M1 Thermal Control

25 August 2003 ATST CoDR Dr. Nathan Dalrymple

Air Force Research LaboratorySpace Vehicles Directorate

Primary Mirror (M1) Thermal Control

• Function: Mitigate mirror seeing

seeing

K

mbar61 106.77

T

Pn −⋅−≅

PP

nT

T

nn Δ

∂∂

+Δ∂∂

= 111

-0.69x10-6 K-1 0.28x10-6 mbar-1

Requirements

1. Minimize mirror seeing

a. Racine experiment: = 0.38 TM - Te) 1.2

b. Iye experiment: greatly reduced by flushing

c. IR HB aerodynamic analysis: = ΔTV, d. Bottom line: requirements on surface-air ΔT 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

Error Budgets

(nm) Error budget Description

500 20 nmDiffraction-

limited

1600 0.05 arcsecSeeing-limited

1000 0.05 arcsec Coronal

IR Handbook Seeing Analysis

Given layer thickness and ΔT, we can estimate .

zlG z

Hd2

0

222 ∫ ′= ρσ

Wavefront variance

Gladstone-Dale parameterFluctuating density Line-of-sight correlation length

Layer thickness

HlT

TGz2

10

22

ρπ

πσφ Δ

≈=

Phase variance

2.01.0 −≅Hlz

Surface-air temperature difference

⎪⎩

⎪⎨

⎧

<−

≥≈

)(1)exp(

)(33.3

s2

D

s

aberrationweak

aberrationstronglz

φφφθ

φφσ

θ

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):

2.0

8.05.05.1

0392.0184.0V

L

V

TLH +

Δ≅

L: upstream heated length (m)ΔT: average temperature difference over upstream length (˚C)V: wind speed (m/s)

Buoyancy term Hydrodynamic term

Assume: If ΔT < 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

Composite 4m mirror seeing estimateRacine [1991] used for natural convection; Zago [1995] used for mixed convection;

Gilbert et al. [1993] used for forced convection

0.00

0.05

0.10

0.15

0.20

0 1 2 3 4 5 6 7 8

V (m/s)

mirror seeing (arcsec)

0.2 K

0.5 K

1.0 K

2.0 K

5.0 K

GEMINI (0.2 K)

An Alternate View

For a particular ΔT, 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

Physical Case 4: Input Profiles

-15

-10

-5

0

5

10

15

20

0 6 12 18 24

t (hours)

0

50

100

150

Sunside air temp (K)

Backside air temp (K)

Absorbed solar flux (W/m^2)

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 ΔT at end of observing day

These results assume calm air.Wind helps both thermal control and seeing.

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0 1 2 3 4 5 6 7 8 9 10 11 12

time (hr)

M1 seeing (arcsec)

Physical Case 4: Input Profiles

-15

-10

-5

0

5

10

15

20

0 12 24 36 48 60

t (hours)

0

50

100

150

Backside air temp (K)

u1 (K)

Absorbed solar flux (W/m^2)

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)

1D,t Finite-Difference Results: Sac Peak Te

Same cooling profile used for both days

t (hr)

Seeing Performance: Sac Peak Te

day day

Good performance over both days

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0 12 24 36 48 60

time (hr)

t (hr)

-500

0

500

1000

1500

2000

2500

3000

3500

0 6 12 18 24

time (hrs)

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 m2 flow area,each side

Covered in greaterdetail in Enclosureslides.

Flushing System Performance (Sample)

Covered in greaterdetail in Enclosureslides.