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NOTE.
In the original negatives of subjects 10 and 12, there are faint dark
rings immediately surrounding some of the stars in the denser parts of the
nebulosity. This effect has no doubt been accentuated in the subsequent
photographic processes. On the plates of these two subjects in the
completed volume, these rings are very distinct and give rise to a
suspicion that the effect has been enhanced by the engraver. A critical
examination of the prints seems to confirm this view. In the original
proofs these rings were inconspicuous and were not noticed. The processes
of steel-facing and printing appear to have increased the effect markedly,
as it is much stronger on the sheets printed for the edition than in any
of the early proofs.
Inasmuch as these effects were not and could not be discovered until the
sheets were assembled in Sacramento for binding, it has not been thought
desirable to delay the issue of the volume for several weeks additional in
order to have new plates and new prints of these subjects made by the
distant engraver.
Lick Observatory,
Mount Hamilton,
November, 1908.
[Illustration: _Plate 10_
_THE GREAT NEBULA IN ORION_]
UNIVERSITY OF CALIFORNIA PUBLICATIONS
PUBLICATIONS
OF THE
LICK OBSERVATORY
PRINTED BY AUTHORITY OF THE REGENTS OF THE UNIVERSITY
VOLUME VIII
SACRAMENTO
W. W. SHANNON SUPERINTENDENT OF STATE PRINTING
1908
AS A TRIBUTE TO THE MEMORY OF
JAMES EDWARD KEELER
and in recognition of his great worth as a man and as an astronomer, the
plates for this volume have been provided by
MR. WILLIAM ALVORD, MR. F. M. SMITH,
MR. ROBERT BRUCE, MISS JENNIE SMITH,
MR. WILLIAM H. CROCKER, MISS MATILDA H. SMITH,
MRS. WILLIAM H. CROCKER, MR. BENJAMIN THAW,
MR. E. J. DE SABLA, MRS. WILLIAM THAW,
MR. J. A. DONOHOE, MR. ROBERT J. TOBIN,
MRS. PHOEBE A. HEARST, THE UNIVERSITY OF CALIFORNIA,
MR. JOHN B. JACKSON, THE STATE OF CALIFORNIA.
MR. E. J. MOLERA,
ORGANIZATION OF THE LICK OBSERVATORY.
HON. CHARLES W. SLACK, HON. WARREN R. PORTER,
HON. WILLIAM H. CROCKER, REV. PETER C. YORKE,
_Committee of the Regents for the Lick Observatory._
BENJAMIN IDE WHEELER, _President of the University_.
W. W. CAMPBELL, _Director and Astronomer_.
R. H. TUCKER,* _Astronomer_.
C. D. PERRINE, _Astronomer_.
H. D. CURTIS, _Mills Acting Astronomer_.
R. G. AITKEN, _Astronomer_.
W. H. WRIGHT, _Astronomer_.
J. H. MOORE, _Assistant Astronomer_.
SEBASTIAN ALBRECHT, _Assistant Astronomer_.
MISS A. M. HOBE, _Carnegie Assistant_.
G. F. PADDOCK, _Mills Assistant_.
MISS L. B. ALLEN, _Carnegie Assistant_.
E. A. FATH, _Fellow_.
J. C. DUNCAN, _Fellow_.
MISS A. E. GLANCY, _Fellow_.
MISS M. E. FRENCH,* _Secretary_.
MISS A. J. VAN COOVER, _Secretary_.
* Absent on leave.
PHOTOGRAPHS OF NEBULAE AND CLUSTERS,
MADE WITH
THE CROSSLEY REFLECTOR,
BY JAMES EDWARD KEELER,
DIRECTOR OF THE LICK OBSERVATORY.
1898-1900.
PREFACE.
When Professor Keeler entered upon the duties of Director of the Lick
Observatory, on June 1, 1898, he planned to devote his observing time for
several years to photographing the brighter nebulae and star clusters, with
the Crossley reflector. The story of his wonderful success with this
difficult instrument is familiar to all readers of astronomical
literature: this form of telescope was in effect born again; and his
contributions to our knowledge of the nebulae were epoch-making.
Professor Keeler's observing programme included one hundred and four
subjects. At the time of his lamented death, on August 12, 1900,
satisfactory negatives of two-thirds of the selected objects had been
secured. The unphotographed objects were mainly those which come into
observing position in the unfavorable winter and spring months. The
completion of the programme was entrusted to Assistant Astronomer Perrine.
The observers were assisted chiefly by Mr. H. K. Palmer, and in smaller
degree by Messrs. Joel Stebbins, C. G. Dall, R. H. Curtiss and Sebastian
Albrecht.
Professor Keeler's photographs enabled him to make two discoveries of
prime importance, not to mention several that are scarcely secondary to
them.
1st.--"Many thousands of unrecorded nebulae exist in the sky. A
conservative estimate places the number within reach of the Crossley
reflector at about 120,000. The number of nebulae in our catalogues is but
a small fraction of this." [The number already discovered and catalogued
did not exceed 13,000. Later observations with the Crossley reflector,
with longer exposure-times and more sensitive plates, render it probable
that the number of nebulae discoverable with this powerful instrument is of
the order of half a million.]
