History of the 13-inch photographic telescope and its use since the discovery of Pluto

ICARUS44, 7-11 (1980)

History of the 13-Inch Photographic Telescope and Its Use Since the Discovery

of Pluto

H. L. G I C L A S Lowell Observatory., Flagstaff, Arizona 86002

Received March 1, 1980; revised July 14, 1980 Immediately after the discovery of Neptune, the possibility of a more distant planet became a valid field of interest. Stimulated by numerous unsuccessful approaches by others, Percival Lowell became interested in the problem. The background that led to the development of several observational programs based on his early theoretical study of all the observed comet orbits in Galle's catalog is outlined. From these experiences, Lowell's successors specified and constructed the 13-inchphotographic search telescope. The use of the telescope for comet and minor planet work following the discovery of Pluto by Clyde W. Tombaugh in 1930 is described, followed by a summary of a 20-year program of proper motion studies by H. L. Giclas, utilizing the early planet search plates as first-epoch observations. Ancillary equipment developed to facilitate observations and reductions of measurement is described. As early as 1902 Dr. Percival Lowell, founder of the Lowell O b s e r v a t o r y , bec a m e interested in the problem of a transNeptunian planet from a study o f the orbits o f all o b s e r v e d c o m e t s as cataloged by J. G. Galle. He did not claim originality of the idea, as others, notably Flammarion in 1879 and Forbes later, had used this a p p r o a c h , as in m o d e m times did Brady with one comet: H a l l e y ' s in 1972 (Brady, 1972). Earliest records o f an actual search in the sky at his o b s e r v a t o r y at Flagstaff date from plates taken by John C. Duncan with the 24-inch refractor in 1905. As the field c o v e r e d by these plates was only 50 arcmin and the magnitude limit was about 13.5 for an e x p o s u r e o f 1 hour, it was not a very efficient search instrument. Following this, a Voigtlander lens (f.l. = 31", scale 4'.5/mm) and a Roettger lens (f.l. = 29", D = 6"3) were used to m a k e the search plates. From the experience gained from these early search attempts, a 5-inch (f.I. = 35") lens was ordered from John A. B r a s h e a r of Pittsburgh. It was used by successive observers, J. C. Duncan, E. C. Slipher, and K. P. Williams, until S e p t e m b e r 1907. This series c o v e r e d the entire invariable plane with overlapping plates made e v e r y 5 ° with

e x p o s u r e s o f 3 hours each, reaching below the 16th magnitude. During this period, Lowell initiated additional analytical investigations in an effort to "finger p o s t , " as he put it, the area to search. For this work he e m p l o y e d William T. Carrigan, a c o m p u t e r for the U.S. Naval O b s e r v a t o r y , to systematically reduce the residuals in longitude of both Uranus and Neptune. Upon completion of the 40-inch reflector, a second search program was initiated with it by C. O. L a m p l a n d in March 191 l, and the search technique was further augmented by the delivery of a Zeiss blink c o m p a r a t o r in July o f that year. L a m p l a n d had r e c o m m e n d e d the purchase of the comparator 5 years earlier. While the magnitude reach of the 40-inch was most satisfactory, its d r a w b a c k , like that of the 24-inch refractor, was the very small field o f good definition. In order then to k e e p the search going with the continued analytical develo p m e n t s , a 9-inch photographic doublet (f.I. = 46"), made by Brashear and the p r o p e r t y of the Sproul O b s e r v a t o r y of S w a r t h m o r e College, was secured on loan. About 1000 plates along the ecliptic were made between April 1914 and July 2, 1916,

O019-1035/80/100007-05502.00/0 Copyright ~ 1980 by Academic Press, Inc. All rights of reproduction in any form reserved.

