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PHOTOGRAPHY, CELESTIAL
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See also:PHOTOGRAPHY, See also:CELESTIAL . The requisites for celestial photography are best explained by a comparison with See also:ordinary photography in several essential points. a. See also:Illumination.—In taking a portrait artificial See also:light is used, being thrown on to the See also:face of the sitter either directly or by reflection. If the See also:day is dull a longer exposure is required, and artificial light may be used when the daylight fails. In photo-graphing the stars there is no question of See also:illuminating them by artificial light; for the strongest searchlight which we could throw in the direction of the heavenly bodies would have no sensible effect. The light used is their own, and its feebleness renders it necessary to make See also:long exposures, the length increasing as we See also:attempt to get images of fainter See also:objects. The invention of the dry See also:plate, by making it possible to give very long exposures; caused a revolution in celestial photography. With the wet plate, exposures were limited to the few minutes during which the film would remain wet; but the dry plate can remain in the See also:telescope for days, See also:weeks or even years if necessary. On the approach of daylight, the cap is put on the See also:camera, or the plate removed into the dark See also:room; but when See also:night returns the plate is put back in the telescope, which is accurately pointed to the same stars, the cap is removed, and the exposure is resumed without any loss from the interruption. b. Magnification.—In taking a portrait we can obtain a large or small See also:size by placing the camera near the sitter or far away. But this method is not available for the heavenly bodies, since we cannot sensibly approach them. To magnify an See also:image we must lengthen the See also:focus of the camera, either directly or in-directly. The See also:direct method is to construct a See also:lens or See also:mirror of long focus; the camera becomes similar in length to a telescope; and indeed resembles a telescope in other respects, except that we take away the See also:eye-piece and put in a photographic plate instead. If, however, we already have a lens of See also:short focus which we wish to use, we may lengthen the focus indirectly by using a secondary magnifier, that is by putting in another lens near the focus of the first. In either See also:case the profitable magnificationis limited, not only by the imperfections of the See also:optical apparatus but by disturbances in the See also:atmosphere. See also:Air currents, either outside or inside the telescope, See also:act as irregular lenses of varying shape, and produce such defects in the image that we gain nothing by enlarging it beyond a certain point. Such air disturbances do not trouble the ordinary photographer at all, or scarcely at all: he is only concerned with a few feet of air, whereas the celestial photographer cannot See also:escape from the See also:necessity of looking through many See also:miles of it. c. Steadiness.—In taking a portrait the photographer is only concerned to See also:fix his camera firmly and to induce his sitter to remain still. The heavenly bodies are in See also:constant See also:motion, though their real and apparent movements are fortunately smooth, except for air disturbances above mentioned. If, there-fore, it were possible to devise perfectly smooth clockwork, we could keep the camera or telescope continually pointed to the required See also:star or stars. But human workmanship has not yet made clockwork of sufficient strength and accuracy to keep a large telescope satisfactorily pointed. The clockwork which had been found See also:good enough for use with visual telescopes was soon found to be quite inadequate for photography. The first method adopted was to bind two telescopes, one visual and the other photographic, firmly together; and by looking through the visual one to keep some See also:object steadily on the crosswires by using the slow motion screws; meanwhile the other telescope was kept properly pointed for taking a photograph. As it was sometimes found that extremely See also:fine movements were required, See also:electrical arrangements were devised, whereby the observer, on simply pressing a See also:button, could accelerate or retard the See also:rate of the clockwork by a See also:minute amount, instead of actually turning the screws by See also:hand. And about the same See also:time the See also:idea arose of making these corrections automatically. This automatic correction is based on the principle that a freely swinging pendulum, which has no See also:work to do, will naturally keep much better time than the clockwork which has to drive a heavy telescope; and if such a pendulum is therefore arranged to send a current every second through certain electro-magnets, apparatus can be devised to detect whether the clockwork is going properly; and to correct it in the right direction, if it is not. One or more of these three methods, which may be called hand-guiding, electrical See also:control, and automatic electric control, are used in taking all celestial photographs. The Photographic Image.—The image of a star cal the plate should be, theoretically, merely a point; but in practice it is a small patch on the plate which grows in size as the exposure is lengthened, while at the same time it becomes darker in the See also:middle. One See also:reason for this is that light is many-coloured, and when we attempt to focus it by a lens, we can only get a very few See also:colours into even approximate focus; the other colours are not brought to focus at all, and See also:form concentric patches of fainter light on the plate, which increase in size with the See also:error of focus. Thus at best our focusing is only a See also:compromise. When the exposure is short, those colours which have most nearly been brought to focus have an effect, while the faint light of the others may produce no sensible impression. It is natural to select for the colours to be brought most sharply to focus those which are most important photographically, viz. those at the See also:violet end of the spectrum. As the exposure proceeds the faint light of the other colours affects the plate by See also:accumulation, and hence the image spreads, while at the same time the central See also:part naturally becomes blacker. A reflecting telescope brings all colours to the same focus; and it might appear, therefore, that images formed with it will not spread in this way. There is, however, another cause of spreading besides that due to See also:colour; neither the reflecting telescope nor the lens can focus all the light received by them for more than one particular star. It is just theoretically possible to construct a mirror which would focus all the light from a star seen in the direction of its See also:axis; but the light from another star seen in a slightly different direction would not be truly focused. since directly we leave the axis, some parts of the mirror have a focus slightly different from other parts; and if the image
