ACCESSORIES
Aside from the ordinary equipment of oculars various accessories
form an important part of the observer’s equipment, their number and
character depending on the instrument in use and the purposes to which
it is devoted.
[Illustration: FIG. 126.—Star Diagonal.]
First in general usefulness are several special forms of eyepiece
equipment supplementary to the usual oculars. At the head of the list
is the ordinary star diagonal for the easier viewing of objects near
the zenith here shown in Fig. 126. It is merely a tube, _A_, fitting
the draw tube of the telescope, with a slotted side tube _B_, at a
right angle, into which the ordinary ocular fits, and a right angled
prism _C_ with its two faces perpendicular respectively to the axes of
the main and side tubes, and the hypothenuse face at 45° to each. The
beam coming down the tube is totally reflected at this face and brought
to focus at the ocular. The lower end of the tube is closed by a cap to
exclude dust.
One looks, by help of this, horizontally at zenith stars, or, if
observing objects at rather high altitude, views them at a comfortable
angle downward. The prism must be very accurately made to avoid injury
to the definition, but loses only about 10% of the light, and adds
greatly to the comfort of observing.
Of almost equal importance is the solar diagonal devised by Sir John
Herschel, Fig. 127. Here the tube structure _A_, _B_, is quite the
same as in Fig. 126 but the right angled prism is replaced by a simple
elliptical prism _C_ of small angle, 10° or less, with its upper face
accurately plane and at 45° to the axes of the tubes, resting on a
lining tube _D_ cut off as shown. In viewing the sun only about 5% of
the light (and heat) is reflected at this upper surface to form the
image at the eye piece.
[Illustration: FIG. 127.—Solar Diagonal.]
Any reflection from the lower polished surface is turned aside out
of the field, while the remainder of the radiation passes through
the prism _C_ and is concentrated below it. To prevent scorching the
observer the lower end of the tube is capped at _E_, but the cap has
side perforations to provide circulation for the heated air. Using such
a prism, the remnant of light reflected can be readily toned down by a
neutral tinted glass over the ocular.
In the telescopes of 3 inches and less aperture, and ordinary focal
ratio, a plane parallel disc of very dark glass over the ocular gives
sufficient protection to the eye. This glass is preferably of neutral
tint, and commonly is a scant 1/16 inch thick. Some observers prefer
other tints than neutral. A green and a red glass superimposed give
good results and so does a disc of the deepest shade of the so-called
Noviweld glass, which is similar in effect.
With an aperture as large as 3 inches a pair of superimposed dark
glasses is worth while, for the two will not break simultaneously from
the heat and there will be time to get the eye away in safety. A broken
sunshade is likely to cost the observer a permanent scotoma, blindness
in a small area of the retina which will neither get better nor worse
as time goes on.
Above 3 inches aperture the solar prism should be used or, if one cares
to go to fully double the cost, there is nothing more comfortable to
employ in solar observation than the polarizing eye piece, Fig. 128.
This shows schematically the arrangement of the device. It depends on
the fact that a ray of light falling on a surface of common glass at an
angle of incidence of approximately 57° is polarized by the reflection
so that while it is freely reflected if it falls again on a surface
parallel to the first, it is absorbed if it falls at the same incidence
on a surface at right angles to the first.
[Illustration: FIG. 128.—Diagram of Polarizing Eyepiece.]
Thus in Fig. 128 the incident beam from the telescope falls on the
black glass surface _a_ at 57° incidence, is again reflected from the
parallel mirror _b_, and then passed on, parallel to its original
path, to the lower pair of mirrors _c_, _d_. The purpose of the
second reflection is to polarize the residual light which through the
convergence of the rays was incompletely polarized at the first.
The lower pair of mirrors _c_, _d_, again twice reflect the light at
the polarizing angle, and, in the position shown, pass it on to the
ocular diminished only by the four reflections. But if the second pair
of mirrors be rotated together about a line parallel to _b c_ as an
axis the transmitted light begins to fade out, and when they have been
turned 90°, so that their planes are inclined 90° to _a_ and _b_ (= 33°
to the plane of the paper), the light is substantially extinguished.
Thus by merely turning the second pair of mirrors the solar image can
be reduced in brilliancy to any extent whatever, without modifying its
color in any way. The typical form given to the polarizing eyepiece is
similar to Fig. 129. Here _t__2 is the box containing the polarizing
mirrors, _a b_, fitted to the draw tube, but for obvious reasons
eccentric with it, _t__1 is the rotating box containing the “analysing”
mirrors _c_, _d_, and _a_ is the ocular turning with it.
Sometimes the polarizing mirrors are actually a pair of Herschel prisms
as in Fig. 126, facing each other, thus getting rid of much of the
heat. Otherwise the whole set of mirrors is of black glass to avoid
back reflections. In simpler constructions single mirrors are used as
polarizer and analyser, and in fact there are many variations on the
polarizing solar eyepiece involving about the same principles.
[Illustration: FIG. 129.—Polarizing Solar Eyepiece.]
In any solar eyepiece a set of small diaphragms with holes from perhaps
1/64 inch up are useful in cutting down the general glare from the
surface outside of that under scrutiny. These may be dropped upon the
regular diaphragm of the ocular or conveniently arranged in a revolving
diaphragm like that used with the older photographic lenses.
The measurement of celestial objects has developed a large group of
important auxiliaries in the micrometers of very varied forms. The
simplest needs little description, since it consists merely of a plane
parallel disc of glass fitting in the focus of a positive ocular,
and etched with a network of uniform squares, forming a reticulated
micrometer by which the distance of one object from another can be
estimated.
It can be readily calibrated by measuring a known distance or noting
the time required for an equatorial star to drift across the squares
parallel to one set of lines. It gives merely a useful approximation,
and accurate measures must be turned over to more precise instruments.
[Illustration: FIG. 130.—Diagram of Ring Micrometer.]
The ring micrometer due, like so much other valuable apparatus, to
Fraunhofer, is convenient and widely used for determining positions.
It consists, as shown in Fig. 130, of an accurately turned opaque
ring, generally of thin steel, cemented to a plane parallel glass or
otherwise suspended in the center of the eyepiece field. The whole
ring is generally half to two thirds the width of the field and has a
moderate radial width so that both the ingress and the egress of a star
can be conveniently timed.