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when the instrument was returned to Swarthmore. A by-product of this search was the publication of 515 asteroid positions. The theoretical investigations culminated with a paper given by Lowell before the American Academy of Science in January 1915 on " A Trans-Neptunian Planet," which was published later that year as an Observatory Memoir. With Lowell's death in 1916 and the uncertainties created by the litigation of his will which followed for several years, very little progress was made on a search. That it was still very much on the minds of the two remaining senior staff members, V. M. Slipher and C. O. Lampland, is seen from the fact that Lampland at the end of World War I inquired, through Ellis Str~mgren in Copenhagen, if Max Wolf of Heidelberg, Germany, would lease one of the 16-inch Bruce photographic objectives to the Observatory. Also about this time, Guy Lowell, the first appointed Trustee of the Observatory after Lowell's death, inquired of the Reverend Joel Metcalf of Taunton, Massachusetts, considered one of the most skilled optical craftsmen in America, if he would assist the Observatory in securing a suitable photographic objective. Nothing came of these inquiries at the time; but after the death of the Reverend Metcalf in 1925, Guy Lowell, with his own personal funds, purchased from the Metcalf estate three pieces of unfinished glass components for a photographic triplet 13 inches in diameter. Before anything could be done with them, however, Guy Lowell died unexpectedly of a stroke in 1927. Roger Lowell Putnam, a nephew of Percival Lowell, then became Trustee and finally that year succeeded in ending the more than l0 years of legal litigation which had hamstrung and demoralized the operation of the Observatory since Percival Loweli's death. As his second undertaking, Roger Putnam assisted Slipher and Lampland in converting the Metcalf glass disks into a suitable search telescope. Funds

from the Lowell Endowment were very limited, so Roger Putnam appealed to his uncle, A. Lawrence Lowell, then president of Harvard University and Percival's brother, to give the funds necessary to finish a lens and build a telescope mounting and dome. Ten thousand dollars was the amount of the gift from A. Lawrence Lowell. After considerable discussion and negotiation, it was decided to award the contract for making the lens to C. A. R. Lundin, the experienced optician with Alvan Clark & Sons, the makers of the 24- and 40- inch telescopes, for the sum of $4000. Mr. Putnam then asked Dr. Slipher if he could build the mounting and dome for the balance of the money donated, to which his answer was "'yes." The work on the mounting and dome began in early 1928; a blueprint of the lens arrived in February with comments from Lundin that it might cover as much as a 20° field, but not with perfectly sharp images to the edge. The final choice was a standard 14 × 17-inch plate which gave a field of 12 × 15 degrees in the sky with a loss of about one magnitude from the center to the edge. The cross-axis telescope mounting was designed and built by Stanley Sykes, an instrument maker trained at Finsbury Tech in England and who, with his brother Godfrey, had built the 24-inch refractor dome in 1896 for Percival Lowell, and who had remained in the employ of the Observatory more or less ever since that time. The mounting and dome were finished in the fall of 1928 on schedule, but it was not until February l I, 1929, that the lens arrived. After almost 2 months of adjustment, testing, and overhauling of the old original Gaertner 24-inch telescope drive clock for use as the drive for the 13-inch, the search was really begun systematically by Clyde Tombaugh on April 6, 1929. So much for the history of the telescope. It is not the domain of this review to retell the exciting story of the discovery of Pluto by Clyde Tombaugh that we are celebrating here today. It has been well done in

