Mirifs: A Mid-Ir Integral Field Spectrograph For Ngst

We present the design of a mid-IR (5-280 mu m) integral field spectrometer for NGST. Details of the opto-mechanical design are given with particular attention being paid to those aspects influenced by the cryogenic, low background space environment in which the instrument needs to work. The instrument consists of three subsections : fore-optics and image slicing integral field units (IFU's), a 5-10 mu m spectrograph and 10-28 mu m spectrograph. Two co-aligned fields of view are separated into two wavelength channels (5-10 and 10-28 mu m) by a dichroic mirror in the fore-optics which also re-image the telescope focal plane onto the slicing mirrors of two IFUS. The short wavelength channel IFU has 18 slices 0.3 arcsec wide and 8.5 arcsec long while the long channel has 10 slices each 0.6 arcsec wide and 6 arcsec long. The IFU design is based on the design developed for the near-IR imager/spectrometer UIST, which is currently under test at the UKATC. The output slits of the IFU's feed two spectrographs which are ail reflective with fixed gratings. The 5-10 mu m channel has a 2K (spectral) x 1K (spatial) pixel detector and works in Ist order giving a resolving power at 10 mu m of R=2000. The 10-28 mu m channel has a 1K x 1K detector and works in orders 2, 3 and 4 giving wavelength ranges of 10-15 mu m (R=1000 - 1500), 15-20 mu m (R=1500 - 2000), 20-28 mu m (R=1000 -1500) respectively.