It depends wholly on the measurement of time as the stars to be
compared drift across the ring while the telescope is fixed, and while
a clock or chronometer operating a sounder is a desirable adjunct
one can do pretty well with a couple of stop watches since only
differential times are required.
For full directions as to its use consult Loomis’ Practical Astronomy,
a book which should be in the library of every one who has the least
interest in celestial observations. Suffice it to say here that
the ring micrometer is very simple in use, and the computation of
the results is quite easy. In Fig. 130 F is the edge of the field,
R the ring, and _a b_, _a′b′_, the paths of the stars _s_ and _s′_,
the former well into the field, the latter just within the ring. The
necessary data comprise the time taken by each star to transverse
the ring, and the radius of the ring in angular measure, whence the
difference in R. A. or Dec, can be obtained.[20]
[20] r the radius of the ring, is given by, r = (15/2)(t′-t) cos Dec.,
t′-t being the seconds taken for transit.
Difference of R. A. = ½ (t′-t)½ (T′-T) where (T′-T) is the time
taken for transit of second star. To obtain differences of declination
one declination should be known at least approximately, and the second
estimated from its relative position in the ring or otherwise. Then
with these tentative values proceed as follows.
Put x = angle _aob_ and _x_′ = angle _a′o′b′_
Also let d = approximate declination of _s_ and
d′ = approximate declination of _s′_
Then sin x = (15/2r) cos d (T′-T)
sin x′ = (15/2r) cos d′ (t′-t) and finally
Difference of Dec. = r (cos x′-cos x), when both arcs are on the same
side of center of ring. If on opposite sides, Diff. = r (cos x′ + cos
x).
[Illustration: _Chamber’s “Astronomy”_ (_Clarendon Press_).
FIG. 131.—Double Image Micrometer. (_Courtesy of The Clarendon Press._)]
There is also now and then used a square bar micrometer, consisting of
an opaque square set with a diagonal in the line of diurnal motion.
It is used in much the same way as the ring, and the reductions are
substantially the same. It has some points of convenience but is
little used, probably on account of the great difficulty of accurate
construction and the requirement, for advantageous use, that the
telescope should be on a well adjusted equatorial stand.[21] The ring
micrometer works reasonably well on any kind of steady mount, requires
no illumination of the field and is in permanent working adjustment.
[21] (For full discussion of this instrument see Chandler, Mem. Amer.
Acad. Arts & Sci. 1885, p. 158).
Still another type of micrometer capable of use without a clock-drive
is the double image instrument. In its usual form it is based on the
principle that if a lens is cut in two along a diameter and the halves
are slightly displaced along the cut all objects will be seen double,
each half of the lens forming its own set of images.
Conversely, if one choses two objects in the united field these can be
brought together by sliding the halves of the lens as before, and the
extent of the movement needed measures the distance between them. Any
lens in the optical system can be thus used, from the objective to the
eyepiece. Fig. 131 shows a very simple double image micrometer devised
by Browning many years ago. Here the lens divided is a so-called Barlow
lens, a weak achromatic negative lens sometimes used like a telephoto
lens to lengthen the focus and hence vary the power of a telescope.
This lens is shown at A with the halves widely separated by the double
threaded micrometer screw B, which carries them apart symmetrically.
The ocular proper is shown at C.
Double image micrometers are now mainly of historical interest, and
the principle survives chiefly in the heliometer, a telescope with the
objective divided, and provided with sliding mechanism of the highest
refinement. The special function of the heliometer is the direct
micrometric measurement of stellar distances too great to be within the
practicable range of a filar micrometer—distances for example up to
1½° or even more.
The observations with the heliometer are somewhat laborious and demand
rather intricate corrections, but are capable of great precision. (See
Sir David Gill’s article “Heliometer” in the Enc. Brit. 11th Ed.). At
the present day celestial photography, with micrometric measurement
of the resulting plates, has gone far in rendering needless visual
measurements of distances above a very few minutes of arc, so that
it is somewhat doubtful whether a large heliometer would again be
constructed.
[Illustration: FIG. 132.—Filar Micrometer. (_Courtesy of J. B.
Lippincott Co._)]
The astronomer’s real arm of precision is the filar micrometer. This
is shown in outline in Fig. 132, the ocular and the plate that carries
it being removed so as to display the working parts. It consists of a
main frame aa, carrying a slide bb, which is moved by the screws and
milled head B. The slide bb carries the vertical spider line mm, and
usually one or more horizontal spider lines, line mm is the so-called
fixed thread of the micrometer, movable only as a convenience to avoid
shifting the telescope.
On bb moves the micrometer slide cc, carrying the movable spider line
nn and the comb which records, with mm as reference line, the whole
revolutions of the micrometer screw C. The ocular sometimes has a
sliding motion of its own on cc, to get it positioned to the best
advantage. In use one star is set upon mm by the screw B and then C is
turned until nn bisects the other star.
Then the exact turns and fraction of a turn can be read off on the comb
and divided head of C, and reduced to angular measure by the known
constant of the micrometer, usually determined by the time of passage
of a nearly equatorial star along the horizontal thread when mm, nn,
are at a definite setting
apart. (Then r = (15(t′-t) cos d)/N where r is the value of a
revolution in seconds of arc, N the revolutions apart of mm, nn, and t
and d as heretofore.)
Very generally the whole system of slides is fitted to a graduated
circle, to which the fixed horizontal thread is diametral. Then by
turning the micrometer until the horizontal threads cut the two objects
under comparison, their position angle with reference to a graduated
circle can be read off. This angle is conventionally counted from 0° to
360° from north around through east.
[Illustration: FIG. 133.—Filar Position Micrometer.]
Figure 133 shows the micrometer constructed by the Clarks for their
24 inch equatorial of the Lowell Observatory. Here A is the head of
the main micrometer screw of which the whole turns are reckoned on the
counter H in lieu of the comb of Fig. 132. B is the traversing screw
for the fixed wire system, C the clamping screw of the position circle,
D its setting pinion, E the rack motion for shifting the ocular, F
the reading glass for the position circle, and G the little electric
lamp for bright wire illumination. The parts correspond quite exactly
with the diagram of Fig. 132 but the instrument is far more elegant in
design than the earlier forms of micrometer and fortunately rid of the
oil lamps that were once in general use. A small electric lamp with
reflector throws a little light on the spider lines—just enough to show
them distinctly. Or sometimes a faint light is thus diffused in the
field against which the spider lines show dark.