HISTORY OF THE PLUTO TELESCOPE great detail by Putnam and Slipher (1932); by Clyde Tombaugh himself (1960); and, more recently and most creditably, by William G. Hoyt (1980) in his book Planets X and Pluto, from which I have drawn heavily for part of the early history. Suffice it to say that after the excitement of the discovery of Pluto on plate Nos. 165 and 171 just 50 years ago today subsided, it was decided to go on with the survey so that it would be possible to say that there were no additional distant planets brighter than the search limit of roughly 16.5 magnitude. An additional 1440 plates were made after the discovery plate for the extended search program. Clyde Tombaugh continued the blink examination of these plates until about eight-tenths of the entire sky paralleling both sides of the ecliptic were searched. As part of his blink examination, Clyde Tombaugh routinely marked all the asteroids. C. O. Lampland (Lampland and Newman, 1932 and continued), with the help of young assistants, beginning in 1932 published approximate positions for 1466 of the approximately 4000 asteroids marked by Tombaugh. H. L. Giclas has continued this program to the present time, supplementing many of these early approximate positions with positions accurately remeasured with a special measuring engine designed to take the large 14 x 17-inch plates. Beginning in 1936, Giclas has used the 13-inch telescope for the observation of comets; short exposures were utilized for accurate position measurements, with longer exposures following the motion for a record of detail in the head and tail of brighter comets. There is now a library of over 2000 comet plates made with this telescope. A special plate drive that covers 50 square degrees and which follows the motion of any object moving up to 3 degrees a day while the telescope is guided at the sidereal rate (Metcalf method) has been built. It is most useful for observing Apolloand Amor-type objects that move so fast they would not be recorded at the normal

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sidereal drive rate. One of each of these types has been discovered with the 13-inch: Anza and 1947XC recovered late last year as 1979XA by E. Helin and now numbered 2201. For special search purposes a variable-rate drive in declination has been installed which, when combined with a variable-frequency control for the drive clock, allows the telescope itself to drive at other than sidereal rate. Several problems in photometry have been solved with the telescope over the years, like BN Monocerotis by F. K. Edmondson and H. L. Giclas in 1944 (1944); RY Lyncis, an Algol-type variable, in 1945 (Giclas, 1945); and nova and supernova magnitudes in 1939 (Giclas, 1939). More recently it was used for a Ph.D. thesis in photometry by Ray McNeil, an Ohio State University student. The original search plate series represents the most homogeneous and complete coverage of the entire sky accessible from this latitude. It is not surprising, therefore, that in 1957 a plan was inaugurated to utilize these plates as the first epoch for a proper-motion survey (Giclas, 1965). This program, after 23 years, is drawing to a conclusion. The entire northern hemisphere and one-quarter of the southern hemisphere were blink-examined independently by two different observers for stars with motions >0'.'2/year. The interval between the two plates blinked at the inception of the program was 28 years, the last few plates over 47 years. Over 800 pages of proper-motion data have been published and distributed, mostly in the Lowell Observatory Bulletin (Vols. IV-VIII). Special supplementary equipment for processing these data was developed. The first piece of equipment acquired at the onset was a precision projection blink built by C. Ridell of Chicago. At the beginning, proper motion and direction were read directly from a calibrated screen, and the coordinates were determined with a theodolite used as an analog device. Later, the whole operation was auto-

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H.L. GICLAS

mated so that the raw measures made by each observer were reduced, compared for consistency, and then combined and printed out in catalog form complete with coordinates in the sky. A useful minor fallout from this program still retained on the projection blink is the capability of measuring approximately (4 arcsec) positions and daily motion of asteroids from two large field plates. Rectangular coordinates for 20 to 40 comparison stars can be measured along with any number of asteroids that appear on one plate; the same asteroids identified on a plate made a day or more before or after can also be measured with respect to the same comparison stars with the exact superposition of the faint stars around the asteroid on both plates. During the first half of the proper-motion program, and in spite of the fine cooperation of the businessmen and city officials of Flagstaff in passing a searchlight and sign illumination code to protect the environment for the three observatories in the area, the scattered light from the growing city seriously affected the performance of the wide-field 13-inch. In spite of the inherent risk involved in moving a telescope where the success of the program depended absolutely on the retention of the exact same collimation of the lens over many years, the telescope was successfully moved from its original location on Mars Hill to a dark-sky site 12 miles southeast of Flagstaff in 1970, thus providing the needed reprieve from deterioration. In order to evaluate the proper-motion program, Table I presents various statistics. This program has supplied a wealth of original raw material for many diversified astrophysical investigations, and it is most gratifying to see the use that has been made of this survey that gives an insight into the nature of many of these low-luminosity objects. One example is G51-15, a star that has an absolute visual magnitude near 17 (Dahn et al., 1972). Perhaps the greatest

