SOLAR B - X-Ray Telescope (XRT)
Smithsonian Astrophysical Observatory|
SOLAR B - X-Ray Telescope (XRT)
Smithsonian Astrophysical Observatory |
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Science Implementation
The scientific requirements discussed in Section 1 translate directly into instrumental requirements for the Solar-B X-ray telescope (XRT):
These objectives can be accomplished with an optimized Wolter I grazing-incidence telescope similar to that used on Yohkoh, although with certain modifications, plus a large format back-illuminated CCD detector and appropriate entrance aperture and focal plane filters.
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The two main methods used to image the corona at soft X-ray wavelengths are grazing-incidence (GI) and normal-incidence (NI). Each of these techniques has strengths and weaknesses, among which are: Solar-B will fly during the declining phase of the current solar cycle, 1--2 years past the next maximum. The corona should therefore be in a state similar to that of Yohkoh in about 1993 or the Skylab mission in 1973, so that coronal holes, CMEs and X-ray bright points will be major observational features. There will be numerous active regions, AR complexes and the associated dynamics and loops interactions. Flares will be observed, although reduced in number from the activity maximum. In addition, XRT must have a response to cool coronal features, such as polar plumes, loop footpoints, and the ``moss'' seen with TRACE, in order to provide overlap with the optical telescope and EUV spectrometer science. The scientific objectives of the mission have taken these factors into account, and lead to the instrumental requirements for the XRT discussed in the next section. The XRT differs from the Yohkoh SXT in three major ways:
In comparison with the Yohkoh SXT, the Solar-B XRT will therefore have six pixels for every SXT pixel, comparable field of view, comparable temperature diagnostic accuracy, and a greater range of temperature coverage. Details of the design are presented in the remainder of this section. The flowdown from these science requirements is shown in table 2
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1. Optics The typical Wolter I design consists of a paraboloid of revolution followed by a hyperboloid of revolution. The focus of the paraboloid is coincident with the back focus of the hyperboloid; the forward focus of the hyperboloid becomes the system focus and the origin of the coordinate system. Solar-B is designed for full sun imaging over the wavelength range 6-60Å , with emphasis placed on the longer wavelength performance. These requirements place significant constraints upon the prescription. Wolter~I designs suffer from field angle dependent coma as well as a curved focal surface. Minimizing the field dependent coma requires minimizing the element length and the graze angle a . However, diffraction from the annular aperture increases the telescope point spread function. Thus, to minimize diffraction effects, which are worse at the longer wavelengths in which we are most interested, we need to maximize the aperture diameter and annulus width. But doing so means that we are increasing a , or L, or both, thereby degrading the off-axis performance. In addition, for constant surface figure error, the fraction of incident energy scattered from the mirrors increases with increasing graze angle. This degrades both on- and off-axis performance. Balance between these competing requirements is achieved by making as large a telescope as spacecraft envelope and weight budgets allow and departing from a pure Wolter~I in favor of a generalized aspheric design that optimizes large field performance. When diffraction is ignored, a smaller aperture, smaller graze angle telescope (Case I) gives better performance due to reduced scattering. This is most noticeable at shorter wavelengths. When diffraction is ``turned on'' the design trade changes dramatically. Now, a larger aperture telescope (Case II) gives significantly better performance, particularly at the long wavelengths of greatest interest. The aperture diffraction dominates encircled energy performance down to ~20Å : the larger telescope design offers better on-axis performance. Off-axis imaging is improved by departing from the Wolter~I design in favor of a generalized (8th order) asphere for each mirror element. The polynomial coefficients are determined by optimizing mirror figure for image quality over a 15 arc-minute radius field, choosing as figure of merit the normalized field averaged rms blur weighted by the low energy effective area, taking into account the maximum field angle to include in the optimization. We use the Simulated Annealing algorithm for continuous spaces, followed by a Downhill Simplex optimization. The Solar-B mirror assembly is comprised of a Zerodur grazing incidence telescope supported by a set of three bipod flexures. An optical telescope is co-aligned and confocal