Commonly either type of illumination can be used and modified as
occasion requires. The filar micrometer is seldom used on small
telescopes, since to work easily with it the instrument should be
permanently mounted and clock-driven. Good work was done by some of the
early observers without these aids, but at the cost of infinite pains
and much loss of time.
The clock drive is in fact a most important adjunct of the telescope
when used for other purposes than ordinary visual observations, though
for simple seeing a smooth working slow motion in R. A. answers well.
The driving clock from the horological view-point is rudimentary. It
consists essentially of a weight-driven, or sometimes spring-driven,
drum, turning by a simple gear connection a worm which engages a
carefully cut gear wheel on the polar axis, while prevented from
running away by gearing up to a fast running fly-ball governor, which
applies friction to hold the clockwork down to its rate if the speed
rises by a minute amount. There is no pendulum in the ordinary sense,
the regularity depending on the uniformity of the total friction—that
due to the drive plus that applied by the governor.
Figure 134 shows a simple and entirely typical driving clock by Warner
& Swasey. Here A is the main drum with its winding gear at B, C is the
bevel gear, which is driven from another carried by A, and serves to
turn the worm shaft D; E marks the fly balls driven by the multiplying
gearing plainly visible. The governor acts at a predetermined rotation
speed to lift the spinning friction disc F against its fixed mate,
which can be adjusted by the screw G.
The fly-balls can be slightly shifted in effective position to complete
the regulation. These simple clocks, of which there are many species
differing mainly in the details of the friction device, are capable of
excellent precision if the work of driving the telescope is kept light.
For large and heavy instruments, particularly if used for photographic
work where great precision is required, it is difficult to keep the
variations of the driving friction within the range of compensation
furnished by the governor friction alone, and in such case recourse
is often taken to constructions in which the fly balls act as relay
to an electrically controlled brake, or in which the driving power is
supplied by an electric motor suitably governed either continuously
or periodically. For such work independent hand guiding mechanism is
provided to supplement the clockwork. For equatorials of the smallest
sizes, say 3 to 4 inches aperture, spring operated driving clocks are
occasionally used. The general plan of operation is quite similar to
the common weight driven forms, and where the weights to be carried are
not excessive such clocks do good work and serve a very useful purpose.
[Illustration: FIG. 134.—Typical Driving Clock. (_Courtesy of The
Clarendon Press._)]
An excellent type of the simple spring driving clock is shown in
Fig. 136 as constructed by Zeiss. Here 1 is the winding gear, 2 the
friction governor, and 3 the regulating gear. It will be seen that the
friction studs are carried by the fly balls themselves, somewhat as in
Fraunhofers’ construction a century since, and the regulation is very
easily and quickly made by adjusting the height of the conical friction
surface above the balls.
For heavier work the same makers generally use a powerful weight driven
train with four fly-balls and electric seconds control, sometimes with
the addition of electric motor slow motions to adjust for R. A. in both
directions.
[Illustration: FIG. 135.—Clark Driving Clock.]
Figure 135 is a rather powerful clock of analogous form by the Clarks.
It differs a little in its mechanism and especially in the friction
gear in which the bearing disc is picked up by a delicately set latch
and carried just long enough to effect the regulation. It is really
remarkable that clockworks of so simple character as these should
perform as well as experience shows that they do. In a few instances
clocks have depended on air-fans for their regulating force, something
after the manner of the driving gear of a phonograph, but though rather
successful for light work they have found little favor in the task
of driving equatorials. An excellent type of a second genus is the
pendulum controlled driving clock due to Sir David Gill. This has a
powerful weight-driven train with the usual fly-ball governor. But the
friction gear is controlled by a contact-making seconds pendulum in the
manner shown diagrammatically in Fig. 137. Two light leather tipped
rods each controlled by an electro magnet act upon an auxiliary brake
disc carried by the governor spindle which is set for normal speed with
one brake rod bearing lightly on it. Exciting the corresponding magnet
relieves the pressure and accelerates the clock, while exciting the
other adds braking effect and slows it.
[Illustration: FIG. 136.—Spring Operated Driving Clock.]
In Fig. 137 is shown from the original paper, (M. N. Nov., 1873), the
very ingenious selective control mechanism. At P is suspended the
contact-making seconds-pendulum making momentary contact by the pin Q
with a mercury globule at R. Upon a spindle of the clock which turns
once a second is fixed a vulcanite disc γ, δ, ε, σ. This has a rim of
silver broken at the points γ, δ, ε, σ, by ivory spacers covering 3° of
circumference. On each side of this disc is another, smaller, and with
a complete silver rim. One, ηθ, is shown, connected with the contact
spring V; its mate η′θ′, on the other side contacts with U, while a
third contact K bears on the larger disc.
The pair of segments σ, γ, and δ, ε, are connected to η θ, the other
pair of segments to η′ θ′. Now suppose the discs turning with the
arrows: If K rests on one of the insulated points when the pendulum
throws the battery C Z into circuit nothing happens. If the disc is
gaining on the pendulum, K, instead of resting on γ as shown will
contact with segment γ, σ, and actuate a relay via V, exciting the
appropriate brake magnet.
[Illustration: FIG. 137.—Sir David Gill’s Electric Control.]
If the disc is losing, K contacts with segment γ, δ, and current
will pass via η′θ′ and U to a relay that operates the other brake
magnet and lets the clock accelerate. A fourth disc (not shown) on
the same spindle is entirely insulated on its edge except at points
corresponding to γ and ε, and with a contact spring like K.
If the disc is neither gaining nor losing when the pendulum makes
contact, current flows via this fourth disc and sets the relay on
the mid-point ready to act when needed. This clock is the prototype
of divers electrically-braked driving clocks with pendulum control,
and proved beautifully precise in action, like various kindred devices
constructed since, though the whole genus is somewhat expensive and
intricate.
