1. Optics Alan Title, HMI-LMSAL Lead, title@lmsal.com
HMI00423A
rev 2003Nov12
Optics
Alan Title, HMI-LMSAL Lead,
Rick Rairden, Optics Lead,

2. Agenda – Optics PDR
Scope: This review covers the lenses, mirrors, and beamsplitters in the imaging system. Broad and narrow band filters are covered in a separate section of the PDR.
Agenda
Optical system requirements
MDI heritage
Optical System
Descriptions of each element
Tolerance approach
Summary
Design status
Next steps

3. Performance Requirements
1 arc-sec diffraction limited Full Sun image at the science sensors
Focus adjustment range of 16.4 mm in equal steps
A full disk solar image for image stabilization via limb sensing
A telecentric beam through the Lyot filter for minimum spectral field effects
Optical system unvignetted
Separate science focal planes for Doppler and vector field measurements
A spectral calibration mode that operates on unimaged sunlight

4. HMI Optics Heritage from MDI
The HMI optical design features common with MDI:
Simple two element refracting telescope to form primary image
Image Stabilization System with a solar limb sensor and PZT driven tip-tilt mirror
The HMI optics improvements from MDI:
Larger aperture, higher spatial resolution
4096x4096 pixel CCDs instead of a single 1024x1024 pixel detector in MDI in order to properly sample the spatial resolution over the entire field of view
Two science focal planes to make both Doppler and vector magnetic field measurements at the required cadence and to provide redundancy

5. HMI Optical Layout

6. Top Level Design Properties
Top Level Optical Design Properties
System focal length 495 cm
System focal ratio f/35.2
Final image scale 24 microns / arc second
Re-imaging lens magnification 2
All lens surfaces (except the primary) are spherical and made to stock test plates
Optical design and analysis done with Zemax® software
( optical system shown unfolded )
Lyot
Michelsons
~ telecentric section 
Re-imaging
CCD
Telescope
Image plane
Shutter
(at pupil)

7. Optical System

8. Physical Layout of Optical Elements
Layout to scale
folding mirror
to CCD
to CCD
polarizing beamsplitter
Limb sensor
Lyot
Oven assembly
active mirror
Telescope assembly
100 cm
( dimension shown for scale )

9. Optical Elements
Full Aperture Filter (filters are discussed in a separate section)
Primary Aspheric Objective Lens
Secondary lens
Focus blocks (6)
Calibration lenses (2)
Polarizing beamsplitter
Blocking Filter
Telecentric Lens
Lyot Filter
Beam shaping lens
Michelson Filter
Reimaging lenses (2)
Beamsplitter
Folding mirrors (4)

10. Telescope 15.0 cm diameter aspheric objective lens
BK7G18 radiation resistant glass
Conic constant ~
2.6 cm diameter secondary lens (plano-concave)
SF6G05 radiation resistant, high index
Focal Length = cm
Focal Ratio = f/17.6
Primary – secondary separation = cm
Separation change with temperature (invar) = micron / °C
Separation change equivalent to one focus block step is 15.9 microns
( Strehl decreases from 1.00 to 0.95 )

11. Focus Blocks Focus blocks are fused silica flats, AR coated
Figure in transmission <  RMS at nm
Parallelism of faces < 0.5 arc minutes
Each focus wheel contains three blocks and an empty position
first wheel: mm, mm, mm, and empty slot
second wheel: mm, mm, mm, and empty slot
Provides 16 focus increments

12. Strehl ratio vs focus block selection
1.00
Focus steps are small enough to provide effectively continuous focusing through the required range
image in focus
0.96
0.98
plus 1/2 focus block
minus 1/2 focus block

13. Polarizing Beamsplitter Assembly
Divides the telescope beam between the science optics and limb sensor
Reflections from blocking filter and limb stabilization sensor are redirected into a light trap ( 1/4 waveplates )
1/4 waveplates
reflected light from limb sensor
to limb sensor
polarized beam to oven
light trap
reflected light from blocking filter
beam in

14. Telecentric and beam control lens group
Telecentric lens provides telecentric beam into the Lyot filter
Beam shaping lens converges rays so that extreme rays are parallel to the optic axis in order to pass unvignetted through the Michelson interferometers
Lenses are fused silica, AR coated
Wavefront error <  RMS at nm (made to standard test plates)
Lyot filter
into Michelsons
telecentric lens
beam control lens

15. Re-imaging Lens Group
Lens system magnifies primary image to achieve required scale on the CCD cameras
Fused silica, AR coated
Located at exit of temperature controlled oven
Magnification 2
Provides effective focal length of 495 cm ( f / 35.2 ) for spatial scale of 0.5 arcseconds per 12 micron pixel
Despace sensitivity +/– 0.65 mm for equivalent to one focus block
~ 680 mm to CCD
~ 28.6 mm beam diameter

16. independent camera shutters
Final Beamsplitter
Beamsplitter divides the beam to the two cameras
fused silica, AR coated, non-polarizing
independent camera shutters
operate in the planes
of image pupils
1/4 waveplate
light trap

17. Calibration lenses

18. Tolerance Development Approach
Determine manufacturing requirement on quality of optical elements
Determine sensitivity of optical elements to initial placement
Determine sensitivity of optics to temperature
Determine temperature-induced tolerances on position
Determine sensitivity of image stability to rotational position of moveable optics
IN PARALLEL
Do trade studies to assign tolerances for :
Each optical component
Positional stability of each mount
Adjustment range of each mount
Temperature range of optics package
Temperature stability of optics package
Run out characteristics of each rotating mechanism
Preliminary estimates are based on the successful MDI optical system

19. HMI Optics Sensitivities

20. Field Curvature and Scale
Field curvature is insignificant – Strehl ratio stays above 0.99
Field Flatness
Magnification Variation

21. Summary The HMI Optical System design Design status is well understood
borrows from MDI/SOHO heritage
meets the performance requirements
physically fits within our package dimensions
can be fabricated to meet our specifications
Design status
layout currently being refined to accommodate calibration mode
drawings for each element, with specifications, are in work
considering same vendor as MDI for telescope lenses (Brashear)
a few vendors under consideration for beamsplitters
several vendors are capable of making simple lenses and mirrors