On the night of UT 030516, several Landolt standard fields were observed 
   with the WTTM, in the B,V,R,I passbands. The 85/15 beam splitter was in 
   use. 
   The standard star observations were processed in the 
   usual way, and photometric solutions were obtained for this night, solving 
   for atmospheric extincion, as well as color terms.

   Let bp, vp, rp and ip be the raw magnitudes in the B,V,R, and I passbands
   respectively. These are of the form

   xp = -2.5*alog10(count rate from a star in Datanumbers (DN)) + 30.00 
             
    in passband X. 

    
    then we can write:

     True mag X = xp + c0 + c1*airmass + c2*colorprime , 
 
    where colorprime are raw colors, like vp-ip, vp-rp, etc.


    The solution for the night of UT030516 are then the coeffs c0, c1 and c2,
    which were determoned for all filters and al possible color 
    combinations. They full solution is given immediately below:



    UT 030516   WTTM  85/15 dichroic   
  ________________________________


 
        MAG = magprime + c0 + c1*airmass + c2*colorprime
 
 
     Passband    colorprime        c0       c1       c2     rms_res  err_mean
 
        V        bp-vp           -4.548   -0.178   -0.148    0.012    0.003
 
        V        vp-rp           -4.593   -0.170   -0.176    0.021    0.005
 
        V        vp-ip           -4.603   -0.193   -0.089    0.020    0.005
 
 
        B        bp-vp           -4.434   -0.386    0.208    0.019    0.005
 
        B        bp-rp           -4.418   -0.379    0.109    0.022    0.006
 
        B        bp-ip           -4.387   -0.371    0.076    0.021    0.005
 
 
        R        bp-rp           -4.889   -0.016   -0.053    0.017    0.005
 
        R        vp-rp           -4.902   -0.016   -0.115    0.017    0.005
 
        R        rp-ip           -4.920   -0.043   -0.117    0.018    0.005
 
 
        I        vp-ip           -5.338   -0.107   -0.019    0.011    0.003
 
        I        bp-ip           -5.334   -0.106   -0.012    0.011    0.003
 
        I        rp-ip           -5.362   -0.097   -0.041    0.011    0.003
 
    

  Rms residuals are 0.02 mag or less, which is typical of a clear (but not 
   pristine photometric) night. The extinction coefficients are also typical 
   of a clear night, but on pristine nights they are up to 30% smaller.

  
   For a star of zero intrinsic color and B=V=R=I=20.0, we can surmise the 
  count rate in photons per second from the above, given that the `gain'
  was set to 1.33 electrons per DN. These rates are derived to be:

     Passband          Count rate
                       (photons/s)   
  
        B                221
        V                205
        R                150
        I                 97








   On the night of UT030602, a similar calibration was done with MIMO, with a
    `gain' setting of 1.4 electrons per DN.  This night was only 21 nights after 
   the WTTM data discussed above -- and the telescope mirrors and optics can 
   be assumed to be in an equivalent state for reflectance and scattering. 
   Thus, a comparison of count rates between the MIMO results of UT030602
   with WTTM results from UT030516 is meaningful comparison of throughput.

    The count rates derived from MIMO observations on UT030602 (for a star 
     with B=V=R=I=20.0 mag) are:


     Passband          Count rate
                       (photons/s)   

        B                 284
        V                 295
        R                 321
        I                 170


   Thus the comparative throughput in WTTM (with the 85/15 beamsplitter) 
    vs MIMO are:

              0.78 in B,  0.69 in V, 0.47 in R, and 0.57 in I


    These differences INCLUDE the effect of the extra reflections to get the 
light into WTTM, as well as the refelections within the WTTM. They also reflect 
any passband and filter throughput differences.