SETTING UP AND HOUSING THE TELESCOPE
In regard to getting a telescope into action and giving it suitable
protection, two entirely different situations present themselves. The
first relates to portable instruments or those on temporary mounts,
the second to instruments of position. As respects the two, the
former ordinarily implies general use for observational purposes, the
latter at least the possibility of measurements of precision, and a
mount usually fitted with circles and with a driving clock. Portable
telescopes may have either alt-azimuth or equatorial mounting, while
those permanently set up are now quite universally equatorials.
Portable telescopes are commonly small, ranging from about 2½ inches
to about 5 inches in aperture. The former is the smallest that can
fairly be considered for celestial observations. If thoroughly good and
well mounted even this is capable of real usefulness, while the five
inch telescope if built and equipped in the usual way, is quite the
heaviest that can be rated as portable, and deserves a fixed mount.
Setting up an alt-azimuth is the simplest possible matter. If on a
regular tripod it is merely taken out and the tripod roughly levelled
so that the axis in azimuth is approximately vertical. Now and then one
sets it deliberately askew so that it may be possible to pass quickly
between two objects at somewhat different altitudes by swinging on the
azimuth axis.
If one is dealing with a table tripod like Fig. 69 it should merely be
set on any level and solid support that may be at hand, the main thing
being to get it placed so that one may look through it conveniently.
This is a grave problem in the case of all small refractors, which
present their oculars in every sort of unreachable and uncomfortable
position.
Of course a diagonal eyepiece promises a way out of the difficulty,
but with small apertures one hesitates to lose the light, and often
something of definition, and the observer must pretty nearly stand
on his head to use the finder. With well adjusted circles, such
are commonly found on a fixed mount, location of objects is easy.
On a portable set-up perhaps the easiest remedy is a pair of well
aligned coarse sights near the objective end of the tube and therefore
within reach when it is pointed zenith-ward. The writer has found a
low, armless, cheap splint rocker, such as is sold for piazza use,
invaluable under these painful circumstances, and can cordially
recommend it.
Even better is an observing box and a flat cushion. The box is merely a
coverless affair of any smooth ⅞ inch stuff firmly nailed or screwed
together, and of three unequal dimensions, giving three available
heights on which to sit or stand. The dimensions originally suggested
by Chambers (_Handbook of Astronomy_, II, 215) were 21 × 12 × 15
inches, but the writer finds 18 × 10 × 14 inches a better combination.
The fact is that the ordinary stock telescope tripod is rather too high
for sitting, and too low for standing, comfortably. A somewhat stubby
tripod is advantageous both in point of steadiness and in accessibility
of the eyepiece when one is observing within 30° of the zenith, where
the seeing is at its best; and a sitting position gives a much greater
range of convenient upward vision than a standing one.
When an equatorial mount is in use one faces the question of adjustment
in its broadest aspect. Again two totally different situations arise in
using the telescope. First is the ordinary course of visual observation
for all general purposes, in which no precise measurements of position
or dimensions are involved.
Here exact following is not necessary, a clock drive is convenient
rather than at all indispensable, and even circles one may get along
without at the cost of a little time. Such is the usual situation with
portable equatorials. One does not then need to adjust them to the
pole with extreme precision, but merely well enough to insure easy
following; otherwise one is hardly better off than with an alt-azimuth.
In a totally different class falls the instrument with which one
undertakes regular micrometric work, or enters upon an extended
spectroscopic program or the use of precise photometric apparatus, to
say nothing of photography. In such cases a permanent mount is almost
imperative, the adjustments must be made with all the exactitude
practicable, one finds great need of circles, and the lack of a clock
drive is a serious handicap or worse.
Moreover in this latter case one usually has, and needs, some sort of
timepiece regulated to sidereal time, without which a right ascension
circle is of very little use.
In broad terms, then, one has to deal, first; with a telescope on a
portable mount, with or without position circles, generally lacking
both sidereal clock and driving clock, and located where convenience
dictates; second, with a telescope on a fixed mount in a permanent
location, commonly with circles and clock, and with some sort of
permanent housing.
Let us suppose then that one is equipped with a 5 inch instrument like
Fig. 168, having either the tripod mount, or the fixed pillar mount
shown alongside it; how shall it be set up, and, if on the fixed mount,
how sheltered?
In getting an equatorial into action the fundamental thing is to place
the optical axis of the telescope exactly parallel to the polar axis
of the mount and to point the latter as nearly as possible at the
celestial pole.
The conventional adjustments of an equatorial telescope are as follows:
1. Adjust polar axis to altitude of pole.
2. Adjust index of declination circle.
3. Adjust polar axis to the meridian.
4. Adjust optical axis perpendicular to declination axis.
5. Adjust declination axis perpendicular to polar axis.
6. Adjust index of right ascension circle, and
7. Adjust optical axis of finder parallel to that of telescope.
Now let us take the simplest and commonest case, the adjustment of
a portable equatorial on a tripod mount, when the instrument has a
finder but neither circles nor driving clock. Adjustments 2 and 6
automatically drop out of sight, 5 vanishes for lack of any means to
make the adjustment, and on a mount made with high precision, like the
one before us, 4 is negligible for any purpose to which our instrument
is applicable.
