1. Materion Barr Precision Optics & Thin Film Coating
The Evolution of Filters for Astronomical Applications: A Manufacturer’s View
Robert W. Sprague, Thomas A. Mooney, John R. Potter,
Kevin R. Downing, Michael J. Tatarek and Ali Smajkiewicz
Materion Barr Precision Optics & Thin Film Coating
Westford, MA U.S.A.

2. Overview Who/what is Materion?
Why do we pursue a small fastidious market?
What has changed in this market from our perspective over the last twenty years?
How the change has influenced our technological development?

3. Materion Barr Precision Optics &Thin Film Coatings Fabricator of Thin Film Coatings
Buellton, CA Westford, MA Windsor, CT Shanghai, PRC
700+ people
100+ deposition systems
ALL Physical Vapor Deposition (PVD)
1 to 1,000,000s of parts
Optical filters from 180 nm to 60 µm
Non-optical Thin Film Structures

4. Buellton, CA Formerly Thin Film Technology (TFT)
Precision thin film coating
Magnetron sputter, IAD and Evaporation
Specialty thin film coatings
Aerospace and medical applications
Infrared filters

5. Windsor, CT Formerly Technimet
Engineered films
Barrier coatings
Roll-to-roll coating
Up to 54” wide
Medical applications
Precision slitting

6. Wai Gai Qiao Free Trade Zone Shanghai Formerly EIS Optics
Optical coatings
Magnetron sputter, evaporation, IAD
Opto-mechanical assemblies
Patterned filters
Wafer level packaging
Large volume commercial applications
Projection display light engines

7. Westford and Tyngsboro , MA Formerly Barr Associates
Evaporation, IAD, magnetron and Ion Beam Sputtering (IBS)
Founded in 1971 by Edward Barr
110,000 ft² (11,800m²)
Wavelength from 150 nm to beyond 60 um
Provide optical filter solutions for virtually all key markets and applications
Purchased by Brush Wellman in 2009
Name changed to Materion in 2011
Location at which the work in this presentation was performed

8. Astronomers Are Always Looking To Improve On Previous Results…
Each instrument is unique. Astronomers use all manner of optical filters.
Wide Band
Bessel set and its derivatives
Narrow Band
Hydrogen line filters
Beam Splitters
Order separation for spectrographs
Notch
Laser Guide star

9. Ground-based Professional Astronomers Have Unique Challenges and Advantages
Looking Through The Atmosphere
Turbulence limits effective aperture
Atmospheric absorption limits spectral regions
Light Pollution
Larger Primary Mirror
MORE LIGHT
See Fainter Objects
See Farther Back In Time
Shorter Exposure
Better resolution

10. Size Evolution Telescopes, Instruments, Filters
Palomar, 1949, 5 meter
Keck , 1993, 10 meter
E-ELT, 2015, 40 meter
1,000 mm
Typical
Instrument
380 mm
Astronomer
MOSFIRE
EAGLE
Filter Size
50 mm
250 mm
500 to 750 mm

11. Technologies Enabling Large Scopes
Spin Casting Up To 8.4 Meters
Steward Observatory Mirror Lab
Light Weight Honeycomb Mirrors
Segmented Primaries
Thirty Meter Telescope (TMT) will have 492 segments
Diffraction-limited observations provide gains in sensitivity that scale as D4 (D is the primary-mirror diameter)
“TMT will provide a sensitivity gain of a factor more than 100 as compared to current 8 m telescopes.” (SCIENCE-BASED REQUIREMENTS DOCUMENT TMT.PSC.DRD CCR18)
Adaptive Optics
Compensate for atmospheric turbulence
Solid State Detectors
Mosaics of large area CCDs

12. We Have Adapted All Aspects Of The Manufacturing Process
Material
Fluorides and Sulphides to Oxides
Methods
Evaporation to IAD, Sputtering
Deposition Systems
Substrate Preparation
Test Equipment
Facilities

