How to size a Newtonian Diagonal

Because Newtonian secondaries are made in certain specific minor axis sizes, the exact formulae determined by the late Ralph Dakin based on geometric optics, although fine for calculating the offset, are unecessary when it comes to selecting an appropriately sized flat.

If we take the following, and ignoring the sagittal depth of the primary:

a = separation flat to prime focus
d = flat minor axis
F = primary focal length
D = primary clear aperture
I = linear field of full illumination

then we deduce:

& from focal ratio:


and putting:
                   where w = working distance (inches)*

*working distance is the distance of the Newtonian focal plane from the tube wall.

we obtain:

It then becomes possible, using equation (3) to plot the linear field diameter of full illumination against the corresponding working distance for any specific flat minor axis, aperture and focal ratio. I have prepared charts for
6-inch; 8-inch; 10-inch & 12-inch apertures, and f/ratios f/4; f/6; f/8; f/10 & f/12.

Select the chart that most closely matches your aperture and f/ratio. Then select an appropriate working distance, and diameter of fully illuminated field of view, and find the corresponding flat minor axis. The size you order or make should not be smaller than the size given from the chart.

The working distance you choose depends on the type of accessories you intend fitting to the focuser, and whether the focuser is a low profile helical or Crayford type or a rack and pinion. The available back focus is the difference between the working distance and the height of the focuser.