HETG: Basic Design

Go to HETG at XRCF

HETG Design

IDL routines were used to generate the data needed as input to the CAD process to design the HETG Support Structure (HESS). Through excellent engineering (Chris Pak and Mike McGuirk) and fabrication (Brenner (PA)) the as-built HETG is very close to the ideal design.

• Parameters used by IDL code are in hess_setup.pro.
• IDL was started with a startup file ( wave_start.pro) that further called hess_prog.pro and hess_setup.pro. Values were kept in common ( hess_common.pro).
• The real calculations were done by hess.pro using the subprocedure hess_equ.pro.
• The calculated values could then be put in a data file by hess_out.pro and plotted by hess_plot.pro (printer setup by pre_print.pro).
• For ray-trace simulations the procedure hess_to_rdb.pro was used to generate the FCP:
  • HESSdesign.rdb
    
    
  This file contains "as-designed" values. Because of the accuracy of the HESS construction, these values will suffice for HETG facet-based model development.

The definitions and uses of the data above are clarified in the Figures and captions below.

HETG as-built, FCPs

The design values above will be adjusted based on the actual HESS metrology (if warranted) and especially to take into account the grating roll values (specifically the MEG mis-aligned gratings). So we have the following planned improvement:

• HESSbuilt.rdb Same as HESSdesign.rdb but with rotated grating-line and dispersion-direction unit vectors to account for the MEG mis-aligned gratings and the statistical mis-alignements (to be derived from XRCF data.) Possibly also adjusted facet locations and angles based on metrology.

CIPs

Based also on this s/w we have created a basic data file hetg*basicNxxxx.rdb which contains some general and higher-level products of direct use in routine data analysis and modeling.

• Designed Rowland Diameter is = 8633.69 mm.
• The vignetting factors for each shell are calculated based on the grating-grating spacing and grating frame dimensions.
• The effective ring radii ( hs_X*TAN(theta_shell) ) or cone angles (theta_shell) which are parameters used to estimate the astigmatism and defocus blur.

Figures

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Figure hess.1 HRMA, Rowland geometry parameters

The HETG design must place facets on the Rowland torus and intercept the rays from the HRMA. Parameters in hess_setup.pro defined the HRMA ray cones. The Rowland torus is defined by the focus location and the Rowland diameter, hs_X, the distance from the focus to the on-axis Rowland circle intercept. The HESS design uses as its X (axial) origin a plane offset from the intercept by a distance of hs_Xoff; this plane is also a physical mechanical surface of the HETG. Thus, to convert the X locations tabulated in HESSdesign.rdb into distances from the (design) focus: add (hs_X - hs_Xoff).

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Figure hess.2 Facet frame parameters

This schematic of an HETG grating frame is viewed along the minus-X direction, that is as seen by an approaching X-ray. The goal of the HESS design is to put the center of the facet active area on the Rowland torus with the facet membrane normal to the HRMA ray through the facet center. The parameters hs_Doff, hs_Dspc, and hs_Dthk (not shown) were used to map the membrane centers to the mounting hole locations. Facet coordinates in the membrane plane are shown here, Xf and Yf.
For ray-tracing, the dot products of Xf and Yf with the {point of intersection of the ray with the facet plane minus the facet center location} gives the the intersection coordinates in frame coordinates, (Xf,Yf)_intersection. Using the geometry above and the values of hs_Alen (in hess_setup.pro) the fate of an X-ray that intersects near the grating can be determined with the steps below (the first satisfied criteria is the result):

Condition	Result
Xf > 0.5*hs_Alen + 25.4*0.070 OR
Xf < 0.5*hs_Alen - 25.4*0.030	outside of this grating's domain
|Xf| < 0.5*hs_Alen AND
|Yf| < 0.5*hs_Alen	interacts with active area of this facet
Yf > 0.5*hs_Alen + 25.4*0.030	passes uneffected (unless Yf much larger)
Yf < 0.5*hs_Alen - 25.4*0.104	might hit HESS or be uneffected
(otherwise)	absorbed by frame