MDI_Configuration_on_SOHO.ps
This section describes the general characteristics and requirements of the MDI-T instrument which consists of two main units - an Optics Package (OP) and an Electronics Package (EP) - and cabling to connect them. Extensive experience gained from the design and operation of the SOHO/MDI instrument will be used in the construction of MDI-T.
MDI-T will use the SOHO/MDI structural model optics enclosure with new mounts fabricated if needed. MDI-T will use the SOHO/MDI flight spare Lyot filter. A spare Lyot will be fabricated for risk reduction. A new Michelson pair will be fabricated with the existing SOHO/MDI spares becoming MDI-T spares. A new electronics enclosure and backplane will be designed, fabricated, and tested. The current SOHO/MDI flight spare filter oven will be evaluated and a replacement fabricated if required. A new beam distribution box will be fabricated as the current beam box is not flight quality. The new box will be designed to accommodate a single axis scanning mirror. A CCD camera of the existing MDI design will be fabricated using the best available chip and the existing enclosure.
MDI-T electronics will be fabricated primarily using electrical designs of SOHO/MDI and the TRACE electrical interfaces designs. Some of the MDI-T Printed Circuit Boards (PCBs) will be slightly different than the SOHO/MDI configuration. In particular, the MDI-T electronics package will be a hybrid of the SOHO/MDI and TRACE electronics packages. The TRACE Control Computer is a modification of the SOHO/MDI DEP (main computer) to support SMEX architecture, specifically the inclusion of a 1553B bus interface for receiving commands and sending housekeeping data and a high speed serial RS422 interface to send image data packets to the telemetry downlink system. The existing TRACE EP needs to be lengthened in order to accommodate the ten Image Processor memory boards. Most of the redundant electronics in the SOHO/MDI EP will be left out of the MDI-T EP in order to reduce mass and complexity.
There are existing Engineering Test Units (ETU) for both SOHO/MDI and TRACE, though neither is completely populated. Some PCBs in these systems are flight spares and will be used directly. Most other boards are flight quality boards and will be populated with flight quality parts. New boards will be fabricated as need. The cost estimate allows for four boards to require new layout and two others to require completely new design. The flight software and firmware will be a combination of the SOHO/MDI and TRACE systems. The electrical Ground Support Software used for TRACE will require only minor modification.
| Aspect | Requirement / value | Comment | |||
| Dimensions (meters) | OP: EP: | 0.87 x 0.40 x 0.25
0.55 x 0.23 x 0.28 | OP: Existing SOHO/MDI OP dimensions.
EP: Existing TRACE EP dimensions increased in length by 25% to maintain the MDI IP memory. | ||
| Mass | OP: EP: Cables: Total: | 26.9 kg 20.0 kg 4.2 kg 51.1 kg | Requirements are based on the mass of the SOHO/MDI OP and the TRACE EP increased by 25%. | ||
| Power | OP: EP: Total: | 19 W 38 W 57 W | Based on knowledge of SOHO/MDI power consumption. Keep alive power is approximately the same as nominal power due to substitution heater requirements. | ||
| Telemetry | 250 kbit/sec continuous
(= 240 kbit/sec + 10kbit/sec) | The helioseismology science requires continuous observing for periods of months to years. Total data loss including planned interruptions of the science must be less than 5%. | |||
| Pointing stability (pitch/yaw) | Jitter: Short term: Mid term: | 0.2"/sec 4"/min 10"/2 month | The solar image is stabilized to <0.1 arc sec by an active system with a bandwidth of ~100 Hz and a range of ±10 arcsec. Figures are for peak excursion from nominal on the timescale given. | ||
| Pointing stability (roll) | Short term: Mid term: | 32"/min 3'/3 days | There is no image stabilization on the roll axis; this requirement is set by the need to keep apparent motion at the solar limb to less than 0.1 pixel per minute. Figures are for peak excursion from nominal on the timescale given. | ||
| Temperature stability at attach points | 1oC/day | The optics must be thermally stabilized to much better than this level. The MDI optics are thermally isolated inside an actively regulated oven; but temperature drift should be designed in all cases to be as low as possible. | |||
| Clear field of view | 22o half cone angle | The scientific field of view is a 0.75o half cone angle; the unobstructed field of view is determined by scattered light requirements. | |||
MDI_Optics_Package_Attachment.ps
The existing SOHO/MDI structural model optics package is the baseline for MDI-T. The mounting legs may have to be modified for Tirana. The current beam distribution box is not flight worthy and must be modified to accommodate the high resolution scanning mirror. The quality of the existing filter oven will be evaluated once the contract starts. The schedule and cost assume replacement is required. Mechanical accommodations for a six position filter wheel and high resolution field scanner must be designed. A new electronics enclosure must be designed, built, and shaken within four months of project start. Optical and mechanism mounts will be designed and fabricated as needed. A SXI heritage six position filter wheel and a scanning mechanism must be designed and constructed immediately to allow time for qualification tests.