2d.--"Most of these nebulae have a spiral structure."
The photographs of the one hundred and four subjects contain the images of
744 nebulae not previously observed. A catalogue of these is published in
the present volume. Their positions, which are thought to be accurate
within 1'', were determined by Messrs. Palmer, Curtiss, and Albrecht.
The main purpose of this volume is to reproduce and make available for
study, the larger and more interesting nebulae and clusters on the
programme, sixty-eight in number. The thirty-six subjects not reproduced
are for the most part small or apparently not of special interest. The
difficulties attending the reproduction of astronomical photographs by
mechanical processes are well-known to all who have made the attempt. It
seems necessary to recognize, at least at present, that delicate details
of structure will be lost, and that contrasts between very bright and very
faint regions will be changed, especially if a good sky background is
preserved; in other words, that the best obtainable reproductions fall far
short of doing justice to the original photographs. Technical studies
should be based upon the original negatives or upon copies on glass.
After considerable experimental work, involving several methods and
several firms, the making of the heliogravure plates and the hand-press
prints was entrusted to The Photogravure and Color Company of New York
City. To this firm's continued interest and willingness to act on
constructive criticism is due much of the excellence of the results.
The expensive reproductions could hardly have been undertaken without the
generous assistance of the donors mentioned on a preceding page.
Professor Keeler's description of the Crossley reflector, of his methods
of observing, and of the chief results obtained, was written only a short
time before his death. It is here republished. Other results of his work
are described in the several papers to which the footnotes refer.
TABLE OF CONTENTS.
The Orion Nebula, _Frontispiece_
The Crossley Reflector of the Lick Observatory, Page 11
List of Nebulae and Clusters Photographed, " 30
Catalogue of New Nebulae Discovered on the Negatives, " 31
Positions of Known Nebulae Determined from the Crossley Negatives, " 42
List of Illustrations, " 45
Illustrations, following " 46
THE CROSSLEY REFLECTOR OF THE LICK OBSERVATORY.[1]
By JAMES E. KEELER.
The Crossley reflector, at present the largest instrument of its class in
America, was made in 1879 by Dr. A. A. Common, of London, in order to
carry out, and test by practical observation, certain ideas of his
respecting the design of large reflecting telescopes. For the construction
of the instrument embodying these ideas, and for some fine astronomical
photographs obtained with it, Dr. Common was awarded the gold medal of the
Royal Astronomical Society in 1884.
In 1885, Dr. Common, wishing to make a larger telescope on a somewhat
similar plan, sold the instrument to Edward Crossley, Esq., F. R. A. S.,
of Halifax, England. Mr. Crossley provided the telescope with a dome of
the usual form, in place of the sliding roof used by its former owner, and
made observations with it for some years; but the climate of Halifax not
being suitable for the best use of such a telescope, he consented, at the
request of Dr. Holden, then Director of the Lick Observatory, to present
it to this institution. The funds for transporting the telescope and dome
to California, and setting them up on Mount Hamilton, were subscribed by
friends of the Lick Observatory, for the most part citizens of California.
The work was completed, and the telescope housed in a suitable observatory
building, in 1895.[2]
On taking charge of the Lick Observatory in 1898, I decided to devote my
own observing time to the Crossley reflector, although the whole of my
previous experience had been with refracting telescopes. I was more
particularly desirous of testing the reflector with my own hands, because
such preliminary trials of it as had been made had given rise to somewhat
conflicting opinions as to its merits.[3] The result of my experience is
given in the following article, which is written chiefly with reference to
American readers. If I have taken occasion to point out what I regard as
defects in the design or construction of the instrument, I have done so,
not from any desire to look a gift horse in the mouth, but in the interest
of future improvement, and to make intelligible the circumstances under
which the work of the reflector is now being done and will be done
hereafter. The most important improvements which have suggested themselves
have indeed already been made by Dr. Common himself, in constructing his
five-foot telescope. The three-foot reflector is, in spite of numerous
idiosyncracies which make its management very different from the
comparatively simple manipulation of a refractor, by far the most
effective instrument in the Observatory for certain classes of
astronomical work. Certainly no one has more reason than I to appreciate
the great value of Mr. Crossley's generous gift.
[Illustration: DOME OF THE CROSSLEY REFLECTOR.]
The Crossley dome is about 350 yards from the main Observatory, at the end
of a long rocky spur which extends from the Observatory summit toward the
south, and on which are two of the houses occupied by members of the
Observatory staff. It is below the level of the lowest reservoir,
"Huyghens," which receives the discharge from the hydraulic machinery of
the 36-inch refractor, and therefore the water engine furnished by Mr.
Crossley for turning the dome can not be used, unless a new water
system--overflow reservoir, pump and windmill--is provided. In this
respect a better site would have been a point on the south slope of
"Kepler,"--the middle peak of Mount Hamilton--just above the Huyghens
reservoir. No addition to the present water system would then have been
needed. The slope of the mountain at this place might cut off the view of
the north horizon, but since the telescope can not be turned below the
pole, this would be a matter of no consequence. Water-power for the dome
is not, however, really necessary.
