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