NICMOS Coronagraph

NICMOS Camera 2 (NIC2) has a coronagraphic observing mode. A hole was bored through the Camera 2 FDA mirror. This hole, combined with a cold mask at the pupil (Lyot stop), provides coronagraphic imaging capability. Internal cold baffling was designed to screen out residual thermal radiation from the edges of the HST primary and secondary mirrors and the secondary mirror support structures (pads, spider, and mounts). An image of a star is formed on the FDA mirror and is re-imaged on the detector. The image of a star in the hole will have diffraction spikes. The hole traps the light from the core of the PSF, reducing the diffracted energy outside of the hole by reducing the high frequency components in the PSF.

The light scattering downstream of the FDA is greatly reduced. The hole edge acts as a new diffraction aperture, and the residual roughness about the hole from the drilling process (Figure 5.1) creates a complex image of the star in the hole. At a radius of 0.3 arcsec, in an idealized PSF, a natural break occurs in the encircled energy profile at 1.6 µm with 93% of the energy in the PSF enclosed. Beyond this radius, the encircled energy profile flattens out toward larger radii.

The light pattern about the coronagraphic hole is not symmetric due in part to the coronagraphic optics and to the Optical Telescope Assembly (OTA) input PSF. The spectral reflections from the roughness about the hole, and imaged in Camera 2, will vary depending upon the location of the target in the hole. There is one azimuth region where the residual light pattern, historically called glint, is brightest. The presence of glint brings the useful coronagraphic radius at the detector to ~0.4 arcsec.

The FDA mirror and the Camera 2 f/45 optics image planes are not exactly parfocal. For nominal Camera 2 imaging, the PAM is positioned to achieve optimal image quality at the detector. For coronagraphic imaging, the PAM is adjusted slightly for optimal coronagraphic performance.

The tilt of the PAM is changed to compensate for translation from the nominal to coronagraphic setting, and to remove off-axis aberrations. The NICMOS dewar anomaly caused the coronagraphic hole to migrate to different locations on the detector. The movement of the hole is not linear. Rather, the hole jitters back and forth along an X-Y diagonal by as much as ±0.5 pixel.