Daniel B. Seaton1 · Amir Caspi1 ·Nathalia Alzate2,3 · Sarah J. Davis4 ·Alec R. DeForest · Craig E. DeForest1 ·Nicholas F. Erickson1 · Sarah A. Kovac1 ·Ritesh Patel1 · Steven N. Osterman1 ·Anna Tosolini1 · Samuel J. Van Kooten1 ·Matthew J. West1© The author(s) ••••Abstract We present results of a dual eclipse expedition to observe the solar corona from two sites during the annular solar eclipse of 2023 October 14, using a novel coronagraph designed to be accessible for amateurs and students to build and deploy. The coronagraph (CATEcor) builds on the standardized eclipseobserving equipment developed for the Citizen CATE 2024 experiment. The observing sites were selected for likelihood of clear observations, for historic relevance (near the Climax site in the Colorado Rocky Mountains), and for centrality to the annular eclipse path (atop Sandia Peak above Albuquerque, New Mexico). The novel portion of CATEcor is an external occulter assembly that slips over the front of a conventional dioptric telescope, forming a shadedtruss externally occulted coronagraph. CATEcor is specifically designed to be easily constructed in a garage or “makerspace” environment. We successfullyobserved some bright features in the solar corona to an altitude of approximately 2.25R⊙ during the annular phases of the eclipse. Future improvements to the design, in progress now, will reduce both stray light and image artifacts; our objective is to develop a design that can be operated successfully by amateur astronomers at sufficient altitude even without the darkened skies of a partial or annular eclipse.
Our hope was that flat field images obtained with the coronagraph assembly attached would help correct for the instrument vignetting function, but we found that neither of our two sets of images provided a consistent measurement of both camera flatness and instrumental vignetting function, so we elected to neglect the latter effect during image calibration. The diffuser-only flats provided an adequate characterization of anisotropy from pixel-to-pixel in the camera, and we used these to flatten the camera response for each image. A few dust particles in the telescope led to artifacts resulting from local imaging of the instrument aperture throughout the images, but the difference between the aperture with the SOLA: manuscript.tex; 3 April 2024; 2:04; p. 12
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Observing the Corona at an Annular Eclipse 10:2310:2510:2710:2910:3110:3310:3510:3710:39 Figure 6. Occulter images showing changes in the background brightness around the time of peak eclipse, in a single polarization channel from our Colorado site. Solar north is approximately 27 counter-clockwise from vertical. Sun behind the occulter and without the occulter assembly was too significant for the diffuser flats to correct, and these artifacts remain in the images.
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We found that the progression of the eclipse led to complex and rapidly evolving artifacts, resulting from the narrow crescent of the Sun illuminating the coronagraph. Figure 6 shows how the pattern of background brightness evolved as the eclipse progressed at Loveland Pass, highlighting how the changing crescent Sun caused changes in the overall background brightness we observed. Because there was no annularity in Colorado, the Sun always appeared as a significantly asymmetrical crescent there, and the images showed a strongly anisotropic pattern of stray light as a result.
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Our hope was that the changing background brightness from the occulter might permit the separation of stray light resulting from diffraction at the occulter edge and other reflections from the corona itself, but due to the complexity of the changing pattern this work is still ongoing. For our preliminary analysis, we therefore focus on the observations from Sandia Crest, where, during annularity the stray light in the images was distributed more isotropically around the occulter, resulting in fewer artifacts within the images and simplifying the analysis and interpretation of the data. Additionally, with a factor of two more obscuration of the Sun, the sky brightness was an equivalent factor darker as well. SOLA: manuscript.tex; 3 April 2024; 2:04; p. 13
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