Angle Measurement Definition
Different image formats are displayed with their origin in different relative locations in the image window. The convention for FITS format is to display the image with the origin in the lower left corner but other formats display the origin in the upper left corner. This means that angles are measured differently in FITS format and other formats. The convention for graphics software is to measure positive angle going clockwise from the positive x axis but it may be different for a FITS image that does have or does not have a World Coordinate System Calibration. Therefore, to accommodate all conventions, angle measurements follow a different sense when measured in different format images:
If
the image has a World Coordinate
System calibration, angle as a Position Angle (PA)
beginning at North=0 and increasing counterclockwise through
East.
If
the image has no WCS calibration or it is from a graphics format
such as TIFF, BMP, or JPG, the angle is measured in the
trigonometric sense starting at 0 degrees pointing to the right and
90 degrees pointing in the direction of increasing row number. A
FITS format image is required by the FITS standard to be displayed
in the traditional Cartesian sense with x to the right and y
upward. Therefore, angle is measured with 0 degrees pointing right
and 90 degrees pointing upward.
FITS Format requires angles to be specified in units of decimal degrees, like 45.53256. However, your script may use degrees or some other format. Since lua converts between strings and numbers, you could specify an angular value in degrees as either 45.53256 or "45.53256." Do not specify angles in DMS or HMS format, like "-35:12:45.33" or "2:05:15.9" unless the specific function expects it. Your script may use whichever units are appropriate, but you must use numeric degrees when sending or receiving coordinates between a script and Mira. Several conversion functions are available; see Date, Time, and Angle Functions.
Pixel Coordinate Definition
Subpixel Coordinate Definition
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