JWST WebbPSF:docs

Appendix: Instrument Property References

We give here references for the instrumental properties assumed in PSF computations, with particular attention to coronagraphic optics. It also notes several places where the current models or available files are limited in some manner that might be improved in a future release.

Note: The WebbPSF software and all of its associated data files are entirely ITAR-free.


The supplied OPDs are the Mission CDR OPD simulation set, produced in March 2010 by Ball Aerospace staff (Paul Lightsey et al.) via the IPAM optical model using Zernike WFE coefficients consistent with Revision V of the JWST optical error budget.

Note: The provided files included no header metadata, and in particular no pixel scale, so one was assumed based on the apparent pupil diameter in the files. The estimated uncertainty in this scale is 1 part in 1000, so users concerned with measurements of PSF FWHMs etc at that level should be cautious.

The current model pixel scale, roughly 6 mm/pixel, is too coarse to resolve well the edge roll-off around the border of each segment. We make no attempt to include such effects here at this time. An independent study using much more finely sampled pupils has shown that the effect of segment edge roll-off is to scatter ~2% of the light from the PSF core out to large radii, primarily in the form of increased intensity along the diffraction spikes (Soummer et al. 2009, Technical Report JWST-STScI-001755)


NIRCam focal plane scale: 0.317 arcsec/pixel (short wave) 0.0648 arcsec/pixel (long wave). From STScI NIRCam web page.

The coronagraph optics models are based on the NIRCam instrument team’s series of SPIE papers describing the coronagraph designs and flight hardware. (Krist et al. 2007, 2009, 2010 Proc. SPIE), as clarified through cross checks with information provided by the NIRCam instrument team (Krist, private communication 2011). Currently, the models include only the 5 arcsec square ND acquisition boxes and not the second set of 2 arcsec squares.

Weak lenses: The lenses are nominally +- 8 and +4 waves at 2.14 microns. The as built defocus values are as follows based on component testing: 7.76198, -7.74260, 3.90240.


NIRspec field of view rotation: 41.5 degrees. Matt Lallo, draft SIAF information; and Ball SI Fields for WFS&C document, J. Scott Knight


NIRISS filter bandpasses are assumed to be precisely identical to NIRCam for the filters in common. The exceptions are F158M, which was a TFI filter, for which I retain the cryo transmission curve as measured by the manufacturer (Barr/Materion), and F380M, which is a new filter still in process of fabrication, for which I include a nominal design filter transmission curve.

Occulting spots: Assumed to be perfect circles with diameters 0.58, 0.75, 1.5, and 2.0 arcsec. Doyon et al. 2010 SPIE 7731. While these are not likely to see extensive use with NIRISS, they are indeed still present in the hardware, so we retain the ability to simulate them.

NRM occulter mask: Provided by Anand Sivaramakrishnan.


MIRIM focal plane scale, 0.11 arcsec/pix: MIRI Optical Bench Assembly (OBA) Design Description, MIRI-DD-00001-AEU, 2.2.1

MIRIM field of view rotation, 4.561 degrees: MIRI Optical Bench Assembly (OBA) Design Description, MIRI-DD-00001-AEU

Coronagraph pupils rotated to match, 4.56 degrees: MIRI-DD-00001-AEU

Coronagraphic FOVs, 30.0 arcsec for Lyot, 24.0x23.8 arcsec for FQPMs: MIRI-DD-00001-AEU 2.2.1

Lyot coronagraph occulting spot diameter, 4.25 arcsec:

Lyot coronagraph support bar width, 0.46 mm = 0.722 arcsec: Anthony Boccaletti private communication December 2010 to Perrin and Hines

Lyot mask files: Anthony Boccaletti private communication to Remi Soummer

LRS slit size (4.7 x 0.51 arcsec): MIRI-TR-00001-CEA. And LRS Overview presentation by Silvia Scheithaur to MIRI team meeting May 2013.

LRS P750L grating aperture mask (3.8% oversized tricontagon): MIRI OBA Design Description, MIRI-DD-00001-AEU

Instrument + Filter Throughputs

Where possible, instrumental relative spectral responses were derived from the Pysynphot CDBS files used for the JWST Exposure Time Calculators (ETCs), normalized to peak transmission = 1.0 (because absolute throughput is not relevant for PSF calculations). Not all filters are yet supported in Pysynphot, however.

Note on MIRI filters: The MIRI instrument team requested that at this time we release only idealized top-hat function filter profiles rather than the measured transmissions. We thus take the properties of these filters from the table at http://www.stsci.edu/jwst/instruments/miri/filters/filters_temp.html . Internal testing at STScI indicates that with this simplification compared against the measured filter profiles, systematic errors in computed PSF FWHMs are typically <1.5% assuming sources with Rayleigh-Jeans spectra at these wavelengths; systematics in encircled energy are generally <1%.

In summary for the following subset of filters we take information from alternate sources other than the CDBS:

Instrument    Filter         Source
-----------  --------        ----------------------------------------------------------------------------------------------------------
NIRCam       F150W2          Top-hat function based on filter properties list at http://ircamera.as.arizona.edu/nircam/features.html
NIRCam       F322W2          Top-hat function based on filter properties list at http://ircamera.as.arizona.edu/nircam/features.html
NIRSpec      F115W           Assumed to be identical to the NIRCam one
NIRSpec      F140X           NIRSpec "BBA" transmission curve traced from NIRSpec GWA FWA Assembly Report, NIRS-ZEO-RO-0051, section 6.3.2
MIRI         all filters     MIRI filters are represented as top-hat functions only at this time, by request of the MIRI team.
FGS          none            Assumed top-hat function based on detector cut-on and cut-off wavelengths.

The above filters’ throughputs do not include the detector QE or OTE/SI optics throughputs versus wavelength (or the throughput of the Germanium FQPM substrates for the MIRI coronagraphic filters). All other filters do include these effects to the extent that they are accurately captured in the Calibration Database in support of the ETCs.

Documentation last updated on August 15, 2014