This document describes the general characteristics and requirements of the MDI instrument as it would exist on Triana. The MDI instrument consists of two main units - an Optics Package (OP) and an Electronics Package (EP) - and cabling to connect them. Extensive experience has been gained from the design and operation of the SOHO/MDI instrument which will be used in the construction of Triana/MDI.
Some of the Triana/MDI Printed Circuit Boards (PCBs) would be slightly different than the SOHO/MDI configuration. In particular, the Triana/MDI electronics package would 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 size of the existing TRACE EP needs to be lengthened in order to maintain the SOHO/MDI complement of 10 Image Processor memory boards instead of the 4 boards in TRACE. Most of the redundant electronics that is in the SOHO/MDI EP could be left out of the Triana/MDI EP in order to reduce mass and complexity.
| 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 from neutral. 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. | |||
The SOHO/MDI Electronics Package has dimensions of 0.45mx0.44mx0.29m, a mass of 28.6 kg, and contains 36 PCB + redundant power converters + motherboard.
The TRACE Electronics Package has dimensions of 0.44mx0.23mx0.28m, a mass of 15.5 kg, and contains 21 PCB + power converters + motherboard.