M1 Thermal Control
description
Transcript of M1 Thermal Control
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.