Many of the spare optics elements from SOHO/MDI are available and will be used. In particular, the existing telescope assembly from the ETU will be qualified for flight and installed in the MDI-T optics package. There is a flight quality Lyot filter, but to reduce risk an additional crystal set will be ordered to fabricate a spare in the event the existing Lyot is found to be inadequate. The existing spare Michelson interferometers have marginal performance. These will be designated the flight spares and a new set of Michelson interferometers will be ordered immediately.
The SOHO/MDI thermal analysis will be adapted for MDI-T to account for spacecraft differences. Early in the program, thermal analysis of SOHO/MDI front window performance and Michelson interferometer temperature gradients will be used to determine if front window and filter oven design changes are required. There will be no backup oven heater controller.
Michelson Tuning Motors: Refurbish and test the two motors used for life testing and are currently in the SOHO/MDI ETU. The bearings will be replaced, and the encoder boards checked to verify they are acceptable for vacuum use.
Filterwheels: Refurbish and test three filterwheel motors from the SOHO/MDI ETU in a fashion similar to the tuning motors. In addition there are two filterwheels in the TRACE ETU that could be used if there is problem with the SOHO/MDI ETU units.
Shutter: Refurbish and test the motor in the SOHO/MDI ETU. The shutter blade and bearings will be replaced.
Front Door: Refurbish and test two motors using parts from the SOHO/MDI structural model (SM). There are motors with assembled gear heads that were removed from the SM before vibration testing. The condition of the white paint on the front door must be tested, and additional new parts will be required.
Alignment Mechanism: The plan is to fabricate two new motors using the existing design. Some new parts will need to be ordered and other manufactured. This is a straightforward task that duplicates previous SOHO/MDI efforts.
New Mechanisms: Two new mechanisms and associated electronics are planned. These are a six position Polarization Analyzer Wheel (PAW) and a high resolution field scanner (HRS). The PAW design is electrically identical the SXI wheel. The HRS is a modification of the TRACE focus mechanism. We have allocated cost and schedule to these items consistent with previous experience in mechanism development. A spare HRS is required for qualification test. Qualification testing of the PAW may leverage SXI experience. Early in the program there will be an integrated design and development risk assessment on both these items and they may be removed for cost and schedule relief.
The power system will be a single string system similar to that used on TRACE. There is no redundancy and there is minimal commanding of individual converters.
A new camera will be fabricated using the existing SOHO/MDI design. A few minor design changes may be required because some parts are no longer available. The cost is based on an entirely new camera using an existing, characterized chip, and the existing enclosure.
The table below shows the electronics board design heritage for MDI-T.
It does not show controllers for the six position PAW or the HRS as
these are new for MDI-T. The cost and schedule have been developed
based on the following: all boards must be fabricated, populated with
components and tested; up to two boards may require new layout as well
as fabrication; up to four boards may require some new design work.
