The HRC-S is a microchannel plate detector on board Chandra and is primarily used for spectroscopic observations with the Low Energy Transmission Grating Spectrometer (LETGS) in place. Photons are detected via signals read out from evenly spaced wires underneath the plates and positions are computed by centroiding around the strongest amplifier signals. This process leads to gaps in between the taps where no events are placed. A deterministic correction is then made during ground processing to these event locations to remove the gaps. We have now developed a new, empirical degap corrections from flight data. We describe the procedure we use, present comparisons between the new degap and lab-data based degap, and investigate the temporal stability of the degap corrections.
The Chandra Low Energy Transmission Grating Spectrometer (LETGS) is
comprised of 3 micro-channel plate (MCP) segments and is primarily
used with the High Resolution Camera spectroscopic array (HRC-S).
In-flight calibration data observed with the LETG+HRC-S show that
there are non-linear deviations in the positions of some lines by as
much as 0.1 Å. These deviations are thought to be caused by spatial non-linearities in the imaging characteristics of the HRC-S detector. Here, we present the methods we used to characterize the non-linearities of the dispersion relation across the central plate of the HRC-S, and empirical corrections which greatly reduce the observed non-linearities by a factor of 2 or more on the central MCP.
The dispersion relation for the Chandra Low Energy Transmission
Grating Spectrometer (LETGS) is known to better than 1 part in 1000
over the wavelength range 5-150 Å. A recent resolution of a data processing software bug that lead to a systematic error in the
computation of photon wavelengths has allowed us to trace further
discrepancies in the dispersion relation to the boundaries between
different microchannel plate segments of the HRC-S imaging detector.
However, data acquired during in-flight calibration with the HRC-S
detector have always shown the presence of additional non-linear
deviations in the positions of some spectral lines by as much as
0.05 Å, which is of the order of a full width half maximum
(FWHM) of a line profile. These latter effects are thought to be caused by spatial non-linearities in the imaging characteristics of the HRC-S detector. Here, we present an improved dispersion relation for the LETG+HRC-S and new methods to help characterize the spatial non-linearities. We also describe an empirical approach that might be used to help improve the position determination of photon events.