with the X-ray telescope, and nested inside the X-ray mirrors. In overall design, the XRT resembles the SXI and AXAF optics. The design yields 55% encircled energy within 1.5 arcsec diameter; this is more than a factor of two better than achieved with SXT. The telescope mirrors will be coated with ion beam sputtered (IBS) iridium over a binding layer of IBS chromium. Estimated effective area at 60Å , excluding obscuration by structure and absorption due to prefilters, is estimated at 5.0~cm2. Both elements will be coated simultaneously to minimize the possibility of contamination between coating cycles. Mirror assembly focal length will be determined using the optical Hartmann Test. The mirror elements will be fabricated by Raytheon Optical Systems, Inc. (ROSI), formerly HDOS. ROSI is world leader in the manufacture of X-ray optics, having recently fabricated the AXAF mirror elements, super-smoothed the TRACE optical elements, and been selected to manufacture the SXI telescopes for GOES-N/O/P. ROSI will provide two mirror assemblies, a structural model and a flight model. The telescope assembly must be rigid and lightweight, while maintaining alignment and minimizing mirror deformations under launch and environmental loads. The XRT uses a monocoque Graphite Fiber-Reinforced Polymer (GFRP) structure which ties the entrance optics assembly to the focal plane CCD/filter/shutter assembly. This structure also supports the electronics and interfaces to the spacecraft optical bench. The mechanisms are highly reliable on XRT: two shutters and a filter wheel. There is also a (non-vacuum) contamination control door. The main structure is a cylindrical center shell and conical end shell sections which provide a high stiffness to weight ratio. The GFRP shells are fabricated of quasi-isotropic GFRP laminates of P75/ERL1962, bonded for assembly using a Hysol EA9394 structural adhesive. NASTRAN modelling of the structure, with anticipated loading, determined that a first mode frequency >40 Hz is achieved with a uniform 0.060-inch GFRP wall, having optimally-placed internal lateral stiffening rings and a four-point base constraint. Internal mechanisms and optical components are supported by kinematic mounts and flexures fastened to the primary structure by screws run into helicoil inserts installed within titanium plugs bonded within the GFRP rib sections. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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The optics assembly includes both the X-ray and the visible light optics mounted to a base plate (ring) which will interface to the XRT housing. The X-ray mirror element will be mounted to an interface base plate via a set of three titanium alloy bipod flexures. These flexures provide a stiff (high first mode), strong (high yield) kinematic support for the mirrors. The flexures are bonded to the mirror element in a stress free configuration. The flexures reduce the sensitivity of the mirror assembly to thermal changes in the base plate or optical bench, as well as thermal stresses in the flexure bodies. Bulk temperature changes and thermal gradients will introduce deformations in the mirror elements due to thermal growth/bending in the mounting pads which couple the flexure body to the mirror element. These deformations are minimized by (1) athermalizing the flexure mount design as much as practically possible and (2) aligning the flexure support points with pre-filter aperture support struts. Because the deformations are azimuthally localized, this reduces the sensitivity of imaging performance to thermal changes by obscuring the part of the telescope that undergoes the greatest distortion. The filter wheel assembly (provided by Lockheed) is a derivative of the TRACE assembly, improved by scaling its size to accommodate the full CCD imaging area. There are two filter wheels in this assembly to accommodate the filters listed in Table 2.2. Each wheel consists of a brushless DC motor with an integral, optical, incremental encoder. The performance characteristics will be comparable to the TRACE filter wheel assembly with ± 1 arcminute repeatability. A third wheel, containing no filters, can be utilized as a moment balance mechanism to compensate for mass moment shifts associated with the rotation of the filter wheels. The focal plane shutter assembly (provided by Lockheed) is a derivative of the TRACE and SXI shutter assemblies, scaled in size to accommodate the full CCD imaging area. This is a brushless DC motor driven disk with encoder position feedback. The disk has a pie-shaped opening and the shutter accommodates exposure times as short as 40ms. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