The cylindrical walls of the dome, 36-1/4 feet inside diameter, are
double, and provided with ventilators. Opening into the dome, on the left
of the entrance, are three small rooms, one of which has been fitted up as
a photographic dark room, and another, containing a sidereal clock and a
telephone, which communicates with the main Observatory, as a study, while
the third is used for tools and storage. There is also a small room for
the water engine, in case it should be used. The dome is at present
supplied with water from only the middle reservoir, Kepler, which is
reserved for domestic purposes and is not allowed to pass through the
machinery.
The dome itself, 38 feet 9 inches in diameter, is made of sheet-iron
plates riveted to iron girders. It also carries the wooden gallery,
ladders, and observing platform, which are suspended from it by iron rods.
The apparatus for turning the dome consists of a cast-iron circular rack
bolted to the lower side of the sole-plate, and a set of gears terminating
in a sprocket-wheel, from which hangs an endless rope. As the dome does
not turn easily, it has been necessary to multiply the gearing of the
mechanism so that one arm's-length pull on the rope moves the dome only
about one inch. In some positions of the telescope the dome can not be
moved more than six or eight inches at a time without danger of striking
the tube, and this slowness of motion is then not disadvantageous. It is
only when the dome has to be moved through a considerable angle, as in
turning to a fresh object, or in photographing some object which passes
nearly through the zenith, that the need for a mechanical means of
rotation is felt.
The observing slit, 6 feet wide, extends considerably beyond the zenith.
It is closed by a double shutter, which is operated by an endless rope.
The upper part, within the dome, is also closed by a hood, or shield,
which serves to protect the telescope from any water that may find its way
through the shutter, and which is rolled back to the north when
observations are made near the zenith. I have recently fitted the lower
half of the slit with a wind-screen, which has proved to be a most useful
addition. It is made of tarpaulin, attached to slats which slide between
the two main girders, and is raised or lowered by halliards, which belay
to cleats on the north rail of the gallery. A more detailed description of
the dome has been given in an article by Mr. Crossley,[4] from which the
reduced figure in Fig. 1[5] has been taken.
The mounting of the three-foot reflector has been very completely
described and illustrated by Dr. Common,[6] so that only a very general
description need be given here. The most important feature of the
mounting is that the telescope tube, instead of being on one side of the
polar axis, as in the usual construction, is central, so that the axis of
the mirror and the polar axis are in the same line when the telescope is
directed to the pole. The declination axis is short, and is supported by a
massive goose-neck bolted to the upper end of the polar axis. The mirror
is placed just _above_ the declination axis. Its weight, and the weight of
the whole tube and eye-end, are counterpoised by slabs of lead, placed in
two iron boxes, between which the goose-neck of the polar axis passes. The
great advantage of this arrangement, and the controlling principle of the
design, is that the telescope is perfectly free to pass the meridian at
all zenith distances. No reversal of the instrument is needed, or is
indeed possible.
[Illustration: THE CROSSLEY REFLECTOR.]
For long-exposure photography, the advantage above referred to is obvious,
but it is attended by certain disadvantages. One of these is that a very
much larger dome is required than for the usual form of mounting. Another
is the great amount of dead weight which the axes must carry; for the
mirror, instead of helping to counterpoise the upper end of the tube, must
itself be counterpoised. When anything is attached to the eye-end (and in
astrophysical work one is always attaching things to the eye-end of a
telescope), from ten to twenty times as much weight must be placed in the
counterpoise boxes below the declination axis. Where room is to be found
for the weights required to counterpoise the Bruce spectrograph, is a
problem which I have not yet succeeded in solving.
In his five-foot reflector, Dr. Common has caused the telescope tube to
swing between two large ears, which project from the upper end of the
boiler-like polar axis, the pivots constituting the declination axis being
near, but above, the lower end of the tube. The mirror, therefore, helps
to counterpoise the upper end of the tube. This I regard as a distinct
improvement. The danger of large masses of metal near the mirror injuring
the definition is, in my opinion, imaginary; at least there is no such
danger on Mount Hamilton, where the temperature variations are unusually
small. Experience with the Crossley reflector, as well as with the other
instruments of the Lick Observatory, shows that the definition depends
almost entirely on external conditions.
My first trials of the reflector, as first mounted at the Lick
Observatory, showed that the center of motion was inconveniently high.
Among other difficulties arising from this circumstance, the spectroscope
projected beyond the top of the dome, so that it had to be removed before
the shutter could be closed. In July, 1898, the pier was therefore cut
down two feet. This brought the eye-end down nearly to the level of the
gallery rail, where it was at a convenient height for the observer when
sitting on a camp-stool, and it made all parts of the mounting more
accessible. Toward the north and south, the range of the telescope, being
limited in these directions by the construction of the mounting, was not
affected by the change, but the telescope can not now be used at such low
altitudes as formerly, near the east and west points of the horizon. The
only occasion likely to call for the use of the reflector in these
positions is the appearance of a large comet near the Sun, and, after some
consideration, I decided to sacrifice these chances for the sake of
increasing the general usefulness of the instrument. Except in rare cases,
all observations are made within three hours of the meridian.
