NIRSpec MOS Operations - Pre-Imaging Using NIRCam

JWST NIRSpec MSA-based observations can optionally acquire "pre-images" with NIRCam in order to plan spectroscopy or target acquisition.

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See also: NIRSpec MOS Operations - Slit Losses

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NIRSpec's multi-object spectroscopy (MOS) mode uses the micro-shutter assembly (MSA), which consists of roughly a quarter million configurable shutters that are 0.20" × 0.46" in size. MSA shutters can be opened in adjacent rows to create customizable and positionable spectroscopy slits ("slitlets") on prime science targets of interest. Because of the very small shutter size, NIRSpec MSA spectral data quality will benefit significantly from accurate astrometric knowledge of the positions of planned science sources. Accurate relative astrometry is most important. The absolute astrometric error will usually be corrected through the use of target acquisition.

Pre-imaging observations for NIRSpec MOS science or MSA-based target acquisition (MSATA) are optional and not required if they can be planned with existing images and catalogs.

Images acquired with the Hubble Space Telescope (HST) usually have an internal relative astrometric accuracy of 5–10 mas that is optimal for planning NIRSpec observations. Tiled mosaics of multiple HST image fields may have slightly decreased accuracy compared to the 5 mas level, but are an improvement over the typical in-field relative accuracy of lower resolution space-based images (i.e., Spitzer Space Telescope) or ground-based cameras. Images from the HST ACS or WFC3 can provide the necessary in-field accuracy for NIRSpec spectroscopy planning. However, there are reasons why other sources of imaging may be necessary: some fields of interest have no or insufficient HST imaging, galactic fields have moderate proper motions that would preclude using HST imaging from earlier epochs, infrared positions may be expected to differ from those at optical wavelengths in some sources, and extragalactic regions observed with HST may lack source detections required for planning NIRSpec observations at wavelengths beyond 2 μm. 

Moreover, Hubble's Wide Field Camera 3 (WFC3) infrared (IR) camera spans a smaller footprint than the NIRSpec MSA; a single WFC3 IR image may not provide the best Catalog source for NIRSpec MOS spectroscopy across the full field of view. Images acquired with the Spitzer Space Telescope cameras or most ground-based imagers do not have adequate astrometric accuracy or field coverage for optimal NIRSpec MOS spectroscopy planning. Thus, optimal NIRSpec MOS planning and data calibration may require pre-imaging observations with JWST's Near-Infrared Camera (NIRCam) to accurately establish source positions for alignment and configuration of the NIRSpec MSAs, and also for MSA Target Acquisition (MSATA) for MOS, IFU and FS science.

To aid in understanding planning constraints and field coverage, a NIRSpec Observation Visualization Tool was created to simultaneously view both the NIRCam pre-imaging footprints and NIRSpec MOS field of view.

What is pre-imaging and when is it needed for MOS observing?

The process of pre-imaging an astronomical field has been developed to support the increased demand for multiplexing spectroscopy. "Pre-imaging" observations are images acquired using the same telescope as the MOS spectroscopy in the same observing cycle, though not necessarily the same instrument. Pre-images for MOS programs are used to define field astrometry for MSATA and aperture slit placement on science objects. Rapid availability and accuracy of MOS planning pre-images is very important for the success of spectroscopy planning. 

The NIRSpec MSA is a fixed grid of micro-shutters; hence, science sources of interest cannot all be perfectly centered within their configured shutters or slitlets. These centering offsets and the very small NIRSpec MSA shutter size can result in significant flux that is lost outside of the slit. Slit throughput loss is a function of wavelength and the relative placement of the science sources within the MSA shutters. Very high quality planning astrometry will help to limit the flux and wavelength calibration errors that result from uncertain spectral source positioning after target acquisition. In-field relative astrometry of 5–10 mas or better is needed for point source observations.

