Differential Emission Measure Reconstruction with the SolarB X-Ray Telescope

Two of the main considerations in the design of the Solar-B X-Ray Telescope are temperature coverage and discrimination. We describe how these factors enter into the design of XRT, as well as the methods we have developed for producing estimates of emission measures. We analyze model DEMs to evaluate our ability to reconstruct DEMs. 1. XRT Design and Temperature Measurements Two of the main considerations in the design of the Solar-B X-Ray Telescope (XRT) are temperature (T ) coverage and temperature discrimination. In order to meet the instrument’s scientific objectives by the observation of the solar corona, XRT is required to be able to distinguish plasma structures across 6.1 < log T < 7.5, with a temperature resolution of ∆(log T ) = 0.2. These requirements affect many aspects of the telescope’s design, such as the channel selections (focal plane filters), the X-ray optical resolution, the white-light rejection, and the quality of the mirror polish. In addition, procedures for estimating plasma temperatures and emission measures are required in order to interpret the observations and address the scientific goals. The purpose of this study is to evaluate one portion of the XRT objectives: the quality of T and emission measure (EM/DEM) reconstruction with XRT. In particular, DEM reconstruction is investigated for its quality versus the number of channels used and the T -range imposed. The temperature sensitivity of XRT is determined by many factors. Some of the more relevant design elements are (1) the reflectance of the telescope as a function of incoming photon energy; (2) the transmission of the entrance aperture prefilters and the focal plane filters as a function of energy; and (3) the response of the focal plane detector as a function of energy. A schematic of the XRT telescope is shown in Fig. 1. The telescope is a grazing-incidence annular mirror, with a nominal average grazing angle of 0.9 degrees. For a given mirror design, the main factor governing the temperature sensitivity is the mirror surface composition. Three mirror coatings (silicon, nickel, and iridium) with various energy cutoffs were considered. For the XRT mirror, iridium was chosen as the coating best suited to meet the mission requirements. A plot showing the telescope X-ray throughput versus wavelength is shown in Fig. 2. The entrance aperture prefilters serve to reject visible light from entering the instrument, and also thereby decrease the heat load, while transmitting more