High image contrast reflecting telescope

A telescope assembly (10,10',10") including a primary mirror (12,12',12") and a secondary mirror (22,22',22") for receiving the primary light beam (50,50',50") reflected from the front side (14,14',14") of the primary mirror (12,12',12") and reflecting the primary light beam (50,50',50") to an eyepiece (28,28',28"). The secondary mirror (22,22',22") is disposed on the optical axis (18,18',18") defined by the primary mirror (12,12',12") and obstructs a portion of the primary light beam (50,50',50") received by the primary mirror (12,12',12"). An intercept mirror assembly intercepts the obstructed portion of the primary light beam (50,50',50") ahead of the rear side (26,26',26") of the secondary mirror (22,22',22") and merges the intercepted light beam into the primary light beam between the front faces (14,14',14",24,24',24") of the primary and secondary mirrors (12,12',12",22,22',22" ) to form an unobstructed view of the on-axis object.

TECHNICAL FIELD 
The present invention relates to improvements in reflecting telescopes. 
More specifically, the present invention relates to an auxiliary optical 
system for all types of reflecting telescopes, including the classical 
Newtonian, a modified Newtonian, and other reflectors which have an 
amplifying secondary mirror such as the classical Cassegrain, Gregorian, 
Maksutov, and Schmidt-Cassegrain. 
BACKGROUND ART 
The reflecting telescope is limited in contrast performance due to the 
spurious light levels induced by the central obstruction caused by the 
secondary mirror. This obstruction causes the impulse response or 
diffraction pattern to have higher side lobe levels than an unobstructed 
aperture. The additional light energy distributed in the region outside 
the main lobe response causes reduced contrast on extended objects such as 
planets and nebula. If the central obstruction were not present, the 
optical performance would be nearly perfect for an on-axis point like 
object (achromatic and ideal diffraction pattern) with contrast limited 
only by light scattered at the primary mirror surface and by deviations 
from an ideal parabolic surface. This is why a good refractor telescope is 
considered superior to the Newtonian or other reflecting telescope 
designs, by some, for planetary and other applications requiring low 
spurious noise levels. 
Examples of prior art reflecting telescopes including auxiliary optical 
systems are U.S. Pat. Nos. 2,628,529 to Braymer, issued Feb. 17, 1953; 
3,598,468 to Perry, issued Aug. 10, 1971; 3,667,827 to Lawrence, issued 
June 6, 1972; and 3,752,559 to Fletcher, et al, issued Aug. 14, 1973. 
The Braymer patent discloses a reflecting telescope including an auxiliary 
optical system in which the primary image is formed by a right cone of 
rays whose base is the effective area of the primary mirror and whose axis 
is coincident with the principal axis of the instrument. The Perry patent 
discloses an optical system for a microscope including a spherical mirror 
tilted a few degrees relative to the optical axis and a plane plate having 
a transparent refractive portion in the path to the mirror and a 
reflective surface on the path from the mirror, with the plate tilted so 
that its refractive portion corrects astigmatism which results from 
tilting of the mirror. The Lawrence patent discloses a tele-objective in 
which the diffraction effects produced by the central obstruction 
effecting the image quality are very small. A relatively small positive 
achromatic doublet provides correction of aberrations of a concave 
spherical primary mirror. The Fletcher, et al, patent discloses a 
Ritchey-Chretien telescope responsive to images located off the telescope 
optical axis and includes a transparent plate positioned in the ray path 
of the image. The flat plate has a tilt angle relative to the ray path 
that compensates substantially for astigmatism introduced by the 
Ritchey-Chretien telescope. 
None of the prior art patents provide a means for correcting for the 
central obstruction presented by the secondary mirror of a reflecting 
telescope. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a telescope 
including primary mirror means having a front and rear sides and defining 
an optical axis for reflecting a primary light beam received from an 
on-axis object and secondary mirror means having a front side and rear 
side for receiving the primary light beam reflected from the primary 
mirror means on the front side and reflecting the primary light beam to an 
eyepiece. The secondary mirror means is disposed on the optical axis and 
obstructs a portion of the primary light beam received by the primary 
mirror means. The invention is characterized by including intercept means 
for intercepting the obstructed portion of the primary light beam ahead of 
the secondary mirror means and merging the intercepted mirror light beam 
imto the primary light beam between the primary and secondary mirrors to 
form an unobstructed view of the object.

