The COSMO coronagraph optical design and stray light analysis Dennis Gallagher*a, Zhen Wub, Brandon Larsona, Peter G. Nelsonc, Phil Oakleya, Scott Sewella Steven Tomczyka aThe National Center for Atmospheric Research High Altitude Observatory-Boulder, Colorado, bNanjing Institute of Astronomical Optics and Technology- Nanjing, China, cSierra Scientific Solutions- Boulder, Colorado ABSTRACT The Coronal Solar Magnetism Observatory Large Coronagraph (COSMO-LC) is a 1.5 meter Lyot coronagraph dedicated to measuring magnetic fields and plasma properties in the solar corona. The COSMO-LC will be able to observe coronal emissions lines from 530-1100 nm using a filtergraph instrument. COSMO-LC will have a 1 degree field of view to ob- serve the full solar corona out to 1 solar radius beyond the limb of the sun. This presented challenges due to the large Etendue of the system. The COSMO-LC spatial resolution is 2 arc-seconds per pixel (4k X 4k). The most critical part of the coronagraph is the objective lens that is exposed to direct sunlight that is five orders of magnitude brighter than the corona. Therefore, it is key to the operation of a coronagraph that the objective lens (O1) scatter as little light as possible, on order a few parts per million. The selection of the material and the polish applied to the O1 are critical in reducing scattered light. In this paper we discuss the design of the COSMO-LC and the detailed design of the O1 and other key parts of the COSMO-LC that keep stray light to a minimum. The result is an instrument with stray light below 5 mil- lionths the brightness of the sun 50 arc-seconds from the sun. The COSMO-LC has just had a Preliminary Design Re- view (PDR) and the PDR design is presented. Keywords: sun, coronagraph, corona, scatter, dust, lens 1. INTRODUCTION The Coronal Solar Magnetism Observatory Large Coronagraph (COSMO-LC) is a 1.5 meter aperture coronagraph that will observe the solar corona from 1.05 out to 2.0 solar radii. The wavelength span for COSMO-LC is (530-1100 nm) which includes key coronal emission lines such as Fe XIV 530.3nm, Fe X 637.4nm and near IR lines such as Fe XIII 1074.7nm, and the He I 1083.0nm chromospheric line. The High Altitude Observatory (HAO), a division of the National Center for Atmospheric Research in Boulder, CO currently operates two 20cm coronagraphs, the white light COSMO-K Coronagraph [1] and the Coronal Multi-channel Polarimeter (CoMP) instrument [2] on Mauna Loa on the big island of Hawaii. These coronagraphs observe the solar corona from 1.05 solar radii out to 3.0 solar radii. A very preliminary de- sign of the COSMO-LC was presented in 2012 [3]. HAO has performed several design trades and studies on various aspects of the COSMO-LC. The work presented in this paper is an extension of one of these studies that explores COS- MO-LC and stray light [4-5]. A key design requirement for a coronagraph is to reduce stray light in the instrument. The sky brightness next to the sun, ~1 arc-minute from the solar limb, on a clear day at locations such as Mauna Loa, located at an altitude of 11300ft, can be as low as a few millionths the brightness of the solar disk. Coronal emission lines are highly variable in brightness, but are typically in the range of 5-100 millionths of the brightness of the solar disk at ~ 1 arcminute from the solar limb. Therefore it is important that the design of a coronagraph have instrumental scatter as low as a few parts per million if observations of the corona are to be made with good signal to noise. Practically all the in- strumental stray light in a coronagraph is due to scattered and diffracted light from the O1 objective lens. The bulk of the scattered light is due to small polishing defects, mirco-roughness, and dust on the surface of the objective that are ex- posed to the direct sunlight. Diffracted light is controlled though the design of the Lyot coronagraph. In a coronagraph, an occulter blocks the intense light from the image of the solar disk. After the occulter, all the optics in a coronagraph are shadowed from the bright sun and can have simple commercial grade polished surfaces to meet stray light requirements. A key decision in the design of the COSMO-LC was the selection of a reflective vs. a refractive O1 objective for the coronagraph optical system. A design trade is presented here where the result was the selection of a refractive optical system for the COSMO-LC due to the stray light advantages of a lens vs. a mirror