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ISAL Team - PowerPoint PPT Presentation

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ISAL Team. Mike Amato System Engineering Jeff Bolognese Structural Analysis Art Bradley Star Field /Fine Guidance Jennifer Bracken ISAL Team Lead Judy Brannen Mechanical Design Mick Correia Mechanical Design Paul Earle Electrical Dennis Evans Optics

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Mike Amato System Engineering

Jeff Bolognese Structural Analysis

Art Bradley Star Field /Fine Guidance

Jennifer Bracken ISAL Team Lead

Judy Brannen Mechanical Design

Mick Correia Mechanical Design

Paul Earle Electrical

Dennis Evans Optics

Rodger Farley Mechanical Systems

Landis Markley Guidance, Navigation, and Control

Wes Ousley Thermal

Mike Roberto ISAL Systems

Carl Stahl Detectors

John Wood ISAL Science Liaison

Eric Young Electro-optical Systems



  • Requirements
  • Trades and Issues
  • Instrument Diagram
  • Orbit Parameters
  • Observation Strategy Summary
  • Optics
  • Detectors
  • Mass, Data Rate
  • Conclusions

Trades and Issues

  • Baselining Hubble type structure rather than lower structure
  • Hubble like and upper structure tripod. Metering structure
  • is stiffer, less obscuration, and more thermally isolated. However,
  • it has higher mass and more complicated baffle integration.
  • 2. Converged on ‘two vane’ shutter design. Advantage is the
  • same illumination time for each pixel, but feedback control
  • is needed. Alternate shutter type did not need feedback control
  • but required very rapid operation to minimize time differences
  • in pixel illumination.
  • 3. Baselining five degrees of freedom secondary mirror adjustment
  • Based on flight design, because secondary mirror is the most sensitive
  • to alignment errors. Tertiary mirror adjustment mechanism does not
  • seem to be needed.
  • 4. May not need thermal control of metering structure, but this could easily
  • be added (approximately 150 W). Plan to use low coefficient of thermal
  • expansion graphite epoxy materials.

Trades and Issues (continued)

5. Do not see unusual stray light problems, but careful baffling is necessary.

6. Baselining 16 bit analog to digital converters. Could also use 12 bit A/D converters with multiple gains.

7. Baselining fixed filters on focal plane which requires stepping across focal plane. Advantage is the reduction in the number of mechanisms (filter wheel

not needed.

8. Future Trade – consider placements of spectrographs so behind focal plane to

reduce stray light.


Orbit Parameters

Modified Chandra orbit for complete observation of

~ northern or southern ecliptic in one orbit and equalizing

spectrograph and focal plane observation times.

Radius of perigee 3.0 Earth radii

Radius of apogee 24.5 Earth radii

Period 3.0 days

Height for data collection =/> 60 x 10^3 km

Time spent near perigee 12 hours

Time for focal plane observations 29 hours

Time using spectrograph 31 hours


Observation Strategy Summary

  • Art Bradley and Landis Markley provided the inputs.
  • Inside electron belts, slew 180 degrees, download data, orbit maintenance,
  • shutter closed
  • Outside electron belts, use ACS guide star CCDs on focal plane for fine
  • pointing control when shutter is open
  • Observation time = 200 seconds
  • Focal plane shutter is then closed for 20 seconds while focal plane is read out,
  • drift is fixed, and pointing is changed by ¼ of CCD position
  • 5. Sky observations on focal plane are repeated for 480 steps in one direction,
  • covering about 5 degrees in 29 hours

Data Rate

  • Focal plane average data rate during an observation 40 M bits/sec
  • (144 CCD arrays with 1600x1600 pixels, 44 HgCdTe arrays with 2000x2000 pixels,16 bits per pixel, 220 seconds to complete observation plus read out focal plane)
  • Average data rate during spectrograph observation 73 K bits/sec
  • (one HgCdTe array with 1000 x 1000 pixels, 16 bits per pixel)
  • Data collected per orbit (assuming about half focal plane observations and half spectrograph observations, assume data compression by factor of 2)
  • Focal plane telemetry per orbit 2.1 T bits
  • Spectrograph telemetry per orbit 4.0 G bits
  • Housekeeping telemetry per orbit 47 M bits
  • 4. If focal plane data taken all the time, and data compressed by factor or two,
  • total data collected during orbit about 4.2 T bits.