Quick Reference
This page summarizes the predicted capabilities of LWS. 

Estimated Physical and Optical Characteristics

Location Forward Cassegrain module, Keck I;
shares module with NIRC
Secondary mirror f/25, gold-coated
Dewar orientation ``Uplooker'' dewar (no tertiary mirror) 
Dewar hold time ~24 hr (LHe dewar)
>30 hr (LN dewar) 
Rotator 540° rotation
Guiding system One visible-wavelength offset guider available; 61×46 arcsec FOV
Scale at detector 0.0011 arcsec/µm; 0.0847 arcsec/75 µm px 
Detector type Boeing Si:As moderate-flux array 
Detector format 128×128 px (75 µm px) 
Data format 6-dimensional fits file (see figure):
Flexure <0.2 arcsec (<3 px) during 3 hour transit near zenith
Apertures 2 longslits, occulting disk, ``clear'' (imaging)
Dispersive elements 1 low- and 1 moderate-resolution grating
Imaging option Dispersive elements removed from light path
Filters 16 slots; 14 filters provided
User filters Not available
Collimator focal length 324 mm
Camera focal length 230 mm
Pupil size 13 mm

Estimated Imaging Performance

Field of view 10.84×10.84 arcsec
Detector pixel scale 0.0847 arcsec/px (75 µm/px) 
Wavelength range (imaging) 3.5-25 µm
Standard filters
Name Bandpass¹ [µm]
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
17.9 16.9-18.9
17.65 17.3-18.2
18.75 18.3-19.2
SiC 10.5-12.9
N wide 3 8.1-13.0 (not usable for imaging)
Spec 10 >6.94 µm (long pass)
Spec 20 >13.71 µm (long pass)
Image size at 10 µm 0.25 arcsec FWHM (instantaneous)
0.30 - 0.35 arcsec FWHM ( typical for longer integrations) 
Expected count rate 11,000 ADU/Jy·sec at 11.7 µm filter from sky
Sensitivity to faint stellar objects 2 17mJy 2 at 11.75 µm (SiC filter) 
100 mJy at 17.65 µm (17.65 filter)
Mode changes 2 min changeover between imaging and spectroscopy


  1. Wavelength range over which filter transmission exceeds 80% of peak.
  2. Signal level yielding S/N=1 in 1 sec (on source time) at stated wavelength over a 1 µm bandwidth, integrated over the detector area extending from the point source center to the first null in the Airy pattern. Quoted flux is light incident on telescope. On source time is typically 3 times shorter than elapsed time due to chop/nod inefficiencies.
  3. N_wide is NOT usable for imaging programs. The increased bandwidth of Nwide  results in high background flux that saturates the detector

Estimated Spectroscopic Performance

Total slit length 10.24 arcsec
Usable slit length¹ 7.0 arcsec
Detector scale 0.080 arcsec per px (75 µm px) 
Wavelength range (spectra) 3.5-25 µm
Order blocking filters N wide (8.1-13.0 µm)
Spec 20 (>13.71 µm long pass) 
Slit widths 3 pix,  6 pix, and 12 pix wide 
Current gratings 8 g/mm (10.0 µm blaze); R=100
50 g/mm (19.5 µm blaze); R=1400 
Number of grating slots 2 slots available, both used
Corresponding dispersions LRES:     0.037 µm/px;
HRES:    0.0024 µm/px (10 µm, N=2) 
HRES:    0.0048 µm/px (20 µm, N=1)
Spectral coverage, single exposure (128 px) LRES: 4.7 µm
HRES (@10.0µm): 0.31 µm
HRES (@20.0µm): 0.62 µm
FWHM resolution R=100 and R=1400 (3 px slit)
Count rate at 10µm TBD
Sensitivity to faint stellar objects² 150 mJy


  1. Slit length subtended by detector pixels.
  2. Signal level yielding S/N=1 per spectroscopic resolution element in 1 sec on source at 11.7 µm using the small (3 pixel = 0.25 arcsec) slit in conjunction with the low-resolution grating and the broadband "N" (8-13 µm) filter at a wavelength of 10 µm. Chop-nod observing mode is assumed. Quoted flux is light incident on telescope. Note that the observing efficiency is approximately 30% in spectroscopic mode; i.e., 30% of the time is spent in the "off beam" and 40% is lost to overhead associated with telescope moves.

Go to: LWS Home Page - Instruments Home Page - Keck Home Page

Last modified: 8 Apr  2002