The steps in getting the sub-assembly efficiency predictions are summarized here and described in more detail below:
Similar steps are carried out for the LETG, using instead the procedure letg_cip_etc.pro.
For general operating principles of the X-GEF see the X-GEF section of the Sub-assembly Measurements description. The analysis of the X-GEF data has been carried out at MIT using IDL code. The X-GEF analysis proceeds through a variety of steps as indicated by the key-word-gated sections of the top level procedure: xgef_analysis.pro.
The X-GEF model parameters are collected into shell-by-shell data files:
[For LETG the bar FWHM, height, and slope have been determined by NIR spectrophotometer method. These values were used by leg_to_multi.pro to create similar rdb files for the LETG gratings:
|Shell||Period-Width-height-slope file||multi-grating File|
Using the 'multi.rdb files of detailed bar shape parameters, the grating average efficiency into different orders on a shell-by-shell basis is calculated using John Davis's multi.sl routine. At MIT, the calculations were done using John Davis's eff s/w using the .sl files in /spectra/d6/effic_tables (see the file do_em_all). The input to these calculations is the bar parameters for each facet which are given in the *_multi.txt files (the 'multi.rdb files without the header information).
The resulting efficiencies are the "membrane efficiency" - that is the efficiency of the grating membrane itself (i.e., for LETG this includes the support structure but not the effect of the circular facet shape(see below, however); for HETG gratings it includes the polyimide but not the effect of gaps between the frames.)
These membrane efficiency files are processed by cip_weights_and_errors.pro to produce the final products: shell efficiency files which include the vignetting terms, and HEG/MEG/HETG mirror weighted efficiency files. Error files are created for these as well.
For the LETG, the "membrane" efficiency files include one factor of 0.809, in order to produce files with the efficiency into the central "0,0" order only, an additional factor of 0.81 is included along with the facet vignetting terms (which shell average to 0.8346) due to the circular facets; these calculations are performed by letg_cip_etc.pro.
The results files ('effic.rdb) are available at MIT (/nfs/spectra/d6/CIP/) and SAO (TBD).
|Grating Shell||Efficiency File|
The 'effic.rdb files have a row per energy. The first column of each row is the energy in keV and subsequent columns are the predicted diffraction efficiencies in order order, e.g., -11, -10, ... , -1, 0, +1, ..., +10, +11. The HETG shells are calculated for orders -11 to +11 and the LETG shells are calculated for orders -25 to +25. Each HETG file is 1.5 Mb and the LETG files are 3.3 Mb each.
The efficiencies tabulated above are the average for a given shell, energy, and order. In order to predict flight effective areas or to compare these to the measured XRCF efficiencies it is useful to apply mirror-shell weighting.
Define the following:
Then the expected effective area and efficiency for the grating spectrometer at energy "E", order "m", and into an aperture "ap" are given by:
Grating effective area (E,m,ap) = sum over n [ A_n(E,ap) * V_Gn * Eff_Gn(E,m) ] Grating sum over n [ A_n(E,ap) * V_Gn * Eff_Gn(E,m) ] efficiency (E,m,ap) = ---------------------------------------------- sum over n [ A_n(E,ap) ]where the sums are over appropriate n in the case of HEG or MEG, and the aperture size is assumed to be much larger than grating-induced PSF effects. The efficiencies generally mentioned for flight performance are for large values of "ap" (e.g. 500 um diameter or 10 arc seconds diameter.