Figure 27: An image of HRMA/LETG/HRC-S of the Mg-K lines from the electron impact point source at the XRCF. This image inclues 8 to 9 orders (+2 to +11, -3 to -11) of diffraction of the Mg-K lines. The central portion is blocked by the HRC ``butter knife" shutters. In addition to the Mg-K lines, there are also diffracted carbon and oxygen K-lines. The cross-dispersion diffraction pattern is due to the fine support structure. The gaps outside the 1st order of the C-K are the gaps between three microchannel plates of the HRC-S. The C-K line width is wider than the Mg-K lines due to solid state effects of the X-ray source target. Postscript version of the above image.
Brinkman et al and Predehl et al have given a detailed calibration description of the LETG, with the emphasis on the EA (effective area) and LSF (line spread function) in their SPIE papers[1,2]. In this article, we will use the XRCF data to illustrate several features of the LETGS.
Figure 27 shows an image of LETG/HRC-S when the X-ray source is the EIPS (electron impact point source) with a Mg anode. This measurement is intented for measuring Mg-K LSF and EA of the LETGS at higher orders. The central portion was blocked by the HRC ``butter knife" shutters, in order to avoid high count rate of 0th, +1st, and -1st orders. From the right half of the image, one can see the diffraction orders from +2nd up to +11th order of Mg-K lines; a similar spectrum is shown on the left half. We can actually see +16th and -16th order of the Mg-K lines from the whole HRC-S image (this image does not include 1/3 of the outer microchannel plates of the HRC-S). In addtion to the diffraction orders from the Mg-K lines, one can also see diffracted orders of O-K and C-K lines. These lines are from the Mg anode used. Figure 28 shows the spectrum from this image. The Mg-K lines have been labelled by their order number, and lines for other elements were labelled with both the line and the order.
Figure 28: The spectrum extracted from the image shown in figure 27.
In addition to the spectral features along the dispersion direction, there is also diffraction along the cross-dispersion direction. The cross dispersion is from the fine support structure of the LETG grating facet. The LETG grating facets do not have a substrate, but they have gold fine support and coarse support structures (for detailed information please refer to the Proposers' Guide). We summarize their properties in the following table.
|Period||9912.5Å||25.4 m||2000 m|
|Bar Width||5150Å||2.5 m||70 m|
These support structures also behave as gratings. The fine support is perpendicular to the grating bars, so its dispersion direction is perpendicular to main grating. This is the cause of the cross-dispersion structure associated with each line. According to the simple grating equation, the longer the wavelength, the farther apart each diffraction order is. Therefore one can easily distinguish the C-K(44.8 Å) from O-K(23.6 Å) and Mg-K(9.89 Å) lines from figure 27. The coarse support structure has a triangular shape. Figure 29 shows a 6-fold symmetry in the diffraction pattern from Penning source lines (160 Å).
The diffraction efficiency of each diffraction order due to the support structures is on the order of less than 1%. Depending on the wavelength of the lines, the diffraction structure may or may not be resolved, and this characteristic will be modelled based on calibration measurements. The higher diffraction order efficiencies of the main grating have been measured at several energies. Predehl et al  have summarized overall effective area calibration. Figure 6 in that paper shows the model predictions of the EA for each order of the Mg-K lines, and the measureed results (using a flow proportional counter as the focal plane instrument).
Figure 29: A portion of the HRMA/LETG/HRC-S image from the Penning source. Two big spots in the middle are from Al-IV (160.07 and 161.69A). The 6-fold symmetry patterns are from the coarse support structure.
1 Brinkman et al, Proc. SPIE 3113, 1997
2 Predehl et al, Proc. SPIE 3113, 1997
Both papers are on the webpage: