Cryogenic thermal mask for space-cold optical testing of space optical systems

Transcription

1 Cryogenic thermal mask for space-cold optical testing of space optical systems Dae Wook Kim ames H. Burge LOFT group College of Optical Sciences Univ. of Arizona

2 Introduction How do the next generation space optical systems look like?

3 6.5m WST Large aperture size -More light collecting -Higher resolution Cryogenic system -working in space -IR telescopes ~7K Image from No A/S -Located at Lagrange

4 Motivation We want to test the final performance before the launch.

5 LOTIS Vacuum Chamber LOTIS 6.5m collimator 6.5m WST Images from LOTIS at Completion of Collimator Integration R.M. Bell*, G.C. Robins, C. Eugeni, G. Cuzner, and S.B. Hutchison Lockheed Martin Space Systems Company Proc. of SPIE Vol D-

6 Test the whole optical system in a space-cold cryogenic vacuum chamber with a collimator.

7 Test the whole optical system in a space-cold cryogenic vacuum chamber with a collimator. The thermal radiation from the ambient e.g. 3K) collimator will warm up the cryogenic optical system under test.

8 Proposed Solution: Cryogenic Thermal Mask Can we block the thermal transfer while passing the test wavefront without significant degradation?

9 Thermal plate - Array of holes - Graybody with emissivity ε - Thermally controlled at desired temperature T

10 Cryogenic Thermal Mask - Series of thermal plates with array of holes - Placed between the collimator and the optical system under test - Temperature of thermal plates is independently controlled to gradually match the temperature difference two space.

11 Simplified Thermal Transfer Model = ε σ T 4 [ W / m ] 4 net _ + ) = ε σ T + net _+ ) + ε net _

12 = _ 4 _ 4 _ 3 _ ) ) ) ) ) ) ) ) ) ) ) ) C H net net net net net net net net T T T T T T T T σ σε σε σε σε σε σε σ ε ε ε ε ε ε ] / [ 4 _ 4 _ m W net net C + = ] / [ m W net net H + = Thermal Transfer Equation for a given CTM parameters Thermal Loads to the cold cryogenic optical system) space and to the hot space ambient collimator)

13 )]}, [ ), { ) )], )**, [ )]** [ )}], ), { ) [ ), y x U F F f y I f x I comb f y x D somb y x U I y I x comb F F D y x cyl F F y x U I y I x comb D y x cyl F F y x U hole f x f y eff eff eff CTM hole f x f y CTM f x f y hole CTM f x f y focal eff eff eff eff eff eff eff eff = = = = = = = = + + = + λ ξ λ η λ ξ λ η λ ξ λ η λ ξ λ η λ λ λ Fraunhofer diffraction theory for the test beam propagation Because of the diffraction from the periodical hole array in the CTM, multiple diffraction orders are generated.

14 Multiple diffraction orders at the focal plane Condition to spatially block unwanted orders D λ f λ eff eff Airy = << = Dsystem I f K I << D system

15 Performance Will it work thermally and optically?

16 Nominal CTM parameter Parameters Symbol Value Temperature of hot space T H 3 K Temperature of cold space T C 35 K Wavelength Λ μm Diameter of hole D hole. m Interval between holes I. m Spacing between plates S.5 m Number of thermal plates N 3 Diameter of optical system D system 6.6 m Number of hole-sets N hole-set ~855 Obscuration ratio Α ~.99

17 Part A. Thermal Performance Analysis

18 Thermal Performance Analysis I Emissivity and T T =3K and T 3 =35K) Less thermal loads - More blackbody-like first and third plate - More reflective second plate T needs to be compromised for Δ C and Δ H

19 Thermal Performance Analysis II T 3 and T T =5K and ε =ε 3 =.9, ε =.) T 3 is useful knob to tweak the thermal load to the optical system side Δ C T is useful knob to tweak the thermal load to the collimator side Δ H

20 Thermal Performance Analysis III Floating T ε =ε 3 =.9, ε =.) Floating T is still gives good thermal performance e.g. <3mW/m ). T does not need to be thermally controlled.

21 Part B. Optical Performance Analysis

22 Optical Performance Analysis I Tolerance in a single hole in the CTM Tolerance - Misalignment: δx = 5 μm, δy = 5 μ m - Hole diameter: δd hole = μ m

23 Optical Performance Analysis II Normalized amplitude error and phase error a = hole U hole _ perturbed x, y) dx dy hole U hole _ perfect hole U hole _ perfect x, y) dx dy x, y) dx dy %) p = phase angle of hole U hole _ perturbed x, y) dx dy phase angle of U hole _ π hole perfect x, y) dx dy waves) The normalized amplitude error a and the phase error p were defined to quantitatively asses the optical errors in the test beam compared to the perfect case.

24 Optical Performance Analysis III Optical errors from the whole CTM One of the tolerance analysis histograms for 855 holes in the nominal CTM Tolerance: δx = 5 um, δy = 5 um, and δdhole = um)

25 Optical Performance Analysis IV Tolerance vs. optical errors Within the realistic tolerance ranges, the induced RMS phase error was <.6 waves, which may be sufficient for most optical testing applications.

26 Summary & Future Works What do we have now, and what needs to be done next?

27 The analysis shows a good thermal and optical performance of the nominal CTM. - A three plate CTM with holes occupying ~% of the thermal plate area and with two black and a polished intermediate thermal plates caused thermal loading less than 3mW/m for both the 3K ambient and the 35K cryogenic sides of the system. - The induced RMS phase error with some realistic CTM tolerance values was <.6waves, which may be sufficient for most optical testing applications. There are remaining future works. - The normalized amplitude error a and the phase error p may not exactly represent the actual errors in the test beam after passing through a CTM. - The simplified thermal model is only the first order calculation. - An actual experimental demonstration using a sub-scale CTM will be made to answer those issues.

28 Thank you.

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