Title: the Jwst Point Spread Function: Calculation Methods and Expected Properties

We present a discussion of the methods that can be used to model the JWST Point Spread Function (PSF) and illustrate some of the potential impacts of the PSF properties on the scientific performance of the telescope. We summarize the quantitative PSF measurements typically used in astronomical imaging and evaluate those quantities on model NIRCam PSFs. These PSFs are generated using optical path difference (OPD) maps based on the most recent ("Revision T") JWST optical error budget. We discuss the dependence of the PSF on wavelength and compare the expected NIRCam PSF with PSFs from the HST/ACS, HST/NICMOS, and Spitzer/IRAC instruments. While high PSF quality in the near-IR is directly constrained by JWST requirements, we find that even at shorter wavelengths (e.g., ! 0.7 µm) the expected JWST PSF quality compares well with existing facilities. We evaluate PSFs for 10 different realizations of the error budget and show that PSFs with a wide range of properties are consistent with the present JWST design requirements. To allow evaluation of PSF properties under more general circumstances we also present a simple method to approximate the change in the OPD map and the resulting PSF induced by specific changes in the position of optical elements in the telescope+instrument system. 2.0 Introduction Previous experience with other space telescopes, and in particular the Hubble Space Telescope (HST), has shown that the ability to obtain groundbreaking discoveries relies heavily on the quality and understanding of the telescope's point spread function (PSF). The critical elements are: (a) that the PSF is of the highest possible quality; (b) that the PSF is as stable as possible; and (c) that the PSF can be accurately modeled and understood during the data analysis stage. HST has a stiff monolithic temperature-controlled primary mirror. Changes in the HST PSF therefore arise almost exclusively due to variations in the distance of the secondary mirror from the primary mirror. These