LSST camera meeting: Strawman optical design update

1
LSST camera meeting Strawman optical design
update

• Lynn Seppala
• July 14, 2004

2
Agenda

• Specifications for the Strawman design
• Strawman design now uses spherical lenses
• Diameter of L1 is 1.6 m May design was 1.54 m
  with aspheric lenses
• Aspheric departure on secondary increases to 85
  vs. 38 microns
• Optical performance of Strawman design
• Image size of lt0.22 arc-second is achieved for
  V-R-I bands
• Integrated throughput is 56.5
• Spherical lenses lead to many fabrication and
  assembly advantages
• Simple null tests for lenses
• Simple verification for corrector lens assembly
• Simple verification for stand-alone three-mirror
  telescope
• Optical prescription of Strawman design

3
Optical layout

• Strawman design specifications
• Three-mirror telescope dimensions
• Camera and corrector lens assembly dimensions
• Lens and filter dimensions

4
LSST Strawman design short-tube 3.5 degree
diameter field

• Modified Paul-Baker, Willstrop Mercenne-Schmidt
  or Laux design
• 8.36 m aperture primary mirror f / 1.25
• 3.5 degree full field of view
• Three aspheric mirror design
• Primary and tertiary mirror are a continuous
  surface 4.0 cm separation between beams
• Three fused silica spherical corrector lenses
• L1 is 1.6 m diameter
• Five interchangeable fused silica filters,
  13.5-22.0 mm thick
• CCD in vacuum, L3 is vacuum barrier
• 25 mm space CCD-L3
• Vertex of secondary mirror is 1.1 m from CCD
  array
• Effective etendue 230 (m-deg)2
• including all losses obstruction and
  vignetting, coatings and detector fill

5
LSST Strawman design short-tube, 3.5 degree field
8.36 m
5.146 m to joint in mirrors
3.2 m
0.224 m
1.1 m
6.26 m
6
LSST short-tube 3.5 degree design layout
1600
1120.346
7
Parameters of the three corrector lenses and
filter
3.5 degree short design with spherical optics and vacuum on L3, revised 6/9/04 3.5 degree short design with spherical optics and vacuum on L3, revised 6/9/04 3.5 degree short design with spherical optics and vacuum on L3, revised 6/9/04 3.5 degree short design with spherical optics and vacuum on L3, revised 6/9/04 3.5 degree short design with spherical optics and vacuum on L3, revised 6/9/04 3.5 degree short design with spherical optics and vacuum on L3, revised 6/9/04

Property Units L1 L2 L3 Filter
Aperture radius mm 780 531 345 371
Freeboard mm 20 19 20 19
Outer diameter mm 1600 1100 730 780
S1 spherical radius mm 2739.4 5198.6 3625.5 5630.3
S2 spherical radius mm -3803.2 -2058.5 -17192 -5630.3
Sag of S1 mm 119.417 29.176 18.420 13.524
Sag of S2 mm -85.092 -74.836 -3.875 -13.524
Sag of centroid mm 102.254 52.006 11.148 13.524
Center thickness mm 68.312 30 60 16.433
Virtual edge thick. mm 33.987 75.660 45.455 16.433
Actual edge thick. mm 30.791 68.897 45.455 16.433
Aprox. volume m3 0.1034 0.0500 0.0221 0.0079
Aprox. mass kg 227.5 110.0 48.6 17.3
Material cost 1.00/cc 227,500 110,000 48,600 17,300
Note SR is convex, -SR is concave Note SR is convex, -SR is concave Note SR is convex, -SR is concave
8
Optical performance

• Imaging performance across 5 spectral bands
• 80 and 50 image sizes across field for all
  bands
• Best focus curves for B, R and Z bands
• Focal shifts less than / - 4 microns
• Throughput analysis
• Obstruction on axis is 0.626 linear transmission
  0.608
• Vignetting at full field is 14.6
• Integrated throughput is 56.5
• Geometrical etendue including central obstruction
  and vignetting is 300 m2 deg2
• 90 detector fill and 85 coating losses drop
  effective etendue to 230 m2 deg2

9
LSST plans on using the 5 spectral bands, B to
Z, U band included for reference

• AR coatings on lenses must span the entire
  spectral range R lt 1 to 2
• AR coating on 2nd surface of filter R lt 0.5
  should span region where T gt 10
• High transmission coating on 1st surface of
  filter should produce sharp cutoff to reduce
  double-pass ghosting

Spectral band Wavelength nm
U 357.7 / - 32.3
B 436 / - 49.5
V 537 / - 47
R 644 / - 75.5
I 807.5 / - 75
Z 940 / - 100
10
Strawman short, 3.5 degree field LSST

• V-R-I bands have 80 energy collected in lt0.22
  arc-seconds diameter images

11
Strawman short, 3.5 degree field LSST

• V-R-I-Z bands have 50 energy collected in lt0.13
  arc-seconds diameter images
• Seeing usually refers to 50 energy collection

