The High Altitude Observatory (HAO) at the National Center for Atmospheric Research (NCAR) The National Center for Atmospheric Research is sponsored by the National Science Foundation. Any opinions, findings and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. COSMO Large Coronagraph Preliminary Design Review Optical Design Dennis Gallagher National Center for Atmospheric Research Boulder, Colorado November 16-17, 2015

The key design requirement for a coronagraph is to keep stray light to a minimum. (<5 ppm) Bernard Lyot described a simple design that achieved this requirement back in 1930 –Use a simple singlet lens (super-polished), no coatings (pinholes) –Block the solar image with an occulter. –Reimage the objective and block diffraction artifacts due to finite apertures in the system (Lyot aperture stop) –Find a location with skies brightness <1-2 ppm (mountain tops) COSMO Optical design

A mirror will have 8 times more scattered than a lens with similar polish. Mirror vs. Lens O1. Lens Mirror

Optical design parameters for the filter graph Note :The coronagraph O1 lens and other optics will transmit out to ~2000nm, so COSMO can be used to work beyond 1083nm

COSMO-LC is Basically a Lyot coronagraph Designed by Zhen Wu, Nanjing Institute of Astronomical Optics & Technology and modified slightly by Dennis Gallagher, HAO Solar image (Occluted) Lyot Stop Pre filter Modulator Polarizing Beam Splitter Lyot Filters Focal planes O1 Aft optics 7.5 meters

Occulter O1 focus Lens #1, #2 pair Lens #3 Lens #4 Lens #5 Lens #6 Lyot Stop Lens #7- 11 Band Pass Filter Modulator Polarizing Beam splitter LiNbO3 Lyot Filters Sensor 12 powered lenses All lenses made from common Ohara and Schott glasses All aft optics use spherical/flat surfaces Aft Optics 2.1 meters

To O1 Concept of Operation Solar focus to occulter 188 mm range Focus of O1 on Lyot stop 15.2 mm range Optics sit on three “stacked” linear stages to adjust system to operate from nm Final focus 3.5 mm range Variable occulter change in diameter mm diameter. 5% oversize 960 arc- sec sun.

1.5 meter O1 Lens F/ meter EFL Corning HPSF 7980 high purity synthetic amorphous silicon dioxide Aspheric surface (~.5mm departure from best fit sphere) Radius= mm Conic= Spherical surface Radius=24650 mm Inclusions per any 100cm^3 volume < 0.03 mm^2 ; Max size <0.1mm This is similar to the best BK7 glass Homogeneity 3 ppm (maxium of ~ 0.5µm surface corrections for variations in lens blank index)

Optical performance at oculter 5% over sized occulter edge is mm beyond image of sun O1 must have an aspheric surface to meet COSMO requirements 200 µm 10 mm Asphere All Spherical 200µm vs. 10mm scale

Light spill past occulter. Only a range of wavelengths can be blocked by the oculter at a time Blue focus Red focus Yellow focus Requires band pass filter to have excellent out of band rejection < This was achieved for K-coronagraph. occulter

Focus of O1 at Lyot Stop 530nm 656nm 1083nm 200µm Lyot stop is a fixed aperture of 90.1 mm or 85% of the 106mm image of the O1 Magnification of O1 is X to within 0.67% from 530 to 1083nm.

Final focus ~3400mm EFL COSMO is not required to be a diffraction limited system 530±0.3nm 656±0.4nm 1083±0.6nm 530nm 656nm 1083nm Monochromatic Lyot filter will scan over narrow wavelength range for Doppler analysis while all COSMO-LC focus stages remain fixed. 40µm

Other Optical MTF through field 1 X Nyquist (2 pixel) 2 X Nyquist 4 X Nyquist 15 µm pixel Distortion <0.1% Flat field Ensquared energy 88%

Performance over wavelength The COSMO-LC final focus image is uniform for all wavelengths from 530 to 1100nm

Optical tolerances One pixel Optical design has essentially 100% probability of meeting 84% energy in 2X2 arcseconds region (33X33µm or 2.2X2.2 pixels) when tolerances are met.

Largest error contributors Lens #1 Lens #11.04% radius.08% radius Group 1 Group 2 Group 3

The High Altitude Observatory (HAO) at the National Center for Atmospheric Research (NCAR) The National Center for Atmospheric Research is sponsored by the National Science Foundation. Any opinions, findings and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Scattered Light Dennis Gallagher

Sources of scatter light –Surface roughness of the O1 (super polish) –Dust on the O1 surface (Keep lens very clean) –Sky scatter (Observe from high altitudes) After the occulter the optics in a coronagraph can have average surface polish with little impact to overall stray light. –The condition of the O1 dominates instrument stray light Internally occulted Coronagraphs and scattered light

Surface Roughness K-Cor O1 Data taken with the 4D Technologies Nanocam Interferometer Average of 10 data point from both side of the optic 7A RMS for spatial periods mm

K-Cor BSDF and Surface PSD Harvey B0=12 L= (50 arcsec) S=-2 7 Å RMS

FRED optical model Traced 63 million rays

Dust dominates scattered light over time We looked at the accumulation of dust on the MK4 coronagraph lens by monitoring increased scatter before and after cleaning of the O1 Lens. Used the MIL-1246C dust distribution model to match the scattering seen in MK4 data. Scattering from Dust particles

MK4 Total brightness before and after cleaning of the O1 lens

COSMO requirement <5ppm at 1.1 Solar radius

An optical design for COSMO shows it will meet the COSMO science requirements. The special C7980 material for the O1 lens is available from Corning Arizona Optical Systems is currently fabricating meter class C7980 lenses for LSST. Tolerance analysis shows the optical system can be fabricated. Using existing conditions from the K-coronagraph at MLSO, stray light from surface polish and dust can be maintained to levels required for COSMO operations. Conclusion