The JWST Point Spread Function: Calculation Methods and Expected Properties

1. TIPS/JIM Meeting The JWST Point Spread Function:Calculation Methods and Expected Properties Russell B. Makidon Stefano Casertano, Colin Cox, & Roeland P. van der Marel Telescopes Group & JWST OTE / WFS&C Team Space Telescope Science Institute June 21, 2007

2. Importance of PSF Quality and Stability • The ability to obtain groundbreaking discoveries relies heavily on the quality and understanding of the telescope’s point spread function (PSF). • The Point Spread Function (PSF) describes the response of an imaging system to a point source or point object. • Critical elements: • that the PSF is of the highest possible quality • that the PSF is as stable as possible • that the PSF can be accurately modeled and understood during the data analysis stage. Russell B. Makidon

3. Changes in the HST PSF • HST has a stiff, monolithic, temperature-controlled primary mirror. • Changes in the HST PSF arise almost exclusively due to variations in the distance of the secondary mirror from the primary mirror. • Changes occur at the level of microns on orbital and secular timescales • Orbital “breathing” due to thermal variations associated with day-night transitions; Multi-year changes due to OTA desorption (150 microns since launch) • See M. Lallo et al. (2005), Instrument Science Report TEL 2005-03 Russell B. Makidon

4. Changes in the HST PSF: SM Motion During an Orbit Russell B. Makidon

5. Changes in the HST PSF: SM Position v. Time Russell B. Makidon

6. Challenges for JWST PSF Stability • The situation will be quite different for JWST. • The 6.5 m primary mirror consists of 18 semi-rigid segments. • Each segment has 7 controllable degrees of freedom (tip, tilt, clocking, piston, two translations, and radius of curvature) • The secondary mirror has an additional 6 degrees of freedom (no radius of curvature correction). • JWST is passively cooled, but will never be fully in thermal equilibrium. • Thermal variations combined with of 132 degrees of freedom will yield a much higher-dimensional parameter space of JWST PSFs than for HST. Russell B. Makidon

7. Requirements on JWST WFE and WFS&C • A good, stable PSF is critically important for many types of science envisioned with JWST. • Requirements on image quality exist at a high level • Govern diffraction limit of JWST, change in encircled energy, and wavefront error (WFE) over the FOV (OBS-1607, OBS-88, OBS-90, OBS-1599) • Wavefront sensing and control (WFS&C) will enable correction of misalignments in the primary mirror segments and secondary mirror. • will ensure that the PSF never exceed the requirement of 131 nm RMS WFE over the Optical Telescope Element (OTE) field of view (FOV). • Many different PSFs are consistent with the JWST WFE budget. Russell B. Makidon

8. PSFs Determined by Aperture Shape and WFE • To lowest order the PSF of an imaging system is determined by two things: the aperture shape and the wavefront errors. • Often, the shape of the aperture is well known and relatively simple • circular or annular apertures • more complex apertures increasingly common (JWST and Keck) • Errors in the wavefront arise from a variety of sources • imperfections in the system’s optics (static or semi-static) • atmospheric variations (as in the case for ground-based observations) • can be extremely difficult to determine. Russell B. Makidon

9. PSF as a Function of Aperture Shape • For an ideal system, the PSF can be calculated based on shape of the aperture (Fourier Transform) • Circular Aperture yields Airy Function • PSF shown with logarithmic grayscale stretch from 1.0e-7 to 1.0e-2; total = 1.0 Russell B. Makidon

10. PSF as a Function of Aperture Shape • Hexagonal Aperture, 6.5 m point-to-point • Six-fold symmetry in PSF • Flux in circular symmetric rings diffracted into “spikes” at 60 intervals Russell B. Makidon

11. PSF as a Function of Aperture Shape • JWST “Tricontagon” outline • 6.5 m flat-to-flat • No segment gaps, though “missing” segment at center • Rough six-fold symmetry maintained, but more complicated profile Russell B. Makidon

12. PSF as a Function of Aperture Shape • JWST “Tricontagon” with segment gaps • Adds more structure to previous PSF, though general morphology same Russell B. Makidon

13. PSF as a Function of Aperture Shape • Full JWST entrance aperture with SM support obstructions • Addition of bright diffraction bar across horizontal and along 60 and 120 lines Russell B. Makidon

14. Key PSF Parameters with Aperture Shape Russell B. Makidon

15. Optical Modeling Tools Used on JWST Project • Many different tools available to model JWST optical systems • Ray tracing codes: ZEMAX, CODE V, OSLO, MACOS, ASAP • Wavefront manipulation: PROPER, JWPSF, MACOS • Integrated modeling: ITM (Ball proprietary) • JWPSF (James Webb Point Spread Function) developed in-house (Cox and Hodge 2006) • Extensive experience with ZEMAX exists at STScI; familiarity with PROPER • STScI purchasing single CODE V license • All JWST optical models delivered to project as CODE V macros Russell B. Makidon

