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M1 Thermal Control

M1 Thermal Control. 25 August 2003 ATST CoDR. Dr. Nathan Dalrymple Air Force Research Laboratory Space Vehicles Directorate. -0.69x10 -6 K -1. 0.28x10 -6 mbar -1. Primary Mirror (M1) Thermal Control. Function: Mitigate mirror seeing. seeing. Requirements. Minimize mirror seeing

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M1 Thermal Control

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  1. M1 Thermal Control 25 August 2003 ATST CoDR Dr. Nathan Dalrymple Air Force Research Laboratory Space Vehicles Directorate

  2. -0.69x10-6 K-1 0.28x10-6 mbar-1 Primary Mirror (M1) Thermal Control • Function: Mitigate mirror seeing seeing

  3. Requirements • Minimize mirror seeing • Racine experiment: q = 0.38 (TM - Te) 1.2 • Iye experiment: q greatly reduced by flushing • IR HB aerodynamic analysis: q = q(DT, V, l) • Bottom line: requirements on surface-air DT and wind flushing Ref: Racine, Rene, “Mirror, dome, and natural seeing at CFHT,” PASP, v. 103, p. 1020, 1991. Iye, M.; Noguchi, T.; Torii, Y.; Mikama, Y.; Ando, H. "Evaluation of Seeing on a 62-cm Mirror". PASP 103, 712, 1991

  4. Error Budgets

  5. Layer thickness Given layer thickness and DT, we can estimate q. Wavefront variance Fluctuating density Line-of-sight correlation length Gladstone-Dale parameter Surface-air temperature difference Phase variance Blur angle IR Handbook Seeing Analysis Strong/weak cutoff ~ 2 rad Ref: Gilbert, Keith G., Otten, L. John, Rose, William C., “Aerodynamic Effects” in The Infrared and Electro-Optical Systems Handbook, v. 2, Frederick G. Smith, Ed., SPIE Optical Engineering Press, 1993.

  6. Hydrodynamic term Buoyancy term IR Handbook Seeing Analysis (cont.) Layer thickness (mks units): L: upstream heated length (m) DT: average temperature difference over upstream length (˚C) V: wind speed (m/s) Assume: If DT < 0 then buoyancy term does not contribute to layer thickness.

  7. Convection Types and Loci Wind is good.

  8. Diffraction-Limited Error Budget Blue contours: rms wavefront error (nm) l = 500 nm Acceptable operating range, assuming no AO correction. AO correction will extend the “green” range.

  9. Seeing-Limited Error Budget Blue contours: 50% encircled energy (arcsec) l = 1600 nm Acceptable operating range

  10. Coronal Error Budget Blue contours: 50% encircled energy (arcsec) l = 1000 nm Acceptable operating range

  11. An Alternate View For a particular DT, V combination, read over on the vertical axis to find seeing

  12. Mirror Thermal Control • Time-dependent problem • Backside cooling • Controlled frontside temperature time lag through substrate knobs

  13. M1 Thermal Loading • Time-dependent problem; this is one snapshot

  14. Thermal Control System Concept Desiccant chamber included in cell to dry air

  15. Flow Loop Concept A: Closed cycle, liquid coolant (heats or cools)

  16. Flow Loop (cont.) Concept B: Open cycle, air coolant (only cools)

  17. 1D,t Finite-Difference Model Inputs: Ideal Day • Desired set point: 1–3 ˚C below ambient temperature

  18. 1D,t Finite-Difference Model Results: Ideal Day M1 temperature OK over most of day Fix with profile optimization

  19. Seeing Performance: Ideal Day Very good performance until positive T at end of observing day These results assume calm air.Wind helps both thermal control and seeing.

  20. 1D,t Finite-Difference Inputs: Sac Peak Te • 23 – 25 June 2001 (60 hr run) • Desired set point: 1–3 ˚C below ambient temperature t (hr)

  21. 1D,t Finite-Difference Results: Sac Peak Te Same cooling profile used for both days t (hr)

  22. Seeing Performance: Sac Peak Te day day t (hr) Good performance over both days

  23. Heat Removal Rate: Ideal Day Peaks at 3200 W • Next steps: • Fan and system curves • Heat exchanger specs • Chiller specs • Time response of fluid volume

  24. 2D,t NASTRAN Results • Response to 2002 workshop comments • Result: actuator thermal “print-through” negligible

  25. Flushing System Concept Covered in greaterdetail in Enclosureslides. 168 m2 flow area,each side 42 vent gates

  26. Flushing System Performance (Sample) Covered in greaterdetail in Enclosureslides.

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