaller areas, users can select individual modules and/or subarrays.
Some considerations when designing this APT program
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" box. This increases the overheads slightly, but makes up for some of the extra overhead by allowing for more flexibility in mosaicking, as described in the next paragraph.
The filter wheels on JWST have limited lifetimes and users must take care to minimize the number of filter wheel moves required by their programs. Normally, this means ensuring that dithers are used in place of mosaics. This is because dither patterns are executed in their entirety before changing to the next filter, while mosaics cycle through all filters at a single position, potentially requiring a large number of filter wheel moves (see the NIRCam Dithers and Mosaics Overview Page). Since this program has divided its observations up by filter, this is not a concern: each observation will include all dithers and mosaics for a single filter, requiring no filter wheel moves within the observation. Altogether, the program requires only up to 7 filter wheel moves, or 5 moves if the observations are executed in sequence.
The primary dither pattern employed here is the INTRAMODULE pattern, which is designed to cover detector gaps in the SW channel. Three INTRAMODULE dithers is the minimum required to cover all SW gaps. (Here the user might also consider four dithers of the INTRAMODULEBOX pattern, resulting in coverage of a rectangular region.)
This program also uses a 3-point subpixel dither pattern to improve the spatial resolution of the images. Since this program employs coordinated parallels with MIRI, it uses a coordinated parallel subpixel dither pattern, which is designed to provide good sampling for both instruments when used in parallel. Here, we use the 3-POINT-WITH-MIRI-F770W option since all MIRI observations here will be with F770W. Note the coordinated parallel subpixel dither options are not available unless the coordinated parallel box is checked in the Observation.
Main article: NIRCam Filters
In the "Filters" box, specify the requiredfilters, 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 Table 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 the deepest imaging. To achieve the required depth, this program uses 7-9 Groups/Int (depending on the filter) and 2 Integrations/Exp. 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 30 in some filters and 50 ks in others. See the ETC Guide for explanations about the selected exposure times.
The parallel MIRI observations are specified in the "MIRI Imaging" tab. The program uses only the F770W filter and the FAST readout pattern. Note the SLOW readout pattern can be used to reduce data volume (see below). The number of groups and integrations are set to match (or be slightly less than) the NIRCam exposure times (Table 2).
Table 1. Exposure parameters for prime NIRCam imaging observations
NIRCam SW Filter
|NIRCam LW Filter|
|NIRCam Exposure Time (s)|
NIRCam Exposure Time (hr)
Table 2. Exposure parameters for parallel MIRI imaging observations
|Observation #||MIRI Filter|
|MIRI Exposure Time (s)|
MIRI Exposure Time (hr)
Main articles: JWST Mosaic Overview
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 two 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 two wide stripes of increased (2×) depth where the modules overlap. Figure 6 shows the Aladin visualization. The NIRCam mosaic covers approximately 25 square arcmin, and the MIRI mosaic covers approximately 7square arcmin.
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. In 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.
Run Visit Planner
Main article: APT Visit Planner
To determine the schedulability window of our proposed observations, we ran the Visit Planner Tool. The position angle requirement restricts the observations to a 32 day window in September/October.
APT 27.1 gives warnings about the data volume that may further impact the scheduling. The data volume per visit is restricted to 58 GB maximum, which is the limit of the solid state recorder onboard the telescope (not including some space reserved for telemetry data). JWST downlinks in two four-hour windows each day. Each window can transfer 28 GB of data. Individual visits can exceed 28 GB, but these may be difficult to schedule since they require two downlink windows to transfer all of the visit data to the ground. This program has four visits per observation (28 visits total for seven observations), and each visit requires 36–46 GB of data and 8–12 hours of observing time. APT gives warnings for every visit of this program because they exceed 29 GB (half the 58 GB capacity). These large data requirements, long visits, and restrictive position angles result in some difficulty in scheduling.
Run Smart Accounting
Main article: APT Smart Accounting
After running Smart Accounting to reduce overheads in APT 27.1, the total charged time is 272.66 hours, of which 218.2 hours is science time with NIRCam and MIRI in parallel. The efficiency is therefore 80.0%.