16. Calculation of PSFs using JWPSF • Calculated PSFs using Fourier Transform method, as implemented in JWPSF software • pervious PSFs: apertures without wavefront errors • subsequent PSFs: use JWST aperture and optical error budget realizations provided by Ball Aerospace (Optical Error Budget “Revision T”) • Optical Path Difference (OPD) describes the difference between a perfect wavefront and an aberrated wavefront • all points on the wavefront no longer in phase • result is a degraded PSF Russell B. Makidon

17. PSF as a Function of OPD • At left: perfect JWST aperture • At right: one realization of JWST Rev T optical error budget • OTE + ISIM + NIRCam with reserves • OPD at left: 0 nm RMS • OPD at right: 110 nm RMS • PSFs at  = 2.0 µm Russell B. Makidon

18. PSF as a Function of OPD • Two realizations of JWST Rev T optical error budget • OPD at left: 110.3 nm RMS • OPD at right: 109.6 nm RMS • Measurable triangularity in PSF core at left; relatively circular core in PSF at right. • PSFs at  = 2.0 µm Russell B. Makidon

19. NIRCam PSFs at 2.0µm Russell B. Makidon

20. PSF as a Function of OPD and Wavelength • Two realizations of JWST Rev T optical error budget • OPD at left: 110.3 nm RMS • OPD at right: 109.6 nm RMS • PSFs at top: F070W • PSFs at bottom: F200W • PSFs shown on same angular scale; same logarithmic grayscale Russell B. Makidon

21. PSF Variation with Wavelength • Quantified radial PSF profile and encircled energy for broadband NIRCam filters • Cases shown for single input OPD • PSFs approach ideal for long wavelengths; still very good at short wavelengths Russell B. Makidon

22. PSF Variation with Wavelength • Encircled energy within 0.15 arcsec shows peak at  = 2.0 µm (diffraction limit) • PSFs core width continues to improve toward short wavelengths (despite absence of requirements) Russell B. Makidon

23. Now and the Near Future • Currently able to do relatively simple optical analyses using a combination of JWPSF and ZEMAX • PROPER available, though STScI experience is limited • has been used to support NIRCam coronagraph studies • segmented primary; generation of OPDs • Obtaining CODE V as a means to vet current models. • All JWST optical models delivered to the Project as CODE V macros Russell B. Makidon

24. ZEMAX Model: Monolithic JWST and NIRCAM • Monolithic primary with full NIRCam optical train • Useful to adjust positions of optical elements, and determine OPDs due to defocus, misalignment, etc. • Add changes to Ball OPDs Russell B. Makidon

25. Using ZEMAX Predictions with JWPSF • Left: PSF in F187N for single Rev T OPD error realization. • Right: PSF in F187N for same Rev T OPD error realization • added 0.2 waves of defocus to Ball-supplied OPD map using the predictions ZEMAX model with monolithic JWST primary mirror Russell B. Makidon

26. PSF as a Function of OPD and Wavelength • Left: PSF in F187N for single Rev T OPD error realization. • Top from JWPSF • Bottom from PROPER • Right: PSF in F187N for same Rev T OPD error realization • Top: ZEMAX model with monolithic JWST primary mirror; 0.2 waves defocus • Bottom: PROPER with 0.15 waves of defocus • Zernike normalization issue? Russell B. Makidon

27. Conclusions • Presented and explored various methods to calculate the PSF http://www.stsci.edu/jwst/externaldocs/technicalreports • Presented PSF properties of astronomical interest given current understanding of telescope design • Understanding JWST PSF will be a challenge • First step toward providing an understanding of the PSF useful for JWST observers • Start of development of tools of use to S&OC developed to address tradeoffs between PSF quality and operations scenarios • JWST compares favorably with HST at wavelengths as short as 0.70 microns; far exceeds capabilities of NICMOS at NIR wavelengths. Russell B. Makidon

28. Comparison: HST/ACS with JWST NIRCam • ACS image of HUDF in V, I, and Z-bands (left) and simulated JWST NIRCam image in F070W, F090W, and F115W (right) Images courtesy M. Stiavelli Russell B. Makidon

29. Comparison: HST/ACS with JWST NIRCam • ACS image of HUDF in V, I, and Z-bands (left) and simulated JWST NIRCam image in F070W, F090W, and F115W (right) Images courtesy M. Stiavelli Russell B. Makidon