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
■ 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
Shanghai, PRC
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
Typical
Instrument
Astronomer
Filter Size
50 mm
MOSFIRE
EAGLE
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
14
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
8
7
6
OPTICAL DENSITY
5
4
3
2
1
0
340 355 370 385 400 415 430 445 460 475 490 505 520 535 550 565 580 595 610 625 640
WAVELENGTH (NM)
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
7
6
ABS
5
4
3
2
1
0
300
400
500
600
700
800
Wavelength (nm)
%T
LAO_130321 %T
100
90
80
70
60
50
40
30
20
10
0
300
400
500
600
700
800
Wavelength (nm)
900
1000
1100
900
1000
1100
Uniformity of Green Filter
0.40%
Defiation from target ceter wavelength
0.35%
0.30%
0.25%
0.20%
0.15%
0.10%
0.05%
0.00%
0
50
100
150
Distance from Center (mm)
200
250
300
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
Three band rugate on Sapphire
100%
Transmission (%)
90%
MODEL
Measured
Uncoated
80%
70%
60%
50%
40%
30%
20%
10%
0%
300
800
1300 1800 2300 2800 3300 3800 4300 4800 5300 5800 6300
Wavelength (nm)
Bandwidths can be Large or Small
Range of Rugates produced at Barr Associates
100%
90%
BW ~ 2%
BW~ 106%
80%
70%
Transmission
60%
50%
40%
30%
20%
10%
0%
400
900
1400
1900
Wavelength (nm)
2400
2900
Single Notch at 45 degrees AOI
100
90
80
70
45° Random verage Polarization
Calcualted T @45 (S+P)/2
50
40
30
1
20
0.9
10
0
400
450
500
550
600
650
Waveelngth (nm)
700
750
800
850
900
Fraction of High Index m aterial
Transmission
60
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
2000
4000
6000
8000
Thickness (nm)
10000
12000
14000
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
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1050
1100
1150
1200
1250
1300
1350
1400
1450
1 nm band width 81 rejection bands OD 3
1.3 mm of coating
0.95
0.9
0.85
0.8
0.75
0.7
0.65
Per Cent High Index
0.6
0.55
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0 28464 77307 132083 192221 252983 313156 374798 434070 494531 555129 615473 675936 736427
Metric Thickness (nm)
802684 866573 927176 1015695.8125
1110747 1182702 1254702
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