The modern tendency in driving apparatus for telescopes, particularly
large instruments, is to utilize an electric motor for the source of
power, using a clock mechanism merely for the purpose of accurately
regulating the rate of the motor. We thus have the driving clock in
its simplest form as a purely mechanical device worked by a sensitive
fly-ball governor. The next important type is that in which the clock
drive is precisely regulated by a pendulum clock, the necessary
governing power being applied electrically as in Fig. 137 or sometimes
mechanically.
Finally we come to the type now under consideration where the
instrument itself is motor driven and the function of the clock is
that of regulating the motor. A very good example of such a drive is
the Gerrish apparatus used for practically all the instruments at the
various Harvard observatory stations, and which has proved extremely
successful even for the most trying work of celestial photography.
The schematic arrangement of the apparatus is shown in Fig. 138. Here
an electric motor shown in diagram in 1, Fig. 138, is geared down to
approximately the proper speed for turning the right ascension axis of
the telescope. It is supplied with current either from a battery or
in practice from the electric supply which may be at hand. This motor
is operated on a 110 volt circuit which supplies current through the
switch 2 which is controlled by the low voltage clock circuit running
through the magnet 3. The clock circuit can be closed and opened at two
points, one controlled by the seconds pendulum 5, the other at 7 by the
stud on the timing wheel geared to the motor for one revolution per
second. There is also a shunt around the pendulum break, closed by the
magnet switch at 6. This switch is mechanically connected to the switch
2 by the rod 4, so that the pair open and close together.
The control operates as follows: Starting with the motor at rest, the
clock circuit is switched on, switches 2, 6 being open and 7 closed.
At the first beat of the pendulum 2, 6 closes and the current, shunted
across the loop containing 5, holds 2 closed until the motor has
started and broken the clock circuit at the timer. The fly-wheel
carries on until the pendulum again closes the power circuit via 2,
6, and current stays on the motor until the timer has completed its
revolution.
This goes on as the motor speeds up, the periodic power supply being
shortened as the timer breaks it earlier owing to the acceleration,
until the motor comes to its steady speed at which the power is applied
just long enough to maintain uniformity. If the motor for any cause
tends to overspeed the cut-off is earlier, while slowing down produces
a longer power-period bringing the speed back to normal. The power
period is generally ¼ to ½ second. The power supplied to the motor
is very small even in the example here shown, only 1 ampere at 110
volts.
[Illustration: FIG. 138.—Diagram of Gerrish Electric Control.]
The actual proportion of a revolution during which current is supplied
the motor is therefore rigorously determined by the clock pendulum,
and the motor is selected so that its revolutions are exactly timed
to this clock pendulum which has no work to do other than the circuit
closing, and can hence be regulated to keep accurate time. The small
fly-wheel (9), the weight of which is carefully adjusted with respect
to the general amount of work to be done, attached to the motor shaft,
effectively steadies its action during the process of government.
This Gerrish type has been variously modified in detail to suit the
instruments to which it has been applied, always following however the
same fundamental principles.
[Illustration: FIG. 139.—Gerrish Drive on 24 inch Reflector.]
An admirable example of the application of this drive is shown in Fig.
139, the 24 inch reflector at the Harvard Observatory. The mount is
a massive open fork, and the motor drive is seen on the right of the
mount. There are here two motors, ordinary fan motors in size. The
right hand motor carries the fly-wheel and runs steadily on under the
pendulum control. The other, connected to the same differential gear as
the driving motor, serves merely for independent regulation and can be
run in either direction by the observer to speed or slow the motion in
R. A. These examples of clock drive are merely typical of those which
have proved to be successful in use for various service, light and
heavy. There are almost innumerable variations on clocks constructed on
one or another of the general lines here indicated, so many variations
in fact that one almost might say there are few driving clocks which
are not in some degree special.
The tendency at present is for large instruments very distinctly toward
a motor-driven mechanism operating on the right ascension axis, and
governed in one of a considerable variety of ways by an actual clock
pendulum. For smaller instruments the old mechanical clock, often
fitted with electric brake gear and now and then pendulum regulated, is
capable of very excellent work.
The principle of the spectroscope is rudimentarily simple, in the
familiar decomposition of white light into rainbow colors by a prism.
One gets the phenomena neatly by holding a narrow slit in a large piece
of cardboard at arms length and looking at it through a prism held with
its edge parallel to the slit. If the light were not white but of a
mixture of definite colors each color present would be represented by
a separate image of the slit instead of the images being merged into a
continuous colored band.
With the sun as source the continuous spectrum is crossed by the dark
lines first mapped by Fraunhofer, each representing the absorption by
a relatively cool exterior layer of some substance that at a higher
temperature below gives a bright line in exactly the same position.
The actual construction of the astronomical spectroscope varies greatly
according to its use. In observations on the sun the distant slit is
brought nearer for convenience by placing it in the focus of a small
objective pointed toward the prisms (the collimator) and the spectrum
is viewed by a telescope of moderate magnifying power to disclose more
of detail. Also, since there is extremely bright light available, very
great dispersion can be used, obtained by several or many prisms, so
that the spectrum is both fairly wide, (the length of the slit) and
extremely long.
In trying to get the spectrum of a star the source is a point,
equivalent to an extremely minute length of a very narrow slit.
Therefore no actual slit is necessary and the chief trouble is to get
the spectrum wide enough and bright enough to examine.
The simplest form of stellar spectroscope and the one in most common
use with small telescopes is the ocular spectroscope arranged much like
Fig. 140. This fits into the eye tube of a telescope and the McClean
form made by Browning of London consists of an ordinary casing with
screw collar _B_, a cylindrical lens _C_, a direct vision prism _c_,
_f_, _c_, and an eye-cap _A_.
The draw tube is focussed on the star image as with any other ocular,
and the light is delivered through _C_ to the prism face nearly
parallel, and thence goes to the eye, after dispersion by the prism.
This consists of a central prism, _f_, of large angle, made of
extremely dense flint, to which are cemented a pair of prisms of light
crown _c_, _c_, with their bases turned away from that of _f_.
We have already seen that the dispersions of glasses vary very much
more than their refractions so that with proper choice of materials and
angles the refraction of _f_ is entirely compensated for some chosen
part of the spectrum, while its dispersion quite overpowers that of the
crown prisms and gives a fairly long available spectrum.
