Appendix: Available Optical Path Difference (OPD) files¶
For each of the five instruments (four SIs plus FGS) there are three provided OPD files. These represent wavefronts as follows:
- The OTE and ISIM intrinsic WFE
- The above, plus a slight defocus to blur the image slightly to approximate image motion.
- The above #2, plus additional WFE due to SI internal optics.
The latter is the largest WFE, and is the default file used in simulations unless another is explicitly chosen. For NIRCam only there is a second, duplicate set of these files with slightly improved WFE based on an optimistic case scenario for instrument and telescope alignment.
The provided OPDs are based on the observatory design requirements, and were developed for the Mission Critical Design Review. The represent the nominal case of performance for JWST, and have not yet been updated with as-built details of mirror surface figures, etc. We intend to make updated OPD files available once suitable reference data have been provided to STScI. For now, see Lightsey et al. 2014 for recent predictions of JWST’s likely performance
Note that the trick of adding some (nonphysical) defocus to blur out the PSF is computationally easy and rapid, but does not give a high fidelity representation of the true impact of image jitter. This is particularly true for coronagraphic observations. Future versions of WebbPSF will likely provide higher fidelity jitter models.
The units of the supplied OPD files are wavefront error in microns.
|File||Instrument||RMS WFE||Includes OTE + ISIM OPD?||Image motion (as defocus)?||SI OPD?|
|OPD_RevV_nircam_115.fits||NIRCam||115.0||Yes, optimistic case||No||No|
|OPD_RevV_nircam_132.fits||NIRCam||132.0||Yes, optimistic case||Yes||No|
|OPD_RevV_nircam_150.fits||NIRCam||150.0||Yes, optimistic case||Yes||Yes|