1. David Woody Caltech Owens Valley Radio Observatory
System Engineering
David Woody
Caltech
Owens Valley Radio Observatory
13,14 July 2005
Feasibilty/Concept Study Mid Term Status Review

2. Approach Error budgets Interaction of components and systems
Need carefully defined
requirements
specifications
design goals
environment parameters
Want measurable engineering specifications

3. status Halfway through one year study
Most of the system engineering work has been on the surface error budget
Most image quality issues are encapsulated in the ½ WFE
Top down and bottom up analysis proceeding
Have identified critical systems issues
Segment sizes
Segment technology
Surface maintenance system
Have identified viable options for the primary surface
Pointing and tracking error budget will be dealt with next
Limits on what optical guiding can accomplish
No sub-millimeter cameras yet
Need careful definition of operational pointing modes and requirements
Will estimate limits set by atmospheric turbulence (water vapor)
Pointing
WFE
Radiometer fluctuations

4. Context for CCAT Basic physical limits shown in Von Hoerner plot
Modified by new materials, technology and dome
CFRP, etc.
Homology
Dome
No solar heating
Minimal wind
Still beyond passive structures
Passive would represent large risk at this point
Telescopes close to the limits already employ CFRP and high degree of homology
Active surface reduces risk at increased cost and complexity

5. von Hoerner plot
CCAT
ALMA

6. Approach to error budget
Rough top down, net 10micron RMS 1/2WFE
Divide evenly among
Segment surface error
Surface maintenance system
Primary support (BUS), outside loop bandwidth
Wave front measurement
Remaining optics and miscellaneous
Initially assume 4-5 microns for each component
Error budget is redistributed as bottom up errors become available

7. Bottom up errors from scaling
Critical parameter is the size of the segments, r
Segment errors
Gravity r~2
Thermal r~2
Resonant frequency 1/r~2
Sensors accuracy
Edge sensor ~1/r
Length sensors accuracy r~0
Tip/tilt sensors ~1/r
System complexity
Number~1-2 = 1/r2-4

8. Scaling applies to a technology
Different coefficients for different technologies
Have data from previous telescopes
Generic laminated disk segment supported at three points
Front and back plate material and thickness
Core material, filling fraction and thickness
Material properties
Density
Elasticity
CTE
Thermal conductivity
(note: some materials, such as CFRP, can be optimized)
Passive load
Peak-to-peak and RMS surface error

9. Three point support, fig. from SNAP 2m

10. CFRP

11. Basic surface maintenance design
Edge sensors
Ensures continuity between panels
Easily corrects for small scale errors
Full surface correction is demanding
Keck type system puts extreme demands on segment performance and gets more difficult as the number of segments increases
Large scale measurement options
Tip/tilt of some or all segments
Absolute distance measurement to secondary of some or all segments
Stable relative distance measurement to secondary of some or all segments
Measurement of underlying primary support structure

12. Edge sensor sensitivity, Keck

13. Edge sensor sensitivity, CCAT

14. Distance & tip-tilt sensor sensitivity, CCAT

15. Wave Front Error Budget

16. Active surface challenges
Open loop
Inaccuracies in simulation model
Errors in measured parameters
Difficult to account for actuator errors
Closed loop
Need full simulation to understand the effects of various errors in a feedback system
Complex non-linear system with potential singularities
Need affordable, accurate and reliable sensors