4. Filters and Plasma Diagnostics The filters used in the XRT perform three functions: 1) to reduce the heat load inside the telescope, 2) to reduce the visible light at the focal so that it is a small fraction of the X-ray intensity, and 3) to provide spectral diagnostics so that coronal plasma temperatures can be determined. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Of the available filter materials for the XRT entrance aperture, the best one appears to be aluminum. This is primarily because it is an excellent reflector in the visible, while its K- and L-absorption edges are at 8Å and 171Å , thus coinciding nicely with the locations of the most important coronal emission lines. For visible-light blocking, aluminum provides roughly a 1/e reduction in transmission for each 100Å of path length. The Al may be supported either on a wire mesh or on an organic thin film substrate. A typical high-quality mesh strong enough to hold the Al has an open area fraction of ~80%; organic films, such as Polyimide, can be as thin as 600Å for the narrow entrance aperture annulus of a GI telescope, although we propose 2000Å for safety. The XRT X-ray throughput for these two entrance filter types is shown in Fig. 2.1, with the Yohkoh SXT response shown for comparison. The SXT was highly filtered, in keeping with the emphasis on flares in that mission. For Solar-B, we desire a response geared more toward lower temperatures, which in practice means longer wavelengths. The balance between short and long wavelength sensitivity in grazing incidence telescopes can be adjusted over a large range by the choice of entrance filter. In Fig. 2.1 we show a standard thin aluminum on mesh, and an aluminum on polyimide filter. The Al-on-mesh is made 1400Å thick, whereas 400Å Al on Polyimide allows heat-rejection and visible light imaging by the XRT. The greatly enhanced low-T throughput of the mesh filter is largely due to the EUV >171Å, which has poor image quality ocal plane filters pairs. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
The thin filters near the focal plane must perform two functions: to reduce the visible-light throughput by ~12 orders of magnitude and to allow maximal X-ray transmission for performing plasma diagnostics. Given the collecting area of the telescope compared with the size of the focal plane image of the Sun, and the 4 orders of magnitude visible attenuation of the prefilter, we require a further 8 orders of magnitude reduction by the focal plane filters.
Typical high-quality thin film metallic filters free-standing and/or on a mesh provide the required visible light attenuation, so a single focal plane filter is adequate. However, these filters are subject to pinhole formation during launch and mission operations. It is therefore wise to have two filter wheels at the focal plane, so that more filters are available and in order to allow two filters to be placed in series in the light path. A visible light image may be formed with the XRT by placing a neutral-density filter in the filter wheel; use of a blue filter minimizes the diffraction limitation and allows sunspots and the limb to be imaged for aspect determination and image coalignment.
The focal plane filters we have selected provide a broad range of temperature responses. An open position on each wheel allows for greater flexibility, redundancy in case of pinholes or damage to the filters during launch or operations, and a visible-light neutral-density filter obtaning visible images with the GI telescope. Examples of the diagnostic ratios obtained with these filters are shown in Fig. 2.2.
The focal plane detector for the XRT must provide a high resolution soft X-ray image with low noise and large dynamic range. It should also be sensitive to visible light, in order to provide co-registration with the optical telescope. Exposure times ranging from less than one second up to a few minutes are be possible, as are sub-arrays (``windowing'') and reduced resolution (summed) images. The data acquisition system uses a single 2K by 2K pixel CCD array with pixel sizes of 13.5m ´ 13.5m (model number EEV42-40 manufactured by EEV Ltd of the UK). The CCD is back illuminated for high quantum (~50-70%) efficiency at the XRT wavelengths, it offers a wide spectral range (3 –400Å) and is capable of software selectable readout rates between 20 and 1000 kHz.
The XRT is a full-Sun instrument and is co-aligned with the Optical Telescope. It therefore will at all times observe the target area of the OT, while maintaining the ability to detect large-scale coronal phenomena. A regular synoptic program of Sun-centered observations will be formulated to provide a long-term database for evolutionary studies and for mission planning. For active regions, the expected exposure times in the XRT are ~1s or shorter. Based on the Solar-B spacecraft performance data presently available, and XRT's short exposure times, we anticipate that the short-term jitter will be enough smaller than our pixel size that no internal motion compensation is needed. iIt is essential that the XRT be capable of coalignment with the optical telescope. A blue neutral-density filter near the focal plane will produce a moderate resolution, high-accutance image formed by the grazing-incidence telescope.
