back to Period Analysis
The gratings on the flight HETG were each measured in the LR setup. The locations of the reflected and diffracted beams are measured from the grating under test and also from a fixed-in-the-instrument reference grating. Thus, all gratings are measured relative to fixed reference gratings (HEG or MEG). The periods of these reference gratings were assigned based on the laser wavelengths and the absolute diffraction angles (as measured by the NEAT rotation stage.) The LR system has a high differential sensitivity and repeatability, however, its assigned periods are not expected to be absolutely accurate.
In order to set the absolute period of our reference gratings, we sent HEG and MEG calibration samples (on Silicon wafers) to John Kramar et al. at NIST (jkramar@NIST.GOV). They measured these HEG and MEG samples. Then, these samples were measured by Dick Elder in the LR setup. From these measurements a calibration factor establishing the absolute period of our LR data is obtained:
|Sample||NIST Period||NIST Error (1 sigma)||LR Period||Calibration Factor||Error|
These calibration factors and errors are applied to the measured average periods of the flight HETG and TOGA gratings:
|Grating||LR Average Period||LR-NIST Period||LR-NIST Error|