1911 Encyclopædia Britannica/Photography, Celestial
PHOTOGRAPHY, CELESTIAL. The requisites for celestial photography are best explained by a comparison with ordinary photography in several essential points.
a. Illumination.—In taking a portrait artificial light is used, being thrown on to the face of the sitter either directly or by reflection. If the day is dull a longer exposure is required, and artificial light may be used when the daylight fails. In photographing the stars there is no question of 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 long exposures, the length increasing as we attempt to get images of fainter objects. The invention of the dry 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 telescope for days, weeks or even years if necessary. On the approach of daylight, the cap is put on the camera, or the plate removed into the dark room; but when 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 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 image we must lengthen the focus of the camera, either directly or indirectly. The direct method is to construct a lens or 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 eye-piece and put in a photographic plate instead. If, however, we already have a lens of 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 case the profitable magnification is limited, not only by the imperfections of the optical apparatus but by disturbances in the atmosphere. Air currents, either outside or inside the telescope, 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 escape from the necessity of looking through many miles of it.
c. Steadiness.—In taking a portrait the photographer is only concerned to fix his camera firmly and to induce his sitter to remain still. The heavenly bodies are in constant motion, though their real and apparent movements are fortunately smooth, except for air disturbances above mentioned. If, therefore, it were possible to devise perfectly smooth clockwork, we could keep the camera or telescope continually pointed to the required 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 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 object steadily on the cross wires 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 fine movements were required, electrical arrangements were devised, whereby the observer, on simply pressing a button, could accelerate or retard the rate of the clockwork by a minute amount, instead of actually turning the screws by hand. And about the same time the idea arose of making these corrections automatically. This automatic correction is based on the principle that a freely swinging pendulum, which has no 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 control, and automatic electric control, are used in taking all celestial photographs.
The Photographic Image.—The image of a star on 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 middle. One 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 colours into even approximate focus; the other colours are not brought to focus at all, and form concentric patches of fainter light on the plate, which increase in size with the error of focus. Thus at best our focusing is only a 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 violet end of the spectrum. As the exposure proceeds the faint light of the other colours affects the plate by accumulation, and hence the image spreads, while at the same time the central 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 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 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 kite. As the exposure is prolonged the small kite-shaped figure gradually increases in size from the point towards the 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 practical problem of great importance to have this distance as great as possible, so that the field of good definition may be large. Estimating in terms of angular distance from the centre of the field, the reflecting telescope has a good field of not more than 40′; a telescope with one compound lens (the ordinary refractor) a field of about 1°, while if two compound lenses are used (as is the case in portrait photography) the field may be very greatly extended, 10° or 15° having been successfully covered. This is naturally a very great advantage of the “doublet” over other forms of telescope, an advantage which has only recently been fully realized. But there is a compensating drawback; to get a large field we must either use a large plate, which is liable to bend or to have a permanent curvature; or if we use a small plate the picture will be on a small scale, so that we lose accuracy in another way.
Star Charts may thus be made by photography with any desired combination of these advantages. The Cape Photographic Durchmusterung is a photographic survey of the southern hemisphere by means of 250 plates each covering 5° × 5° taken at the Royal Observatory, Cape of Good Hope; the plates being afterwards measured at Groningen in Holland by Professor J. C. Kapteyn who recorded the places to 0s.1 and 0′.1. A much higher degree of accuracy is aimed at in the international scheme for a map of the whole sky undertaken jointly by eighteen observatories in 1887. The plates are only 2° × 2°, and each of the eighteen observatories must take about 600 to cover its zone of the sky once, 1200 to cover it twice. Exposures of 6 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 réseau 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 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 resolution of the executive committee, is not to exceed ±0⋅20″. An additional scheme for a 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. 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 history of remarkable objects can be read backwards when once attention is drawn to them. Thus the history of the asteroid 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 discovery, and the 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 glass prism in front of the object glass of a telescope the light from each star can be extended into a spectrum: and a chart can thus be obtained showing not only the relative positions, but the 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 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 surface
of the plate. The point N is of fundamental importance in the geometry of the star chart and it is natural to 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 adjustment, the object glass should be “squared on” to the tube by a 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 plane.
