GB2407391A - Reflector telescope - Google Patents

Abstract

A reflector telescope has an elliptical primary reflector 103 and a secondary reflector 105 with a negative lens 104 in between. Light reflected from the primary reflector is passed through the negative lens and is diverted by the secondary reflector to a positive lens 106 at the side of the telescope tube. The positive lens focuses the light rays onto an eye piece 100 or ocular or onto a photographic emulsion or electronic detector. The telescope reduces spherical aberration and can be configured to have a focal ratio significantly greater than the focal ratio of the primary reflector.

Description

24079 1 Reflector Telescope The present invention relates to a reflector

telescope and particularlybut not exclusively, to a catadi optri c reflector tel escope employing a spherical primary reflector having a relatively short focal length and which reduces or substantially eliminates the problems conventionally associated with the use of spherical mirrors in reflector telescopes.

Reflectortelescopes generally employ a concave primaryreflectorwhich collects and focuses light incident upon the reflector surface through the telescope's aperture and a smaller secondary reflector to direct the reflected light to the telescope's eyepiece. Normally, the primaryreflector is provided by a paraboloidal mirror which has certain advantages in terms of imaging, the most important teeing that all parallel rays of light incident on the mirror are convergent et the focal point ofthe mirror, irrespective ofthe distance ofthe point of incidence from the central axis ofthe mirror. The result is that the focussing ability ofparabolic orparabolicalmirrors is generallygood, even if the focal length of the primary mirror is short.

However, the use of paraboloidal mirrors in reflectortelescopes does have certain disadvantages.

Firstly, the image is stronglydistorted towards the edge ofthe field of view- an aberration ofthe reflectorknown as comain which the image of apoint Iyingoffthe axis ofthe reflector has acomet shaped appearance making such telescopes unsuitable for photography. Moreover, coma is more stronglypronounced as the focal lengthoftheprimaryreflectorisreduced so that long focal length reflectors must be used if the effect is to be minimised, thus resulting in a longer overall length of the telescope. In addition, the manufacture of paraboloidal mirrors is difficult and expensive.

The above disadvantages are overcome by the use of a spherical primaryreflector inplaee of a paraboloidal one. Spherical mirrors are considerably easier and cheaper to manufacture than paraboloidal mirrors and they do not generate coma distortion and are therefore suitable for photographic purposes.

However, the use of sphericalprimary reflectors in reflectingtelescopes also presents anumberof disadvantages. While coma distortion is not present, spherical mirrors suffer from a defect known as spherical aberration inwhich the rays of light incident on the mirror come to a focus in slightly differentpositions ratherthan acommon focal point. Full size correctors are therefore normally required to compensate for this defect, as in the case of, for example, the Maksutov telescope and the Schmidt camera. Naturally, this results in manufacturing difficulties and increased expense.

Moreover, spherical aberration varies inverselywith the cube ofthe focal length ofthe mirror so that, again, the manufacture of compact telescopes is extremely difficult. Increasing the focal length oftheprimaryreflectorin order to compensate for spherical aberration increases the length ofthe telescope. These instruments also normally have averylargecentral obstruction, which degrades the image by transferring some of the energy in the Airey disc to the diffraction rings. Maksutov and Schmidt/Cassegrain telescopes also have a third reflection normally through a hole in the primary mirror and thus have an inconvenient observing position.

In general, therefore, the disadvantages of spherical reflectors outweigh those of paraboloidal mirrors and i t is the latter that are therefore more commonly used in reflector telescopes. If the di sadvantages associated with spherical mirrors could be reduced or substantially eliminated, however, then the use of spherical mirrors in reflector telescopes would be of considerable advantage.

EP-A-1,272,886 to the current applicant describes a reflector telescope which employs a spherical primary reflector but which reduces or substantially eliminates spherical aberration whilst being both compact and relatively inexpensive to produce.

The present invention seeks to provide an improved reflector telescope.

Accordingly, there isprovided a reflector telescope comprising: a primary reflector; a secondary reflector; first correcting means; and second correctingmeans; characterized in that: the primary reflector comprises a reflector figured to an elliptical shape; the first correcting means comprises negative or diverging lens means interposed between the primary reflector and the secondary reflector; and the second correcting means comprises positive or converging lens means interposed between the secondary reflector and an eye-piece or ocular of the telescope.