To adapt the mounting to the latitude of Mount Hamilton, a wedge-shaped
casting, shown in the illustration, had been provided, but through some
error, arising probably from the fact that the telescope had been used in
two different latitudes in England, the angle of the casting was too
great. When the pier was cut down its upper surface was therefore sloped
toward the south, in order to compensate the error in the casting. Plate
VII shows the instrument very nearly as it is at the present time.
The polar axis of the Crossley reflector is a long, hollow cylinder,
separated by a space of about one-eighth of an inch from its concentric
casing. The idea was to fill this space with mercury, and float the
greater part of the thrust of the axis, the function of a small steel pin
at the lower end being merely to steady the axis. But this mercury
flotation, as applied to the Crossley telescope, is a delusion, as I think
Mr. Crossley had already found. The mercury, it is true, relieves the
thrust to some extent, but it greatly increases the already enormous side
pressure on the steel pin at the bottom, thus creating a much greater evil
than the one it is intended to remedy. The workmen who set up the mounting
inform me that the small bearing at the lower end of the polar axis is
badly worn, as I should expect it to be. Instead of putting mercury into
the space intended for it, I have therefore poured in a pint or so of oil,
to keep the lower bearing lubricated. For the reasons indicated above, the
force required to move the telescope in right ascension is perhaps five
times greater than it should be. The lower end of the polar axis ought to
be fitted with ball bearings to take the thrust, and with a pair of
friction wheels on top; but it would be difficult to make these changes
now. It should be observed that the disadvantages of the mercury flotation
are considerably greater at Mount Hamilton than at the latitude for which
the telescope was designed.
[Illustration: THE CROSSLEY REFLECTOR.]
As already stated above, the range of the telescope is limited on the
south by the construction of the mounting. The greatest southern
declination which can be observed is 25 deg.. In England this would doubtless
mark the limit set by atmospheric conditions, but at Mount Hamilton it
would be easy to photograph objects 15 deg. farther south, if the telescope
could be pointed to them.
The original driving-clock having proved to be inefficient, at least
without an electric control, a new and powerful driving-clock was made by
the Observatory instrument maker, from designs by Professor Hussey. In its
general plan it is like that of the 36-inch refractor. The winding
apparatus, contained in the large casting of the original mounting, has no
maintaining power, and can not easily be fitted with one. The clock could
in no case be wound during a photographic exposure, on account of the
tremors attending the operation, but it would be somewhat more convenient
to have the stars remain on the plate during the winding. With a little
practice, however, one can wind the clock without actually stopping it,
though the object must afterwards be brought back to its place by means of
the slow motion in right ascension.
Two finders have recently been fitted to the Crossley reflector. One has
an object-glass of four inches aperture and eight feet six inches focal
length, with a field of about 1 deg. 2', which is very nearly the photographic
field of the main telescope. Its standards are bolted to one of the corner
tubes of the reflector. The other finder has a three-inch objective and a
large field. It had not been mounted when the photograph for the plate was
made.
When a telescope is used for photographing objects near the pole, with
long exposures, the polar axis must be quite accurately adjusted, for
otherwise the centers of motion of the stars and of the telescope will not
agree, and the star images will be distorted. It is true that with a
double-slide plate-holder, like the one used with the Crossley reflector,
one star--namely, the guiding star--is forced to remain in a fixed
position with respect to the plate; but the differential motion of the
other stars causes them to describe short arcs, or trails, around this
star as a center. A considerable part of the spring of 1899 was spent in
efforts to perfect the adjustment of the polar axis, an operation which,
on account of the peculiar form of the mounting, offers unusual
difficulties.
In the first plan which was tried, the reflector was used as a transit
instrument. The inclination of the declination axis was determined with a
hanging level which had been provided by Mr. Crossley, the hour circle and
polar axis being very firmly clamped. The clock correction being known
from the records kept at the Observatory, the collimation and azimuth
constants were found by the usual formulae. This method failed to give
satisfactory results, and it was found later that the declination and
polar axis were not exactly at right angles.
There is only one part of the sky on which the telescope can be reversed;
namely, the pole. A method which promised well, and on which some time was
spent, consists in photographing the pole (the declination axis being
horizontal) by allowing the stars near it to trail for ten or fifteen
minutes, then turning the polar axis 180 deg. and photographing the pole again
on the same plate. Half the distance between the images gives the error of
the polar axis, which, if the plate is properly oriented, is easily
resolved into horizontal and vertical components; while the distance of
each image from the center of the plate is this error increased or
diminished by twice the deviation of the telescope axis. In this case the
vertical component depends upon the reading of the declination circle, and
the horizontal component gives the error of collimation. This method
failed, however, to give consistent results, mainly on account of
instability of the mirror, and was abandoned.
The use of the large mirror for purposes of adjustment was finally given
up, and the axis was adjusted by observations of _Polaris_ with the long
finder, in the usual manner. In order to reach the star at lower
culmination the finder tube had to be thrown out of parallelism with the
main telescope.
