Casegrain telescope with spherical mirror surfaces

A telescope of the Cassegrain type comprising:

• a) a spherical primary mirror,
• b) a plurality of lens elements constituting a corrector lens group, and
• c) a spherical secondary mirror, said secondary mirror being a reflective coating formed on a refractive surface of one of said lens elements of said corrector lens group;
• said primary and secondary mirrors and said lens elements of said corrector lens group being disposed coaxially with respect to each other along an optic axis;
• said primary mirror having a specified focal length defining a front focal plane of said primary mirror;
• means located at said front focal plane of said primary mirror to define an entrance pupil for said telescope;
• said reflective coating being formed on the refractive surface of the lens element of said corrector lens group closest to said primary mirror along said optic axis so that said reflective coating faces said primary mirror;
• said primary and secondary mirrors being configured and positioned with respect to each other along said optic axis so that optical power for said telescope is provided by said primary and secondary mirrors, and so that contributions to astigmatism attributable to said primary and secondary mirrors substantially cancel each other so as to leave only an inconsequential residual value of astigmatism for said telescope;
• said lens elements of said corrector lens group being configured and positioned with respect to each other along said optic axis so that contributions to other remaining geometrical aberrations of said telescope attributable to said primary and secondary mirrors are reduced by said corrector lens group, wherein:
• i) said primary and secondary mirrors and said lens elements of said corrector lens group are configured and positioned with respect to each other so that said telescope focuses a distant object to an image plane;
• and
• ii) said primary and secondary mirrors and said lens elements of said corrector lens group are configured and positioned with respect to each other so as to have a focal length of approximately 248 millimeters according to a design form as follows:
• __________________________________________________________________________ Thickness or Radius Separation Glass Surface (mm) (mm) n.sub.d V.sub.d __________________________________________________________________________ Entrance pupil s = 4.487 (25 mm) R.sub.1 = -51.7405 L.sub.1 t = 1.000 1.51009 63.46 R.sub.2 = -148.0587 s = 0.653 R.sub.3 = -67.5501 L.sub.2 t = 1.851 1.62004 36.37 R.sub.4 = -50.0000 s = 55.365 M.sub.1 R.sub.5 = -125.0000 s = 55.365 M.sub.2 R.sub.6 = R.sub.4 = -50.0000 Exit pupil (s = -25.000) Image s = 100.000 __________________________________________________________________________

A telescope of the Cassegrain type comprising:

• a) a spherical primary mirror,
• b) a plurality of lens elements constituting a corrector lens group, and
• c) a spherical secondary mirror, said secondary mirror being a reflective coating formed on a refractive surface one of said lens elements of said corrector lens group;
• said primary and secondary mirrors and said lens elements of said corrector lens group being disposed coaxially with respect to each other along an optic axis;
• said primary mirror having a specified focal length defining a front focal plane of said primary mirror;
• means located at said front focal plane Of said primary mirror to define an entrance pupil for said telescope;
• said reflective coating being formed on the refractive surface of the lens element of said corrector lens group closest to said primary mirror along said optic axis so that said reflective coating faces said primary mirror;
• said primary and secondary mirrors being configured and positioned with respect to each other along said optic axis so that optical power for said telescope is provided by said primary and secondary mirrors, and so that contributions to astigmatism attributable to said primary and secondary mirrors substantially cancel each other so as to leave only an inconsequential residual value of astigmatism for said telescope;
• said lens elements of said corrector lens group being configured and positioned with respect to each other along said optic axis so that contributions to other remaining geometrical aberrations of said telescope attributable to said primary and secondary mirrors are reduced by said corrector lens group, wherein:
• i) said primary and secondary mirrors and said lens elements of said corrector lens group are configured and positioned with respect to each other so that said telescope focuses a distant object to an image plane with a substantially flat field;
• and
• ii) said primary and secondary mirrors and said lens elements of said corrector lens group are configured and positioned with respect to each other so as to have a focal length of approximately 158 millimeters according to a design form as follows:
• __________________________________________________________________________ Thickness or Radius Separation Glass Surface (mm) (mm) n.sub.d V.sub.d __________________________________________________________________________ Entrance pupil s = 51.898 (25 mm) R.sub.1 = -72.9714 L.sub.1 t = 2.000 1.43700 90.70 R.sub.2 = -90.5128 s = 1.000 R.sub.3 = -1262.1112 L.sub.2 t = 2.000 1.56732 42.84 R.sub.4 = 89.9431 s = 1.000 R.sub.5 = 90.2546 L.sub.3 t = 8.000 1.62014 63.49 R.sub.6 = -250.000 s = 1.000 M.sub.1 R.sub.7 = -250.0000 s = 50.000 M.sub.2 R.sub.8 = R.sub.6 = -250.0000 Exit pupil (s = -125.000) Image s = 65.217 __________________________________________________________________________

