Converting arcseconds to radians

Well, 180 degrees = pi radians, so 1 degree = pi/180 radians.

1 degree = 60 arcminutes. Each arcminute is made up of 60 arcseconds, therefore 1 degree = (60)(60), or 3600 arcseconds.

Equating the above, you now know that 3600 arcseconds = pi/180 radians. Divide both sides by 3600 (to get 1 arcsecond) and you'll get 1 arcsecond = pi/[(180)(3600)], or 1 arcsecond = pi/648000. To get that in a decimal form, just divide pi by 648000 on your calculator. You can get increasingly small amounts by dividing that quantity even further. (So, for .001 arcseconds, divide pi/648000 by 1000, for example.)