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Mission Overview

JWST Extragalactic Medium-band Survey

Primary Investigator: Christina Williams, Sandro Tacchella, Michael Maseda

HLSP Authors: Christina Williams, Sandro Tacchella, Chris Willott, Michael Maseda, Ben Johnson, Brant Robertson

Released: 2023-06-20

Updated: 2023-06-20

Primary Reference(s): Williams et al. 2023

DOI: 10.17909/fsc4-dt61

Citations: See ADS Metrics

Read Me

Source Data:

  • JWST GO Proposal 1963
JEMS survey footprints
Figure 1: The JEMS footprints (RGB) on the Hubble Legacy Field map of GOODS-S (grayscale).  The blue circle indicates the ancillary MUSE deep data and the orange box outlines the Ultra Deep Field.


The JWST Extragalactic Medium-band Survey (JEMS) is a deep Cycle 1 imaging survey program using 5 medium-band filters at 2 and 4 micron (F182M, F210M, F430M, F460M, F480M).  With two NIRCam detector footprints (using all 5 filters) and one NIRISS imaging footprint (using F430M and F480M filters), the data total 15.6 square arcminutes with integration times 13915-27830 seconds, including full coverage of the Hubble Ultra Deep Field (HUDF), building on the deepest multi-wavelength public datasets available anywhere on the sky. These science-ready mosaics, which reach 5σ point source limits (ABmag) of ∼29.2 in 2μm filters and ∼28.2-28.6 at 4μm. Our chosen filters create a JWST imaging survey in the UDF that enables novel analysis of a range of spectral features potentially across the redshift range of 0.3 < z < 20, including Paschen-α, Hα+[N II], and [O III]+Hβ emission at high spatial resolution. Figure 1 shows the JEMS footprints (RGB) on the Hubble Legacy Field map of GOODS-S (grayscale).  

Sensitivities Measured in 0.3” Diameter Apertures (Williams et al. 2023)

NIRCam Filter

1st Release 5-sigma (ABmag) 


1st Release 5-sigma (ABmag) 


















Data Products

Image mosaics are presented with a pixel scale of 0.030” and are named according to the following convention:



  • <inst> is the instrument, one of "nircam" or "niriss".
  • <filter> is the filter, one of "f182m", "f210m", "f430m", "f460m", "f480m".
  • <version> is the version number ("v1.0").

Data file types:

_drz.fits Image mosaic

Data Access

JEMS images are available in the MAST Portal (web-based, cross-mission search interface) and Astroquery (Python package to search for and download files from Python scripts you write), and via the direct download table below. Set the Provenance Name filter to JEMS in the Portal Advanced Search to find all or individual images.

from astroquery.mast import Observations
all_obs = Observations.query_criteria(provenance_name="jems")
data_products = Observations.get_product_list(all_obs)
  • A web-based interface for cross-mission searches of data at MAST or the Virtual Observatory
  • Search for and retrieve JEMS data products programmatically.

Direct Download

Image files can be downloaded directly from the table below based on the filter used.

Image Mosaics by Filter
NIRCam (2.0 GB each) NIRISS (1.4 GB each)
F430M F430M
F480M F480M



A python script is provided that will adjust the WCS from the ancillary Hubble Legacy Field imaging (Illingworth et al. 2016; Whitaker et al. 2019) in order to be co-analyzed with JEMS, as outlined in the Appendix of Williams et al. 2023. Note that MAST offers this script as a courtesy to the JEMS team, and offers no warranty for it's use.


Please remember to cite the appropriate paper(s) below and the DOI f you use these data in a published work. 

Note: These HLSP data products are licensed for use under CC BY 4.0.