The present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure l is a simplified diagram of a conventional Newtonian reflecting telescope; Figures 2a and 2b are raydiagrams illustrating/he reflection characteristics of asphericalreflector; and Figure 3 is a section through a preferred form of telescope according to the invention.

in figure I, a simplified diagram of a conventional Newtonian reflecting telescope is shown generally at l O. Thetelescopeconsists of a generally elongate, cylindrical tube 12, open et one end thereof (or otherwise provided with an optical window) to allow the passage of light rays from a distant object into the telescope. A primary reflector in the form of a concave mirror 14, usually paraboloidal in shape, is mounted at the other end ofthe tube with its reflective surface directed towards the open end or optical window.

A sccondaryrellectorin the form of agenerallyplanarmirror 16 is mounted within the tube at a 1 5 position between the open end ofthc tube and the primary reflector 14 with its reflective surface directed generally towards the primary reflector 14 but angled at 45 away from the central axis CA of the primary reflector 14. A lens 18 is mounted coaxially within a tube 20 arranged substantially at right-angles to the main tube 12 and serves as an ocular or eyepiece.

in use, light rays from a distance object enter the tube 12 via the open end or optical window with the rays being generallyparallel. The parallel rays are incident upon theprimaryreflector 14 and are reflected to converge on the focal point FP oftheprimaryreflector 14. In order to maximise angular magnification of the telescope, the primary reflector 14 has a focal length which is considerably longer than the distance between the primary reflector 14 and the secondary reflector 16.

The light rays reflected by the primary reflector 14 are therefore intercepted by the secondary reflectorpriorto convergence and are turned through90 end reflected towards the ocular 18. The plane surface ofthe secondarymirror 16 causes the reflected rays to convergepriorto incidence upon the ocular such that they are diverging when they strike the ocular 18. The ocular 18 refracts the secondary rays such thattheyarc emitted from the ocular 18 substantiallyparallel. The parallel rays can thus be viewed by the retina.

As stated above, it would be advantageous to replace the paraboloidal reflector 14 with a spherical re n ector in order to reduce the effects of coma inherent with paraboloidal reflectors. However, the use of a such a spherical reflector would give rise to the problems indicated above. More particularly, spherical aberration could cause some ofthe incident light to converge before being intercepted by the secondary reflector with the effect that some of the rays reflected from the primary reflector would be diverging on incidence with the secondary mirror and would not, therefore, be reflected towards the ocular. The resulting image would be highly distorted.

Figures 2a and 2b illustratethereflectiveproperties of asphericalmirror20. Parallel light rays that are incident upon the mirror 20 at a position close to the central axis CA ofthe mirror are reflected through the focal point F as in the case of a paraboloidal mirror. However, rays which are incident upon the mirror at positions further from the central axis CA are reflected such that they miss the focal pomt. In fact, the further from the central axis the incident rays strike the mirror, the greater the distance bywhich the reflectedraywill miss the focalpoint. Rays incident on the mirror at positions further from the central axis will be reflected such that they cross the central axis at positions close to the surface of the mirror itself.

As shown in figure 2a, spherical mirrors having a long focal length, for example f8 and above, and those having relatively small apertures, for example 0. lm (4") or less, are not so greatly effected bythis problem, although it is still noticeable, since the spread ofthe focal or convergence points ofthe reflected rays is relativelysmall. However, the use of reflectors having long focal lengths necessitates a long overall length of the telescope.

Use of a spherical reflectorhaving a shorter focal length clearlyreduces the length ofthe telescope but exacerbates the problem of spherical aberration. As shown in Figure 2b, rays incident upon a reflector having a short focal length at a distance far from the central axis ofthe mirror can even be reflected back onto the mirror itselfbefore being Finally reflected towards the secondary mirror.

This produces severe distortion of the image.

The above problems associated with the use of spherical reflector have been reduced in the telescope described in the applicant's copending application EP-A-1,272,886.

