Transcript Max_SysReqsThruSciCases.ppt

Plan to develop system
requirements through science cases
Claire Max
Sept 14, 2006
NGAO Team Meeting
I hope to “marry” two approaches to
science-driven design of NGAO
1) Requirements driven:
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Scientists develop “science requirements”
Hand off to technical folks
Technical folks design AO system to meet the requirements
Determine if meeting the requirements is feasible (dollars, technical
risk, schedule, ...)
Iterate science requirements and AO design
2) Technology driven:
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Technical advances make one or more new approaches to AO possible
Scientists go off and assess what new science can be done with the
new technologies
Decisions based on “is the new science worth the cost and risk”
Iterate on which new technical approaches to implement
Both approaches have pros and cons
• Requirements driven:
– Pros:
– Better-defined from the technical point of view
– Clear goals against which designs can be evaluated
– Cons:
– Can lead to costly and/or risky designs, if one or two difficult
requirements torque around the whole process
• Technology driven:
– Pros:
– Easier to deal with “the art of the possible”
– Ask “what important science can I do with this new
technology?”
– Cons:
– Poorer understanding of specific design-trades that might
really optimize performance for key science cases
How can we have the best of both worlds?
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Suggested approach:
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Last spring we showed generically how 12 important science areas
would benefit from NGAO
For next couple of months, we want to focus on the subset of
science cases that push AO design the hardest
– Start by considering 4 key areas today
The process in brief, for each key area:
1. Understand and document science requirements x and y, including
how they interact with requirement z
2. Iterate with performance budget for various AO architectures
3. Develop and document observing scenarios
4. Understand and document requirements on science instruments.
Develop prioritized instrument list. Iterate with instrument
selection and error budgets.
5. Deliver Science Requirements Document
For example:
spectroscopy of high-z galaxies
• Strongest requirements driver for encircled energy
• AO performance requirements and trades:
– Encircled energy fraction against sky coverage
– If you are willing to accept larger tip-tilt errors (lower
encircled energy within a given area), you can achieve a higher
sky coverage fraction
– Spatial resolution (lenslet or “slitlet” size) against backgrounds due
to AO system and telescope
– If you use a larger lenslet or “slitlet”, you are admitting more
background light. So to achieve given sensitivity, need to run
the AO system colder (lower background)
• Science instrument requirements and trades:
– Strongest driver for multi-IFU instrument (MCAO or MOAO)
– MOAO: potentially higher Strehl, but fewer (more costly) IFU heads
– MCAO: lower Strehl, but can cover more galaxies at once
Next
• Plan to develop system requirements through
performance budgets