A multi-purpose telescope objective having a Petzval type lens configuration and including an iris diaphragm.

BACKGROUND OF THE INVENTION 
Refractor telescopes having air-spaced doublet objectives perform well with 
an f/5 relative aperture when used for low power purposes. However, at 
high power, they are limited by zonal spherical aberration and secondary 
chromatic aberration. The latter results in a purple fringe due to the 
fact that the common blue-red focus does not coincide with the 
yellow-green focus. These aberrations encountered at high power usage can 
be minimized by providing a relatively small relative aperture, e.g. f/10, 
or by using very expensive glass types for the lens elements. 
SUMMARY OF THE INVENTION 
This invention relates to refractor telescopes and more particularly to a 
multi-purpose telescope employing a Petzval type lens for the objective. 
It is the object of the present invention to provide a refractor telescope 
which has improved secondary color correction, minimal zone spherical 
aberration, and minimal sphero-chromatism, the latter being a chromatic 
variation of spherical aberration. 
It is a further object of the invention to provide a refractor telescope 
which performs better with an f/4 relative aperture than a telescope 
employing an air-spaced doublet with an f/5 relative aperature when the 
same type glass is used in both. 
It is still another object of the invention to provide a multi-purpose 
refractor telescope which can be used for low power wide angle views, or 
high power high resolution views, or for photographic purposes. 
In carrying out the invention, a telescope is provided having a Petzval 
type lens for the objective and an iris diaphragm located adjacent the 
rear achromat of the objective. 
Features of the invention: the ability to adjust the exit pupil so as to 
match the telescope aperture to air turbulence conditions and thereby 
optimize image resolution; minimizing residual secondary spectrum which 
otherwise might be annoying when viewing very bright objects; easy control 
of image brightness; and the ability to adjust the exit pupil to avoid the 
most aberrated portion of the observer's eye and thereby improve his 
visual acuity. 
Other features of the invention may be gained from the foregoing and from 
the description of a preferred embodiment of the invention which follows.

DETAILED DESCRIPTION OF THE INVENTION 
A telescope 10 constructed according to the present invention is shown 
schematically in FIG. 1. The objective lens comprises a pair of spaced 
achromatics 11 and 12 mounted in the telescope barrel 13. An iris 
diaphragm 14 is provided, and while it may be placed anywhere between the 
two achromats, it preferably is located adjacent achromat 12. In this 
position it will be the smallest and least expensive commensurate with its 
mechanical design, and it will be conveniently located for adjustment by 
an observer peering through the eyepiece 15 of the telescope. Neither the 
eyepiece nor the specific configuration of the iris diaphragm form any 
part of the present invention, hence they are shown schematically and not 
in detail. 
The telescope objective, shown in more detail in FIG. 2, is a Petzval type 
lens. It comprises two widely spaced achromats 11 and 12 having an 
effective focal length that is appreciably shorter than the physical 
length of the objective measured from the front vertex of achromat 11 to 
the focal plane of the lens. Thus, the telescope objective having the 
parameters hereinafter set forth will have an effective focal length of 
508.6 mm. and an overall length of approximately 755 mm. 
The front achromat 11 includes lens elements LI and LII, while the rear 
achromat 12 includes lens element LIII and LIV. Lens elements LI and LIII 
are made of BK-7 glass having a refractive index of 1.5168 and an Abbe 
number of 64.2. Lens elements LII and LIV are made of SF-12 glass having a 
refractive index of 1.6477 and an Abbe number of 33.8. All air-glass 
surfaces are preferably provided with an anti-reflective coating. 
The lens elements for an f/4 relative aperture Petzval type lens will have 
the following parameters where the radii, lens thicknesses and 
separations, clear apertures, and outside diameters are given in 
millimeters. 
______________________________________ 
Lens Lens Lens Clear 
Ele- Thick- Separa- 
Aper- Outside 
ment Radii ness "d" tion "s" 
ture Diameter 
______________________________________ 
r1=+411.43 
LI d1=16.0 126 132 
r2=-544.15 
s1=6.0 
r3=-501.7 
LII d2=12.0 126 132 
r4=-6890.0 
s2=480.0 
r5=+253.263 
LIII d3=8.0 60 64 
r6=-253.263 
s3=0.0 
r6=-253.263 
LIV d4=8.0 60 64 
r7=-1386.0 
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In FIG. 3, the longitudinal chromatic aberration of the Petzval lens shown 
in FIG. 2 is conventionally plotted to show the focal point deviation, in 
millimeters, for the three wavelengths specified by the c, d, and f 
Fraunhofer lines. When compared to the aberration of an f/5 air-spaced 
doublet, shown in FIG. 4, it is seen that the f/4 objective of the present 
invention compares very favorably with the f/5 doublet. Moreover, the 
chromatic aberration is relatively flat over the full range of relative 
apertures, thereby providing excellent resolution and color correction at 
any iris setting. It is further noted that, for photographic purposes, the 
Petzval objective has been optimized for 35 mm. film format. 
Having thus described the invention, it is clear that what may appear to be 
other embodiments could be provided without departing from the spirit and 
scope of the invention. Thus, other Petzval type objectives having 
different parameters could be provided. For example, lens 11 could be 
cemented, or both achromats could be air-spaced, or cemented. Hence it is 
intended that the foregoing specification and the accompanying drawing be 
interpreted as illustrative rather than in a limiting sense.