Astrometry Project
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?
Before announcing 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 image.
Googling 'astrometry' produced very little helpful information until I found this site. At last there was some mathematics that was not beyond my capability!
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.
Also 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 miraculous!

The 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 be made.

The unknown star was at position (2028.63, 679.69).
  The derived coordinates were:
Cartes du Ciel gave the star coordinates as:
6h 37m 58.06s, 16° 18' 29.47''
6h 37m 58.09s, 16° 18' 28.60''
  You may have noticed that four stars have coordinates displayed. This was to test this 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 previous number.
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.