A telescope of the Cassegrain type comprising:

• a) spherical primary mirror,
• b) plurality of lens elements constituting a corrector lens group, and
• c) a spherical secondary mirror, said secondary mirror being a reflective coating formed on a refractive surface of one of said lens elements of said corrector lens group;
• said primary and secondary mirrors and said lens elements of said corrector lens group being disposed coaxially with respect to each other along an optic axis;
• said primary mirror having a specified focal length defining a front focal plane of said primary mirror;
• means located at said front focal plane of said primary mirror to define an entrance pupil for said telescope;
• said reflective coating being formed on the refractive surface of the lens element of said corrector lens group closest to said primary mirror along said optic axis so that said reflective coating faces said primary mirror;
• said primary and secondary mirrors being configured and positioned with respect to each other along said optic axis so that optical power for said telescope is provided by said primary and secondary mirrors, and so that contributions to astigmatism attributable to said primary and secondary mirrors substantially cancel each other so as to leave only an inconsequential residual value of astigmatism for said telescope;
• said lens elements of said corrector lens group being configured and positioned with respect to each other along said optic axis so that contributions to other remaining geometrical aberrations of .said telescope attributable to said primary and secondary mirrors are reduced by said corrector lens group, wherein:
• i) said primary and secondary mirror and said lens elements of said corrector lens group are configured and positioned with respect to each other so that said telescope is afocal;
• and
• ii) said primary and secondary mirrors and said lens elements of said corrector lens group are configured and positioned with respect to each other according to a design form as follows:
• __________________________________________________________________________ Thickness or Radius Separation Glass Surface (mm) (mm) n.sub.d V.sub.d __________________________________________________________________________ Entrance pupil s = 18.685 (25 mm) R.sub.1 = -36.2810 L.sub.1 t = 5.359 1.62004 36.37 R.sub.2 = -48.5420 s = 1.071 R.sub.3 = -87.8614 L.sub.2 t = 5.359 1.51009 63.14 R.sub.4 = -50.0000 s = 42.681 M.sub.1 R.sub.5 = -150.0000 s = 42.681 M.sub.2 R.sub.6 = R.sub.4 = -50.0000 Exit pupil (s = -25.000) __________________________________________________________________________

A telescope of the Cassegrain type comprising a spherical primary mirror and a spherical secondary mirror, and also comprising a plurality of lens elements functioning as a corrector lens group, said telescope being produced by a process characterized in that:

• (A) a specified optical power for said telescope and correction for astigmatism of said mirrors are provided primarily by determining relative positions of said mirrors, said relative positions of said mirrors being substantially independent of relative positions of the lens elements of said corrector lens group, and
• (B) correction for other geometrical aberrations of said telescope is provided primarily by said corrector lens group
• said process comprising the steps of:
• a) selecting a design form for a conventional Cassegrain-type telescope having a concave spherical primary mirror and a convex spherical secondary mirror, said primary and secondary mirrors of said conventional Cassegrain-type telescope being coaxially disposed with respect to each other along an optic axis, said concave spherical primary mirror of said conventional Cassegrain-type telescope having a specified focal length defining a front focal plane of said primary mirror, said primary and secondary mirrors of said conventional Cassegrain-type telescope providing said specified optical power, said design form for said conventional Cassegrain-type telescope serving as a "starting design form" with which to begin designing said telescope;
• b) locating an entrance pupil for said telescope at said front focal plane of said concave primary mirror of said conventional Cassegrain-type telescope in said "starting design form";
• c) developing an "intermediate design form" for said telescope by:
• (i) positioning a plurality of lens elements coaxially with respect to each other along said optic axis in said "starting design form", said plurality of lens elements in combination with each Other having negligible optical power, said lens elements functioning as said corrector lens group, all refractive surfaces of said lens elements of said corrector lens group being spherical surfaces;
• (ii) removing said secondary mirror of said conventional Cassegrain-type telescope from said "starting design form";
• and
• (iii) providing a reflective coating on a central portion of a refractive surface of the lens element of said corrector lens group that is closest to said concave primary mirror in said "starting design form" so that said reflective coating faces said concave primary mirror, said reflective coating replacing said secondary mirror of said conventional Cassegrain-type telescope that has been removed from said "starting design form", said reflective coating functioning as a secondary mirror for said telescope, said concave primary mirror together with said collector lens group and said reflective coating serving as optical elements of said "intermediate design form";
• d) adjusting separation between said concave primary mirror and said secondary mirror along said optic axis in said "intermediate design form" so as to provide correction for astigmatism of said mirrors, adjustment of the separation between said primary mirror and said secondary mirror introducing a change in optical power for said telescope in said "intermediate design form";
• and
• e) adjusting geometrical parameters of said corrector lens group in said "intermediate design form" so as:
• (i) to restore said specified optical power for said telescope, and
• (ii) to correct for other geometrical aberrations of said telescope.