However, the applicant has now found that replacing the spherical reflector with an elliptical reflector produces a further improvement in the quality ofthe image, particularly when primary reflector mirrors are of short focal ratio i.e. less than f3.

Figure 3 shows a section through a preferred form ofteleseope aeeordingtotheinventionwhieh aims to allow the use of elliptical redactors. Denoted generallyat 100, the telescope comprises a generallycylindrical primarytube 101 having an aperture in the form of an optical window 111 at one end 101 a thereof. The other end 101bofthetube 101 isoptieallyelosedbymeansofaeap ormounting 102. An elliptical primary reflecting mirror 103 is mounted on the cap 102 bymeans of an adhesive or other mechanical means with its reflecting surface directed generally towards the aperture 111. I n the preferred embodiment described, the elliptical primary reflecting mirror 103 (hereafter primary mirror) has a focal length of between f2 and f3. Other advantageous, though less preferred, focal lengths for the primary mirror are between fat and fs.

An elliptical, planarsecondarymirror 105 is mounted within the tube 101 in general alignment with the central axis CA ofthe tube and the primarymirror 103. The elliptical, planar seeondarymirror (hereaftersecondarymirror) 105 has its reflective surface directed generallytowards theprimary mirror 103 but is angled atapproximately45 awayfrom the axis oftheprimarymirror 103 to cause light rays parallel to the central axis CA oftheprimarymirrorto be reflected through 90 towards the side of the tube 101, as in the case of the earlier described Newtonian telescope. In the preferred embodiment, the secondary mirror is held imposition within the tube 101 bymeans of a mechanical spidermounting (not shown). An alternative method of mounting the secondary mirror would be via a flat, full-aperture optical window.

A f] rst correcting means in the form of a diverging or negative lens 104 is positioned between the primarymirror 103 and the secondarymirror 105, substantiallycoaxialwith the tube 101. In the preferred embodiment, the negative lens 104 is mounted on the same mounting as the secondary mirror l 05 but, clearly, alternative mounting means maybeprovided. In the preferred embodiment, the lens 104 is an achromatic doublet.

Second correcting means in the form of a converging or positive lens 106 is mounted in an aperture 107 in ornearthe side wall ofthe tube 101 and is arranged generally et right angles to the negative lens 104 and substantiallyin alignment with the central axis ofthe secondarymirror 105. The positive or converging lens 106 is mounted in the aperture l 07 by a suitable adhesive or by any other suitable mechanical means.

A secondarytube 109 extends from the side ofthe primary tube 101, substantiallyperpendicular thereto and coaxial with the aperture 107. An eyepiece or ocular lens (not shown) is mounted within the secondary tube 109 in the manner of a conventional Newtonian telescope as described with reference to figure 1.

Operation of the telescope of figure 3 will now be described.

Light rays from a distant object enter the aperture ofthe telescope substantiallyparallel. These are shown generallyat 108a. These rays 108atravel through the tube 101 and are incident upon the reflective surface ofthe primary mirror 1 03. Since the primary mirror 103 is concave, the rays 1 08a are reflected such that they converge towards the central axis CA ofthe primary mirror 103 and are shown at 1 08b, 1 08c. However, as stated above, rays 1 08a which strike the reflective surface of the primary mirror 103 at a distance from the central axis CA of the mirror will be reflected such that theyconverge more steeplywith central axis ofthemirrorthan those rays which strike the surface of the mirror closer to the central axis. In other words, parallel rays striking the mirrorclose to its perimeterwill converge to a focalpoint FP on the central axis CA ofthe primary mirror closer to the surface ofthe mirror than the focal point FP2 Of rays which strike the mirror close to its central axis.

The negative lens 104 is positioned at a distance from the primary mirror 103 such that all reflected rays from the primary mirror are converging when they are collected by the negative lens 104. In other words, the negative lens 104 is positioned at a point closer to the mirror than the focal point FP' of the most radially outward of the reflected rays.

l O The negati ve lens 104 serves to refract the incoming, converging rays 108b,108e reflected from the primary mirror 103 such that the rays 108d emerging from the negative lens 104 are approximately parallel.

