coy Camera Offset Equations
The [vmi][cz]i files contain images of the Sun. Usually, the pole of the Sun is approximately aligned (by the camera rotator in the instrument) with the x-axis of the camera image.
The coy files contain coefficients for polynomials (of hour angle) that provide the correction (in degrees) needed to precisely align the pole of the Sun with the x- or y-axis of the camera.
The corrections for all network sites (including the engineering test unit in Tucson) for one network day are in one file. The date for which the correction applies is contained as part of the file name: mrcoyYYMMDD; e.g., mrcoy960318 is the file for March 18, 1996.
The first line in the file contains the date when the file was generated and the name of the program (and version number) that created it.
Site N a0 a1 a2 a3 a4 a5 where Site is the 2-letter site abbreviation N is the number of terms in the polynomial expansion a0 to a5 are the polynomial coefficients (normally columns for a4 and a5 are all zeroes) To compute the camera offset angle (i.e., the direction to Solar North) for a given site and date as a function of time one: 1) select the mrcoy file for the appropriate date, 2) find the site abbreviation in column 1 ("Site"), 3) read the number of coefficients from column 2 ("N") 4) read the coefficients from the next N columns, and 5) then compute: offset(Site,time) = 90.0 - (a0 + a1*HA(Site,time) + a2*HA(Site,time)**2 + ....) where the offset is in degrees measured clockwise from the y-axis and HA(Site,time) is the local hour angle for a given image measured in radians (HA goes from -PI/2 to +PI/2). HA can usually be found as a header parameter in the image data products; e.g. HA = 0.892798 / Hour angle (radians)
The polynomials for the interval between the start of network operations until 950709 are from the reduction of drift scan data acquired at the end of each deployment. Consequently, the polynomial for a given site does not change from one network day to the next.
From 950710 forward, the polynomials are obtained from the COPIPE data reduction stage that compares contemporaneous images from multiple sites by correlating a small annular ring extracted from the velocity images and fixing the polynomials for specific sites on specific dates to the results of the drift scan reductions.
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