urn:nasa:pds:context:instrument:keck.10m_keck1.lws_ir 1.0 1.10.0.0 Product_Context 2020-02-18 1.0 Initial export from OLAF urn:nasa:pds:context:telescope:keck.10m_keck1 instrument_to_telescope Jones, B. and R.C. Puetter, 'Keck long wavelength spectrometer', Proceedings of the SPIE - The International Society for Optical Engineering, 1946, Pages 610-621, 1993. Keck I Long Wavelength Spectrograph (IR) Imager Spectrometer Instrument Overview =================== The long-wavelength spectrometer (LWS) is a mid-infrared imaging spectrometer that operates at the Keck I telescope of the W. M. Keck Observatory. The instrument operates with the f/25 secondary mirror and is mounted at the forward Cassegrain focus of the telescope. It features a Boeing 128 x 128 pixel Si:AS BIB array, imaging within a 10.2 x 10.2 arcsecond field, and spectroscopy at R=100 or 1400 over the 3-25 micron range. Chopping and chop-nod operating modes are available. More information on the LWS is provided in the paper by Jones and Puetter, 1993 [JONES&PUETTER1993], and at the web site for the LWS instrument: http://www2.keck.hawaii.edu/inst/lws/lws.html Instrument Characteristics ========================== The physical and optical characteristics and estimated imaging and spectroscopic performance of the LWS are provided below. These properties are reproduced from the Keck Telescope and Facility Instrument Guide, available at http://www2.keck.hawaii.edu/observing/kecktelgde/ktelinstupdate.pd f. This guide is included as documentation for this archive, by permission of the Keck facilities management. Physical and optical characteristics ------------------------------------ Dewar hold time : ~24 hours for LHe, >30 hours for LN Guiding system : One visible-wavelength offset guider Scale at detector : 0.0011 arcsec/micron or 0.080 arcsec/pixel Detector type : Boeing Si:As moderate-flux array Detector format : 128 x 128 pixels Pixel size : 75 microns Data format : Six-dimensional FITS file [X,Y,chopbeams,chopsets,nodbeams,nodsets] See below, under 'Data Acquisition' Apertures : 2 longslits, occulting disk, and 'clear' for imaging Dispersive elements : 1 low- and 1 moderate-resolution grating Filters : 14 filters provided Collim. focal length : 324 mm Camera focal length : 230 mm Pupil size : 13 mm Estimated Imaging Performance ----------------------------- Field of view : 10.24 x 10.24 arcsec Detector pixel scale : 0.080 arcsec/pixel (75-micron pixel) Wavelength range : 3.5-25 microns Standard filters : Name Bandpass in microns L 3.5-4.2 M 4.4-5.0 8.0 7.5-8.2 8.9 8.4-9.2 9.9 9.4-10.4 10.7 10.0-11.4 11.7 11.2-12.2 12.5 12.0-13.0 17.65 17.3-18.2 17.9 16.9-18.9 18.75 18.3-19.2 SiC 10.5-12.9 Spec 10 >6.94 (long pass) Spec 20 >13.71 (long pass) Image size, 10 micron : 0.25 arcsec FWHM (instantaneous) 0.30-0.35 arcsec FWHM (longer exposures) Expected count rate : 11,000 ADU/Jy sec from sky with 11.7 micron filter Sensitivity to faint : 17 mJy at 11.75 micron with SiC filter stellar objects 100 mJy at 17.65 micron with 17.65 filter Estimated Spectroscopic Performance ----------------------------------- Total slit length : 10.24 arcsec Usable slit length : 7.0 arcsec Detector scale : 0.080 arcsec/pixel (75-micron pixel) Wavelength range : 3.5-25 micron Order blocking filters : Name Bandpass in microns N wide 8.1-13.0 Spec 20 >13.71 (long pass) Slit widths : 3, 6 and 12 pixels wide Current gratings : 8 g/mm (10.0 micron blaze); R=100 50 g/mm (19.5 micron blaze); R=1400 Number of grating slots : 2 slots available, both used Corresponding dispersions : LRES: 0.037 micron/pixel HRES: 0.0024 micron/pix (10 micron,N=2) HRES: 0.0048 micron/pix (20 micron,N=1) Spectral coverage, : LRES: 4.7 micron single exposure HRES at 10.0 micron: 0.31 micron HRES at 20.0 micron: 0.62 micron FWHM resolution R=100 and R=1400 (3 pixel slit) Count rate at 10micron : TBD Sensitivity to faint : 150 mJy stellar objects Operational Mode ================ Chop-nod is the standard mode of data acquisition for the LWS. Chopping or tilting of the secondary mirror is performed to cancel sky radiation and nodding of the telescope is performed to cancel the radiation contribution from the telescope. The chop-nod method for the LWS uses nod and chop throws of equal amplitudes but in opposite directions. The result is that for each nod position, one of the chop beams is in the same x-y location on the detector. The resulting image, after applying a double difference process, requires no further shift or add processing. Data Acquisition ================ The data acquisition system for the LWS allows the user to choose the save-to-disk frequency (maximum 5 Hz) and the total integration time. The save frequency and the integration time determine the number of chop sets, also called save sets, and the number of nod sets. These sets are saved as six-dimensional FITS files: FITS NAXIS Definition ----- ------------------------------------------------------- 1 The number of image columns (X) 2 The number of image rows (Y) 3 The number of chop beams 4 The number of chop frame pairs saved per nod 5 The number of nod beams 6 The number of final chop-nod sets An illustration of the timing of data acquisition and the ordering of chop-nod frames within a six-dimensional FITS file is provided in the DOCUMENT/LWS_FITS.JPG file. Software -------- Before reducing LWS data, the six dimensions of a raw FITS image must be properly combined into a co-added, chop-nod, two- dimensional image. Software to aid this process is available from the Keck website for the Infrared Reduction and Analysis Facility (IRAF) or for the Interactive Data Language (IDL): http://www2.keck.hawaii.edu/inst/lws/wmkolws.html (IRAF) http://www2.keck.hawaii.edu/inst/lws/lws-idl.html (IDL) Once an image is converted to a two-dimensional array, standard optical or infrared processing packages can be used for further reductions.