The cylindrical lens _C_ merely serves to stretch out the tiny round
star image into a short line thereby giving the resulting spectrum
width enough to examine comfortably. The weak cylindrical lens is
sometimes slipped over the eye end of the prisms to give the needed
width of spectrum instead of putting it ahead of the prisms.
A small instrument of this kind used with a telescope of 3 inches to 5
inches aperture gives a fairly good view of the spectra of starts above
second or third magnitude, the qualities of tolerably bright comets and
nebulæ and so forth. The visibility of stellar spectra varies greatly
according to their type, those with heavy broad bands being easy to
observe, while for the same stellar magnitude spectra with many fine
lines may be quite beyond examination. Nevertheless a little ocular
spectroscope enables one to see many things well worth the trouble of
observing.
[Illustration: FIG. 140.—McClean Ocular Spectroscope.]
With the larger instruments, say 6 or 8 inches, one can well take
advantage of the greater light to use a spectroscope with a slit, which
gives somewhat sharper definition and also an opportunity to measure
the spectrum produced.
An excellent type of such an instrument is that shown in Fig. 141,
due to Professor Abbé. The construction is analogous to Fig. 140. The
ocular is a Huyghenian one with the slit mechanism (controlled by a
milled head) at A in the usual place of the diaphragm. The slit is
therefore in the focus of the eye lens, which serves as collimating
lens. Above is the direct vision system J with the usual prisms which
are slightly adjustable laterally by the screw P and spring Q.
At N is a tiny transparent scale of wave lengths illuminated by a faint
light reflected from the mirror O, and in the focus of the little lens
R, which transfers it by reflection from the front face of the prism
to the eye, alongside the edge of the spectrum. One therefore sees the
spectrum marked off by a bright line wave-length scale.
[Illustration: FIG. 141.—Abbé Ocular Spectroscope.]
The pivot K and clamp L enable the whole to be swung side-wise so that
one can look through the widened slit, locate the star, close the slit
accurately upon it and swing on the prisms. M is the clamp in position
angle. Sometimes a comparison prism is added, together with suitable
means for throwing in spectra of gases or metals alongside that of
the star, though these refinements are more generally reserved for
instruments of higher dispersion.
To win the advantage of accurate centering of the star in the field
gained by the swing-out of the spectroscope in Fig. 141 simple
instruments like Fig. 140 are sometimes mounted with an ordinary ocular
in a double nose-piece like that used for microscope objectives, so
that either can be used at will.
Any ordinary pocket spectroscope, with or without scale or a comparison
prism over part of the slit, can in fact be fitted to an adapter and
used with the star focussed on the slit and a cylindrical lens, if
necessary, as an eye-cap.
Such slit spectroscopes readily give the characteristics of stellar
spectra and those of the brighter nebulæ or of comets. They enable one
to identify the more typical lines and compare them with terrestrial
sources, and save for solar work are about all the amateur observer
finds use for.
For serious research a good deal more of an instrument is required,
with a large telescope to collect the light, and means for
photographing the spectra for permanent record. The cumulative effect
of prolonged exposures makes it possible easily to record spectra
much too faint to see with the same aperture, and exposures are often
extended to many hours.
Spectroscopes for such use commonly employ dense flint prisms of about
60° refracting angle and refractive index of about 1.65, one, two, or
three of these being fitted to the instrument as occasion requires.
A fine example by Brashear is shown in Fig. 142, arranged for visual
work on the 24 inch Lowell refractor. Here A is the slit, B the prism
box, C the observing telescope, D the micrometer ocular with electric
lamp for illuminating the wires, and E the link motion that keeps the
prism faces at equal angles with collimator and observing telescope
when the angle between these is changed to observe different parts of
the spectrum. This precaution is necessary to maintain the best of
definition.
When photographs are to be taken the observing telescope is unscrewed
and a photographic lens and camera put in its place. If the brightness
of the object permits, three prisms are installed, turning the beam
180° into a camera braced to the same frame alongside the slit.
For purely photographic work, too, the objective prism used by
Fraunhofer for the earliest observation of stellar spectra is in wide
use. It is a prism fitted in front of the objective with its refracting
faces making equal angles with the telescope and the region to be
observed, respectively. Its great advantages are small loss of light
and the ability to photograph many spectra at once, for all the stars
in the clear field of the instrument leave their images spread out into
spectra upon the photographic plate.
Figure 143 shows such an objective prism mounted in front of an
astrographic objective. The prism is rotatable into any azimuth about
the axis of the objective and by the scale i and clamping screw r can
have its refracting face adjusted with respect to that axis to the
best position for photographing any part of the spectrum. Such an
arrangement is typical of those used for small instruments say from 3
inches to 6 inches aperture.
[Illustration: FIG. 142.—Typical Stellar Spectroscope.]
For larger objectives the prism is usually of decidedly smaller angle,
and, if the light warrants high dispersion, several prisms in tandem
are used. The objective prism does its best work when applied to true
photographic objectives of the portrait lens type which yield a fairly
large field. It is by means of big instruments of such sort that the
spectra for the magnificent Draper Catalogue have been secured by the
Harvard Observatory, mostly at the Arequipa station. In photographing
with the objective prism the spectra are commonly given the necessary
width for convenient examination by changing just a trifle the rate of
the driving clock so that there is a slight and gradual drift in R. A.
The refracting edge of the prism being turned parallel to the diurnal
motion this drift very gradually and uniformly widens the spectrum to
the extent of a few minutes of arc during the whole exposure.
When one comes to solar spectroscopy one meets an entirely different
situation. In stellar work the difficulty is to get enough light, and
the tendency is toward large objectives of relatively short focal
length and spectroscopes of moderate dispersion. In solar studies there
is ample light, and the main thing is to get an image big enough to be
scrutinized in detail with very great dispersion.
[Illustration: FIG. 143.—Simple Objective Prism.]
Especially is this true in the study of the chromospheric flames
that rim the solar disc and blaze over its surface. To examine these
effectively the spectroscope should have immense dispersion with a
minimum amount of stray light in the field to interfere with vision of
delicate details.
