Here you can download Astrocalc, an Excel file (33 Kb) with three worksheets which you can use to perform useful calculations and astronomical tasks.
The first worksheet computes the field covered by a lens or mirror of your choice on a 35 mm film and on a SBIG ST237 CCD camera.
The second worksheet illustrates the optic rules for lens projection, with formulas for both positive (eyepiece) and negative (Barlow lens) projection.
The third worksheet has drawings and practical guidelines to build a Hartmann focusing mask, a tool I find very useful to reach a sharp focus with my 8 inch fast newtonian.
If you want you can download only the lens projection sheet in Jpg format (45 Kb).

Download Astroclc.xls or only Projctns.jpg