Cal folks,  

 The -120 ACIS pixel size released by Mark Bautz

 23.987 +- 0.001 microns

is about 540 ppm change from the 24.000 value.  For HETGS
I was seeing a need to change the wavelength scale by
420 ppm, 570 ppm (MEG, HEG - with preference to follow
the HEG 570 ppm value)
(http://space.mit.edu/HETG/technotes/abs_lam_0007/abs_lam_0007.html )
 -- so the HETGS data agree well with this pixel size change
and imply that                                                

  ***--->  the HETG Rowland spacing is as expected.

 In the near term we have changed the Rowland spacing to 8637.0
in lieu of changing the pixel size - the ratios
 23.987/8632.48 = 0.00277869
and 
 24.0/8637.0 =    0.00277874
agree to ~ 20 ppm and so the current reprocessing should indeed 
produce HETGS (and LETG-ACIS-S, unless there is an LETG spacing
error) wavelengths the same as if we'd had the new ACIS pixel
size in place (and hence in the distant next reprocessing the
wavelengths will not change much due to pixel size changes...).

  -Dan D  9/22/2000