12
Strawman R band Best focus across detector
varies by / - 3 mm
13
Strawman R band Best focus across detector
varies by / - 3 mm
0.7 field
On-axis
Full-field
14
Strawman Z band Best focus across detector
varies by / - 5 mm
15
Strawman B band Best focus across detector
varies by / - 3.6 mm
16
Strawman geometric etendue excluding spider
300 m-deg2

• Geometrical etendue

p/48.36 m 3.5 deg2 0.565 298 m-deg2

• Detector fill factor 90and coating transmission
  lt85 drop effective etendue to 228 m-deg2

17
The strawman design with all spherical lenses has
many advantages

• Optical surfaces are easier to fabricate
• Potential to reduce fabrication time
• Lenses are easier to test during fabrication
• Potential to achieve a simple null test
• Less uncertainty after fabrication
• Three-mirror telescope, by itself, and camera
  assembly, by itself, are both well-corrected
  on-axis
• Simple null test during assembly of three-mirror
  telescope
• Simple null test during assembly of camera optics
  
• Less uncertainty after each stage of assembly

18
Null test for L1 uses a 1.7 m diameter spherical
mirror R 3.803 m to achieve a wavefront
error lt0.02 waves PV _at_ 633 nm
To interferometer

• Lens should be mounted in same cell used in
  LSST any transmitted wavefront errors due to
  gravity deformations are taken out

3.262 m
L1 Spherical mirror R 3.803 m,
same as concave surface of L1
19
Null test for L2 uses spherical lenses and the
same mirror R 3.803 m to achieve a
wavefront error lt0.07 waves PV _at_ 633 nm
To interferometer
200 mm diameter spherical null lenses

• Lens should be mounted in same cell used in LSST
  any transmitted wavefront errors due to gravity
  deformations are taken out

5.24 m
L2
Spherical mirror R3.803 m, same as concave
surface of L1
20
Null test for L3 uses a negative lens and the
same mirror R 3.7 m to achieve to achieve
a wavefront error lt0.09 waves PV _at_ 633 nm

• Collimated 100 mm diameter output beam to
  interferometer

• Lens is thick and will be used as a vacuum
  barrier in LSST
• Any transmitted wavefront errors due to gravity
  deformations are small compared to vacuum effects
• Any transmitted wavefront due to vacuum-induced
  deformations are neglible because L3 is so close
  to the image plane

2.7 m
L3 Spherical mirror R 3.803 m, same radius as
concave surface of L1
21
Null test for filters uses the same 1.7 m
diameter spherical mirror R
3.7 m to achieve a null wavefront lt0.04 waves
_at_ 633 nm
To interferometer

• Lens should be mounted in same cell used in
  LSST any transmitted wavefront errors due to
  gravity deformations are taken out

3.800 m
B-band filter, 22 mm th Spherical mirror R
3.803 m, same radius as concave surface of L1
22
Three-lens corrector has near diffraction-limited
images over 1 mm diameter field

• Strehl ratio double-pass 0.31 _at_ 633 nm
• Valuable assembly aid and alignment to verify
  lens centering and lens tilt

Mirror diameter 1.1 m, R 1400 mm sphere
15.75 mm FS plate
23
Three-mirror telescope, without the camera
package in place, is well-corrected on-axis

• Three-mirror telescope has lt0.14 arc-sec images
  over 2 mm diameter field 40 arc-sec
• Telescope can be initially aligned without the
  camera assembly in place

24
Optical prescription of Strawman short-tube 3.5
degree design

• Optical layout for R band
• Radii, spacing and thicknesses
• Aspheric data
• Filter dimensions and adjustments for filter
  exchange

