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At Penn State, two new instrument component technologies, namely silicon gratings and gaussian-shaped pupil masks, have been developed and are ready for producing high quality components for all three NGST IR instruments. Fabrication of silicon grisms with sizes up to 2 inches in dimension has become a routine process at Penn State thanks to newly developed techniques in chemical etching, lithography, and post-processing. The newly etched silicon grisms have a typical rms surface roughness of ~ 9 nm with the lowest of 0.9 nm, significantly lower than our previous ones (~ 20-30 nm) and have ~ 0.035 wave wavefront distortion at 0.6328 μm, indicating diffraction-limited performance in the entire infrared wavelengths (1.2 -10 μm) where silicon has excellent transmission. These processes have also significantly eliminated visible defects due to grating mask break during chemical etching. For the best grisms, we have less than 1 defect per cm2. The measured total integrated scatter is less than 1% at 0.6238 mm, indicating similar or lower scatter in the IR when grisms are operated in transmission. Silicon grisms and silicon immersion gratings will both boost spectral resolving power by more than 3 times for NGST near-IR MOS and mid-IR camera and spectrograph without pushing current instrument design. The higher dispersed spectra can be selected either by a filter or a low resolution grism cross-disperser. Our current grating techniques allow us to make gratings with a groove period from a few microns to more than 100 microns. For the first order grism, the theoretical grating efficiency is beyond 80% with a single layer of AR coating. The immersion gratings will have similar grating efficiency. Based on our previous measurements of a silicon echelle grism, this kind of grism can provide ~ 60% efficiency when they are operated in high orders.
We have also developed Gaussian-shaped pupil masks for high contrast imaging with the NGST IR cameras. Depending on its final mirror configuration, this kind of mask can offer 10-6 contrast imaging as close as 5 lambda/D to a bright point source. The advantage of using this mask instead of a conventional graded Lyot coronagraph is that it is much easier to implement by simply inserting it at a pupil location to reach deep null. Therefore, the observing efficiency can be significantly improved. A prototype of this kind of mask has been tested at the Mt. Wilson 100inch telescope with adaptive optics and demonstrates 10-3-10-4 contrast at ~ 5 λ/D at the initial observations. The contrast level is comparable to an IR coronagraph in the same IR instrument, but is about one order of magnitude worse than the scattered light levels caused by the mirror surface. We have also studied other mask coronagraph designs for high contrast imaging. The hybrid and band-limited designs show great promise for further improving image contrast. The NGST IR cameras with new coronagraph designs will allow high contrast imaging for extra-solar planets and substellar companions around nearby stars.
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