The parallel rays 108d emerging from the negative lens 104 are incidentupontheplanarmirror 105 and are caused to rotate through 90 by the 45 angle of the secondary mirror 105.

The parallel rays 108e reflected from the secondary mirror 105 arecollectedbythepositive lens 1 OG which focuses the rays towards an image plane denoted at 110. In the embodiment of figure 3, theeyepieceorocular(not shown) is positioned within the secondary tube 109 et a distance from the positive lens 106 greater than the focal length of the lens 106 such that the light rays collectedbythe eyepiece are diverging. The eyepiece thus focuses the diverging rays onto the rethink of a user in the manner of a conventional eyepiece. Alternatively, a photographic emulsion or electronic detector, such as a charged- coupled device, may be positioned at the image plane l l O for photographic purposes. The lens 106 is preferably achromatic.

The conic constant ofthe elliptical reflectorcanbe in the range up to 0.9. Table Ibelow gives the spot size for anumberofconic constants for a431 mm (17 inch) reflector over an angle of 0.25 degrees subtended at the focal point. The spot size is given at three positions where 0 = the eentre, 0.7 is 0.7 ofthe distance towards the edge ofthe field and I = the edge ofthe 0.25 degree field.

The diffraction limit = 2.32 m.

Table I

spot size (pm) Conic constant At centre 0.7 _ Spherical 1.88 4.15 8.32 0.1 1.74 3.9 5.71 0.2 1.64 3.85 3.63 0.3 1.6 3.44 2.57 0.4 1.66 3.33 2.79 0.5 1.84 3.43 3.75 0.75 2.86 4.75 6.54 It will be appreciated that the present inventionprovides a telescope having all ofthe advantages associated with the employment of a spherical primaryreflectorbutwhichreduces or substantially eliminates the problems conventionally associated with the use of such a reflector.

l 5 In particular, it will be clear to those skilled in the art that the use and configuration ofthe negative lens 104 and the positive lens 106 increases the effective focal length ofthe primarymirror 103 and hence the focal ratio ofthe telescope. In practice, the combination ofthe negative lens 104, the mirror 105 and the positive lens 106 can increase the effective focal ratio ofthe telescope 100 by up to 5 times ormorc. This large increase in effective focalratio allows the actual focal length of the primary mirror 103 to be reduced which, as a consequence, reduces the overall length ofthe telescope. The use of a short-focus primaryreflectortogetherwith the focal ratio-increasing lenses permits the overall tube length ofthe telescope to be short allowing the telescope to be able to be described as compact.

Prior art telescopes which employ spherical primary reflectors, such as the above mentioned Maksutov and Schmidt/Cassegrain telescopes, usually employ secondary reflectors which cause the light rays to be reflected through ahole in the primary mirror. The rays are therefore reflected along the length of the tube at least three times, resulting in a degradation in the image collected.

The effect ofthe secondaryreflectorcauses an amplification oftheprimaryreflector's focal ratio and overall focal ratios ofthese telescopes are usually around fig, which is inconvenientlylarge. A focal ratio of f6 or below is usually preferred.

The present invention advantageously employs aprimaryreflectorhaving a focal length ofbetween 1: and f5 and, more preferably, a focal length of between f2 and f3. However, the disadvantages associated with the use of a short focal lengthprimaryreflectorinterms of increased spherical aberration is considcrablyreduced or substantially eliminated bythe use and positioning ofthe negative lens 104. The exact position of the negative lens 104 is not crucial although certain considerations must be taken into account when determining the optimum position.

As willDc appreciated bythose skilled in the art, the nearerthenegative lens 104 is placed to the primarymirror, the better refractive performance it will provide. However, in order to intercept l 5 all ofthc reflected rays from the primary mirror, the closer to the mirror the negative lens 104 is positioned, the larger in diameter it is required to be. There is therefore a trade-offbetween placing the lens 104 close enough to the primarymirror 103 to provide good refractive performance whilst being far enough away from the mirror to enable it to be relatively small in diameter yet still collect all of the light reflected from the primary reflector.

