Various tests of the NICMOS coronograph show that it performed well and substantially reduced scattered light compared to direct images. The NICMOS onboard software and flight software allowed for automatic location of the coronographic hole and adjust the focus to optimize the performance of the coronograph.
Hole-location Software Upgrade
The coronograph on NICMOS consists of a laser- ablated hole in the NIC2 mirror on the field divider assembly (FDA). One of the effects of the dewar expansion and subsequent contraction has been a change in the relative geometry of the NIC2 detector relative to the FDA. For the coronograph, this has caused the apparent location of the coronographic hole to drift relative to the fixed acquisition aperture on NIC2. Because it is necessary to center a star in the coronographic hole to within 1/4 pixel or better, a NICMOS onboard software upgrade has been implemented which allows to accurately locate the coronographic hole at the beginning of each observation.
Separate Coronograph Focus
The deformation of the NICMOS dewar led to changes in focus for all three cameras. The Pupil Alignment Mechanism (PAM) is an adjustable mirror that can used to adjust the focus of the NICMOS cameras. While the PAM was designed to adjust the telescope focal plane onto the field divider assembly (FDA) which separates the incoming light into the three individual NICMOS cameras, it has mostly been used to focus the telescope beam onto the NICMOS detectors. By moving the PAM mirror to focus on the NIC2 detector, however, the image at the FDA (whith the coronagraphic hole) is no longer focussed which leads to increased scattered light levels.
To minimize this effect, tests were performed at a variety of PAM positions. The results show the greatest reduction in scattered light for a PAM position of +3.38 mm which places a focussed image at the FDA. This position was added to the flight software, and is now the default for all coronographic observations. The resulting slight defocus in the detector plane can be neglected