Hubble Space Telescope Finder Images and Catalogs

Hubble Space Telescope (HST) images and associated catalogs can provide the necessary astrometric accuracy to create finder images for planning JWST NIRSpec MOS science and MSA target acquisition (MSATA).

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One of the main observing modes of NIRSpec is the multi-object spectroscopy (MOS) mode using the micro-shutter assembly (MSA). The MSA consists of roughly a quarter million configurable shutters that are 0.20" × 0.46" in size. The shutters can be opened in adjacent rows to create flexible and positionable slits ("slitlets") on prime science targets of interest.

Because of the very small shutter size, the NIRSpec MOS spectral data quality and target acquisition accuracy will benefit significantly from accurate astrometric knowledge of the positions of planned science sources and MSATA reference stars. The NIRSpec target acquisition process removes small uncertainties in catalog absolute astrometric accuracy. The internal relative accuracy of the catalog is the important quality. The necessary accuracy may be achieved using existing Hubble Space Telescope data when anchored to Gaia for absolute orientation.

Finder images from HST data

Images acquired with the Hubble Space Telescope (HST) have the optimal relative astrometric accuracy of 5–10 mas for planning NIRSpec observations. This high level of astrometric accuracy is achievable using “drizzling” techniques implemented in the DrizzlePac package to combine images acquired with HST. Tiled mosaics of multiple HST image fields may have slightly decreased accuracy compared to the 5 mas level, but still represent an improvement over the typical in-field relative accuracy compared to lower resolution space-based images (i.e., Spitzer Space Telescope) or ground-based cameras.  Anchoring the images and catalogs to Gaia may be necessary to achieve the absolute orient accuracy of 60" needed for MSA target acquisition (MSATA).

NIRSpec has a field of view spanning 3.6' × 3.4' and is highly sensitive over its wavelength range (0.6 to 5.3 μm). Optimal MOS plans will be created from existing HST images that span the majority of this field. NIRSpec MOS observations can be planned using optical or infrared images as the sources for the astrometry. HST images used for planning NIRSpec MSA spectroscopy should preferentially have been acquired within the past decade for optimal spectroscopic planning due to appreciable proper motions (~10 mas or more) in Galactic field stars.

Recommended HST instruments

Table 1 shows the current instruments onboard HST that are suitable to create HST finder images. The images obtained with those instruments have the required spatial resolution to create a proper NIRSpec finder image.

Table 1. Imagers onboard HST



Pixel size

Advanced Camera for Surveys

Wide Field Channel


Wide Field Camera 3



Wide Field Camera 3



Figure 1 shows the footprints of the HST imaging cameras: ACS (left), WFC3 UVIS (top right), and WFC3 IR (bottom right).

ACS and WFC3 UVIS imagers cover a moderate fraction of the NIRSpec MSA footprint. Dithered observations using these cameras that map a slightly wider field can serve as optical image and catalog sources for NIRSpec MOS planning.

The WFC3 IR field of view spans slightly more than one MSA quadrant, and so overall spans only about ~1/3 of the available NIRSpec MOS multiplexing space. Additionally, WFC3 IR has coarser spatial pixel sampling (Table 1) than ACS or the UVIS camera. As a result, a single pointing of the WFC3 IR imager is likely not an ideal source of images and catalogs for NIRSpec MOS planning and will result in a loss of multiplexing opportunity from field coverage and probable decreased calibration accuracy from astrometric accuracy. However, dithered and drizzled mosaics of multiple WFC3 IR pointings acquired to optimize spatial sampling may provide improved field coverage and astrometric accuracy that is acceptable for NIRSpec planning. Verification of catalog accuracy and orientation requirements should be checked for WFC3 IR images and mosaics.

Figure 1. The footprints of the HST imaging cameras and the NIRSpec MSA field of view

Sky projection of the HST ACS (left) and WFC3 (right) detectors. The figure in the center presents the projection on the sky of the JWST NIRSpec micro-shutter assembly using the same scale.

Accessing HST data

HST finder images of potential NIRSpec targets can be found on MAST (Barbara A. Mikulski Archive for Space Telescopes).

