1、 MAKING YOUR OWN TELESCOPE Allyn J.Thompson Por to the timeof the telescope,mans view of the celestial universe was woefully restricted when compared with what now can be enjoyed on any clear evening with ordinary binoculars.There were visible to him then only the naked-eye objects,the sun and the m
2、oon,five of rhe planets,and on a clear night stars down to about the 6th magnitude, some 2000 in all. A few hazy spots could also be seen,and there would be an occasional comet.Completely unknown were the outer planets,satellites of the planets,Saturns rings,and infinite numbers of stars and galaxie
3、s. Yet,working without optical aid,early observers managed to make some amazingly accurate charts of the visible stars,and amassed the observations from which the laws of planetary motion were deduced.The principal instrument used in establishing star and planet positions was the quadrant,a device h
4、aving a graduated arc,and a pointer that pivoted about its center.With it Tycho Brahe,Danish astrinimer,and one of the keenest of all observers,was able to record the positions of star to within one minute of arc-about 1/30 the diameter of the moon.This was an amazing feat,when it is considered that
5、 one minute of arc is about the limit of visual acuity. Then,in 1608,seven years after Tychos death,the telescope was brought upon the scene by a Dutch spectacle maker,Jan Lippershry,to whom its invention is credited.Theinvention marked one of the great progressive triumphs of man,enabling him to re
6、ach farther and eer farther out into space.It was not much of a telescope,this first refractor,consisting of two spectacle lenses perhaps an inch in diameter,one convex and the other concave,and magnifying possibly two or three times.Lippershey,whose name historians spell in various ways,managed to
7、combine two such instruments into a unit,and thus also made the first binocular telescope. The Galilean Telescpoe.Very soon,spectacle makers and scientists up and down Europe,learning of lippersheys invention,were making similar instruments.Notable among the scientists was Gakukei Galilei,the great
8、Italian physicist and astronomer,who fitted a plano-convex and a plano-concave spectacle lens into opposite ends of a lead tube,making a telescope that magnified three times.”They appeared three times nearer and nine times larger in surface than to the naked eye,”wrote Galileo.He experimented furthe
9、r and improved this erecting telescope as well as was possible with simple lenses,carrying the magnification up to 30 or more.This was about the limit of its usefulness,however,on account of the great reduction in the size of its field of view. The general arrangement of Galileos telescope is shown
10、inFIg.2.Ordinarily,rays from a distant object AB would,after refraction through the objective lens O,meet to form an inverted image ba in the focal plane,but by interposing the concave eye lens E in front of that plane the rays are caused to become divergent,as though they had proceeded from the poi
11、nts AB,where a virtual image of the object is formed.This image is erect and magnified；the amount of magnification is the ratio of angle c to angle c. As the eye pupil can hardly embrace all of the rays emerging from lens E,only part of the actual field shown can be utilized.Also.the exit pupil is l
12、ocated inside the instrument.Tfield of view thus depends on the size of the eye oupil,and on the diameter of the objective lens.The Galilean telescope is found today in the form of opera and field glasses,but employing quite moderate magnification:2 to 3 power in the opera glass,and 3 to 6 power in
13、the field glass. The Keplerian Telescope.An improvement on Galileos telescope was made in 1611 by Johannes Kepler,a German astronomer and former pupil of Tycho,who suggested that the converging rays from the objective be allowed to come th a focus,and that the resultant image be magnified with a con
14、vex lens.Fig.3 shows the advantage of this new arrangement.T rats,upon emergence from the eye lens,are now converging；hence more of them and a wider field of view can be taken in by the eye.Projected backward through the eye lens,the rays appear to proceed from BA,where a virtual image,inverted and
15、enlarged,is formed.As before,the amount of magnification is in the ratio of angle c to angle c.Considerably higher magnification can be had with this inverting telescope. But with increasing magnification,the inherent defects of a lens,notably chromatic and spherical aberration were likewise increas
16、ed.The aberrations could be diminished to a considerable extent by lengthening the focus of the objective lens. Consequently,in efforts to reduce these aberrations,enormous proportions were reached,instruments of 130 and 150 feet in length being constructed.Lens diameters up to six inches and more w
17、ere attained.Non-spherical surfaces were also attempted in an endeavor to overcome spherical aberration.With these extremely long telescopes,working fields of only two or three minutes of arc must have been the rule.For comparison,the angular diameter of the planrt Jupiter is almost one minute of ar
18、c,so the trials and patience of these 17th-century astronomers in aiming their exceedingly long instruments can be appreciared. Magnification is a secondary condary consideration of the telescope；its chief function is to collect light,The eye alone gathers a limited amount of light,hence the lununos
19、ity of an object determines its visibility；also,the unaided eye can resolve only a limited amount of detail.An objective lens of the same diameter as the pupil of the eye would not improve vision,regardless of the amount of magnification employed,except that thtough this enlargement the detail in an
20、 object would be made more apparent.A l-inch objective lens,assuming it to be about 3.5 times the diameter of the eye pupil,collects about 13times as much light,and correspondingly fainter objects become visible.The amoun of detail seen is also increased,due to the greater aperture.So it id evident
21、that the early boservers needed larger objectives for greater light-gathering and resolving power. But since spherical aberration increases with the square of the aperture,the only way in which it could be kept under control was to lengthen the focus,but there was a practial limit to what lengths co
22、uld be handled.Moreover,a larger field of view was greatly desired and this could accrue only with the use of shorter focal lengths.While spherical aberration could be pretty well eliminated by the use of two suitably curved lenses of the same kind of glass,there still remained chromatic aberration
23、to be contended with. In the hope of combining lense of different glasses in such a way as to overcome chromatic aberration,Sir Isaac Newton attempted to determine if refraction and dispersion were the same in all optical media.Although his experiment was inconclusive,from it New ton assumed that re
24、fraction and dispersion were proportional to each other,and he decided that nothing could be done to improve the refractor.He therefore directed his energies to the formation of images from concave reflecting surfaces,which are perfectly achromatic, Practical experments with reflectors had already b
25、egun in 1639,but it was not until 1663 that they gained any prominence.In that year a Scottish mathematician,James Gregory,at the age of 24,published a treatise entitled Optica Promota.In this he gave a description of a compound reflecting telescope employing two concave specula.The larger one was t
26、o be perforated,and to have a paraboloidal surface； the smaller was to be ellipsoidal. The arrangement is shown in Fig.6.Notice that the ellipsoidal mirror s is place beyond the focal point F of the primary,which is also of the foci of the ellipsoid.From this position,the secondary mirror returns th
27、e rays to form an erect image at its other focus f. High magnification could be had with this instrument,the second reflection amplifying the focal length of the primary in the ratio of fs to Fs. Construction of the telescope was undertaken,but whatever chance it may have had of performing creditably was lost by polishing the polishing egent. The unyielding lap was an insurmountable barrier to parabolizing,interest apparently ebbed,and about 60 years were to elapse before a workable model was finally produced.