NIRCam is the workhorse imaging camera at 1–5 μm wavelengths for JWST. It is the preferred camera for high quality images for JWST NIRSpec spectroscopy and target acquisition planning. NIRCam will provide imaging for high precision source catalogs that can be used to plan follow-up spectroscopy with NIRSpec. For this reason, same-cycle imaging with NIRCam (referred to as "NIRCam pre-imaging") is available to support and plan programs that use the NIRSpec MSA for spectroscopy or target acquisition. 

In JWST observing cycle 2 and beyond, NIRCam images acquired in previous cycles will be excellent resources for NIRSpec observation planning. These images are not acquired using the same process as the NIRCam pre-imaging defined here. The NIRCam pre-imaging process discussed here refers primarily to planning images acquired in the same observing cycle as the NIRSpec spectroscopy. 

Observing process for NIRSpec spectroscopy with NIRCam pre-imaging

See Also: NIRSpec MOS and MSATA Observing Process

JWST NIRSpec MSA-based observations (MOS or MSATA) require a fixed Aperture Position Angle (APA) assignment in order to plan the spectroscopy or MSA target acquisition. Across the 3.4' × 3.6' field of view of NIRSpec, very small deviations away from a planned orient can cause science sources to move out of their MSA shutters. As a result, NIRSpec programs that use the MSA must have a follow-up planning process after the proposal submission and program acceptance. The MSA observations will need to be updated for “flight-ready” status once pre-imaging observations are obtained and a fixed APA and observation execution window are assigned. The process for MOS observation planning has been fully described in the article NIRSpec MOS and MSATA Observing Process, and includes a timeline for MOS observation planning with NIRCam pre-images included.

NIRCam pre-imaging planning

Figure 2 shows the four NIRSpec MSA quadrants projected onto the sky; for comparison, the NIRCam imager fields of view—both the long and short wavelength channels—are also shown. The NIRCam long and short wavelength detectors will image the same field of view but for illustration, are shown separately. It is evident that the NIRSpec field of view is large in comparison to that of NIRCam. For this reason, creating a finder image with NIRCam in order to cover the NIRSpec MOS field of interest will require some planning, including dither patterns and mosaicking.

Figure 2. Sky projection of the relative sizes of the NIRSpec MSA (left) and NIRCam fields (right) fields of view for both wavelength channels

Sky projection of the relative sizes of the NIRSpec MSA field of view (left) and NIRCam fields (right) for both wavelength channels (not at the same spacecraft orient). Size of the apertures are given in arcseconds. The NIRCam short wavelength channel (SWC) and long wavelength channel (LWC) simultaneously observe the same imaging field; they are presented offset here to highlight the areas sampled by the 10 detectors (8 in SWC, 2 in LWC).

NIRCam options for pre-imaging: filters, dithers and mosaics

The Near Infrared Camera (NIRCam) is the 1–5 μm imager on JWST and will provide imaging with excellent sensitivity for precision source catalogs that can be used to plan follow-up spectroscopy with NIRSpec. NIRCam acquires simultaneous images in two channels—the short wavelength channel (SWC) and long wavelength channel (LWC)—using 10 detectors. The two channels have a wide range of filter options available including wide, medium, and narrow width filters that are matched to emission features. The NIRCam F115W filter available in the SWC is very well matched in profile to the NIRSpec F110W filter, which is one of the three filters available for MSATA.

The NIRCam instrument has a footprint that spans an extent of 5.1' × 2.2' on the sky. There is a gap of ~40" between the two NIRCam modules. The field of the SWC is sampled by eight detectors with 32 mas/pixel and 5"  gaps in between individual module detectors, and the LWC uses two detectors with 65 mas/pixel (see Figure 2). 

NIRCam SWC and LWC data can be acquired with dithers and mosaics to create images that cover the more complete observing footprint of the NIRSpec field of view. There are a few available options that can achieve the full field coverage of the NIRSpec MSA footprint. Table 1 shows three recommended dither patterns that can be used to provide near continuous (>95%) field coverage of the NIRCam footprint (covering detector gaps) in both the SWC and LWC channels. All of these dither patterns come from the FULL dither option for NIRCam, which provides the patterns designed to span the detector gaps and cover the full field of the instrument. At present, an additional FULLBOX dither pattern suitable for tiling an area for NIRSpec follow-up observations has been defined, called 8NIRSPEC. Further details on this and other NIRCam tiling patterns can be found at NIRCam Primary Dithers.

