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NICI Focal Plane Masks
The NICI Focal Plane Mask Wheel lies outside the dewar, ahead of the pupil masks and the beam splitter. The AO system brings light to a focus on the masks. NICI is configured for the coronagraphic imaging mode by selecting one of the occulting masks in this wheel, and for standard imaging mode by selecting the Clear mask. The available masks are listed in the table below.
|NICI Focal Plane Masks|
|Position||Mask Name||Description||Gemini ID|
|1||Clear||Clear CaF2 substrate*||G5710|
|2||F0.90||0.90 arcsec radius||G5711|
|3||F0.65||0.65 arcsec radius||G5712|
|4||F0.46||0.46 arcsec radius||G5713|
|5||F0.32||0.32 arcsec radius||G5714|
|6||F0.22||0.22 arcsec radius||G5715|
*All the coronagraph masks are occulting spots deposited on Calcium Fluoride substrates. The Clear mask is a substrate of the same thickness, in order to maintain focus on the science detectors, with no occulting spot. Each mask, including Clear, has a different pattern of imperfections such as scratches and dust specks that appear in focus on the science detectors, and therefore require separate flat field exposures. Separate flat fields are also required each time a mask is moved in a dither sequence.
The occulting spots are not completely opaque; their minimum transmissivity is about 1 part in 300. Stars centered under the mask are therefore dimmed by approximately 6.2 magnitudes.
Scans of the F0.32 and F0.46 mask are shown in the figures below. The scan data are available via the following links.
The focal plane masks are mounted in a wheel and therefore describe an arc on the science camera detectors as the wheel is rotated. The trajectory of the F0.32 occulting spot on the Red channel (Holmes) detector is plotted in the figure below. The position of the spot may be adjusted along the arc using the parameter d in the Observing Tool NICI Offset component. The figure indicates the F0.32 spot position for several values of d; note that the d = 0 position is deliberately offset from the array center to avoid the quadrant boundaries.