The filters and CCD camera will be calibrated at SAO’s in-house facility constructed for the AXAF HRC program. We have available several X-ray pipes fitted with sources such as a micro-focus, Henke, and multi-anode soft X-ray sources, gas-flow proportional counters, monochromators, and analysis software. The prefilters and focal plane filters will be calibrated in the same manner as were the HRC and TRACE filters; thickness of all component elements can be determined by filter-in/filter-out throughput measurements over X-ray energies from 72 eV to 1486 eV. The TRACE solar simulator filter test chamber will be used for filter lifetime and outgassing tests. Arrangements have been made for the telescope to be fully calibrated at MSFC using the AXAF X-ray Calibration Facility. Both throughput and point response function will be determined.
The XRT is capable of generating data at a far higher rate than can be accommodated by the spacecraft, so that careful planning of exposure sequences, and judicious use of data compression, windowing and averaging methods is required. Our present suggestions in these areas have been discussed in earlier sections. Operation of the XRT will be controlled by uplink of a command table, typically every 24 hours, although several days of operation can be programmed in any given upload. A baseline synoptic program will also be permanently resident in the control computer, to be executed if no other commands are received. Onboard monitoring of Housekeeping data will be available to determine whether temperatures and voltages are within safe limits, and regular monitoring of the CCD exposure levels will be used to determine safe exposure times. Standby and safe-hold modes are incorporated, with monitoring and commanding available as needed via ground station contact. Post-launch support for the XRT involves three main activities: 1) instrument operations; 2) coordination of XRT with OT and EIS; and 3) health and safety monitoring of the XRT. These tasks will be the responsibility of the SAO MO&DA Lead and Mission Scientists. Instrument operations involve the generation of scientific and calibration command sequences, typically on a daily basis; these are produced in the XRT workstation, checked for accuracy and safety, then transmitted to the ISAS control computer for upload. File size for these transmissions will be modest, making use of stored tables and sequences onboard. Daily observing plans are generated as part of the overall Solar-B mission planning, and are done in conjunction with the planned operations for the other instruments.
C. Data Analysis and Archiving
As with other ISAS science missions, we anticipate that the entire Solar-B telemetry stream will be archived on the Sirius or equivalent system at ISAS. Following the Yohkoh model, we propose that a single reformatting program which resides on an ISAS workstation generate the Level-Zero data for all scientific instruments and Solar-B spacecraft shared data bases. Such a unified system minimizes duplication of effort and telemetry processing, promotes common treatment of the data and greatly facilitates coordinated planning and analysis. While the individual PI teams retain full control of the instrument specific Level-Zero definitions, common attributes in the data sets are exploited and access to Solar-B data such as attitude and ephemeris is provided to all teams in a consistent manner. The estimated size of the long term archive for all of Solar-B is 3.5 GB per day, including associated data bases and catalogs. Based upon Yohkoh experience, we estimate that the long term archive generation can occur 2 to 4 weeks after data acquisition. When all of the available DSN and KSC data for a given day is verified resident on ISAS/Sirius, the reformatter would generate the long term archive. The baseline plan consistent with current ISAS capabilities and infrastructure would be to write two master versions of this long term archive to 4mm tapes.
Assuming that reasonable cost sharing between ISAS and the Solar-B instrument teams is negotiated, we propose that the long term archive for all of Solar-B is made available on a single DVD per UT-based day. The current pace of evolution, popularity, capacity and apparent reliability of DVD makes that an attractive choice as the Solar-B distribution media. The large capacity of DVDs coupled with the increasing capacity/cost ratio of DVD juke boxes will likely make it an excellent choice for online storage of large Solar-B data sets. A unified distribution , based on time division instead of instrument division, minimizes overhead for each science team and enables Solar-B coordinated data analysis. In this area, the Yohkoh (unified) distribution approach has proven vastly superior to SOHO.
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Reference
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Acknowledgement. This was work was supported by a contract from
NASA/MSFC to SAO.
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