In using a spectroscope like Fig. 142, if one turned the slit toward
the landscape, the instrument being removed from the telescope and the
slit opened wide, he could plainly see its various features, refracted
through the prism, and appearing in such color as corresponded to
the part of the spectrum in the line of the observing telescope. In
other words one sees refracted images quite distinctly in spite of the
bending of the rays. With high dispersion the image seen is practically
monochromatic.
Now if one puts the spectroscope in place, brings the solar image
tangent to the slit and then cautiously opens the slit, he sees the
bright continuous spectrum of the sky close to the sun, plus any light
of the particular color for which the observing telescope is set, which
may proceed from the edge of the solar disc. Thus, if the setting is
for the red line of hydrogen (C), one sees the hydrogen glow that plays
in fiery pillars of cloud about the sun’s limb quite plainly through
the opened slit, on a background of light streaming from the adjacent
sky. To see most clearly one must use great dispersion to spread this
background out into insignificance, must keep other stray light out of
the field, and limit his view to the opened slit.
[Illustration: FIG. 144.—Diagram of Evershed Solar Spectroscope.]
To these ends early solar spectroscopes had many prisms in tandem,
up to a dozen or so, kept in proper relation by complicated linkages
analogous to the simple one shown in Fig. 142. Details can be found
in almost any astronomical work of 40 years ago. They were highly
effective in giving dispersion but neither improved the definition nor
cut out light reflected back and forth from their many surfaces.
Of late simpler constructions have come into use of which an excellent
type is the spectroscope designed by Mr. Evershed and shown in diagram
in Fig. 144. Here the path of the rays is from the slit through the
collimator objective, then through a very powerful direct vision
system, giving a dispersion of 30° through the visible spectrum, then
by reflection from the mirror through a second such system, and thence
to the observing telescope. The mirror is rotated to get various parts
of the spectrum into view, and the micrometer screw that turns it gives
means for making accurate measurement of wave lengths.
There are but five reflecting surfaces in the prism system (for the
cemented prism surfaces do not count for much) as against more than
20 in one of the older instruments of similar power, and as in other
direct vision systems the spectrum lines are substantially straight
instead of being strongly curved as with multiple single prisms. The
result is the light, compact, and powerful spectroscope shown complete
in Fig. 145, equally well fitted for observing the sun’s prominences
and the detailed spectrum from his surface.
[Illustration: FIG. 145.—Evershed Solar Spectroscope.]
In most of the solar spectroscopes made at the present time the prisms
are replaced by a diffraction grating. The original gratings made by
Fraunhofer were made of wire. Two parallel screws of extremely fine
thread formed two opposite sides of a brass frame. A very fine wire was
then wound over these screws, made fast by solder on one side of each,
and then cut away on the other, so as to leave a grating of parallel
wires with clear spaces between.
Today the grating is generally ruled by an automatic ruling engine
upon a polished plate of speculum metal. The diamond point carried by
the engine cuts very smooth and fine parallel furrows, commonly from
10,000 to 20,000 to the inch. The spaces between the furrows reflect
brilliantly and produce diffraction spectra.[22]
[22] For the principle of diffraction spectra see Baly, Spectroscopy;
Kayser, Handbuch d. Specktroskoie or any of the larger textbooks of
physics.
When a grating is used instead of prisms the instrument is commonly
set up as shown in Fig. 146. Here _A_ is the collimator with slit upon
which the solar image light falls, _B_ is the observing telescope, and
_C_ the grating set in a rotatable mount with a fine threaded tangent
screw to bring any line accurately upon the cross wires of the ocular.
[Illustration: FIG. 146.—Diagram of Grating Spectroscope.]
The grating gives a series of spectra on each side of the slit,
violet ends toward the slit, and with deviations proportional to 1,
2, 3, 4, etc., times the wave length of the line considered. The
spectra therefore overlap, the ultra violet of the second order being
superimposed on the extreme red of the first order and so on. Colored
screens over the slit or ocular are used to get the overlying spectra
out of the way.
The grating spectroscopes are very advantageous in furnishing a wide
range of available dispersions, and in giving less stray light than
a prism train of equal power. The spectra moreover are very nearly
“normal,” _i.e._, the position of each line is proportional to its
wave length instead of the blue being disproportionately long as in
prismatic spectra.
In examining solar prominences the widened slit of a grating
spectroscope shows them foreshortened or stretched to an amount
depending on the angular position of the grating, but the effect is
easily reckoned.[23]
[23] The effect on the observed height of a prominence is h = h′ sin
c/sin t, where h is the real height, h′ the apparent height, c the
angle made by the grating face with the collimator, and t that with
the telescope (Fig. 146).
If the slit is nearly closed one sees merely a thin line, irregularly
bright according to the shape of the prominence; a shift of the slit
with respect to the solar image shows a new irregular section of the
prominence in the same monochromatic light.
These simple phenomena form the basis of one of the most important
instruments of solar study—the spectro-heliograph. This was devised
almost simultaneously by G. E. Hale and M. Deslandres about 30 years
ago, and enables photographs of the sun to be taken in monochromatic
light, showing not only the prominences of the limb but glowing masses
of gas scattered all over the surface.
The principle of the instrument is very simple. The collimator of a
powerful grating spectroscope is provided with a slit the full length
of the solar diameter, arranged to slide smoothly on a ball-bearing
carriage clear across the solar disc. Just in front of the photographic
plate set in the focus of the camera lens is another narrow sliding
slit, which, like a focal plane shutter, exposes strip after strip of
the plate.
The two slits are geared together by a system of levers or otherwise
so that they move at exactly the same uniform rate of speed. Thus
when the front slit is letting through a monochromatic section of
a prominence on the sun’s limb the plate-slit is at an exactly
corresponding position. When the front slit is exactly across the sun’s
center so is the plate slit, at each element of movement exposing a
line of the plate to the monochromatic image from the moving front
slit. The grating can of course be turned to put any required line
into action but it usually is set for the K line (calcium), which is
photographically very brilliant and shows bright masses of floating
vapor all over the sun’s surface.
Figure 147 shows an early and simple type of Professor Hale’s
instrument. Here A is the collimator with its sliding slit, B the
photographic telescope with its corresponding slide and C the lever
system which connects the slides in perfectly uniform alignment.
