WILLIAMS COLLEGE ECLIPSE
The NEWKIRK camera which Jonathan Kern designed and built to obtain these images uses a 4" quartz/fluorite doublet of 60.25" f.l. fed by an 8" quartz coelostat. Below are unprocessed exposures obtained through the radially symmetric, neutral density filter located in the focal plane which compensates for the steep decline in coronal radiance with increasing distance. (the NEWKIRK filter, after Gordon Newkirk, former director of the High Altitude Observatory). It is fabricated by evaporating a metal film onto glass in a high vacuum. The basic requirement of such a filter is that it compensate as accurately as possible for the radial decrease in brightness as one goes away from the limb of the sun in the film plane of the telescope. Thus, the transmission should vary as the reciprocal of the function describing the K+F+sky brightness. This calls for an optical density of 10 -3 at the limb (10 f/stops, or a factor of 1000) decreasing very rapidly to a density of near unity at 4 solar radii. (See curve) The solar image at focus was .570" in diameter. The plate scale is .0175"/ minute of arc, yielding a photographic field of 2.12 by 2.68 degrees on 120 format roll-film (image size 2.23" x 2.81") . The central spot you see in each image is the calibration window, which remains centered on the sun. Because there is perceptible motion of the moon with respect to the sun, the NEWKIRK filter may appear decentered. It is not. These images are also side-reversed, because of the single reflection introduced by the coelostat mirror.
Click on each image for a full screen view
Density function of NEWKIRK filter