Enter for each star its RA Dec, and the x y pixel coordinates of its image centroid

The calculator determines a mathematical transformation between x,y pixel coordinates in your image and RA,Dec sky coordinates. The calculator uses the Right Ascension (RA) and Declination (Dec) coordinates and the x,y pixel coordinates of a set of stars in the image to determine this transformation.

You need accurate RA, Dec coordinates for the stars, taken from an accurate stellar catalog (e.g., the Hubble Space Telescope Guide Star Catalog (GSC), or the US Naval Observatory Astrometric Standards Catalog (USNO-A2.0), the latter is the recommended catalog since it is available on the web with a number of useful utilities --- see the explanation).

For all RA, Dec entries stick with coordinates for one "epoch" --- "2000" coordinates, for example. Then the resulting coordinates for the object will be in that coordinate system.

For the x, y coordinates you need to determine accurate pixel coordinates (to a fraction of a pixel) of the "center of light" (the centroid) of the star's image. To determine these centroid coordinates you can use SIP's Instrumental Magnitude and Centroid selection under the Analyze menu item.

The RA, Dec format is hh mm ss.sss... ±dd mm ss.sss...; see the example entry. The hh mm ss.sss... values are the RA, where hh is the number of hours (e.g., 23, or 04), mm is the number of minutes (e.g, 31), and ss.sss... is the number of seconds, including any decimal fraction of a second (e.g., 34.44, or 42.56345). The ±dd mm ss.sss... values are the Dec, where ±dd is the number of degrees, including a sign (e.g, +14, or -23), mm is the number or arcseconds (e.g., 35, or 12), and ss.sss... is the number of arcseconds, including any decimal fraction of an arcsecond (e.g, 23.45). The only rule in entering these values is that they each value must be separated from the others by one or more spaces.

The x, y entry format is xx.xxx... yy.yyy... (e.g., 327.67 123.45). Once again, the only rule is that these two values must be separated from each other by one or more spaces.

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