The field of the object glass will in general be curved: so that the points of best focus for different stars lie on a surface such as AG (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 ring on the plate. The star A will thus be represented y 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 line Ca drawn from the star to the optical centre cuts the plate. If this can be done, then the chart represents the geometrical projection of the heavens from the point C on to the plane PL. The stars are usually conceived as lying on the celestial sphere, with an arbitrary 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 property of projections that a straight line projects into a straight line; and in the present instance we may add that every straight line corresponds to a great circle on the celestial sphere. Hence if we measure any rectilinear coordinates (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)/(1-kx−ly), Y＝(dx+ey+f)/(1−kx-ly), | (1) |
the numerators being any linear functions of (x, y) but the denominators being the same linear function. When x＝0, y＝0, then X=c and Y=f, which are thus the co-ordinates of the origin of (xy) on plate (XY). The co-ordinate of the origin of (XY) on plate (xy) can be shown to be (k, l) if proper units of length be chosen.
As a particular case the co-ordinates
| x＝cot δ cos α, y＝tan δ sin α | (2) |
represent the rectangular co-ordinates of a star of RA and declination a and δ, projected on the tangent plane at the north pole. If the same star be projected on the tangent plane at the point (A, D), then its rectangular co-ordinates (δ, η) will be
| ξ =tan (a−A) sin q sec (q−D), η＝tan (q−D), | |||
| where tan q＝tan δ sec (a−A), | (3) |
the axis of η being directed towards the pole. It can readily be verified that (ξ, η) can be expressed in terms of (x, y) by relations of the form (1). The co-ordinates (ξ, η) have been named “standard co-ordinates” and represent star positions on an ideal plate free from the effects of refraction and aberration. For plates of not too large a field, 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 (ξ, η) by linear relations. The linearity of these relations is obviously not disturbed by the choice of origin of axes and of orientation; in which the effects of procession and mutation for any epoch may be included. Hence to obtain the standard co-ordinates (ξ, η) of any object on a plate it is only necessary to know the position of the plate centre (the point N in fig. 1) and the six constants in the relations
| ξ＝Ax+By+C, η＝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 (ξ, η) are known: but in practice it is better to use as many “known” stars as possible. These equations are well adapted to solution by least squares or any equivalent 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. Common (1841–1903), F.R.S., of Ealing, led the way in 1883 with a successful picture of the great nebula in Orion, taken with a 3 ft. concave mirror by Calver. Dr Isaac Roberts (1829–1904) was the first to show the real structure of the great nebula in Andromeda, by a photograph also taken with a reflector. In the clear atmosphere of the Lick Observatory in California, small nebulae were photographed in great numbers by Professor J. E. Keeler (1857–1900): and it was shown what a large percentage were spiral in form. Prof G. W. Ritchey, at the Yerkes Observatory, has followed up these successes with a 2-ft. reflector, and is constructing a 5-ft., to be erected on Mt Wilson (Cal.); but he has also shown that pictures of clusters are best taken with a telescope of long focus, such as the great Yerkes refractor; and incidentally that this telescope, although intended for visual work, can be adapted to photography by using a “colour screen” just in front of the plate, which sifts out the rays not brought to focus.
Photography of the Moon.—G. W. Ritchey has used the same device of a colour screen for the moon, and obtained even better pictures than those obtained at Paris, which were previously the best. The positions of a large number of craters and other points have been measured by Dr J. H. G. Franz and S. A. Saunder on photographs, and a new epoch in lunar topography has thereby been created.
Photography of the Planets.—Some striking successes have been obtained at the Lowell Observatory, Flagstaff, Arizona: by cutting down the aperture of the object-glass some of the delicate markings, called canals, on the planet 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. Barnard and others. Here, as in the case of nebulae, the photograph is 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 appearance of violent shattering, and if successive pictures can be obtained at such times we may learn something of the nature of such disturbances.
Solar Photography.—The light of the sun is so intense that the 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 limb, owing to the absorption of light by the solar atmosphere; and the bright faculae (which are thus inferred to lie above the 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 spectroheliograph, which is an instrument for photographing in the light of one very definite colour—say a single hydrogen line. The faculous appearances can be photographed 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. But for a full account of such matters reference must be made to the articles Sun and Spectroheliograph.
Authorities.—Various papers in the Monthly Notices of the Royal Astronomical Society and in the Astrophysical Journal. Also the bulletins and circulars of the Harvard, Lick and Yerkes Observatories; and of the Executive Committee for the Astrographic Catalogue (published by Gauthier Villars for the Paris Académie des Sciences). See also more especially a paper by G. W. Ritchey in the Decennial Papers of the University of Chicago, reprinted in vol. ii. (1903) of the Yerkes Observatory Publications. (H. H. T.)