system. Ground-based and Airborne Telescopes VI, edited by Helen J. Hall, Roberto Gilmozzi, Heather K. Marshall, Proc. of SPIE Vol. 9906, 990654 · © 2016 SPIE · CCC code: 0277-786X/16/$18 · doi: 10.1117/12.2235234 Proc. of SPIE Vol. 9906 990654-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 08/10/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx In this paper we will discuss scattered light from the optical surfaces of a reflecting and refracting optical system along with scattered light from dust on the optical surfaces. The performance of the COSMO-LC in terms of image brightness due to scattered light from the O1 over the field of view is presented. The design of the main O1 objective lens is pre- sented along with polishing and cleanliness requirements. An overview of the current PDR design of the COSMO-LC is also presented. 2. LENS VS. MIRROR The trade between using a lens or a mirror is a very important one for an instrument with the stray light requirements of the COSMO-LC. Micro-roughness due to residual polishing defects (surface finish) and dust scatter light differently for a mirror and a lens. In this section we describe scattered light from surface finish on a mirror and a lens. We also de- scribe the effects of dust on a mirror and a lens and how dust increases scatted light. For the 1.5 meter COSMO-LC the cost of the lens would be significantly more than for a mirror, both in raw material and fabrication cost, another reason to carefully explore the lens vs. mirror selection of the objective lens for the COSMO-LC. 2.1 Scattering from surface finish The discussion of scattered light from optical surfaces has been well described in the literature. Works by [6] describe a method of relating the surface Power Spectral Density (PSD) to the Bidirectional Scattering Distribution Function (BSDF) with units of 1/sr. This relation is given by Equation 1. The integration of the BSDF is the Total Integrated Scat- ter TIS. Equation 1 BSDF(ß-ß0) = n S ( ) � ∆ � � TIS = (ß-ß0) ß The illumination wavelength is λ, is the incident angle,∫ is the scattering angle, ß-ß0 is the difference between the specular and the scattering angles, and S ( ) is the two dimensional (2-D) PSD of the optical surface. ∆n is the differ- ence between the index of refraction between the two media. For a mirror ∆n=2 and for a typical lens ∆n≈0.5. For the 2 COSMO-LC the incidence and scattering angles are small, so the cosine terms and, are set to ~1 in Equation 1. The ß-ß0 relation is used because scatter is somewhat invariant to incidence angle. Scatter is determined from the specular angle ß0. In our case ß0 0. The relation between BSDF and S ( ) in Equation 1 is valid for surfaces where the RMS roughness is much less than λ. To apply the units used in this paper Equation 1 can be rearranged to give Equation 2 2 where the PSD S ( ) is≅ in units of (Angstroms^2 mm^2) and is in units of Angstroms. 2 BSDF × n Equation 2 S ( )~ Å mm �× � � ∆ = sqrt( S ( ) ) � � The PSD, or S ( ) , is a quantity that can be measured directly∫ using a 2D interferometer. σ is the RMS surface rough- ness. The scale of features on the optical surface scatter light into angles that can be determined by the simple grating 2 equation. The BSDF is a description of how a surface will scatter light through angle. The BSDF of a surface can be found directly by measuring how light is scattered off a surface through angle in an instrument called a scatterometer. These data can then be used to derive the 2-D Power Spectral Density (PSD) for the surface using equation 1. The term 4 n 16 in Equation 1 becomes for a lens surface n=1.5, and for a mirror surface. One can 2 2 2 2 see that for a given surface PSD a mirror will display ~8-10 (n 1.45-1.5) times more scattering than a lens due to sur- ∆ � 4 � 4 � 4 face� finish. This� takes into account that� a lens �has two surfaces where a mirror� has a single � surface. The PSD is a physi- cal description of the optical surface and is wavelength independent.≅ The BSDF, or scattered light, has a 1 dependence. 2 � � � Proc. of SPIE Vol. 9906 990654-2 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 08/10/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx 2.2 Scattering from dust To further explore the comparison between a lens and a mirror we look at how dust adds to scatter light in a lens and mirror optical system. In practice no optical system is free of dust and this is especially true for an instrument located at a remote mountain top.