the jwst point spread function: calculation methods and expected properties tips/jim meeting russell...

The JWST Point Spread Function:Calculation Methods and Expected

Properties

TIPS/JIM Meeting

Russell B. MakidonStefano Casertano, Colin Cox, & Roeland P. van der Marel

Telescopes Group & JWST OTE / WFS&C TeamSpace Telescope Science Institute

June 21, 2007

June 21, 2007 Russell B. Makidon TIPS/JIM Meeting

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.


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


Changes in the HST PSF: SM Motion During an Orbit


Changes in the HST PSF: SM Position v. Time


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.


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.


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.


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



• Hexagonal Aperture, 6.5 m point-to-point

– Six-fold symmetry in PSF

– Flux in circular symmetric rings diffracted into “spikes” at 60 intervals



• 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



• JWST “Tricontagon” with segment gaps

– Adds more structure to previous PSF, though general morphology same



• Full JWST entrance aperture with SM support obstructions

– Addition of bright diffraction bar across horizontal and along 60 and 120 lines


Key PSF Parameters with Aperture Shape

Percentage Flux Ellipticity Parameters

Shape FWHM Radius

of 1st min

within

first

min

within

50 mas

within

150 mas

outside 1

arcsec

e1 e2 Total

Circle 0.064” 0.079” 84.4 79.1 91.7 0.8 0.0000 0.0000 0.0000

Hexagon 0.070” 0.086” 83.9 74.6 91.0 0.9 0.0060 0.0000 0.0060

Tricontagon 0.067” 0.080” 74.2 68.1 88.9 1.2 -0.0043 0.0000 0.0043

Tricontagon

w/gaps

0.069” 0.083” 72.4 65.3 87.0 1.8 -0.0025 0.0000 0.0025

JWST Pupil 0.067” 0.080” 70.8 64.9 85.6 2.8 -0.0145 0.0000 0.0145


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


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


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


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


NIRCam PSFs at 2.0µm

Sharpness

Flux inside

0.15”

(total = 1.0)

Ellipticity

mean min, max e1 e2

F070W 0.155 0.026 0.107, 0.194 0.72 0.01 0.046 0.154 -0.031 0.147

F200W 0.076 0.002 0.073, 0.082 0.80 0.01 0.014 0.024 -0.010 0.030


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


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


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)


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


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


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


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?


Conclusions

• Presented and explored various methods to calculate the PSFhttp://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.


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

the jwst point spread function: calculation methods and expected properties tips/jim meeting russell...

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