We are in the process of determining the throughput of DEIMOS in spectroscopic mode
             for all the gratings and grating tilts of relevance. What you will find in the links below are down-
             loadable plots and tables with the results of observations of the star BD+28 4211, taken under
             photometric conditions during the months of June through September/2002.

                       Our throughputs are defined as follows:

             Throughput =  Nd/Nt

                        where:

             Nd = Number of photons detected
             Nt = Number of photons hitting the telescope's primary mirror

                        The latter is given by:

             Nt = Nst * 10 ** (-0.4*airmass*ext(lambda))

                         where Nst is the star's photon flux, in photon/sq.cm/s, taken from the HST/CALSPEC
             database of standard star fluxes, and ext(lambda) is the mean monochromatic extinction coefficient
             for Mauna Kea.
 
 

                       Throughput data taken with various combinations of grating tilts and order-blocking filters
              for the aluminum 600 and 900 l/mm gratings, and a more limited, preliminary set of data for the
              gold 830 and 1200 l/mm gratings are available. A more comprehensive data set for the latter
              two gratings and for the aluminum 1200 l/mm grating are in the process of being collected and
              reduced, and will be made available soon. By following the links below, you'll be able to view plots,
              or download postscript files and ascII tables with the measured throughputs.

                       We note that these throughputs are higher by a factor of 1.056 than the values from an earlier
              release, due to a correction in the value adopted for the collective area of the Keck telescope.
 

            Aluminum 600 l/mm

            Aluminum 900 l/mm

            Gold 830 l/mm:      download ps      view plot

            Gold 1200 l/mm:    download ps      view plot
 
 

                       We also made a theoretical throughput estimate for each of the grating/filter settings,
             which is compared to the data in some of the Figures above. This model throughput is the
             result of the product of the efficiencies of all the components in the optical path from the
             telescope's primary mirror, down to the CCD:
 

              Throughput = Telescope_Mirrors * Front_Window * Collimator * Tent_Mirror *
                                        Camera * Grating * CCD
 
 

                       We give more details below on the input data for each of the components above:
 

                     1) Telescope Mirrors

                       The Keck telescope mirrors are aluminum-coated. We adopted a reflectivity curve
             for fresh aluminum taken from Thermo Oriel.  Aluminum gets old, though, mostly due to
             oxidation and dust accumulation, and this has a large impact on its reflecting properties.
             We accounted for the aging effect by following a prescription suggested by Mike Bolte. In
             Figure 1 we show a plot of the fresh aluminum reflectance as a function of wavelength, and
             the multiplicative factor applied to that curve to account for aging. In Figure 2, we display
             the final reflectance curve adopted for the combination of the three Keck mirrors, taking
             into account the aging effect. Admittedly, the above correction for the aging effect is quite
             crude. Therefore, the final reflectance shown in Figure 2 needs to be taken with a grain of
             salt. For instance, measurements by Bill Brown, from Lick, show that just washing the mirrors
             can boost the reflectance by up to 10%, so that periodic variations as large as that are to be
             expected.
 
 

                      2) DEIMOS Front Window
 
 
 

                      3) Collimator
 
 
 

                      4) Tent Window
 
 
 

                      5) Camera
 
 
 

                      6) Gratings
 
 
 

                      7) CCDs