The source of power is a very accurately regulated water pressure
cylinder mounted parallel with the collimator. The result is a complete
photograph of the sun taken in monochromatic light of exactly defined
wave length and showing the precise distribution of the glowing vapor
of the corresponding substance.
Since the spectro-heliograph of Fig. 147, which shows the principle
remarkably well, there have been made many modifications, in particular
for adapting the scheme to the great horizontal and vertical fixed
telescopes now in use. (For details of these see Cont. from the Solar
Obs. Mt. Wilson, Nos. 3, 4, 23, and others). The chief difficulty
always is to secure entirely smooth and uniform motion of the two
moving elements.
[Illustration: FIG. 147.—Hale’s Spectro-heliograph (Early Form).]
So great and interesting a branch of astronomy is the study of variable
stars that some form of photometer should be part of the equipment of
every telescope in serious use for celestial observation. An immense
amount of useful work has been done by Argelander’s systematic method
of eye observation, but it is far from being precise enough to disclose
many of the most important features of variability.
The conventional way of reckoning by stellar magnitudes is conducive to
loose measurements, since each magnitude of difference implies a light
ratio of which the log is 0.4, _i.e._, each magnitude is 2.512 times
brighter than the following one. As a result of this way of reckoning
the light of a star of mag. 9.9 differs from one of mag. 10.0 not by
one per cent but by about nine. Hence to grasp light variations of
small order one must be able to measure far below 0.1^_m_.
[Illustration: FIG. 148.-Double Image Stellar Photometer.]
The ordinary laboratory photometer enables one to compare light sources
of anywhere near similar color to a probable error of well under 0.1
per cent, but it allows a comparison between sharply defined juxtaposed
fields from the two illuminants, a condition much more favorable to
precision than the comparison of two points of light, even if fairly
near together.
Stellar photometers may in principle be divided into three classes. (1)
Those in which two actual stars are brought into the same field and
compared by varying the light from one or both in a known degree. (2)
Those which bring a real star into the field alongside an artificial
star, and again bring the two to equality by a known variation,
usually comparing two or more stars via the same artificial star;
(3) those which measure the light of a star by a definite method of
extinguishing it entirely or just to the verge of disappearance in a
known progression. Of each class there are divers varieties. The type
of the first class may be taken as the late Professor E. C. Pickering’s
polarizing photometer. Its optical principle is shown in Fig. 148. Here
the brightness of two neighboring objects is compared by polarizing
at 90° apart the light received from each and reducing the resulting
images to equality by an analyzing Nicol prism. The photometer is
fully described, with, several other polarizing instruments, in H. A.
Vol. II from which Fig. 148 is taken.
A is a Nicol prism inserted in the ocular _B_, which revolves carrying
with it a divided circle _C_ read against the index _D_. In the tube
_E_ which fits the eye end of the telescope, is placed the double
image quartz prism _F_ capable of sliding either way without rotation
by pulling the cord _G_. With the objects to be compared in the same
field, two images of each appear. By turning the analyzing Nicol the
fainter image of the brighter can always be reduced to equality with
the brighter image of the fainter, and the amount of rotation measures
the required ratio of brightness.[24] This instrument works well for
objects near enough to be in the same field of view. The distance
between the images can be adjusted by sliding the prism _F_ back and
forth, but the available range of view is limited to a small fraction
of a degree in ordinary telescopes.
[24] If A be the brightness of one object and B that of the other, α
the reading of the index when one image disappears and β the reading
when the two images are equal then A/B = tan²(α-β). There are
four positions of the Nicol, 90° apart, for which equality can be
established, and usually all are read and the mean taken. (H. A. II,
1.)
The meridian photometer was designed to avoid this small scope. The
photometric device is substantially the same as in Fig. 148. The
objects compared are brought into the field by two exactly similar
objectives placed at a small angle so that the images, after passing
the double image prism, are substantially in coincidence. In front of
each of the objectives is a mirror. The instrument points in the east
and west line and the mirrors are at 45° with its axis. One brings
Polaris into the field, the other by a motion of rotation about the
telescope axis can bring any object in or close to the meridian into
the field alongside Polaris. The images are then compared precisely
as in the preceding instance.[25] There are suitable adjustments for
bringing the images into the positions required.
[25] For full description and method see H. A. Vol. 14, also Miss
Furness’ admirable “Introduction to the Study of Variable Stars,” p.
122, et seq. Some modifications are described in H. A. Vol. 23. These
direct comparison photometers give results subject to some annoying
small corrections, but a vast amount of valuable work has been done
with them in the Harvard Photometry.
The various forms of photometer using an artificial star as
intermediary in the comparison of real stars differ chiefly in the
method of varying the light in a determinate measure. Rather the best
known is the Zöllner instrument shown in diagram in Fig. 149. Here
_A_ is the eye end of the main telescope tube. Across it at an angle
of 45° is thrown a piece of plane parallel glass _B_ which serves to
reflect to the focus the beam from down the side tube, _C_, forming the
artificial star.
[Illustration: FIG. 149.—Zöllner Photometer Diagram.]
At the end of this tube is a small hole or more often a diaphragm
perforated with several very small holes any of which can be brought
into the axis of the tube. Beyond at _D_, is the source of light,
originally a lamp flame, now generally a small incandescent lamp, with
a ground glass disc or surface uniformly to diffuse the light.
Within the tube _C_ lie three Nicol prisms _n_, _n__{1}, _n__{2}.
Of these _n_, is fixed with respect to the mirror B and forms the
analyser, which _n__{1} and _n__{2} turn together forming the
polarizing system. Between _n_1_ and _n_2_ is a quartz plate _e_ cut
perpendicular to the crystal axis. The color of the light transmitted
by such a plate in polarized light varies through a wide range.
By turning the Nicol _n_2_ therefore, the color of the beam which
forms the artificial star can be made to match the real star under
examination, and then by turning the whole system _n_2_, _E_, _n_1_,
reading the rotation on the divided circle at _F_, the real star can be
matched in intensity by the artificial one.
[Illustration: FIG. 150.—Wedge Photometer.]