DETAILED DESCRIPTION OF THE DRAWINGS 
A telescope constructed in accordance with the present invention is 
generally shown at 10 in FIG. 1. Like structures between the different 
embodiments are shown as primed numbers. 
The telescope shown in FIG. 1 is a classical Newtonian telescope. The 
telescope includes a primary mirror 12 having a front reflective side 14 
and a rear side 16. The primary reflective mirror 12 defines an optical 
axis indicated at 18. The primary mirror 12 reflects a primary light beam 
indicated by arrows 50 received from an on-axis object. The light beams 
are parallel to the optical axis 18 and are perpendicular to a series of 
planes, one of which being schematically indicated at 20. 
A secondary mirror is generally indicated at 22. The secondary mirror 22 is 
canted 45 degrees relative to the optical axis 18. The secondary mirror 22 
has a front side 24 and a rear side 26. The secondary mirror 22 receives 
the primary light beam 50 reflected from the front side 14 of the primary 
mirror 12 and reflects the primary light beam 50 to an eyepiece 28. The 
secondary mirror 22 is disposed on the optical axis 18 and obstructs a 
portion of the primary light beam 50 received by the primary mirror 12. 
The light blockage caused by the secondary mirror 22 is avoided by the 
present invention. The invention includes intercept means for intercepting 
the obstructed portion of the primary light beam 50 ahead of the rear side 
26 of the secondary mirror 22 and merging the intercepted light beam into 
the primary light beam 50 between the front faces 14, 24 of the primary 
and secondary mirrors 12, 22 to form an unobstructed view of the on-axis 
object. The intercept means intercepts the otherwise blocked light beam 
before the light beam reaches the rear face 26 of the secondary mirror 22 
and then merges the otherwise obstructed light beam back into the primary 
light beam 50 prior to the primary light beam 50 reaching the eyepiece 28. 
More specifically, the intercept means includes a first intercept mirror 30 
mounted adjacent the rear side 26 of the secondary mirror 22 for 
reflecting the obstructed portion of the primary light beam 50 off of the 
optical axis. A redirecting mirror 32 is mounted off of the optical axis 
18 for redirecting the obstructed portion of the primary light beam 
reflected by the intercepting mirror 30 in the direction of the primary 
mirror 12. The assembly includes a merging mirror 34 for merging the 
obstructed portion of the primary light beam reflected from the 
redirecting mirror 32 into the primary light beam received by the eyepiece 
28. A combination lens assemlby generally indicated at 36 converges the 
obstructed portion of the primary light beam into the primary light beam 
received by the eyepiece 28. 
The primary mirror 12 has a predetermined focal length and the lens 36 has 
a focal length equal to the focal length of the primary mirror 12. In 
other words, the lens 36 has a focal length equal to the focal length of 
the primary mirror 12. The merging mirror 34 serves to merge the 
converging light from the lens 36 into the center of the primary light 
beam thus forming an effective fully illuminated aperture. The optical 
path length from the reference plane 20 to the focal points of the primary 
mirror 12 and the lens 36 must be equal. This implies that the distances 
in terms of the number of wave lengths of light is equal for the two 
optical paths at any particular frequency of visible light. 
The primary mirror 12 is a parabolic mirror. The intercept mirror 30 is a 
flat mirror mounted on the optical axis 18 adjacent the rear side 26 of 
the secondary mirror 22. The reflecting mirror 32 is a flat mirror mounted 
off of the optical axis 18. A flat flass plate 38 is used as a support for 
the secondary mirror 22 and the intercepting mirror 30 through supports 40 
and 42 diagrammatically shown, respectively. 
The eyepiece lens 28 provides a primary focal adjustment. The assembly 10 
includes secondary focal adjustment means for adjusting for path length 
compensations due to ambient environmental variations, such as changes in 
ambient temperature. The secondary focal adjustment means can include an 
adjustment mechanism shown schematically at 46 for translating the 
reflecting mirror 32 to bring the intercept means into correct phasing 
with the primary and secondary mirrors 12, 22. 
A modified Newtonian design telescope is generally shown at 10' in FIG. 2. 