The FULL3 TIGHT pattern consists of three offset positions that are optimally designed to cover the gaps in the SWC and provide good coverage and depth in the central area of the dithered image. The FULL3 pattern is similar to FULL3 TIGHT, but the offsets in the vertical direction are larger which provides wider overall field coverage, but more extended detector gaps toward the edges. The FULL6 pattern provides deep field coverage in the central regions of the field and over a more extended footprint than either of the 3-point patterns can provide. The fixed offsets defined for each dither pattern are listed in Table 1. Figure 3 shows two examples of these dither patterns; the FULL3 TIGHT and FULL6 dither coverage, overplotted with the NIRSpec MSA footprint (at the same observatory orient angle). In order to completely cover the NIRSpec MSA field of view, one of the dither patterns, below, plus a mosaic tile position may be necessary.

The FULLBOX 8NIRSPEC dither pattern covers a large area: 6' × 5' to serve as pre-imaging for NIRSpec MSA spectroscopy. It consists of eight pointings with offsets listed in Table 1. Note that the fifth dither and the return to the start are very large dithers and will result in visit splitting for any target. 

Table 1. Recommended NIRCam dither patterns for pre-imaging

Dither pattern name

Horizontal offset

Vertical offset


-58′′,  0′′,  +58′′ 

-7.5",  0" , +7.5"


-58", 0", +58"

-23.5", 0" ,+23.5" 

FULL6-72", -43", -14", +15", +44", +73"-30", -18", -6", +6", +18", +30"
FULLBOX 8NIRSPEC-24.6", -24.4", 24.6", 24.4",  24.6",  24.4", -24.6", -24.4"-64.1", -89.0", -88.8", -63.9", 64.1", 89.0", 88.8", 63.9"

NIRCam imaging acquired with the described FULL dither options from Table 1 may be insufficient for planning spectroscopy over the full NIRSpec field of view. However, NIRCam observations can also be mosaicked or tiled to create a wider image, beyond the size of the NIRCam footprint from dithering alone. The 8NIRSPEC dither pattern is large enough to accommodate the MSA footprint.

A selection of one of the dither options presented in Table 1 combined with a 2 × 1 mosaic pattern with NIRCam can be used to provide full coverage of the NIRSpec field of view at any observing position angle (See Figure 4).

The NIRSpec Observation Visualization Tool was created to investigate field coverage between the NIRCam and NIRSpec MSA footprints in support of the pre-imaging process.

Figure 3. The NIRCam FULL3 TIGHT and FULL6 dither patterns

The HST image of the Tarantula Nebula (PI: E. Sabbi) overplotted with (left) a NIRCam SWC FULL3 TIGHT dither pattern, and (right) a NIRCam FULL6 dither pattern with the LWC shown. NIRSpec MSA quadrant views are over plotted in both cases.
Figure 4. Coverage of a NIRCam image with FULL6 dither and a 2 × 1 mosaic

The Tarantula Nebula image with a two tile NIRCam LWC mosaic shown. Overplotted in green and yellow is the NIRSpec MSA quadrant field of view at two possible spectroscopy execution orients. The FULL6 pattern with a 2 × 1 mosaic covers a wider extent field than the MSA quadrant footprint.


Beck, T. et al. 2016 SPIE 9910, 12
Planning JWST NIRSpec MSA spectroscopy using NIRCam pre-images

Coe, D. 2017, JWST-STScI-005798
More Efficient NIRCam Dither Patterns

Latest updates
    Moved some content, including process and timeline, in previous version of this article to the updated NIRSpec MOS and MSATA Observing Process .

    Removed section on slit losses and added tip box concerning special requirements

    Added new 8NIRSPEC dither pattern in text and table
Originally published