The base-plate having no definite center of rotation in azimuth, and the
wedges and crowbars used for moving it being uncertain in their action, a
watch telescope, provided with a micrometer eyepiece, was firmly secured
to the mounting throughout these operations, in such manner that a mark on
the southern horizon could be observed through one of the windows of the
dome. The errors of the polar axis were finally reduced to within the
limits of error of observation.
The movable hour circle and driving wheel of the Crossley reflector has
two sets of graduations. The driving screw having been thrown out of gear,
the circle is turned until the outer vernier indicates the sidereal time,
whereupon the driving screw is thrown into gear again. The inner vernier
is then set to the right ascension of the object which it is desired to
observe. As an inconsistency, of minor importance, in the design of the
mounting, I may note that the slow motion in right ascension changes the
reading of the outer vernier instead of that of the inner one. In
practice, however, no inconvenience is caused by this construction.
In the early experiments and photographic work with the Crossley
telescope, irregularities in driving were a source of great annoyance. Dr.
Roberts, in laying down the conditions which should be fulfilled by a good
photographic telescope, says that a star should remain bisected by a
thread in the eyepiece for two minutes at a time. The Crossley telescope
was so far from fulfilling this condition that a star would not keep its
place for two consecutive seconds; and the greatest alertness on the part
of the observer did not suffice to ensure round star images on a
photographic plate. It was obvious that the fault did not lie with the
driving clock; in fact, many of the sudden jumps in right ascension, if
explained in this way, would have required the clock to run backward;
nevertheless the clock was tested by causing its revolutions to be
recorded on a chronograph at the main Observatory, together with the beats
of one of the standard clocks. For this purpose a break-circuit attachment
was made by Mr. Palmer. The errors of the clock were in this way found to
be quite small.
The principal source of the irregularities was found in the concealed
upper differential wheel of the Grubb slow motion. This wheel turned with
uncertain friction, sometimes rotating on its axis, and sometimes
remaining at rest. After it was checked the driving was much better, and
was still farther improved by repairing some defective parts of the train.
Small irregularities still remain. They seem to be partly due to
inaccuracies in the cutting of the gears, or of the teeth of the large
driving wheel, and partly to the springing of the various parts, due to
the very considerable friction of the polar axis in its bearings. The
remaining irregularities are so small, however, that they are easily
corrected by the screws of the sliding plate-holder, and with reasonable
attention on the part of the observer, round star images are obtained with
exposures of four hours' duration.
The large mirror, the most important part of the telescope, has an
aperture of three feet, and a focal length of 17 feet 6.1 inches. It was
made by Mr. Calver. Its figure is excellent. On cutting off the cone of
rays from a star, by a knife-edge at the focus, according to the method of
Foucault, the illumination of the mirror is very uniform, while the star
disks as seen in an ordinary eyepiece are small and almost perfectly
round. They are not, I think, quite so good as the images seen with a
large refractor; still, they are very good indeed, as the following
observations of double stars, made recently for this purpose, will show.
Several close double stars were examined on the night of April 17, 1900,
with a power of 620. The seeing was four on a scale of five. The
magnitudes and distances of the components, as given in the table, are
from recent observations by Professor Hussey with the 36-inch refractor.
Star. Mag. _d._ Result of Obs.
[Greek: Omega Sigma] 208
([Greek: phi] _Urs. Maj._) 5.0, 5.5 0''.35 Not resolved; too
bright.
[Greek: Omega Sigma] 249, AB 7.2, 8.0 0 .54 Easily resolved.
[Greek: Omega Sigma] 250 7.7, 8.0 0 .44 Resolved.
[Greek: Omega Sigma] 267 8.0, 8.2 0 .30 Just resolved at best
moments.
Although the theoretical limit of resolution for a three-foot aperture is
not reached in these observations, I do not think the mirror can do any
better.
The small mirror, or flat, at the upper end of the tube, is circular, the
diameter being nine inches. Its projection on the plane of the
photographic plate is therefore elliptical; but the projection of the
mirror and its cell on the plane of the great mirror is very nearly
circular.
The small mirror, acting as a central stop, has the effect of diminishing
the size of the central disk of the diffraction pattern, at the expense of
an increase in the brightness of the system of rings. To this effect may
be due, in part, the inferiority of the reflector for resolving bright
doubles, as compared with a refractor of the same aperture. For
photographic purposes, it is evident that the mirror is practically
perfect.
The upper end of the tube can be rotated, carrying with it the flat and
the eye-end. Whenever the position is changed, the mirrors have to be
re-collimated. In practice it is seldom necessary to touch the adjusting
screws of the mirrors themselves. The adjustment is effected by means of
clamping and butting screws on the eye-end, and a change of the line of
collimation, with respect to the finders and the circles, is avoided. The
operation is generally referred to, however, as an adjustment of the
mirrors.