Once you have the calibrated data in you computer, you may want to select the files that are needed to build the finder image. The STScI calibration pipelines generate several data products that are indicated by the filename suffix. Complete information on data types for HST observations can be found in the ACS Data Handbook and in the WFC3 Data Handbook

Table 2. Description of common HST suffix names

RAWRaw uncalibrated image from a single exposure in units of DN
ASNAssociation table for an observation set
FLTCalibrated, flat-fielded individual exposure in units of electrons
FLCCalibrated, flat-fielded and CTE-corrected images in units of electrons
DRZCalibrated, geometrically-corrected, dither-combined image (created by AstroDrizzle) in units of electrons/s
DRCCalibrated, geometrically-corrected, CTE-corrected, dither-combined image in units of electrons/s

The drizzled products (DRZ, DRC) produced by the automatic pipeline are good starting points. Note that these images are only generated for single visits, and several visits might be available for your field of view. The FITS files that you need for creating a mosaic are the FLC files (if they exist) or the FLT files. Also, software for correcting for charge transfer inefficiencies exists for both ACS/WFC and WFC3/UVIS. Visit the ACS website or the WFC3 website for more information. 

Creating a finder image from calibrated data

The official STScI suite of tasks that allow users to align and mosaic HST images is DrizzlePac which can be found at

This website contains the latest information on installation, features, resources, and some useful examples. There are 2 main codes in the DrizzlePac suite: TweakReg and AstroDrizzle.

AstroDrizzle uses the world coordinate system (WCS) information to align the images. For the most part, this works well for images taken in the same orbit. Images in one visit over several orbits usually align well because the same guide star pair is used. Images from different visits, however, cannot be aligned based on WCS alone because guide star catalog positions have uncertainties as high as 0.3" to 0.5". Therefore, the TweakReg task was developed to fine-tune the alignments, using point sources common to each image to determine offsets, scale differences, and rotations.

The HST catalog's absolute astrometry will be corrected by the MSA target acquisition process. For the purpose of NIRSpec MSA observations, a catalog derived from a drizzled HST image does not require perfect absolute astrometry; what's important is that the relative astrometry among sources stays within the recommended ranges for the type of NIRSpec observation. Full details are available in the NIRSpec MSA Target Acquisition article.  

If you are unfamiliar with creating mosaics using the DrizzlePac suite of codes, simple examples are provided in the drizzlepac website.

Creating your own catalogs

See Also: NIRSpec MPT - CatalogsMPT Catalogs - ExamplesNIRSpec MSATA Reference Star Selection Recommended StrategiesNIRSpec MSA Target Acquisition

Several tools, some of which are listed in Table 3, are available for extracting positions of sources from astronomical images. The main goal is to create a catalog for input to the MSA Planning Tool for planning NIRSpec MOS observations.  The catalog requires excellent relative positional accuracy, and needs to be complete over the region of the sky that will be observed.

Users are encouraged to verify the relative astrometric precision of the catalogs that serve as on-sky observation planning sources for NIRSpec MSA.

Table 3. Useful tools to create source catalogs from astronomical images

DAOStarFinderSoftware for crowded field stellar photometryAstropy versionStetson, P.B. 1987, PASP 99, 191
IRAFStarFinderIRAF task in the STSDAS package that automatically detects stellar objects in a list of imagesAstropy versionstarfind
SExtractorSoftware that builds a catalog of objects from an astronomical image.Astromatic.netBertin, E. & Arnouts, S. 1996, A&AS, 117, 393: 

Hubble Source Catalog

The Hubble Source Catalog (HSC) is designed to optimize science from the Hubble Space Telescope by combining the tens of thousands of visit-based source lists in the Hubble Legacy Archive (HLA) into a single master catalog. The HSC is easily accessible through the HST MAST Archive.

A use case of the HSC with the goal to determine whether the catalog astrometry is accurate enough for measuring positions for a NIRSpec MOS optimal target acquisition is presented in the MAST webpage titled Example #10: Using the HSC to determine positions for a JWST NIRSpec Multi-Object Spectroscopic (MOS) observation.


Stetson, P. B. 1987, PASP, 99, 191
DAOPHOT: A Computer Program for Crowded-Field Stellar Photometry

Bertin, E. & Arnouts, S. 1996, A&AS, 117, 393: 
SExtractor: Software for source extraction


STScI ACS website
STScI WFC3 website

Latest updates
    Updated a link to starfind. Added links to other articles, and clarified the text about making your own catalog.
Originally published