For this design, the secondary mirror 22' is not at a 45 degree angle to 
the optical axis 18' as in the classical Newtonian mirror shown in FIG. 1, 
but is positioned at a 90 degree angle to the optical axis and reflects 
the light from the primary mirror 12' directly toward a perforation 44 in 
the primary mirror 12'. 
Both the Newtonian telescope shown in FIG. 1 and the modified Newtonian 
telescope shown in FIG. 2 include secondary mirrors 22, 22' which aer 
non-amplifying mirrors. In these assemblies, the lens 36, 36' are disposed 
between the reflecting mirror 32, 32' and the merging mirrors 34, 34'. The 
secondary mirrors 22, 22' in both embodiments are flat mirrors. In the 
Newtonian telescope shown in FIG. 1, the merging mirror 34 is a flat 
mirror mounted on the optical axis 18 between the front side 14 of the 
primary mirror 12 and the front side 24 of the secondary mirror 22. The 
eyepiece 28 is mounted off of the optical axis 18. The lens assembly 36 
includes a combination lens assembly mounted between the reflecting mirror 
32 and the merging mirror 34, the lens 36 converging the light which is 
eventually merged by the merging mirror 34 to the primary light beam 50. 
In the modified Newtonian design mirror as shown in FIG. 2, the secondary 
mirror 22' is not at a 45 degree angle to the optical axis 18' as in the 
classical Newtonian telescope shown in FIG. 1, but is positioned at a 90 
degree angle to the optical axis 18' and reflects the light from the 
primary mirror 12' directly toward the perforation 44 in the primary 
mirror 12'. The eyepiece 28' is disposed behind the primary mirror 12'. 
This arrangement is not normally used because the obstruction must be at 
least 1/2 the diameter of the primary mirror 12' to permit access to the 
focal point for visual use. This limitation is overcome as the merging 
mirror means 34' includes a first flat merging mirror 32' off of the 
optical axis 18' receiving the reflected obstructed portion of the primary 
light beam from the intercepting mirror 30' and a second flat merging 
mirror 48 between the front faces 14', 24' of the primary and secondary 
mirrors 12', 22' and facing the primary mirror 12' for receiving the 
reflected light from the first flat merging mirror 34' and merging the 
light with the primary light beam reflected by the secondary mirror 22' to 
the eyepiece 28'. Unlike the Newtonian design telescope shown in FIG. 1 
wherein the merging mirror 34 merges the obstructed light beam into the 
primary light beam on the secondary mirror 22, in the embodiment shown in 
FIG. 2, the second merging mirror 48 merges the obstructed light beam with 
the primary light beam reflected directly to the eyepiece 28'. Again, the 
lens assembly 36' includes a combination lens disposed between the 
reflecting mirror 32' and the first merging mirror 34'. The lens assembly 
36' has the same focal length as the primary mirror 12' and is achromatic. 
The application of the present invention to the Cassegrain, Gregorian, 
Schmidt-Cassegrain and Maksutov are shown in FIG. 3. For the Maksutov and 
Schmidt-Cassegrain designs, the supporting window 38" assumes the optical 
shape required for the designs, and, for all the designs the secondary 
mirror 22' is not flat as shown in FIGS. 1 and 2 but is convex for all 
designs except for the Gregorian design, where it is concave ellipsoidal. 
Referring to FIG. 3, the secondary mirror 22" is an amplifying mirror. The 
primary mirror 12" includes a central perforation 44' and an eyepiece 28" 
mounted therebehind. The merging mirror 34" is a first flat merging mirror 
34" disposed off of the optical axis 18" for receiving reflected light 
from the reflecting mirror 32" and further includes a second flat merging 
mirror 48' mounted between the front faces 14" and 24" of the primary and 
secondary mirrors 12" and 22". The second flat merging mirror 48' faces 
the primary mirror 12". The lens 36" includes a combination lens disposed 
on the optical path 18" in front of the intercepting mirror 30". The 
position of the lens assembly 36" has been moved to the entrance of the 
optical system just ahead of the obstruction point caused by the secondary 
mirror 22". Since all of the primary mirror systems have shorter optical 
paths than their effective focal lengths as a result of the amplifying 
secondary mirrors 22", the lens 36" must have a longer focal length in 
order to match the focal length of the primary mirror system. The optical 
length of the intercepting mirror means is used entirely for converging 
the incoming light with none of the path used only for redirecting the 
incoming light. The telescope designs having amplifying secondary mirrors 
22" allow all of the mirrors of the intercept means to be smaller than the 
intercepted obstructed portion of the primary light beam. 