For adjusting the mirrors there are two collimators. One of these is of
the form devised by Mr. Crossley.[7] It is very convenient in use, and is
sufficiently accurate for the adjustment of the eye-end when the telescope
is used for photographic purposes, inasmuch as the exact place where the
axis of the large mirror cuts the photographic plate is not then a matter
of great importance, so long as it is near the center. Moreover, as stated
farther below, the direction of the axis changes during a long exposure.
The other collimator is of a form originally due, I think, to Dr.
Johnstone Stoney. It consists of a small telescope, which fits the
draw-tube at the eye-end. In the focus of the eyepiece are, instead of
cross-wires, two adjustable terminals, between which an electric spark can
be passed, generated by a small induction machine, like a replenisher,
held in the observer's hand. The terminals are at such a distance inside
the principal focus of the objective, that the light from the spark, after
reflection from the flat, appears to proceed from the center of curvature
of the large mirror. The rays are therefore reflected back normally, and
form an image of the spark which, when the mirrors are in perfect
adjustment, coincides with the spark itself. The precision of this method
is very great. It is in fact out of proportion to the degree of refinement
attained in other adjustments of the reflector, for a slight pressure of
the hand on the draw-tube, or movement of the telescope to a different
altitude, instantly destroys the perfection of the adjustment. I have
provided these collimators with an adapter which fits the photographic
apparatus, so that one can adjust the mirrors without having to remove
this apparatus and substitute for it the ordinary eye-end carrying the
eyepieces.
For visual observation the Crossley telescope is provided with seven
eyepieces, with powers ranging from 620 downward. The lowest power is only
60, and consequently utilizes only 12 inches of the mirror, 9 of which are
covered by the central flat. It is therefore of little value, except for
finding purposes. The next lowest power utilizes 28 inches of the mirror.
The other eyepieces call for no remark.
But, while the Crossley reflector would doubtless be serviceable for
various kinds of visual observations, its photographic applications are
regarded as having the most importance, and have been chiefly considered
in deciding upon the different changes and improvements which have been
made.
The interior of the dome is lighted at night by a large lamp, which is
enclosed in a suitable box or lantern, fitted with panes of red glass, and
mounted on a portable stand. In order to diffuse the light in the lower
part of the dome, where most of the assistant's work is done, the walls
are painted bright red; while to prevent reflected light from reaching the
photographic plate, the inner surface of the dome itself, the mounting,
and the ladders and gallery are painted dead black. The observer is
therefore in comparative darkness, and not the slightest fogging of the
plate, from the red light below, is produced during a four-hours'
exposure. On the few occasions when orthochromatic plates are used the
lamp need not be lighted.
Experiments have shown that the fogging of the photographic plate, during
a long exposure, is entirely due to diffuse light from the sky, and is
therefore unavoidable. For this reason the cloth curtains which lace to
the corners of the telescope tube, enclosing it and shutting out light
from the lower part of the dome, have not been used, since their only
effect would be to catch the wind and cause vibrations of the telescope.
They would probably have little effect on the definition, and at any rate
could not be expected to improve it.
For photographing stars and nebulae the Crossley reflector is provided with
a double-slide plate-holder, of the form invented by Dr. Common.[8] This
apparatus, which had suffered considerably in transportation, and from
general wear and tear, was thoroughly overhauled by the Observatory
instrument-maker. The plates were straightened and the slides refitted. A
spring was introduced to oppose the right ascension screw and take up the
lost motion--the most annoying defect that such a piece of apparatus can
have--and various other improvements were made, as the necessity for them
became apparent. They are described in detail farther below.
The present appearance of the eye-end is shown in the illustration. The
plate-holder is there shown, however, on one side of the tube, and its
longer side is parallel to the axis of the telescope. This is not a good
position for the eye-end, except for short exposures. In practice, the
eye-end is always placed on the north or south side of the tube, according
as the object photographed is north or south of the zenith. The right
ascension slide is then always at right angles to the telescope axis, and
the eye-end can not get into an inaccessible position during a long
exposure.
As the original wooden plate-holders were warped, and could not be
depended upon to remain in the same position for several hours at a time,
they were replaced by new ones of metal, and clamping screws were added,
to hold them firmly in place. The heads of these screws are shown in the
plate, between the springs which press the plate-holder against its bed.
To illuminate the cross-wires of the guiding eyepiece, a small electric
lamp is used, the current for which is brought down from the storage
battery at the main Observatory. The coarse wires have been replaced by
spider's webs,[9] and reflectors have been introduced, to illuminate the
declination thread. A collimating lens, placed at its principal focal
distance from the incandescent filament of the lamp, makes the
illumination of the wires nearly independent of their position on the
slide, and a piece of red glass, close to the lens, effectually removes
all danger of fogging the plate. The light is varied to suit the
requirements of observation by rotating the reflector which throws the
light in the direction of the eyepiece.
[Illustration: DOUBLE-SLIDE PLATE-HOLDER OF THE CROSSLEY REFLECTOR.]
In long exposures it is important for the observer to know at any moment
the position of the plate with reference to its central or zero position.
