In Jean Texdreau's Telescope Making book, he publishes a formula but like the Newt software there are considerations that can become well "involved".

You can check some of my old posts here going back several years under crockett.

One consideration is the eyepieces you are going to use or film size to determine your spot illumination. A few years ago the Newt program did not allow for 100% illumination for the full spot. But it may now.

A reasonably good 60° Apparent Field of View Eyepiece will need an illuminated spot the same diameter as its focal length. In other words a 40mm eyepiece with a 2" barrel should have a 40mm illuminated spot.


If that is your goal, layout a cross sectional drawing (back of gift wrapping papper or kraft wrapping paper is OK)without the diagonal, showing the primary mirror and the the focal point with the full spot diameter. Draw a line from each side of the mirror to the corresponding side of the spot. Measure back from the spot along the centerline a distance equal to 1/2 the outside tube diameter plus the hight of the focuser with the drawtube extended minus about 1/2 inch. Draw a line through this point 45° from the centerline. Measuring the length of this new line between the lines extending from mirror to spot will give you the major axis size for the diagonal (close). This will also give you your component spacing. I started on this reply this morning but had ISP problems.