TABLE 1 SUMMARY OF LOWELL OBSERVATORY PROPER-MOTION PROGRAM Number of stars listed Number of different stars a Newly discovered (50%) New motion > l'.'00/year New motion >0'.'50/year New pairs common motion New companions added to known motion stars White dwarf suspects >lE20/year White dwarf suspects <0"20/year Minor planets Comets Very red stars (GR-) <0"20/year Finding charts published

16,788 I 1,786 5,923 64 875 156 I0 I 197 1,712 1,546 5 504 14,119

Combining duplicate measures from overlapping regions.

impetus to the understanding of these objects was the photometry, radial velocity, and spectral classification work of Eggen and Greenstein (1965a,b, 1967) in their three A s t r o p h y s i c a l J o u r n a l papers, followed by the determination of the parallaxes of many of the objects by the U.S. Naval Observatory (Riddle et al., 1970; Routly, 1972; Harrington et al., 1975; Dahn et al., 1976). A detailed inventory is beyond the scope of this paper, but to mention a few of the most singular objects: There is the first helium emission-line star (G61-29), identified by Burbidge and Strittmatter (1971), which turned out to be an eclipsing binary as well (Warner, 1972). Many highfrequency, oscillating, variable polarized white dwarfs were found, as was a pair of mainsequence red dwarfs (G208-44, 45) of absolute magnitudes 15.0 and 15.6, respectively, and approximately 0.1 solar mass at a distance of 4.7 parsecs (Harrington and Dahn, 1974). Another interesting one is GD428, a dwarf Cepheid of the lowest intrinsic luminosity of all known dwarf Cepheids, with the shortest period (about 55 minutes), 0.2 solar mass, and an absolute visual magnitude of 5.5 at a distance of 100 parsecs (Berg and Duthie, 1977). Of the

HISTORY OF THE PLUTO TELESCOPE 1900 w h i t e d w a r f s u s p e c t s listed, t w o - t h i r d s o f t h o s e s a m p l e d are c o n f i r m e d . E v e n with the t e r m i n a t i o n o f the p r o p e r m o t i o n p r o g r a m , the t e l e s c o p e is still v e r y m u c h in use b y s e v e r a l staff m e m b e r s . In a d d i t i o n to the o n g o i n g p r o g r a m o f p o s i t i o n m e a s u r e m e n t of comets and asteroids by E. Bowell a n d H. G i c l a s , it has b e e n u s e d a s t r o m e t r i c a l l y to a c c u r a t e l y c h e c k posit i o n s o f stars that m a y be o c c u l t e d b y a s t e r o i d s o r a p l a n e t like S a t u r n . R e c e n t l y a c o a r s e g r a t i n g has b e e n d e s i g n e d b y O. G. F r a n z for f u r t h e r r e f i n e m e n t o f a s t r o m e t r i c r e d u c t i o n s o f p o s i t i o n s o f faint stars from the b r i g h t e r s t a n d a r d r e f e r e n c e star catalogs for the o c c u l t a t i o n p r o g r a m s . A n d p e r h a p s , in s u m m a r y , in h o n o r i n g C l y d e T o m b a u g h , the first u s e r o f the teles c o p e , the m o s t fitting t r i b u t e we c a n give this t e l e s c o p e is that it has fulfilled the h o p e that V. M. S l i p h e r e x p r e s s e d w h e n he ans w e r e d R o g e r P u t n a m ' s letter in 1927 acc e p t i n g the gift for its c o n s t r u c t i o n from A. L a w r e n c e L o w e l l , w h e n he said: We hope this instrument will have a useful career and that President Lowell will find satisfaction in the results attained through its use. I b e l i e v e it has fulfilled this h o p e a n d s o m e h o w feel s e c u r e that, in the h a n d s o f g e n e r a t i o n s to c o m e , it will c o n t i n u e its " u s e f u l Ca r e e r. ' '

