Guidelines for proposals where JWST and HST overlap in capabilities

JWST and HST have unique, complementary capabilities in wavelength coverage and instrumentation. However, both offer observing modes in the 0.6–1.7 πœ‡m wavelength range with an overlap in capabilities. It is important to give careful consideration to which facility to propose to for programs that extend into this parameter space. These considerations apply particularly to joint JWST-HST proposals, which allow users to request HST observations in support of a JWST GO program. Proposers should check for archival HST data. In most cases, the HST observations will be complementary but it is possible to request overlapping observations if those optimize the science (e.g. extending temporal coverage). Ultimately the TAC will decide if the proposed observations have been justified sufficiently.

ModeHSTJWST
Imaging

Far-red: WFC3-UVIS & ACS

Near-IR: WFC3-IR: 0”.13 /pixel

NIRCam: 0”.03 / pixel

NIRISS: 0”.06/pixel  (parallel only)
Spectroscopy

WFC3-IR grisms

R ~ 210 - 130

NIRISS WFSS

R ~ 150

NIRSpec low resolution mode

R ~ 300-30

Limiting sensitivity for S/N = 10 in 10ks (point source)

WFC3 F775W: 37 nJy

WFC3 F814W: 32 nJy

WFC3 F110W: 30 nJy

WFC3 F160W: 58 nJy

NIRCam F070W: 20.5 nJy

NIRCam F090W: 14.6 nJy

NIRCam F115W: 12.9 nJy

NIRCam F150W: 10.8 nJy


JWST has unique capabilities in the 0.6-1.7 πœ‡m wavelength range, including coronagraphy, integral field spectroscopy, time series observations, and moderate resolution (R ~ 1000-3000) spectroscopy. However, as the table above shows, HST and JWST overlap in offering broadband imaging and low-resolution spectroscopy in the 0.6-1.7 πœ‡m wavelength range. Generally speaking, for observation programs spanning that wavelength range, if HST can achieve the science in a relatively efficient manner, it will be difficult to justify the use of JWST. For imaging observations below ~1.0 πœ‡m, the sensitivity of JWST is only slightly better than HST. In this range, the use of JWST over HST may require particularly compelling arguments. For grism spectroscopy and imaging observations longwards of ~1.0 πœ‡m, the performance of JWST significantly exceeds that of HST, and it may be easier to argue for the use of JWST, except in cases of very shallow exposures.

Further, in programs where a combination of optical and near-infrared observations are required, there may be justifiable scientific reasons to obtain all observations with a single observatory. For example, an imaging program covering bands at ~0.5, ~0.8, and ~1.25 πœ‡m may be better scheduled on HST. Conversely, a different imaging program covering ~0.9, ~3.6, and ~4.5 πœ‡m bands may be simpler to schedule as JWST-only, even if the 0.9 πœ‡m band could be reasonably obtained with HST. There may also be cases where the limited field of regard of JWST limits temporal coverage of a target, necessitating complementary observations with HST.

In general, proposals including observations below 1.7 πœ‡m should include an explicit discussion, including appropriate ETC/APT calculations, of the rationale for selecting the proposed facility.