Lone Star Observatory Optical System Specifications

(All specifications confirmed by laser interferometery)

Classic Cassegrain Mirrors Configured by Jerry Brunache, Carlsbad, Ca

Primary Mirror
Diameter 24 in/0.61 m
Type of glass Pyrex
Original glass blank thickness 4.25
Final thickness of shaved blank 4.00 in
F/ratio of primary 5.30
Focal length of primary 127.22in/3,231mm
Diameter of center core hole 7in
Weight of cored primary mirror 131.19lbs
Coating HiVac Enhanced Aluminum w/dielectric overcoat
Coating applicator QSP Optical
Percent light reflectivity 96%
Beveled edges Less than 0.125 in
Secondary Mirror
Diameter 8.25in
Type of glass Pyrex
Thickness 1.375 in
Secondary magnification 2.997
Secondary radius of curvature 115.125 in
Conic constant on secondary -3.978
Image size on secondary 8.05in
Weight of secondary 5.51lbs
Coating HiVac Enhanced Aluminum w/dielectric overcoat
Percent of light reflectivity 96%
Overall Optical System
Cassegrain f/ratio 15.889
Focal length 381.33 in/9,686 mm
Back focus distance 25.81 in (primary vertex to image)
Primary/Secondary separation 88.938 in
Radius center of curvature 254.125 in
Magnification loss 0.10
Light loss 10%
Diffraction effect 32%R
Image size 3.40in
Light gathering ability 8,019 times human eye
Theoretical resolution capability 0.190 seconds of arc
Theoretical max power @ 50 times aperture 1,200X
Useable max power @ 30 times aperture 720X
Minimum useable magnification @ 3.5 times aperture 84X
Theoretical magnitude of faintest star visible Mag 16.9
Widest field of view (lowest power wide field eyepiece) 18 min of arc
Optical System With 2X Telecompressor Installed
System f/ratio w/Telecompressor installed 7.945
Focal length w/Telecompressor installed 190.67/4,843mm
Diameter of Telecompressor 3.165 in
Length of assembled Telecompressor 4in
Number of lens elements in Telecompressor 4
Image size with Telecompressor installed 1.75in
Back focus with Telecompressor installed 19.66 in
Widest field of view (lowest power, wide field eyepiece 36 min of arc
Wave Front Accuracy
“The axial wavefront error of the system was measured over several 20-inch diameter sub-apertures. The system tests were performed at a wavelength of 6328 Angstroms using a 1/20th wave 20-inch clear aperture optical flat and a scatter plate interferometer. The worst of these measured slightly better than 0.070 lambda peak-to-valley, yielding a total system wavefront error of lambda/10.2 peak-to-valley (slightly better than 1/10th wave). The system is actually better than the peak-to-valley wavefront numbers indicate. Using the Root Mean Square (RMS) wavefront error as a measure of quality, 0.070 lambda is usually considered diffraction limited. The system actually measured 0.023 lambda, three times better. Mounted and aligned properly, the system performance will be limited by atmospheric conditions under virtually all circumstances.”

Don Small, Senior Optical Engineer, Perkin-Elmer