For this purpose scales with indexes are attached to both slides; but as
they can not be seen in the dark, and, even if illuminated with red light,
could not be read without removing the eye from the guiding eyepiece, I
have added two short pins, one of which is attached to the lower side of
the right ascension slide, and the other to its guide, so that the points
coincide when the scale reads zero. These pins can be felt by the fingers,
and with a little practice the observer can tell very closely how far the
plate is from its central position. It would not be a very difficult
matter to improve on this contrivance, say by placing an illuminated
scale, capable of independent adjustment, in the field of the eyepiece,
but the pins answer every purpose. The declination slide is changed so
little that no means for indicating its position are necessary.
In this apparatus, as originally constructed, the cross-wires of the
guiding eyepiece were exactly in the plane of the photographic plate. The
earlier observations made with the Crossley reflector on Mount Hamilton
showed that this is not the best position of the cross-wires. The image of
a star in the guiding eyepiece, which, when in the middle of its slide, is
nearly three inches from the axis of the mirror, is not round, and its
shape varies as the eyepiece is pushed in or drawn out. In the plane of
the photographic plate (assumed to be accurately in focus), it is a
crescent, with the convex side directed toward the center of the plate.
This form of image is not suitable for accurate guiding. Outside this
position the image changes to an arrow-head, the point of which is
directed toward the axis, and this image can be very accurately bisected
by the right ascension thread. As the construction of the apparatus did
not allow the plane of the cross-wires to be changed, the wooden bed of
the plate-holder was cut down, so as to bring the wires and the plate into
the proper relative positions.
After some further experience with the instrument, still another change
was made in this adjustment. It was found that the focus often changed
very perceptibly during a long exposure, and while the arrow-head image
above described was suitable for guiding purposes, its form was not
greatly affected by changes of focus. Between the crescent and the
arrow-head images there is a transition form, in which two well-defined
caustic curves in the aberration pattern intersect at an acute angle. The
intersection of these caustics offers an excellent mark for the
cross-wires, and is at the same time very sensitive to changes of focus,
which cause it to travel up or down in the general pattern. The bed of the
plate-holder was therefore raised, by facing it with a brass plate of the
proper thickness.
Why the focus of the telescope should change during a long exposure is not
quite clear. The change is much too great to be accounted for by expansion
and contraction of the rods forming the tube, following changes of
temperature, while a simple geometrical construction shows that a drooping
of the upper end of the tube, increasing the distance of the plate from
the (unreflected) axis of the mirror, can not displace the focus in a
direction normal to the plate, if it is assumed that the field is flat.
The observed effect is probably due to the fact that the focal surface is
not flat, but curved. During a long exposure, the observer keeps the
guiding star, and therefore, very approximately, all other stars, in the
same positions relatively to the plate; but he has no control over the
position of the axis of the mirror, which, by changes of flexure, wanders
irregularly over the field. The position of maximum curvature, therefore,
also varies, and with it the focus of the guiding star relatively to the
cross-wires, where the focal surface is considerably inclined to the field
of view. It is certain that the focus does change considerably, whatever
the cause may be, and that the best photographic star images are obtained
by keeping the focus of the guiding star unchanged during the exposures.
This is done by turning the focusing screw of the eye-end.
In making the photographs of nebulae for which the Crossley telescope is at
present regularly employed, it was at first our practice to adjust the
driving-clock as accurately as possible to a sidereal rate, and then, when
the star had drifted too far from its original position, on account of
changes of rate or of flexure, to bring it back by the right-ascension
slow motion, the observer either closing the slide of the plate-holder or
following the motion of the star as best he could with the right-ascension
screw. Lately a more satisfactory method, suggested by Mr. Palmer, has
been employed. The slow motion in right ascension is of Grubb's form,[10]
and the telescope has two slightly different rates, according to whether
the loose wheel is stopped or allowed to turn freely. The driving-clock is
adjusted so that one of these rates is too fast, the other too slow. At
the beginning of an exposure the wheel is, say, unclamped, and the guiding
star begins to drift very slowly toward the left, the observer following
it with the screw of the plate-holder. When it has drifted far enough, as
indicated by the pins mentioned farther above, the wheel is clamped. The
star then reverses its motion and begins to drift toward the right; and so
on throughout the exposure. The advantages of this method over the one
previously employed are, that the star never has to be moved by the slow
motion of the telescope, and that its general drift is in a known
direction, so that its movements can be anticipated by the observer. In
this way photographs are obtained, with four hours' exposure, on which the
smallest star disks are almost perfectly round near the center of the
plate, and from 2'' to 3'' in diameter.
The star images are practically round over a field at least 1 inch or 16'
in diameter. Farther from the center they become parabolic, but they are
quite good over the entire plate, 3-1/4 by 4-1/4 inches.
From these statements it will be seen that small irregularities in driving
no longer present any difficulties. But certain irregular motions of the
image still take place occasionally, and so far it has not been possible
entirely to prevent their occurrence.
It was found that the declination clamp (the long slow-motion handle
attached to which is shown in the illustration) was not sufficiently
powerful to hold the telescope firmly during a long exposure. A screw
clamp was therefore added, which forces the toothed-declination sector
strongly against an iron block just behind it, thus restoring, I think,
the original arrangement of the declination clamp as designed by Dr.
