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

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

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

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

von Hoerner plot CCAT ALMA

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

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

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

Three point support, fig. from SNAP 2m

CFRP

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

Edge sensor sensitivity, Keck

Edge sensor sensitivity, CCAT

Distance & tip-tilt sensor sensitivity, CCAT

Wave Front Error Budget

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