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

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

Requirements 1.Minimize mirror seeing a.Racine experiment:  = 0.38  T M - T e ) 1.2 b.Iye experiment:  greatly reduced by flushing c.IR HB aerodynamic analysis:  =  T  V,  d.Bottom line: requirements on surface-air  T and wind flushing Ref: Racine, Rene, “Mirror, dome, and natural seeing at CFHT,” PASP, v. 103, p. 1020, Iye, M.; Noguchi, T.; Torii, Y.; Mikama, Y.; Ando, H. "Evaluation of Seeing on a 62-cm Mirror". PASP 103, 712, 1991

Error Budgets (nm) Error budgetDescription nm Diffraction- limited arcsec Seeing- limited arcsecCoronal

IR Handbook Seeing Analysis Given layer thickness and  T, we can estimate . Wavefront variance Gladstone-Dale parameter Fluctuating densityLine-of-sight correlation length Layer thickness Phase variance Surface-air temperature difference Blur angle 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.

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

Convection Types and Loci Wind is good.

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

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

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

An Alternate View For a particular  T, V combination, read over on the vertical axis to find seeing

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

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

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

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

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

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

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

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.

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

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

Seeing Performance: Sac Peak T e day Good performance over both days t (hr)

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

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

Flushing System Concept 42 vent gates 168 m 2 flow area, each side Covered in greater detail in Enclosure slides.

Flushing System Performance (Sample) Covered in greater detail in Enclosure slides.