If you are very lucky you may take a
photograph of the night sky and find an object in the image that does not
appear in the sky catalogues. Perhaps it is an asteroid or you may have
discovered a new comet or nova?
your discovery to the world you need to be able to tell people what the
coordinates are so that they can verify your observation.
Numerous software packages allow this to be done, but some are
expensive, and others are very fussy in only allowing FITS format images and
you have to tell the software several difficult to find properties of the
Googling 'astrometry' produced very
little helpful information until I found
this site. At
last there was some mathematics that was not beyond my
I set about writing an Excel
spreadsheet that would take the coordinates of three known stars on the image
plus the pixel positions of the stars and from that the coordinates of any
other object on the image could be derived.
needed were the coordinates of the centre of the image. This does not
have to be very accurate, and you could probably use the coordinates of the
star nearest the centre, but there is an easy way to find an accurate value.
Simply join 'Flickr' which is a site for
sharing photographs. You then join the 'astrometry group'. Any picture
that you add to the astrometry group will be analysed and you will be told the
exact centre coordinates, and also the significant objects present in the image
will be identified. Field width and arcseconds per pixel are also found. You do
not need to tell it what scale or rotation you have. Quite
first attempt produced a speadsheet that gave very accurate RA values, but the
DEC values were always badly wrong. Checking the maths and the spreadsheet
formulae numerous times produced no better result. Fortunately there was an
email address on the site and Dr Kevin McLin kindly replied. There were indeed
errors in the equations which were duly corrected.
Two points worth noting: a) the internet is sometimes wrong, b)
don't be too shy to send off an email!
Below is one of the images used for
testing the spreadsheet. The bright star is Alhena, correctly identified by
Flickr, and the centre coordinates are displayed to an alarming number of
decimal places. Coordinates for three stars were found from Cartes du Ciel.
Pixel positions were measured with Photoshop. An important point is that
Photoshop puts (0,0) at the top left. Astrometric methods require x,y pixel
measure measurements relative to the centre of the image so a correction has to
star was at position (2028.63, 679.69).
derived coordinates were:
Cartes du Ciel gave the star coordinates as:
||6h 37m 58.06s,
16° 18' 29.47''
6h 37m 58.09s, 16°
may have noticed that four stars have coordinates displayed. This was to
website which requires four marker stars. The accuracy of the three star
spreadsheet was just as good as the four star method.
A web enabled version of the
spreadsheet is available here. To use
it, click in the blue cells already
containing numbers, and type your new values. There is no need to delete the
Press TAB to move from
cell to cell.
Flickr gives values for the centre
of the image in the form 67.4865347, 34.54667.
To convert to format 4h 29m 56.77s, 34° 32' 48.01'' use
the convertor at the right of the spreadsheet.