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.

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?

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

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

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

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

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

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

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

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

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.05.001.CCR18) Adaptive Optics Compensate for atmospheric turbulence Solid State Detectors Mosaics of large area CCDs

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

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

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) - 700 mm is big Sensitivity of design Stability of the deposition process

H beta Narrow Band Filter Diameter: 70 mm +/- 0.2 mm Clear aperture: 65 mm minimum diameter CWL = 486.136 +/- 0.03 nm FWHM <= 0.05 nm (0.01%) Peak T% > 45% (Goal > 50%) Transmission variation < 5% over clear aperture TWF < 0.25 waves P-V @ 430 nm over 65 mm CA min (see note) Operating temp: 18-20ºC AOI = 0 degrees, collimated beam Out-of-band blocking OD4 from 340-640 nm

Our Measurements Blocking

Our Measurements Transmission uniformity

Customers Measurement Transmission

Customers Measurement uniformity map Black color in this map corresponds to a central wavelength of 486.115 nm (and below) White color to a central wavelength of 486.155 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

Study the Sun Spots High resolution video image View the video at: http://www.nso.edu/press/H-Beta

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

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

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

Pan-STARRS Filters Comparison of Pan-STARRS filter set measured at Barr Associates and in use. Barr’s measurements are the lower curves. http://svn.pan-starrs.ifa.hawaii.edu/trac/ipp/wiki/PS1_Photometric_System

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

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

Uniformity of Green Filter

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

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

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

Bandwidths can be Large or Small

Single Notch at 45 degrees AOI

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

1 nm band width 81 rejection bands OD 3

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

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