This is viewed via the lens _G_ and two tiny points of light appear
in the field of the ocular due respectively to reflection from the
front and back of the mirror _B_, the latter slightly fainter than
the former. Alongside or between these the real star image can be
brought for a comparison, and by turning the polarizer through an angle
α the images can be equalized with the real image. Then a similar
comparison is made with a reference star. If A be the brightness of the
former and B of the latter then
A/B = sin²α/sin²ββ
The Zöllner photometer was at first set up as an alt-azimuth instrument
with a small objective and rotation in altitude about the axis _C_.
Since the general use of electric lamps instead of the inconvenient
flame it is often fitted to the eye end of an equatorial.
Another very useful instrument is the modern wedge photometer, closely
resembling the Zöllner in some respects but with a very different
method of varying the light; devised by the late Professor E. C.
Pickering. It is shown somewhat in diagram in Fig. 150. Here as before
O is the eye end of the tube, B the plane parallel reflector, C the
side tube, L the source of light D the diaphragm and A the lens forming
the artificial star by projecting the hole in the diaphragm. In actual
practice the diameter of such hole is 1/100 inch or less.
[Illustration: FIG. 151.—Simple Polarizing Photometer.]
The light varying device W is a “photographic wedge” set in a frame
which is graduated on the edge and moved in front of the aperture by a
rack and pinion at R. There are beside colored and shade glasses for
use as occasion requires. The “photographic wedge” is merely a strip
of fine grained photographic plate given an evenly graduated exposure
from end to end, developed, and sealed under a cover glass. Its
absorption is permanent, non-selective as to color, and it can be made
to shade off from a barely perceptible density to any required opacity.
Sometimes a wedge of neutral tinted glass is used in its stead.
Before using such a “wedge photometer” the wedge must be accurately
calibrated by observation of real or artificial stars of known
difference in brightness. This is a task demanding much care and is
well described, together with the whole instrument by Parkhurst (Ap.
J. 13, 249). The great difficulty with all instruments of this general
type is the formation of an artificial star the image of which shall
very closely resemble the image of the real star in appearance and
color.
Obviously either the real or artificial star, or both, may be varied
in intensity by wedge or Nicols, and a very serviceable modification
of the Zöllner instrument, with this in mind was recently described by
Shook (Pop. Ast. 27, 595) and is shown in diagram in Fig. 151. Here A
is the tube which fits the ordinary eyepiece sleeve. E is a side tube
into which is fitted the extension D with a fitting H at its outer
end into which sets the lamp tube G. This carries on a base plug F a
small flash light bulb run by a couple of dry cells. At O is placed
a little brass diaphragm perforated with a minute hole. Between this
and the lamp is a disc of diffusing glass or paper. A Nicol prism is
set a little ahead of O, and a lens L focusses the perforation at the
principal focus of the telescope after reflection from the diagonal
glass M, as in the preceding examples. I is an ordinary eyepiece over
which is a rotatable Nicol N with a position circle K. At P is a third
Nicol in the path of the rays from the real star, thereby increasing
the convenient range of the instrument. The original paper gives the
details of construction as well as the methods of working. Obviously
the same general arrangement could be used for a wedge photometer using
the wedge on either real or artificial star or both.
The third type of visual photometer depends on reducing the light of
the star observed until it just disappears. This plan was extensively
employed by Professor Pritchard of Oxford some 40 years ago. He used
a sliding wedge of dark glass, carefully calibrated, and compared two
stars by noting the point on the wedge at which each was extinguished.
A photographic wedge may be used in exactly the same way.
Another device to the same end depends on reducing the aperture of the
telescope by a “cat’s eye,” an iris diaphragm, or similar means until
the star is no longer visible or just disappearing. The great objection
to such methods is the extremely variable sensitivity of the eye under
varying stimulus of light.
The most that can be said for the extinction photometer is that in
skillful and experienced hands like Pritchard’s it has sometimes given
much more consistent readings than would be expected. It is now and
then very convenient for quick approximation but by no courtesy can
it be considered an instrument of precision either in astronomical or
other photometry.[26]
[26] The general order of precision attained by astronomical
photometers is shown in the discovery, photographically, by
Hertzsprung in 1911, that Polaris, used as a standard magnitude for
many years, is actually a variable. Its period is very near to four
days, its photographic amplitude 0.17 and its visual amplitude about
0.1, _i.e._, a variation of ± 5 per cent in the light was submerged in
the observational uncertainties, although once known it was traced out
in the accumulated data without great difficulty.
The photometer question should not be closed without referring the
reader to the methods of physical photometry as developed by Stebbins,
Guthnick and others. The first of these depends on the use of the
selenium cell in which the electrical resistance falls on exposure
of the selenium to light. The device is not one adapted to casual
use, and requires very careful nursing to give the best results, but
these are of an order of precision beyond anything yet reached with an
astronomical visual photometer. Settings come down to variations of
something like 2 per cent, and variations in stellar light entirely
escaping previous methods become obvious.
The photoelectric cell depends on the lowering of the apparent electric
resistance of a layer of rarified inert gas between a platinum grid and
an electrode of metallic potassium or other alkali metal when light
falls on that electrode. The rate of transmission of electricity is
very exactly proportional to the illumination, and can be best measured
by a very sensitive electrometer. The results are extraordinarily
consistent, and the theoretical “probable error” is very small, though
here, as elsewhere, “probable error” is a rather meaningless term
apt to lead to a false presumption of exactness. Again the apparatus
is somewhat intricate and delicate, but gives a precision of working
if anything a little better than that of the selenium cell, quite
certainly below 1 per cent.
Neither instrument constitutes an attachment to the ordinary
telescope of modest size which can be successfully used for ordinary
photometry, and both require reduction of results to the basis
of visual effect.[27] But both offer great promise in detecting
minute variations of light and have done admirable work. For a good
fundamental description of the selenium cell photometer see Stebbins,
Ap. J. =32=, 185 and for the photoelectric method see Guthnick A. N.
=196=, 357 also A. F. and F. A. Lindemann, M. N. =39=, 343. The volume
by Miss Furness already referred to gives some interesting details of
both.
[27] Such apparatus is essentially appurtenant to large instruments
only, say of not less than 12″ aperture and preferably much more. The
eye is enormously more sensitive as a detector of radiant energy than
any device of human contrivance, and thus small telescopes can be well
used for visual photometry, the bigger instruments having then merely
the advantage of reaching fainter stars.