Example FITS file GONG Header

B0: Solar latitude of the sub-earth point in degrees.

CD1_1: Conversion factor for x-axis. To get an initial estimate for the location of the solar image on the CCD, the limb finding algorithm first computes the centroid of the image.

CENTROXC: is the x-component of the centroid.

CENTROYC: is the y-component of the image centroid.

C_MA: This is an improved estimate of the major axis length as determined using HGEOM (Hankel Geometry, see Toner & Jefferies, 1993, Ap.J. 415, 852-861.

C_MI: This is an improved estimate of the minor axis length as determined using HGEOM (Hankel Geometry).

FILLED: Because of the way the GONG Project processes its imaged data, every minute (or time step, if observations are being recorded at some other cadence) needs a place holder. Therefore, if an image is determined to be "bad" for some reason, it is flagged by inserting the keyword FILLED. Originally, FILLED = no meant the image was bad, and FILLED = yes meant that the image had been replaced by some other image, or an interpolated image was used (however, interpolating to replace bad images was never actually implemented). The upshot of all of this is that any user of GONG images should first check for the existence of the keyword FILLED, and if it exists, consider that image as bad and go on to the next one.

FNDLMBXC: The x-coordinate of the center of the solar disk (in pixels). Zero is the leftedge of the left-most column of the image. Therefore, 0.5 is halfway through the first column and 1.0 is at the right edge of the first column/left edge of the second column. There is negligible dead space between columns.

FNDLMBYC: The y-coordinate of of the center of the solar disk (in pixels). Zero is at the bottom edge of the bottom row, 0.5 is half way through the bottom row, 1.0 is at the top edge of the first row/bottomedge of the second row.

FNDLMBMI: The semi-minor axis length (in pixels) estimated by fitting the limb of the solar intensity image with an ellipse.

FNDLMBMA: The semi-major axis length (in pixels) estimated by fitting the limb of the solar intensity image with an ellipse.

FNDLMBAN: The position angle of the fitted ellipse in degrees measured counter-clockwise from the positive y-axis (as shown below). Estimated Geometry Errors

The keywords XC_ERR. YC_ERR, MA_ERR, MI_ERR, AN_ERR, CMA_ERR, CMI_ERR are the estimamted, 1-sigma, errors on: FNDLMBXC, FNDLMBYC, FNDLMBMA, FNDLMBMI, FNDLMBAN, C_MA, and C_MI. The units are pixels for all but AN_ERR which is in degrees.

Processing history.

L0: Solar longitude of the sub-earth point in degrees.

OFFSET: Angle between the x-axis of the camera and solar pole in degrees.

PIXLENX: The pixel length along the x-direction. The units are arbitrary. PIXLENX (and PIXLENY) were introduced in order to perform the image remapping properly on GONG Classic data (which had rectangular pixels: The smaller dimension was defined to be 1.0, thus making the other dimension 1.2778. For GONG+ data both pixlenx and pixleny equal 1.0

PIXLENY: The pixel length along the y-direction.

P_ANGLE: Solar pole-position angle in degrees.

SEMIDIAM: Semi-diameter of the Sun in radians.

TS_START: Date-time of the start of a time series.

TS_END: Date-time of the end of a time series.

X_CENTER: This keyword is entered at the site by the instrument. It is a rough estimate for the x-center of the image and is used to help keep the image centered on the CCD. Do NOT use this keyword!!

Y_CENTER: This keyword is entered at the site by the instrument. It is a rough estimate for the y-center of the image and is used to help keep the image centered on the CCD. Do NOT use this keyword!!

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