Optics and filters

1
Optics and filters

• Lynn Seppala

2
Baseline Feb 2003 design modified for 3.5 degree
field angle long design Paul-Baker / Willstrop
Mersenne-Schmidt configuration
Tertiary center of curvature is near secondary
vertex

• Initial Paul-Baker design used parabola-sphere-sph
  ere for the three mirrors, with collimated light
  between secondary and tertiary
• Design has low asphericities but camera location
  is a problem camera lens assembly is longer and
  larger to achieve 0.2 arc-sec images

3
Ming design short design extends Seppala
modifications to configuration similar to Laux
three mirror telescope
Tertiary center of curvature is far from vertex
of secondary
Primary and tertiary mirrors might be cast as a
single component
Field of view of 3.5 degrees
Location of camera may be better for mounting

• Initial attempts at 3 degree field showed uniform
  64 throughput but severe asphericities
• Recent designs f/7 divergent beam from
  secondary and larger camera lenses show
  significant reduction in aspheric departures and
  better camera location tradeoff is somewhat
  reduced throughput

4
Status of lenses and filters

• Optical designs have been generated for both
  short and long designs for field angles of
  3.0, 3.5 and 4.0 degrees
• All designs have 80 images sizes of lt 0.2
  arc-sec in the V-R-I bands and Image size of at
  least 0.24 arc-sec in the B and Z band
• All designs have similar refractive optics near
  the camera
• Maximum diameter and length increase for
  increasing field angle
• Smallest camera package is 1.34 m in diameter by
  0.81m in length
• Largest camera package is 1.86 m in diameter by
  1.07 m in length

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Comparison of short designs and long baseline
designs for 3.0, 3.5 and 4.0 degrees

• All designs have 80 images sizes of lt 0.2
  arc-sec in the V-R-I bands and Image size of at
  least 0.24 arc-sec in the B and Z band
• A reasonable balance of throughput, aspheric
  departures and camera location were assumed,
  pending a specific systems requirements document
  detailing the scientific requirements

6
Status of lenses and filters

• Optical designs have been generated for both
  short and long designs for field angles of
  3.0, 3.5 and 4.0 degrees
• All designs have 80 images sizes of lt 0.2
  arc-sec in the V-R-I bands and Image size of at
  least 0.24 arc-sec in the B and Z band
• All designs have similar refractive optics near
  the camera
• Maximum diameter and length increase for
  increasing field angle
• Smallest camera package is 1.34 m in diameter by
  0.81m in length
• Largest camera package is 1.86 m in diameter by
  1.07 m in length
• Optical component drawings have been generated
  for a short design with a field of view of 3.5
  degrees initial cut
• Null optical tests have been designed using
  spherical lenses for both aspheric lenses L1 and
  L2
• Optical drawing packages have been sent to Kodak
  and Tinsley for comments and budgetary estimates
  OCLI will be included for filters and AR coatings
• Inquiries at LLNL and OCLI indicate feasibility
  of anti-reflection coatings of lt 1 average
  reflectivity for zero to 28 degrees angle of
  incidence
• Input regarding coating and materials of narrow
  band filters will be solicited

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Component drawings are based on a 3.5 degree
short design

• All numbers in these drawings-- mechanical (D R,
  DT, wedge) , optical materials ( optical
  grade, defects ), surface (scratch/dig, figure,
  mid-frequency and roughness) and coating
  (reflectivities or transmission) -- are
  place-holders only

