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

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

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

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

HMI Optical Layout

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)

Optical System

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 )

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)

Telescope 15.0 cm diameter aspheric objective lens BK7G18 radiation resistant glass Conic constant ~ 0.70321 2.6 cm diameter secondary lens (plano-concave) SF6G05 radiation resistant, high index Focal Length = 246.5 cm Focal Ratio = f/17.6 Primary – secondary separation = 48.64 cm Separation change with temperature (invar) = 0.778 micron / °C Separation change equivalent to one focus block step is 15.9 microns ( Strehl decreases from 1.00 to 0.95 )

Focus Blocks Focus blocks are fused silica flats, AR coated Figure in transmission < 0.02  RMS at 632.8 nm Parallelism of faces < 0.5 arc minutes Each focus wheel contains three blocks and an empty position first wheel: 10.2 mm, 6.8 mm, 3.4 mm, and empty slot second wheel: 2.55 mm, 1.70 mm, 0.85 mm, and empty slot Provides 16 focus increments

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

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

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 < 0.02  RMS at 632.8 nm (made to standard test plates) Lyot filter into Michelsons telecentric lens beam control lens

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

independent camera shutters Final Beamsplitter Beamsplitter divides the beam 50-50 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

Calibration lenses

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

HMI Optics Sensitivities

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

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