Summary of HETGS Effective Area with P1H1-P2H2 Mirrors
			3/13/92  D. Dewey

  This table compares HETGS performance using only two mirror shells,
P1H1+P2H2, with the baselined HETGS performance.  Specifically I'm calculating:
      ( MEG*(P1H1) + HEG*(P2H2)  ) divided by
       ( MEG*(P1H1+P2H2+P3H3) + HEG*(P4H4+P5H5+P6H6)  )
These ratio curves contain a variety of structure but can be pretty well
summarized by their average value (on log scale, i.e., geometric mean) in
a few energy bands.

  The mix of HEG and MEG type gratings is retained for purposes of comparison
(however, see note below.)  The HRMA model (data from Elsner) is for 7 Angstrom
surface roughness and 1 arc sec encircled diameter.  The same ACIS efficiency
is used for all models.


   Energy band          Coating (P1H1/P2H2)             Baseline Area
      keV        Ni/Au     Ni/Ni     Au/Au     Ir/Ir        (cm2)
   ----------- -------------------------------------     -----------
    0.4 - 0.8     38 %      38 %      30 %      31 %        0 -  15

    0.8 - 2.0     44 %      40 %      47 %      50 %       15 - 150

    2.0 - 4.0     32 %      45 %      25 %      28 %      150 -  55

    4.0 - 6.0    (17)      ( 8)      (20)      (30)        55 -  28

    6.0 - 8.0      -         -         -       (<5)        28 -   7
   ----------- -------------------------------------     -----------
   Average
     0.4-4 keV    38 %      41 %      34 %      36 % 
   ----------- -------------------------------------

    Cutoff E    6.0 keV   4.8 keV   6.0 keV   6.4 keV
    (1 cm2)
   ----------- -------------------------------------

  Note: In terms of efficiency the distinction between HEG/MEG gratings is
based on material and thickness (and not period):
   HEG = 1 micron thick gold
   MEG = 0.5 micron thick silver
The HEG and MEG have comparable efficiencies 0.4-0.8 keV,  MEG is ~x2 better
in the range 1-2 keV, and HEG is  much (>x2) better above 3.5 keV.