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Filter drawing tabulated for six filters,
U-B-V-R-I-Z
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Details of fabrication for the filters
STEP 1 - BLANK FABRICATION 1.1 MATERIAL WILL BE
CORNING 7940 FUSED SILICA, GRADE 2/C
OR EQUIVALENT. STEP 2 -SURFACE FABRICATION 2.1
EDGE CHIPS SHALL NOT EXCEED 0.25 mm. 2.2 QUALITY
OF SURFACE "S1" AND "S2" TO BE LESS THAN OR EQUAL
TO 40 / 20 SCRATCH / DIG OVER THE CLEAR
APERTURE. 2.3 WEDGE BETWEEN "S1" AND "S2" SHALL
BE LESS THAN 30 ARC-SEC (TOTAL INDICATED RUNOUT
0.106 mm) 2.4 EDGES AND BEVELS WILL BE INSPECTION
POLISHED (FREE FROM GRAY TO THE NAKED EYE UNDER
BRIGHT ILLUMINATION. 2.5 DRAWING NUMBER, TAB, AND
SERIAL NUMBER TO BE APPLIED BY SANDBLAST WITH A
TYPED TEMPLATE. VIBRATORY OR SCRIBE TYPE MARKING
ARE NOT ACCEPTABLE. ANY OTHER METHOD MUST
BE APPROVED BY LLNL.
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Surface specification over clear aperture
ROUGHNESS SPECIFICATION FOR CLEAR APERTURE a)
MID-SPATIAL FREQUENCY ROUGHNESS 3.0 nm RMS
DEFINED BY INTEGRATING THE PSD OVER A RANGE OF
0.030/mm TO 1.0/ mm b) HIGH SPATIAL FREQUENCY
ROUGHNESS 3.0 nm RMS DEFINED BY INTEGRATING THE
PSD FOR FREQUENCIES gt 1.0/ mm 2.7 FOR COLLIMATED
INCIDENT LIGHT AT 633 nm IN THE INDICATED NULL
TEST, THE SINGLE PASS PEAK-TO-VALLEY WAVEFRONT
IN TRANSMISSION OVER THE CLEAR APERTURE SHALL BE
LESS THAN 0.70 WAVES AT 633 nm, AFTER TIP, TILT
AND FOCUS TERMS ARE REMOVED. THE GRADIENT OF THE
WAVEFRONT SHALL BE LESS THAN ? / 8.
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Tabulated filter data for the six filters
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Example of coating specification for transmission
filter and anti-reflection coating

• STEP 3 - COATING FINISHED SURFACES.
• 3.1 THE ANGLE OF INCIDENCE FOR SURFACES "S1"
  SHALL BE 28.0
• DEGREES TO 7.0 DEGREES INCLUSIVE. THE ANGLE OF
  INCIDENCE
• FOR SURFACES "S2" SHALL BE 27.0 DEGREES TO 8.0
  DEGREES INCLUSIVE.

3.2 table of spectral bands
3.3 SURFACE "S1" AND SURFACE "S2" BROAD BAND
ANTIREFLECTION COATING -- COATING SPECIFICATION
SHALL APPLY FOR ALL WAVELENGTHS FROM 386 nm TO
1040 nm OVER THE ENTIRE CLEAR APERTURE. THE
COATING GOAL IS TO HAVE THE AVERAGE REFLECTIVITY
TO BE LESS THAN 1.0 PER SURFACE OVER
EACH SPECTRAL BAND AND LESS THAN 1.0 PER
SURFACE FROM 386 nm TO 1040 nm. 3.4 HARDNESS AND
ADHERENCE REQUIREMENTS PER MIL-M-13508 (3.8)
AND MIL-C-675 (3.8.5).
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Eight layers of alternating high and low index
materials TiO2/SiO2
Figure 3 Spectral curve at 20 degrees angle of
incidence with 50 case Monte Carlo error analysis
Chris Stolz, LLNL.
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Lens L1 can be tested using a retro-reflector and
a spherical null lens
NULL TEST
L1
Spherical null lens
1189.20 /- 1.0
To interferometer
L2
Spherical mirror
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Lens L2 can be tested using a retro-reflector and
a spherical null lens
NULL TEST
Spherical mirror retro-reflector, R2026
concave
Spherical Null lens
L2
To interferometer
2934.6
L2
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Summary and near-term action items

• Optical component drawings have been generated
  for a short design with a field of view of 3.5
  degrees initial cut
• Optical components will be used in discussions
  with vendors
• Vendors comments might be useful before a final
  design is established
• Near term action items
• Continue to solicit comments from vendors
• Refine drawings and start to finalize tolerances
• LSST team members should identify possible
  vendors to LLNL
• Optical-mechanical model should be developed soon
• Shutter and filter change mechanism need to be
  developed further
• Model needs to have an initial concept that
  includes CCD array, electronics and wiring,
  cooling, vacuum if needed, image rotator, etc.
• Initial assembly procedure will be developed
• Identify items that may affect final design
  selection