REFERENCES BERG, R. A., AND DUTHIE, J. G. (1977). GD 428: An extreme dwarf Cepheid. Astrophys. J. 215,

L25-L27. BRADY, J. L. (1972). The effect of a trans-Plutonian planet on Halley's Comet. Pub/. Astron. Soc. Pacific 84, 314-322. BURBIt~E, E. M., AND STRITTMATTER, P. A. (1971). G61-29, a helium emission-line star. Astrophys. J. 170, L39-L42. DAHN, C. C., BEHALL, A. L., GUETTER, H. H., PRISER, J. B., HARRINGTON, R. S., STRAND, K. AA., AND RIDDLE, R. K. (1972). The low-luminosity star G51-15. Astrophys. J. 174, L87.

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DAHN, C. C., HARRINGTON, R. S., RIEPE, B. Y., CHRISTY, J. W., GUETTER, H. H., BEHALL, A. L., WALKER, R. W., HEWITT, A. V., AND ABLES, H. D. (1976). Publ. o f the U.S. Naval Observatory, Second Series, Vol. 24, Pt. III. U.S.

Govt. Printing Office, Washington, D. C. EDMONDSON, F. K., AND GICLAS, H. L. (1944). BN Monocerotis: An N-type variable. Astrophys. J. 100, I-7. EGGEN, O. J., AND GREENSTEIN, J. L. (1965a). Spectra, colors, luminosities, and motions of the white dwarfs. Astrophys. J. 141, 83-108. EGGEN, O. J., AND GREENSTEIN,J. L. (1965b). Observations of proper-motion stars. II. Astrophys. J. 142, 925-933. EGGEN, O. J., AND GREENSTEIN, J. L. (1967). Observations of proper-motion stars, 11I. Astrophys. J. 150, 927-942. GICLAS, H. L. (1939). Photographic magnitudes of the supernova in NGC 4636. Publ. Astron. Soc. Pacific 51, 166-168. GICLAS, H. L. (1945). An eclipsing star in Lynx. Astron. J. 51, 2(10-201. GICLAS, H. L. (1966). The Lowell proper motion program. In Vistas in Astronomy (A. Beer, Ed.), Vol. 8, pp. 23-32. Pergamon, New York. HARRINGTON, R. S., DAHN, C. C., BEHALL, A. L., PRISER, J. B., CHRISTY, J. W., RIEPE, B. Y., ABLES, H. D., GUETTER, H. H., HEWITT, A. V., AND WALKER, R. W. (1975). Publ. o f the U.S. Naval Observatory. 2nd ser., Vol. 24, Pt. I. U.S.

Govt. Printing Office, Washington, D.C. HARRINGTON, R. S., DAHN, C. C., AND GUETTER, H. H. (1974). The nearby double star G208-44/45. Astrophys. J. 194, L87. HOYT, W. G. (1980). Planets X and Pluto. Univ. of Arizona Press, Tucson. LAMPLAND, C. O., AND NEWMAN, K. A. (1932). Photographic positions of asteroids. Astr. Nachr. 247, 69-72. PUTNAM, R. L., AND SLIPHER, V. M. (1932). Searching out Pluto---Lowell's trans-Neptunian Planet X. Scientific Monthly 34, 5-32. RIDDLE, R. K., PRISER, J. B., AND STRAND, K. A~. (1970). Publ. o f the U.S. Naval Observatory,

2rid ser., Vol. 20, Pt. III. U.S. Govt. Printing Office, Washington, D.C. ROUTt.EY, P. M. (1972) Publ. o f the U.S. Naval Observatory, Second Series, Vol. 20, Pt. VI. U.S. Govt. Printing Office, Washington, D.C. TOMaAUGH, C. W. (1960). Reminiscences of the discovery of Pluto. Sky & Telescope 19, 264-270. WARNER, B. (1972). G61-29. Intern. Astron. Union Circ. No. 2388.