SOHO/MDI | TRACE | MDI-T | ||
Board | Part Number | Board | Part Number | |
DEP CPU | MDI081059-101 | CC CPU CCD | TRA081059-101 | Trace |
DEP Memory | MDI081062-101 | CC Memory | TRA081062-101 | Trace/MDI |
DEP S/C I/F1 | MDI081068-101 | CC S/C I/F | TRA081068-101 | Trace |
Parallel I/F | MDI081071-101 | CC AE IF | TRA081071-101 | Trace/MDI |
PZT Drive | MDI081074-101 | PZT Drive | TRA081074-101 | Trace/MDI |
Limb Tracker | MDI081078-101 | Limb Tracker | TRA081078-101 | Trace/MDI |
Analog I/F | MDI081084-101 | AE Analog IF | TRA081084-101 | Trace/MDI |
MTM 1 & 2 | MDI081093-101 | Prism 1 & 2 | TRA081093-101 | MDI |
Shutter/PAW | MDI081096-101 | Shutter & Quad | TRA081096-101 | MDI |
CAL 1 & 2 | MDI081099-101 | Fltr Whl 1 & 2 | TRA081099-101 | MDI |
Door & Legs | MDI081102-101 | Focus Mech | TRA081102-101 | MDI |
IP APU | MDI081043-101 | DHC APU | TRA081043-101 | Trace/MDI |
IP DEP I/F | MDI081046-101 | DHC CC I/F | TRA081046-101 | Trace/MDI |
IP CCD I/F | MDI081049-101 | DHC CCD/Tlm | TRA081049-101 | Trace |
Local Mem (3) | MDI081039-101 | Local Mem (3) | TRA081039-101 | Trace/MDI |
Main Mem (10) | MDI081036-101 | Main Mem (4) | TRA081036-105 | Trace/MDI (10) |
The software will be based on the SOHO/MDI and TRACE systems that are quite similar. The command and telemetry databases will be modified from the SOHO format to the SMEX format. The firmware instructions will be modified to output packetized data for the Medium-l data stream to the RS422 and to generate SOHO/MDI style compression in firmware rather than hardware. The TRACE interface simulator may require some modification. That software is not particularly stable and would have been modified had TRACE integration taken longer than it did. The workstation EGSE is usable with only minor changes.
The MDI-T schedule is extremely ambitious. The development process is not without risk and it would, therefore, be inappropriate to not discuss some of those risks.
Many of the existing designs contain parts that are no longer made and for which there is no direct equivalent replacement part. MDI-T electronics development is based on an extremely high degree of design reuse. If too much redesign is required to accommodate replacement parts, the schedule will suffer. We are developing a list of parts; including what is currently in house, what parts can be ordered and what parts are in scarce supply.
There are no spare DMA ASICs. These cannot be replaced on the time scale of the Triana development. Several are evaluation rather than qualified parts, but must be used anyhow. If there are problems with more than a few of these ASICs, the schedule will be in jeopardy.
To our knowledge, there is only one flight quality Harris 80C86 processor in existence for use as the MDI-T Control Computer CPU. If something happens to that chip, the project will be in jeopardy.
The SOHO/MDI spare Michelson interferometers and Lyot filter are planned for the MDI-T program. The project plan calls for development of spares, however, there may not be sufficient time to develop these spares.
We are not planning on any instrument level acoustic testing since there was none on SOHO/MDI. We plan thermal vacuum testing for instrument optical characterization and workmanship testing only. Instrument sunlight testing will take place during the winter when sunny days are at a premium. There is little opportunity to move those tests forward. We are deferring thermal balance to spacecraft level testing. We have costed and scheduled a full instrument vibration test, but may remove it and test only high risk assemblies.
The software interface simulator requires major attention and may require a rewrite. It was never particularly stable and the key programmer has left the company. We have costed and scheduled a major rework effort at the level of the initial development. The sequencer part of the flight software will require significant rework if the HRS and six position PAW are included in the package. We expect are no major difficulties adapting the code.
There is little risk in the camera and sensor area. The camera is a proven design. There are at least two fully characterized sensors that could be used. The CCD evaluation software is under transition to a PC system. We have added some budget for CCD software that will be used either to accelerate the transition or to maintain the old system.
We depend on the availability of several key people who are involved in other projects. This Triana project is time critical, and the expertise of personnel involved the SOHO/MDI and TRACE projects is essential to maintaining the MDI-T schedule.
The facilities in building 252 are adequate for thermal vacuum and ambient functional testing. A new clean tent will be purchased as a fixed asset. Vibration and EMI testing will use local subcontracted facilities. We assume that facilities can be scheduled consistent with program plans.
