Newtonian reflecting telescope calculator      

This web form is based on calculations and formulas taken from the French document: newton-fr.pdf or the English translation of this one: newton-en.pdf. It is specifically designed for amateur Newtonian telescope makers. It allows to setup the dimensions and details relevant to the various components of these telescopes. Before starting, it may be useful, or even necessary, to take a look at the online Help.
Version 1.1 (Aldebaran). Contact:    Web site:

Primary mirror diameter (D):Primary mirror focal length (f):
Secondary mirror minor axis (a): Fully illuminated field diameter (Cpl):
Focal plane/Secondary mirror distance (p):Focuser: , input aperture/focal plane (l):
Unit of Measure:       

Primary mirror
Diameter (D): 12.00 inFocal length (f): 72.00 inLuminosity: 1896 xFocal ratio (F): 6.00
Sagitta (z): 0.13 inAngular resolution: 0.39 secApparent magnitude limit: 15Focus tolerance: +- 40 microns

Secondary mirror
Minor axis (a): 3.01 in   Offset: 0.11 ina': 2.99 inPrimary mirror distance (f-p): 59.02 inObstruction: 25.1 %

Fully illuminated field
Cpl diameter: 1.00 in    (low value ~0.57 in)Angular field of view: 47.7 minutesIllumination: 93.72 %
Focuser vignetting: noi=0.77 in     j=-0.79 in     l <= lmax (~6.54 in)

Telescope front aperture
Primary mirror distance ff - 0.79 inf - 1.57 inf - 2.36 inf - 3.15 inf - 3.94 inf - 11.70 inf - 13.00 inf - 14.30 in
Optimal diameter size (pumax)14.00 in13.98 in13.96 in13.93 in13.91 in13.89 in13.67 in13.64 in13.60 in

Off axis % illumination outside fully illuminated field (with obstruction)
Axis distance (inch)0.510.550.590.630.670.710.750.790.830.870.910.940.98
Average illumination (pu=pumax)93.6392.8891.8090.5289.1687.6886.0884.4782.8181.0579.3277.5575.81
Apparent magnitude lost0.
Average illumination (+focuser, pu=pumax)93.6392.8891.8090.5289.1687.6886.0884.4780.7274.2167.5460.6153.55
Apparent magnitude lost0.

Off axis field curvature
Off axis (inch)
Shift (microns)27172739547089109

Off axis length coma
Off axis (inch)
Length (microns)1626425263738394104