produced is magnified, it is seen to have a shape like that of a face of the plate. The point N is of fundamental importance in the, See also:kite. As the exposure is prolonged the small kite-shaped figure
gradually increases in size from the point towards the See also:head, and this defect is the more pronounced the farther we depart from the centre of the plate. The result is, speaking generally, that the images near the centre of a plate may be fairly small and circular, but at a certain distance from the centre they become distorted and large. It is a See also:practical problem of See also:great importance to have this distance as great as possible, so that the See also: C. Kapteyn who recorded the places to Oa'1 and O''1. A much higher degree of accuracy is aimed at in the See also:international See also:scheme for a See also:map of the whole See also:sky undertaken jointly by eighteen observatories in 1887. The plates are only 20 X 2°, and each of the eighteen observatories must take about 600 to See also:cover its See also:zone of the sky once, 1200 to cover it twice. Exposures of 6 See also:min., 3 min., and 20 sec. are given, the telescope being pointed in a slightly different direction for each exposure; so that each star to about the 9th magnitude shows 3 images, and stars to the 11th or 12th magnitude show 2; which has the incidental advantage of distinguishing stars from dust-specks. A reseau of lines accurately ruled at distances of 5 mm. apart in two directions at right angles is impressed on the plate by artificial light and See also:developed along with the star images; and by use of these reference lines the places of all stars shown with 3 min. exposure are measured with a probable error which, by a See also:resolution of the executive See also:committee, is not to exceed o•2o". An additional scheme for a See also:series of charts enlarged from similar plates with much longer exposure has proved too costly, and only a few observatories have attempted it. Meanwhile Professor E. C. See also:Pickering of Harvard, by using doublet lenses which cover a much larger field at once, has photographed the whole sky many times over. The plates have not been measured, and would not in any case yield results of quite the same accuracy as those of the international scheme; but being systematically stored at the Harvard Observatory they form an invaluable reference library, from which the See also:history of remarkable objects can be read back-wards when once See also:attention is See also:drawn to them. Thus the history of the asteroid See also:Eros, discovered in 1898, was traced back to 1894 from these plates; new stars have been found on plates taken previous to the time of See also:discovery, and the See also:epoch of their blazing up recovered within narrow limits; and the history of many variable stars greatly extended. The value of this collection of photographs will steadily increase with time and growth. Spectroscopic Star Charts.—By placing a See also:glass See also:prism in front of the object glass of a telescope the light from each star can be extended into a spectrum: and a See also:chart can thus be obtained showing not only the relative positions, but the See also:character of the light of the stars. This method has been used with great effect at Harvard: and from inspection of the plates many discoveries have been made, notably those of several novae. The See also:Geometry of the Star Chart.—Let OS in the figure be the object glass with which the photograph is taken, and let its optical centre be C. Let PL be the plate, and draw CN perpendicular to the sur- geometry of the star chart and it is natural to See also:call it the plate centre; but it must be carefully distinguished from two other points which should theoretically, but may not in practice, coincide with it. The first is the centre of the material plate, as placed in position in the telescope. In the figure NL is purposely drawn larger than PN, and this material centre would be to the right of N. The second point is that where the optical axis of the object glass (CG in the figure) cuts the plate. The object glass is drawn with an exaggerated tilt so that CG falls to the right of CN. To secure See also:adjustment, the object glass should be " squared on " to the See also:tube by a See also:familiar operation, so that the tube is parallel to CG: and then the plate should be set normal to the tube and therefore to CG. This is done by observing reflected images, combined with rotation of the plate in its See also:plane. The field of the object glass will in See also:general be curved: so that the points of best focus for different stars See also:lie on a See also:surface such as AGB (purposely exaggerated). The best practical results for focus will thus be obtained by compromise, placing the plate so that some stars, as A, are focused beyond the plate, and others, as B, nearer the object glass: exact focus only being possible for a particular See also:ring on the plate. The star A will thus be represented by a small patch of light, pq on the plate, which will grow in size as above explained. When we measure the position of its image we select the centre as best we can: and in practice it is important that the point selected should be that where the See also:line Ca drawn from the star to the optical centre cuts the plate. If this can be done, then the chart represents the geometrical See also:projection of the heavens from the point C on to the plane PL. The stars are usually conceived as lying