25
Short 3.5 degree strawman LSST design R band
prescription all units are meters
Surface number Radius of curvature Spacing Outer semi-diameter Hole semi-diameter Material Description
-- 4.21197 4.37500 2.48400 AIR Outer baffle
1 -19.20000 -6.03441 4.18000 2.57300 MIRROR Primary
2 -6.03268 6.25850 1.60000 0.82000 MIRROR Secondary
3 -8.57743 -4.03811 2.57300 0.66000 MIRROR Tertiary
-0.824 from tertiary vertex -0.824 from tertiary vertex 2.47000 0 Inner baffle
4 -2.73940 -0.06831 0.80000 0 SILICA L1
5 -3.80320 -0.50822 0.80000 0 SILICA L1
6 -5.19860 -0.03000 0.55000 0 SILICA L2
7 -2.05850 -0.36781 0.53000 0 SILICA L2
8 -5.63032 -0.01643 0.39000 0 SILICA Filter
9 -5.63032 -0.04457 0.39000 0 SILICA Filter
10 -3.62550 -0.06000 0.36500 0 SILICA L3
11 -17.19200 -0.02500 0.36500 0 SILICA L3
26
Short 3.5 degree strawman LSST design all
units are meters
Conic Constant AD, 4th order AE, 6th order AF, 8th order Departure from best fit conic microns Departure from best fit sphere microns
primary -1.254809 0 5.8773E-09 0 0.8 --
secondary -0.284514 0 -1.2799E-05 -9.1574E-07 -- 85
tertiary 0.129492 0 -2.2717E-07 -3.6963E-09 -- 309
Polynomial aspheric coefficients
27
Comparison of short designs and long baseline
designs for 3.0, 3.5 and 4.0 degrees May camera
SLAC meeting
Comparison of data for short and long designs for 3.5 and 4.0 degrees field of view Comparison of data for short and long designs for 3.5 and 4.0 degrees field of view Comparison of data for short and long designs for 3.5 and 4.0 degrees field of view Comparison of data for short and long designs for 3.5 and 4.0 degrees field of view Comparison of data for short and long designs for 3.5 and 4.0 degrees field of view Comparison of data for short and long designs for 3.5 and 4.0 degrees field of view Comparison of data for short and long designs for 3.5 and 4.0 degrees field of view Comparison of data for short and long designs for 3.5 and 4.0 degrees field of view Comparison of data for short and long designs for 3.5 and 4.0 degrees field of view Comparison of data for short and long designs for 3.5 and 4.0 degrees field of view Comparison of data for short and long designs for 3.5 and 4.0 degrees field of view Comparison of data for short and long designs for 3.5 and 4.0 degrees field of view Comparison of data for short and long designs for 3.5 and 4.0 degrees field of view Comparison of data for short and long designs for 3.5 and 4.0 degrees field of view Comparison of data for short and long designs for 3.5 and 4.0 degrees field of view Comparison of data for short and long designs for 3.5 and 4.0 degrees field of view Comparison of data for short and long designs for 3.5 and 4.0 degrees field of view  
Telescope type Field of view Aspheric data Aspheric data Aspheric data Aspheric data Aspheric data Aspheric data Aspheric data Aspheric data Aspheric data              
Telescope type Field of view Primary Primary Secondary Secondary Tertiary Tertiary L1 L2     L1 L2 L3      
    Radius of curvature (m) Primary conic Aspheric departure (microns) Secondary diameter (m) Aspheric departure (microns) Tertiary diameter (m) Aspheric departure (microns) Aspheric departure (microns) System length (m) M2 to CCD (m) L1 diameter (m) L2 diameter (m) L3 diameter (m) Camera length L1 to CCD (m) Throughput on-axis Throughput at full field
Short 3.0 19.20 -1.241 52 3.14 316 4.94 804 146 6.18 1.10 1.32 0.93 0.66 0.91 64 55
Short 3.5 19.20 -1.224 38 3.2 361 5.07 700 98 6.25 1.10 1.54 1.13 0.73 1.06 61 53
Long 3.5 18.33 -0.997 20 3.6 111 5.7 436 300 9.00 4.30 1.64 1.18 0.77 0.97 61 55
Short 4 19.20 -1.270 100 3.45 397 5.07 1370 261 6.11 1.10 1.66 1.20 0.83 1.13 61 52
Long 4 18.29 -1.006 26 3.7 176 5.9 567 400 8.85 4.15 1.86 1.30 0.83 1.07 61 50

• All designs have 80 images sizes of lt 0.2
  arc-sec in the V-R-I bands and lt 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

28
Adjustments during filter exchanges
Fine focus adjust
Coarse focus adjust

• Separate filter for each band
• All filters have same radii of curvature 5.6 m
  concave and convex
• Filter central thickness varies from 22 mm to
  13.5 mm, B to Z bands
• Camera assembly adjustment range of 2.42 mm
• L2 adjustment range of 1.86 mm
• 7.8 mm lens shift 1.0 mm focus
  shift

29
Camera assembly and lens L2 are moved after each
filter exchange Camera assembly can be coarse
adjustment 0.025 mm and L2 can be fine focus
adjustment 7.8 mm lens shift 1.0 mm
focus shift
Spectral band Wavelength nm Instrument motion (mm) L2 motion (mm) Filter thickness (mm)
U optional 357.7 / - 32.3 -2.984 -1.953 -26.6
B 436 / - 49.5 -1.609 -1.200 -22.0
V 537 / - 47 -0.599 -0.445 -18.5
R 644 / - 75.5 0.000 0.000 -16.4
I 807.5 / - 75 0.563 0.438 -14.4
Z 940 / - 100 0.811 0.658 -13.5
30
Summary

• The Strawman design uses reasonable compromises
  to achieve a full diameter field of 3.5 degrees
• Strawman design now uses spherical lenses
• L1 is 1.6 m in diameter vs. 1.54 m in May design
• Secondary asphericity increases from 38 microns
  to 85 microns
• Simple null tests for lenses
• Simple verification for corrector lens assembly
• Simple verification for stand-alone three-mirror
  telescope
• Optical performance of Strawman design is good
• Image size of lt0.22 arc-second is achieved for
  V-R-I bands
• Integrated throughput is 56.5
• Geometrical etendue excluding spider loss 300
  m2deg2
• Expected 90 detector fill and 85 coating
  transmission give effective etendue excluding
  spider loss 230 m2deg2