HXT is a Fourier synthesis type imager consisting of 64 bi-grid modulation subcollimators (SC's). Each SC has a different pitch and/or a position angle of collimator grids, together with a NaI (Tl) scintillation crystal and a detector photomultiplier located behind the SC. The number of hard X-ray photons passing through a single SC is periodically modulated with respect to the incident angle, which gives a modulation pattern of the corresponding SC, and count rate data obtained by each detector which can be regarded as a spatial Fourier component (+ DC level) of a hard X-ray image.
When a flare-mode is triggered, a set of 64 hard X-ray count rate data is accumulated every 0.5 s (= the highest temporal resolution) in four energy bands between 14 and 93 keV (L, M1, M2, and H bands, respectively) and is transferred from HXT to the Data Processor (DP). The data are then telemetered down to the ground and hard X-ray images can be synthesized using image restoration procedures such as the Maximum Entropy Method (MEM).
The field-of-view (FOV) of HXT is about 35¢×35¢, i.e. covering the whole Sun. This means that HXT can detect hard X-rays of flares regardless of their position on the Sun without re-pointing the spacecraft. The basic image synthesis FOV of HXT is 2¢06¢¢×2¢06¢¢ with the angular resolution as high as approximately 5¢¢. A detailed description on the overview of the instrument is given by Kosugi et al. (1991). See Kosugi et al. (1992) for the in-orbit performance of HXT as well as some initial results.
Unlike SXT, HXT is operated in a single observation mode (except for calibration). When the Sun is quiet (i.e. in the quiet-high or quiet-medium mode), only hard X-ray count rate data in the lowest energy band (L band; see section 2-2) are telemetered once in every major frame (e.g. 2-s data accumulation in high-bit rate). When the flare-high mode is (automatically) triggered by the occurrence of a flare, then count rate data in the four energy bands, with a 0.5-s data accumulation, are telemetered to the ground. In this case, four sets of data ((64 SC's × 4 energy bands)× 4) have their telemetry allocation in one major frame.
When the spacecraft is in South Atlantic Anomaly (SAA) or in spacecraft night, high voltage supplies for the detector photomultipliers are reduced and no solar observations are made with HXT.
Since HXT images are synthesized from sets of data from the 64 SC detectors, it is essential that the gain of each detector be set equal to each other with high precision (within 1 %). The gain calibration of HXT is conducted typically once in a month and the gain has been set constant since the beginning of HXT observations in October 1991, which enables us to obtain reliable hard X-ray images in the four energy bands.
In the calibration mode of HXT (HXT-CAL or Pulse Height (PH) mode), X-ray count data in 64 energy channels between 14 and 93 keV for each detector, instead of four energy bands in the case of observation mode (Pulse Count (PC) mode), are obtained. The gain of each detector is monitored by accumulating X-rays from an Am-241 calibration source (whose line profile peaks at 59.5 keV) attached on an aluminum case for the NaI (Tl) crystal.
The energy-channel relationship to which detector gains are adjusted is given as follows:
|
In normal observation mode, the four energy bands, L, M1, M2, and H, have the following energy range:
| Energy Band | PH Ch | Energy Range (keV) |
| L | 1 - 7 | 13.9 - 22.7 |
| M1 | 8 - 15 | 22.7 - 32.7 |
| M2 | 16 - 31 | 32.7 - 52.7 |
| H | 32 - 63 | 52.7 - 92.8 |
NOTE: Ch=0 is not included in the L band.
Even when the Sun is quiet, there are some counts in the four energy bands due to X-rays from the calibration sources, as well as e.g., cosmic X-ray radiation. For a better HXT data analysis, we recommend you carefully subtract non-flare background from flare data you are going to analyze. The typical background (BGD) count rates in the four energy bands are:
| Energy band | BGD count rate (cts/s/SC) |
| L | ~ 1 |
| M1 | ~ 2 |
| M2 | ~ 1 |
| H | ~ 8 |
One must be careful since the above BGD count rates have an orbital dependence; for example, the NaI scintillators are activated during SAA passages and there are some excess counts in the four bands even after exit from SAA. The most suitable way for background subtraction that we recommend at present is to use data just before or after the flare as BGD.
Background subtraction plays an important role in image synthesis as well as spectral analysis with HXT. This is especially true if you are going to analyze not-so-intense flares. Even for intense flares, image synthesis with inappropriate background subtraction will provide you with images with spurious sources scattered around in the image synthesis FOV.
Among the 64 grid pairs of HXT, 48 are called Fourier elements whose slit and
wire widths are the same to each other while 16 are fanbeam elements
whose wire widths are three times larger than their slit widths. See Kosugi et al. (1991)
for more information on the arrangement of the 64 SC's.
Modulation pattern parameters of each SC's, which are necessary for
synthesizing images, are listed in a file whose name is obtained by the
following command:
IDL > file = concat_dir('$DIR_HXT_CAL', 'para3.dat')
In the image synthesis procedure modulation patterns of Fourier SC's are
approximated by sinusoidal patterns while those of fanbeam SC's by triangular
ones.
The geometrical area of HXT, which is determined by the (average) transmission of the SC's, is 57.4 cm2. When considering the spectral energy response of the HXT, the following components need be taken into account:
The spectral energy response for HXT, with the effect of K-escape of tungsten at
69.6 keV and measured energy resolution of NaI (Tl) + photomultiplier tubes
taken into account, are summarized in a file whose name is obtained as follows:
IDL > file = concat_dir('$DIR_HXT_CAL', 'possi4.dat')
A concise summary of the energy spectral response, together with some useful
figures, is provided in ``The Yohkoh HXT Databook (I).''
See the Instrument Guide Appendix section A.2 for a description of the time delay in the HXT data.
Each hard X-ray count rate measurement transferred from HXT to the DP consists of 12-bits. The data are then compressed into 8-bits in the DP according to the following rule:
m = n (n = 0- 15),
m = int(4 x sqrt(n)) (n = 16-4080),
m = 255 (n = 4081-4095),
where int(n) represents the truncated integer value of n. Count rate data for
HXT telemetered down to the ground (as well as the variable data which is
obtained by reading the HXT reformatted database with YODAT) are the compressed
numbers; they need be decompressed before starting any analyses on HXT.
Decompressed HXT data are obtained by the following command:
If you have any question about HXT software and/or instrument, or if you need any advice in HXT data analyses, contact: