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The Camera 2 coronograph comprises two elements. A 170 micron diameter hole has been laser ablated out of the Camera 2 mirror in the NICMOS field divider assembly, which is at the image plane. (Small irregularities within a 10 microns annulus at the edge of the hole may be a source of residual scattered light in the images.) An oversized cryogenic pupil-plane mask screens out residual radiation from the edges of the HST primary and secondary mirrors and the secondary mirror support structures (pads, spider, and mounts.). This mask obscures approximately 15% of the primary mirror area. (Scattering by dust on the primary mirror may affect the overall image contrast, and while this is expected to be a small effect it can only be quantified on-orbit.).
The SMOV measurements of coronographic performance were being carried out as this version of the NICMOS Instrument Handbook was written. A preliminary description of the results of the SMOV tests will be placed on the STScI NICMOS WWW page on August 1, 1997.
Initial indications are that the coronograph meets or exceeds expectations.
The science exposures are then specified using any of the NICMOS observing modes with the target positioned on the NIC2-CORON aperture (which is behind the coronographic spot).
Figure 5.1: Acquisition Process
Very bright targets might cause saturation, leading to poor results in the centroid solution, and in the subsequent placement behind the occulting spot. To avoid this, a narrow band filter may have to be used to cut down the target flux. Since the NICMOS filters are in the pupil plane there should not be a shift introduced by using a different filter than needed for the science observations.
For observations longer than ~5 minutes the probability of cosmic ray hits occurring in the same pixel in each of the two acquisition images is sufficiently high
that observers must instead use an early acquisition image to avoid their observation failing due to a false center determination. Early acquisitions are described in
the next section. In practice, this should not be a severe restriction as in the
F160W filter one will reach a signal-to-noise of 50 at H=17 in only 2-3 minutes.
Reuse Target Offset and Interactive Acquisitions
In crowded fields, or for extended objects, the coronographic acquisition should not be relied on, since by necessity the on-board centering algorithm is rather simple. Whenever you know a priori that this is the situation, or the complexity of the field is unknown, we recommend obtaining an acquisition image before the scientific observation instead, even though this will require slightly more HST observations to accomplish your program. The telescope control system has the ability to re-use the same pair of guide stars as were used for the acquisition exposure, and from the accurate coordinates you have obtained, it is then possible to blind-offset the source onto the coronographic spot (RE-USE TARGET OFFSET). This can be obtained a few orbits or days prior to the science exposure. Alternatively, a real-time, interactive acquisition (INT-ACQ) can be obtained although the number of these are limited and must be justified in the Phase I proposal. This will mainly be necessary for time critical observations. Detector and Coronographic Hole Motion Issues
Since the coronographic hole is located in the field divider assembly (FDA) external to the dewar, the position of the image of the hole on the NIC2 detector will change with any relative motions between the FDA and NIC2. Some motion was expected due to the release of gravity but continuing motion has occurred on both long and short (orbits) timescales. Figure 5.2 graphically shows the offset.
Figure 5.2: A ratio of a recent flat field taken on-orbit and a flat field measured during thermal vacuum testing shows the change in position of the coronographic hole in that interval. Approximately half of this motion was expected relaxation in zero-G, the remainder has resulted from the deformation of the NICMOS dewar. The bright spot marks the location of the spot at the time of the on-orbit flat field; the dark spot shows its location during thermal vacuum testing before launch. More recent observations show the coronographic spot moving back towards its expected on-orbit position. The bright region near the bottom of the detector shows the area of the detector that is vignetted by a mask on the field divider assembly.
While the motion during a single orbit appears to be <0.25 pixels, a method of locating the coronographic hole's image on the NIC2 detector as part of the target acquisition process is underway and will be supported in Cycle 7-NICMOS. PSF Centering
Both the total encircled energy rejection (from the occulted core of the PSF) and the local contrast ratio obtainable in a coronographic image depend on the accuracy of the target centering on the occulting spot. The goal is to center the PSF of the occulted source to a precision of a quarter pixel. The decrease in the fractional encircled energy due to imprecise centering of the core of an idealized PSF in the occulting spot is 0.3 percent for a 1/4 pixel offset, and 4.4 percent for a 1 pixel (75 milliarcseconds) offset at 1.6 microns. The predicted fractional decrease in the encircled energy relative to that for a perfectly centered PSF is plotted against the shift of the center of the PSF from the center of the hole in Figure 5.3.
Figure 5.3: Contrast Decrease Due to PSF De-centering
However, a small error in target centering will create an asymmetric displacement of the PSF zonal structures both in and out of the occulting spot, leading to position dependent changes in the local image contrast ratios. Coronographic Decision Chart
The decision chart given in Figure 5.4 leads you through the selection process to construct a coronographic observation.
Figure 5.4: Coronographic Decision Chart