13. Material Change Prior to 1980’s
Filters were produced with evaporation, mostly resistively heated
Many materials were hygroscopic, filters had to be encapsulated for long life and stable operation
Difficult to create with a very good transmitted wave front
Oxide materials
Lower absorption in the blue and UV
Highly porous and thus susceptible to drift
In the 80’s, “energetic” processes were developed
Ion assisted deposition, magnetron sputtering, Dual Ion Beam Sputtering, ion plating and others
Produced filters with very high packing density, no measurable humidity drift

14. What makes a filter “Big”
Driven by :
Uniformity of spectral characteristics
Narrow filters (bw ~.02% in visible) - big is 70 mm
Broad band (bw a few % or more) mm is big
Sensitivity of design
Stability of the deposition process

15. H beta Narrow Band Filter
Diameter: 70 mm +/- 0.2 mm
Clear aperture: 65 mm minimum diameter
CWL = / nm
FWHM <= 0.05 nm (0.01%)
Peak T% > 45% (Goal > 50%)
Transmission variation < 5% over clear aperture
TWF < 0.25 waves 430 nm over 65 mm CA min (see note)
Operating temp: 18-20ºC
AOI = 0 degrees, collimated beam
Out-of-band blocking OD4 from nm

16. Our Measurements Blocking

17. Our Measurements Transmission uniformity

18. Customers Measurement Transmission

19. Customers Measurement uniformity map
Black color in this map corresponds to a central wavelength of nm (and below)
White color to a central wavelength of nm (or above)
Gray scale is linear, the extreme values (black/white) of the gray scale have not been incurred in the map)
Black ring demarks the clear aperture

20. Study the Sun Spots High resolution video image
View the video at:

21. Broad Band Filter Growth 1997-2004
75 mm for SDSS
Delivered 1997
150 mm for WIYN
Delivered 2004

22. Broad Band Filter Growth 2004-2008
570 mm for Pan-STARRS
Delivered 2008
Pan-STARRS was at the limit of our capabilities.

23. Broad Band Filter Sets Sloan Digital Sky Survey
Bessel- Johnson Filters
Made from color filter glass
Absorption based
Angle insensitive
Size limited by CFG manufacture
Interference Based
Angle sensitive
Bandwidths and position broadly tunable
Size limited by deposition system

24. Pan-STARRS Filters
Comparison of Pan-STARRS filter set measured at Barr Associates and in use. Barr’s measurements are the lower curves.

25. Next Steps Large filters require large deposition systems
Precision filters larger than 560 mm could not be made
Acquired a new chamber based on experience and modeling
System delivered in January 2013
First filter shipped in March 2013

26. Subaru Hyper Suprime Camera Filters All Dielectric Filter Fully Blocked for Si

27. Uniformity of Green Filter

28. Rugates Development supported by Air Force (1997-2004)
Based on sinusoidal refractive index variation
Bandwidth is proportional to amplitude of index variation
Reflectance per cycle is proportional to index contrast
Rejection is by reflection, so more rejections mean more cycles
Spatial period of structure determines wavelength of reflection
Ideally has no harmonics
Works very well for applications requiring narrow rejection bands in broad transmission spectra
Beam splitters for Guide Stars
Light pollution rejection

29. Rugate Cost Drivers Relative Bandwidth (FWHM/CWL)
Reflection per cycle is determined by index contrast
Rejection requirement (OD)
Wavelength
Longer wavelength means longer cycles
Cost ~ Wavelength * OD/RBW

30. Rugate Filters can be Made at any Wavelength from Visible through SWIR

31. Bandwidths can be Large or Small

32. Single Notch at 45 degrees AOI

33. What do they want ?
Remove the ‘Meinel bands’ of the hydroxyl radical (OH) in an ionospheric layer at 90 km.
See what is in between

34. 1 nm band width 81 rejection bands OD 3

35. 1 nm band width 81 rejection bands OD 3 1.3 mm of coating

36. The only way to know your limitations
Conclusion
The only way to know your limitations
is to exceed them!
Astronomers require you keep pushing the envelope of what is possible because they demand the highest performance
The methods then developed can be applied to other projects