Common. This clamp holds the tube very firmly.
The irregularities to which I have referred consist in sudden and
unexpected jumps of the image, which always occur some time after the
telescope has passed the meridian. These jumps are sometimes quite
large--as much as one-sixteenth of an inch or 1. They are due to two
causes: flexure of the tube, and sliding of the mirror on its bed. When
the jump is due to sudden changes of flexure, the image moves very
quickly, and vibrates before it comes to rest in its new position, and at
the same time there is often heard a slight ringing sound from the tension
rods of the tube. There seems to be no remedy for the sudden motions of
this class. The tension rods are set up as tightly as possible without
endangering the threads at their ends or buckling the large corner tubes.
A round telescope tube, made of spirally-wound steel ribbon riveted at the
crossings, would probably be better than the square tube now in use.
Jumps due to shifting of the mirror are characterized by a gentle, gliding
motion. They can be remedied, in part, at least, by tightening the copper
bands which pass around the circumference of the mirror within its cell.
This will be done the next time the mirror is resilvered.
All that the observer can do when a jump occurs is to bring back the image
as quickly as possible to the intersection of the cross-wires. If all the
stars on the plate are faint, no effect will be produced on the
photograph; but stars of the eighth magnitude or brighter will leave short
trails. The nebula, if there is one on the plate, will, of course, be
unaffected.
Before beginning an exposure the focus is adjusted by means of a
high-power positive eyepiece. An old negative, from which the film has
been partially scraped, is placed in one of the plate-holders, and the
film is brought into the common focus of the eyepiece and the great
mirror. The appearance of the guiding star, which varies somewhat with the
position of the guiding eyepiece on its slide, is then carefully noted,
and is kept constant during the exposure by turning, when necessary, the
focusing screw of the eye-end. For preliminary adjustments a ground-glass
screen is often convenient. On it all the _DM._ stars, and even
considerably fainter ones, as well as the nebulae of Herschel's Class I,
are easily visible without a lens.
Plates are backed, not more than a day or two before use, with Carbutt's
"Columbian backing," which is an excellent preparation for this purpose.
During the exposure the observer and assistant exchange places every half
hour, thereby greatly relieving the tediousness of the work, though two
exposures of four hours each, in one night, have proved to be too
fatiguing for general practice. At the end of the first two hours it is
necessary to close the slide and wind the clock.
The brightness of the guiding star is a matter of some importance. If the
star is too bright, its glare is annoying; if it is too faint, the effort
to see it strains the eye, and changes of focus are not easily recognized.
A star of the ninth magnitude is about right. In most cases a suitable
star can be found without difficulty.
In such an apparatus as that described above, the amount by which the
plate may be allowed to depart from its zero position is subject to a
limitation which has not, I think, been pointed out, although it is
sufficiently obvious when one's attention has been called to it. It
depends upon the fact that the plate necessarily moves as a whole, in a
straight line which is tangent to a great circle of the sphere, while the
stars move on small circles around the pole. The compensation for drift,
when the plate is moved, is therefore exact at the equator only.
Let the guiding star have the declination [Greek: delta]_{1}, and let a
star on the upper edge of the plate (which, when the telescope is north of
the zenith, and the eye-end is on the north side of the telescope, will be
the southern edge) have the declination [Greek: delta]_{2}. Then if the
guiding star is allowed to drift from its zero position through the
distance _d_, the other star will drift through the distance _d_ (cos
[Greek: delta]_{2} / cos [Greek: delta]_{1}). If the guiding star is
followed by turning the right-ascension screw, the upper edge of the
plate, as well as the guiding eyepiece, will be moved through the distance
_d_. Hence there will be produced an elongation of the upper star,
represented by
_e_ = _d_ ((cos [Greek: delta]_{2} / cos [Greek: delta]_{1}) - 1)
from which _d_ = (_e_ cos [Greek: delta]_{1}) /
(cos [Greek: delta]_{2} - cos [Greek: delta_{1}]).
Now, in the Crossley reflector, the upper edge of the plate and the
guiding eyepiece are just about 3-2/3 inches, or 1 deg., apart. If _e_ is
given, the above formula serves to determine the maximum range of the
slide for different positions of the telescope.
It has been stated farther above that the smallest star disks, on a good
photograph, are sometimes not more than 2'' in diameter, or in a linear
measure, about 1/20 mm. An elongation of this amount is therefore
perceptible. There are many nebulae in high northern declinations, and
there are several particularly fine ones in about +70 deg.. If, therefore, we
take [Greek: delta]_{2} = 70 deg., [Greek: delta]_{1}, = 71 deg., _e_ = 0.05, and
substitute these values, we find _d_ = 1.0 mm, which is the greatest
permissible range of the plate in photographing these nebulae. Before I
realized the stringency of this requirement, by making the above simple
computation, I spoiled several otherwise fine negatives by allowing the
plate to get too far from the center, thus producing elongated star
images.
There is a corresponding elongation in declination, the amount of which
can be determined by an adaptation of the formula for reduction to the
meridian, but it is practically insensible.