on the celestial See also:sphere, with an arbitrary See also:radius and centre at the observer, which is in this case the object glass: describing such a sphere with C as centre and CN as radius, the lines bCB and aCA project the spherical surface on to a tangent plane at the point N, which we call the plate centre. If we point the telescope to a different part of the sky, we select a different tangent plane on which to project. It is a fundamental See also:property of projections that a straight line projects into a straight line; and in the See also:present instance we may add that every straight line corresponds to a great circle on the celestial sphere. Hence if we measure any rectilinear co-ordinates (x, y) of a series of stars on one plate, and co-ordinates (X, Y) of the same stars on another plate, and (x, y) are connected by a linear relation, so must (X, Y) be. This property leads at once to the equations, X= (ax+by+c)/(i —kx—ly), Y= (dx+ey+f)/(1-kx—ly), (i) the numerators being any linear functions of (x, y) but the denominators being the same linear See also:function. When x= o, y=o, then X=c and Y=f, which are thus the co-ordinates of the origin of (xy) on plate (XY). The co-See also:ordinate of the origin of (XY) on plate (xy) can be shown to be (k, 1) if proper See also:units of length be chosen. As a particular case the co-ordinates x=cot S See also:cos a, y=tan I See also:sin a (2) represent the rectangular co-ordinates of a star of RA and See also:declination a and I, projected on the tangent plane at the See also:north See also:pole. If the same star be projected on the tangent plane at the point (A, D), then its rectangular co-ordinates (1;, n) will be =tan (a—A) sin q sec (q—D), n= tan (q—D), where tan q=tan I sec (a—A), (3) the axis of n being directed towards the pole. It can readily be verified that (f, n) can be expressed in terms of (x, y) by relations of the form (i). The co-ordinates (t, n) have been named "See also:standard co-ordinates and represent star positions on an ideal plate See also:free from the effects of See also:refraction and See also:aberration. For plates of not too large a field, See also:differential refraction and aberration are so small that their product by squares of the co-ordinates may be neglected, and the actual star positions (x, y) are connected with (>;, n) by linear relations. The linearity of these relations is obviously not disturbed by the choice of origin of axes and of See also:orientation; in which the effects of procession and mutation for any epoch may be included. Hence to obtain the standard co-ordinates (, n) of any object on a plate it is only necessary to know the position of the plate centre (the point N in fig. I) and the six constants in the relations t=Ax+By+C, n=Dx+Ey+F, (4) where (x, y) are rectilinear co-ordinates referred to any axes. The constants can theoretically be determined when there are three stars on the plate for which f, n are known: but in practice it is better to use as many " known " stars as possible. These equations are well adapted to See also:solution by least squares or any See also:equivalent See also:device. Photography of Nebulae and Clusters.—Some of the earliest and most striking successes in celestial photography were the pictures of nebulae. Dr A. A. See also:Common (1841-19o3), F.R.S., of See also:Ealing, led the way in 1883 with a successful picture of the great nebula in See also:Orion, taken with a 3 ft. See also:concave mirror by Calver. Dr See also:Isaac See also:Roberts (1829–1904) was the first to show the real structure of the great nebula in See also:Andromeda, by a photo-graph also taken with a reflector. In the clear atmosphere of the Lick Observatory in See also:California, small nebulae were photo-graphed in great See also:numbers by Professor J. E. Keeler (1857–1900): and it was shown what a large percentage were See also:spiral in form. Prof. G. W. Ritchey, at the See also:Yerkes Observatory, has followed up these successes with a 2-ft. reflector, and is constructing a 5-ft., to be erected on Mt See also: The positions of a large number of craters and other points have been measured by Dr J. H. G. See also:Franz and S. A. Saunder on photographs, and a new epoch in lunar See also:topography has thereby been created. Photography of the See also:Planets.—Some striking successes have been obtained at the See also:Lowell Observatory, Flagstaff, See also:Arizona: by cutting down the See also:aperture of the object-glass some of the delicate markings, called canals, on the See also:planet See also:Mars have been photographed; but even these do not approach what can be seen by the eye. Photography of Comets.—Some wonderful pictures have been obtained of comets by Professor E. E. See also:Barnard and others. Here, as in the case of nebulae, the photograph is See also:superior to the eye in detecting faint luminosity, and delicate details of the tail structure have been photographed which could never be seen. In several pictures the tails have an See also:appearance of violent shattering, and if successive pictures can be obtained at such times we may learn something of the nature of such disturbances. See also:Solar Photography.—The light of the See also:sun is so intense that the See also:chief difficulty is to obtain a short enough exposure. When successfully taken, photographs of the surface show the well-known spots and the mottling of the surface. The image sensibly falls off in intensity towards the See also:limb, owing to the absorption of light by the solar atmosphere; and the See also:bright faculae (which are thus inferred to lie above the See also:main absorbing layer) are seen near the limb. But an immense advance in solar photography was made about a dozen years ago by the invention of the See also:spectroheliograph, which is an See also:instrument for photographing in the light of one very definite colour—say a single See also:hydrogen line. The faculous appearances can be photo-graphed with this instrument all over the sun's disk, instead of merely near the limb. The appearance presented varies enormously with the line selected, or (in the case of the wide " lines " in the spectrum, such as the H and K lines) with the particular part of the same line selected. Additional information and CommentsThere are no comments yet for this article.
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