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 CoatingsFabricator 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, CAFormerly Thin Film Technology (TFT)

■ Precision thin film coating

■ Magnetron sputter, IAD and Evaporation

■ Specialty thin film coatings

■ Aerospace and medical applications

■ Infrared filters

Windsor, CTFormerly Technimet

■ Engineered films

■ Barrier coatings

■ Roll-to-roll coating

Up to 54” wide

Medical applications

■ Precision slitting

Wai Gai Qiao Free Trade Zone ShanghaiFormerly 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 , MAFormerly 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

TypicalInstrument

Filter Size 50 mm 250 mm 500 to 750 mm

MOSFIRE EAGLEAstronomer

38

0 m

m

1,000 mm

Technologies Enabling Large Scopes

Spin Casting Up To 8.4 MetersSteward 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 DetectorsMosaics 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 materialsLower 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 MeasurementsBlocking

0

1

2

3

4

5

6

7

8

340 355 370 385 400 415 430 445 460 475 490 505 520 535 550 565 580 595 610 625 640

WAVELENGTH (NM)

OP

TIC

AL

DE

NS

ITY

Our MeasurementsTransmission uniformity

Customers MeasurementTransmission

Customers Measurementuniformity 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

150 mm for WIYN

Delivered 2004

75 mm for SDSSDelivered 1997

Broad Band Filter Growth1997-2004

570 mm for Pan-STARRSDelivered 2008

Broad Band Filter Growth2004-2008

Pan-STARRS was at the limit of our capabilities.

Broad Band Filter SetsSloan Digital Sky Survey

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

Bessel- Johnson Filters

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

LAO_130321 %T

0102030405060708090

100

300 400 500 600 700 800 900 1000 1100

Wavelength (nm)

%T

Subaru Hyper Suprime Camera FiltersAll Dielectric Filter Fully Blocked for Si

0

1

2

3

4

5

6

7

300 400 500 600 700 800 900 1000 1100

Wavelength (nm)

AB

S

Uniformity of Green Filter

0.00%

0.05%

0.10%

0.15%

0.20%

0.25%

0.30%

0.35%

0.40%

0 50 100 150 200 250 300

Distance from Center (mm)

Def

iatio

n fr

om ta

rget

cet

er w

avel

engt

h

RugatesDevelopment 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 Wavelengthfrom Visible through SWIR

Three band rugate on Sapphire

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

300 800 1300 1800 2300 2800 3300 3800 4300 4800 5300 5800 6300

Wavelength (nm)

Tra

nsm

iss

ion

(%

)

MODEL

Measured

Uncoated

Bandwidths can be Large or Small

Range of Rugates produced at Barr Associates

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

400 900 1400 1900 2400 2900

Wavelength (nm)

Tran

smis

sion

BW ~ 2%

BW~ 106%

Single Notch at 45 degrees AOI

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 2000 4000 6000 8000 10000 12000 14000

Thickness (nm)Fr

actio

n of

Hig

h In

dex

mat

eria

l

0

10

20

30

40

50

60

70

80

90

100

400 450 500 550 600 650 700 750 800 850 900

Waveelngth (nm)

Tra

nsm

issi

on

45° Random verage Polarization

Calcualted T @45 (S+P)/2

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

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1050 1100 1150 1200 1250 1300 1350 1400 1450

1 nm band width 81 rejection bands OD 31.3 mm of coating

Metric Thickness (nm)0 28464 77307 132083 192221 252983 313156 374798 434070 494531 555129 615473 675936 736427 802684 866573 927176 1015695.8125 1110747 1182702 1254702

Per

Cent H

igh Index

0.95

0.9

0.85

0.8

0.75

0.7

0.65

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

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