In FIG. 3, a barlow lens can be used in conjunction with the lens 36" to 
achieve the correct focal length and optical delay simultaneously, and to 
obtain more design flexibility. Since the image of a Gregorian telescope 
is erect, a prism must be added to this system to invert the image without 
causing reversion so that the two optical systems have the same image 
orientation. 
As a comparative example, the modified Newtonian design telescope shown in 
FIG. 2 could be based on a 4 inch primary mirror 12', with a 16 inch focal 
length for a focal ratio of f4. The lens assembly 36' would also have a 16 
inch focal length and would be 2 inches in diameter with the corresponding 
focal ratio of f8. For a typical Cassegrainian design as shown in general 
form in FIG. 3, the 4 inch primary mirror 12" could have an 8 inch focal 
length with f2 local ratio and the overall system could have a f6 local 
ratio with a 24 inch focal length and 1 inch diameter secondary mirror 
22". The secondary optical system would have a 1 inch diameter lens with a 
24 inch focal length for a focal ratio of f24. This is a smaller 
obstruction than shown in FIG. 2 and could be increased to provide a focal 
point which is further behind the primary mirror 12". The high f number in 
this example eases the design of the achromatic objective. Given a 
particular application, an overall design trade-off would be conducted to 
optimize the combined system performance. 
For all designs, the merging of the primary optical system with the 
intercept means is only perfect for an on-axis point, but the overall 
performance for a typical extended object would shown improvement over 
some finite angular interval. 
FIGS. 4, 5, and 6 are computer generated plots of the impulse response for 
a circular aperture. The vertical axis is the log of the image intensity 
referenced to the peak of the response. The horizontal axis is given in 
normalized image distance units for the purpose of comparison of impulse 
width among the three figures. FIG. 4 shows the ideal response with first 
side lobes approximately 17 dB below the peak. FIG. 5 is for a 10% 
circular obstruction and FIG. 6 is for a 30% obstruction, where the 
percentage is the diameter of obstruction divided by the diameter of the 
primary mirror multiplied by one hundred (100). It can be seen that the 
energy in the side lobe region is increased as the obstruction is 
increased, thus lowering the contrast. The application of the correction 
obtained by the intercept means of the present invention permits the 
achievement of a response as shown in FIG. 4 given any size obstruction. 
In implementing the present invention, the initial alignment of the primary 
mirror system with the intercept means path lens to a fraction of a 
wavelength are critical, and the relative mechanical stability required to 
maintain the fractional wavelength over temperature and mechanical stress 
variations is an additional consideration. A fine adjustment for path 
length compensation to account for temperature variations could be used in 
addition to the normal focus control as discussed above with regard to the 
secondary focal adjustment mechanism 46, 46', 46". In operation, first the 
image is focused, then the separate control 46, 46', 46" which translates 
the reflecting mirror 32" in any of the embodiments brings the two systems 
into correct phasing. Other mirrors could also provide the same 
adjustment. 
The optical window 38, 38', 38" could be replaced by the conventional 
spider support, but the system performance would be limited by spider 
diffraction. This can still offer a signficant improvement for some 
designs. 
In most cases, the focal ratio of the lens assembly 36, 36', 36", will be 
greater than F10 (focal length/diameter being greater than 10). The high 
focal ratio of the lens assembly 36, 36', 36" is an advantage in 
constructing an achromatic lens which will not degrade the perfect color 
performance of the Newtonian reflector. As mirror technology improves to 
provide less scattered light due to surface imperfections and lens 
technology improves to provide better color correction, the technique 
described herein becomes more effective in achieving a high performance 
system. 
The invention has been described in an illustrative manner, and it is to be 
understood that the terminology which has been used is intended to be in 
the nature of words of description rather than of limitation. 
Obviously, many modifications and variations of the present invention are 
possible in light of the above teachings. It is, therefore, to be 
understood that within the scope of the appended claims wherein reference 
numerals are merely for convenience and are not to be in any way limiting, 
the invention may be practiced otherwise than as specifically described.