Convert X/Y Offsets to RA and Dec

(Latest Revision: 8 May 2009)

This is a tool to convert rectangular offsets to RA and Dec, given a "central" coordinate (for e.g. a cD galaxy in a cluster) to which the offsets are referred. It assumes a flat field with a fixed plate scale. Field rotation is also accommodated, as is sign reversal (e.g. offset increasing to the south).

Data Entry

Enter the right ascension and declination of the reference position. Positions may be specified in sexagesimal or decimal mode in most of the usual notations. NED's Input Position Interpreter (below) has more information on how positions are interpreted and displayed in NED.

Alternatively, enter a valid object name in the second box. If there is an object name entered, it will take precedence over any coordinates in the first box.

Set the radio buttons for increasing X- and Y-offsets, then enter a rotation angle if necessary. A positive angle indicates counterclockwise rotation, while a negative number specifies clockwise rotation.

Choose your image scale in arcseconds, arcminutes, or degrees. For example, for an image scale of 0.22 arcsec/pixel, enter "0.22" in the left box, and select "arcseconds" from the menu in the right box.

If your units are already in arcseconds, arcminutes, or degrees, use "1.0" in the image scale box and choose the appropriate units from the menu in the right box.

Finally, enter your X,Y offsets in either the "X-offsets" and "Y-offsets" boxes, or in the "Comma-separated X,Y offsets" box. If you use comma-separated values, you may append labels if you wish.

Clicking "Submit" will do the calculations, and will return a page showing your input, the output coordinates, and a schematic of the field around your input central coordinates.

NED's Input Position Interpreter

All of the following examples are legal input. In some cases of ambiguous input, the output may not be what you intend, so the interpreter returns a warning message before returning its interpretation. If you use decimal hours or decimal degrees in the RA field, we suggest noting this explicitly as in "14.3256h" or "214.8840d". The examples below show the default behavior of the interpreter.

16.3678 16 hr 22 min 04.08 sec (if in "longitude" field with equatorial RA set) or
16.3678 degrees (otherwise)
16 16 hr (if in "longitude" field with equatorial RA set) or
16.0 degrees (otherwise)
6h3m23s 06 hr 03 min 23 sec
26h7m23s 02 hr 07 min 23 sec (where 02 hr = 26 - 24)
79d23m90s +79 deg 24 min 30 sec
20 9 50 20 hr 09 min 50 sec (if in "longitude" field with equatorial RA set) or
20.16388 (otherwise, with warning message)
20d9'50" +20 deg 09 min 50 sec
20:9:50 20 hr 09 min 50 sec (if in "longitude" field with equatorial RA set) or
20.16388 (otherwise, with warning message)
20 9 20 hr 09 min of time (if in "longitude" field with equatorial RA set) or
20.15 deg (if in "longitude" field, with warning message) or
20 deg 09 min (if in "latitude" field)
20:9 Same as "20 9"

Return to "X-Y Offset to RA/Dec Conversion" page.
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