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Example #10: Using the HSC to determine positions for a JWST NIRSpec Multi-Object Spectroscopic (MOS) observation

(Star Clusters in M51)

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GOAL: Determine whether the HSC Version 2 astrometry is accurate enough for measuring positions for a NIRSpec MOS observation. The astrometric accuracy needs to be better than about 15-25 mas for optimal target acquisition accuracy (see NIRSpec MPT: Catalogs for details). In many cases NIRCam images must be taken with JWST to provide this level of accuracy, but in certain cases where HST observations have been taken it may be possible to use the HSC to provide the target lists, and hence save JWST observing time.

NOTE: While the example here is for NIRSpec MOS observations, the steps provided here can be used to determine if the HSC astrometry is sufficient to support other types of JWST observations (e.g. NIRSpec target acquisition, MIRI Medium Resolution Spectroscopy).

SCIENCE CASE: Provide a target list of star cluster candidates in a region in the north-cental part of M51 with better than ~20 mas astrometric accuracy. In order to do this, we must confirm the folowing:

• The science targets must be in the HSC
• The HSC relative coordinates must be accurate enough
• The HSC coordinates must be in the JWST Guide Star (GAIA) reference frame, and if not we need to determine an accurate "correction"

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Bring up the interactive display, set Require NumImages > 1 (blue), and overlay the HSC catalog (green). Examine the image and see that there are extended sources (e.g. Concentration Index >> 1) that are cluster candidates (orange).

The default number of selection filters only includes down to the F275W filter. Click on the "Edit Filters" (blue), select the W3_F689M filter (green), and hit "Apply" (orange).

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Now select only the WFC3_F689M objects brighter than 22 (blue), and click the "Remove rows with null value" box (green) just below W3_F689M histogram. This will reduce the number of objects to 983 (orange), making the cross matching with the GAIA catalogs (which will be the reference frame for JWST observations) much faster.

We selected this object to center on since it turns out to be a GAIA source. Our goal is to determine how good both the relative and absolute astrometry are for the HSC in this field. We will assume that GAIA represents the "truth" value. Hence we need HSC stars that are also found in the GAIA catalog to perform the analysis. To verify that the HSC object is also in GAIA, click on the cutout image to bring up the HLA interactive display. Open up the HSC controls (blue), select "Require NumImages" to be >1 (green), set the Catalog Overlay Region to be "Visible area" and click on "Set Region" (orange), and select the HSC and GAIA catalogs (yellow). We can now verify that the object is in both catalogs.

Then use the "View charts for these data" (blue) to plot, and set the X and Y series to be the Delta_RA_mas and Delta_Dec_mas columns. We find a nice concentrated knot of points from good matches of isolated point like objects, with a halo of outer points from regions that are crowded. We now find that the inner scatter ball has a declination diameter of about 100 mas (milli arcseconds). We will get a more precise value in steps below.

The light colored points in this figure show the smaller scatter.

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We will exclude points greater than 1 sigma from the average (blue); note that I have converted from decimal degrees to mas. This results in keeping 117 of the original 154 matches, and gives scatters of 24 mas for RA and 25 mas for Dec. Since our requirement is that the HSC coordinates be in the JWST Guide Star (GAIA) reference frame to with ~20 mas, we can use the HSC to define our JWST observation.

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