In practice, the diameter ofthe negative lens 104 is less than 35% ofthe aperture ofthe primary mirror l 03 and more preferably in the range of between 15 and 25% ofthe aperture in order to reduce the diffraction effects and loss of contrast caused byalarge central obstruction. However, it is preferred thatthe negative lens Repositioned closer to theprimarymirror 103 than the first focal pon1t FP ofthe reflected rays 108b, 108c from the primary mirror 103 to ensure that all reflected rays collected by the negative lens 104 are converging.

1 0 The optics within the telescope apart from the elliptical primaryreflectormaybe spherical but not exclusively so, enabling the telescope to be suitable forthe purposes of photography and further greatlyreduces the cost of manufacture "hereof. Furthermore, the arrangementoftheprimary reflector and the negative and positive lenses provides the considerable advantage that a flat field of view is generated at the focal plane 107 and the small central obstruction means that the image has more energy in the Airey disc and reduced energy in the diffraction rings.

It will also be appreciated by those skilled in the art of optical design that the telescope of the present invention can simply, and without the exercise of inventive thought, be configured to ensure any errors are limited to within the Airey disc or, in other words, diffraction limited.

While in the drawings, the negative andpositive lenses 104,106 are shown es single lenses, they arepreferablyin the form of achromatic doublets. However, they may take any suitable form such as single or compound Icnses, triplets ormultilayer lenses providedthat theyprovide the functions described above.

Claims (1)

1. Claims 1 A reflector telescope (100) comprising: a primary reflector

   (103); a secondary reflector (105); first correcting means (104); and second correcting means (106); characterized in that: the primary reflector (103) comprises a reflector figured to an elliptical shape; the first correcting means ( 104) comprises negative or diverging lens means interposed between the primary reflector (103) and the secondary reflector (105); and the second correcting means (106) comprises positive or converging lens means interposed between the secondary reflector (105) and an eye-piece or ocular of the telescope (100).

   2 A telescope as claimed in claim 1 wherein said negative or diverging lens means (104) is positioned substantially co-axially with said primary reflector ( 103) and at a distance therefrom such that all light rays reflected from the primary reflector ( 103) are converging when incident upon the negative or diverging lens means (104).

   3 A telescope as claimed in claim 1 or claim 2 wherein the negative or diverging lens means (104) is a cub-aperture lens having a diameter less then or equal to 35%ofthediameterofthe primary reflector (103).

   4 A telescope as claimed in claim 3 wherein the sub-aperture negative or diverging lens means (104) has a diameter of between 15 and 25% of the aperture of the primary reflector A telescope as claimed in any preceding claim wherein the first and second correcting means (104,106) are arranged to amplify the focal ratio ofthe telescope (100) by at least one and a half times the focal ratio of the primary reflector (103).

   6 A telescope as claimed in anyofclaims 1 to 5 whereintheprimaryreflector(103) has a focal ratio of between f, and fs.

   7 A telescope as claimed in claim 6 whereintheprimaryreflector(103)has a focal ratio of f3.

   1 () 8 A telescope as claimed in any preceding claim wherein the overall focal ratio of the telescope (100) is at least Is.

   9 A telescope as claimed in any preceding claim wherein the secondary reflector (105) comprises aplanarmirrorpositioned substantiallycoaxiallywith theprimaryreflector (103) and angled to divert light reflected from theprimaryreflector(103)to the second correcting means (106).

   A telescope es claimed in claim 9 wherein the secondaryreflector(105) is elliptical in shape.

   11 A telescope as claimed in any preceding claim wherein at least one ofthe first or second correcting means (104, 106) comprises an achromatic doublet.

   12 A telescope as claimed in anypreceding claim wherein at least one ofthe first or second correcting means (104, 106) comprises an achromatic triplet.

   l 3 A telescope as claimed in anyprecedingclaimwherein the secondaryreflector (105) is mounted on a Cat optical window within the telescope.

   14 A telescope as claimed in anyofclaims I to 12 wherein the secondaryreflector(105) is mounted on a mechanical spider or the like within the telescope.

   l 5 A telescope as claimed in any preceding claim wherein the first correcting means ( 106) is co-mounted with the secondary reflector (105) within the telescope.