Adjustments 1, 3 and 7 are left and these should be performed in the
order 7, 1, 3, for sake of simplicity. To begin with the finder has
cross-wires in the focus of its eyepiece, and the next step is to
provide the telescope itself with similar cross-wires.
These can readily be made, if not provided, by cutting out a disc of
cardboard to fit snugly either the spring collar just in front of
a positive eyepiece or the eyepiece itself at the diaphragm, if an
ordinary Huygenian. Rule two diametral lines on the circle struck for
cutting the cardboard, crossing at the center, cut out the central
aperture, and then very carefully stretch over it, guided by the
diametral lines, two very fine threads or wires made fast with wax or
shellac.
[Illustration: FIG. 168.—Clark 5-inch with Tripod and Pier.]
Now pointing the telescope at the most distant well defined object in
view, rotate the spring collar or ocular, when, if the crossing of the
threads is central, their intersection should stay on the object. If
not shift a thread cautiously until the error is corrected.
Keeping the intersection set on the object by clamping the tube, one
turns attention to the finder. Either the whole tube is adjustable
in its supports or the cross-wires are capable of adjustment by
screws just in front of the eyepiece. In either case finder tube or
cross-wires should be shifted until the latter bear squarely upon the
object which is in line with the cross threads of the main telescope.
Then the adjusting screws should be tightened, and the finder is in
correct alignment.
As to adjustments 1 and 3, in default of circles the ordinary
astronomical methods are not available, but a pretty close
approximation can be made by levelling. A good machinist’s level is
quite sensitive and reliable. The writer has one picked out of stock at
a hardware shop that is plainly sensitive to 2′ of arc, although the
whole affair is but four inches long.
Most mounts like the one of Fig. 168 have a mark ruled on the support
of the polar axis and a latitude scale on one of the cheek pieces.
Adjustment of the polar axis to the correct altitude is then made by
placing the level on the declination axis, or any other convenient
place, bringing it to a level, and then adjusting the tripod until the
equatorial head can be revolved without disturbing this level. Then
set the polar axis to the correct latitude and adjustment number 1 is
complete for the purpose in hand.
Lacking a latitude scale, it is good judgment to mark out the latitude
by the help of the level and a paper protractor. To do this level the
polar axis to the horizontal, level the telescope tube also, and clamp
it in declination to maintain it parallel. Then fix the protractor to
a bit of wood tied or screwed to the telescope support, drop a thin
thread plumb line from a pin driven into the wood, the declination axis
being still clamped, note the protractor reading, and then raise the
polar axis by the amount of the latitude.
Next, with a knife blade scratch a conspicuous reference line on the
sleeve of the polar axis and its support so that when the equatorial
head is again levelled carefully you can set approximately to the
latitude at once.
Now comes adjustment 3, the alignment of the polar axis to the
meridian. One can get it approximately by setting the telescope tube
roughly parallel with the polar axis and, sighting along it, shifting
the equatorial head in azimuth until the tube points to the pole star.
Then several methods of bettering the adjustment are available.
At the present date Polaris is quite nearly 1° 07′ from the true pole
and describes a circle of that radius about it every 24 hours. To get
the correct place of the pole with reference to Polaris one must have
at least an approximate knowledge of the place of that star in its
little orbit, technically its hour-angle. With a little knowledge of
the stars this can be told off in the skies almost as easily as one
reckons time on a clock. Fig. 169 is, in fact, the face of the cosmic
clock, with a huge sweeping hour hand that he who runs may read.
[Illustration: FIG. 169.—The Cosmic Clock.]
It is a clock in some respects curious; it has a twenty-four hour face
like some clocks and watches designed for Continental railway time; the
hour hand revolves backward, (“counter-clockwise”) and it stands in the
vertical not at noon, but at 1.20 Star Time. The two stars which mark
the tip and the reverse end of the hour hand are delta Cassiopeæ and
zeta Ursæ Majoris respectively. The first is the star that marks the
bend in the back of the great “chair,” the second (Mizar), the star
which is next to the end of the “dipper” handle.
One or the other is above the horizon anywhere in the northern
hemisphere. Further, the line joining these two stars passes almost
exactly through the celestial pole, and also very nearly through
Polaris, which lies between the pole and δ Cassiopeæ. Consequently if
you want to know the hour-angle of Polaris just glance at the clock and
note where on the face δ Cassiopeæ stands, between the vertical which
is XXIV o’clock, and the horizontal, which is VI (east) or XVIII (west)
o’clock.
You can readily estimate its position to the nearest half hour, and
knowing that the great hour hand is vertical (δ Cassiopeæ up) at I^h
20^m or (ζ Ursæ Majoris up) at XIII^h 20^m, you can make a fairly close
estimate of the sidereal time.
A little experience enables one to make excellent use of the clock
in locating celestial objects, and knowledge of the approximate hour
angle of Polaris thus observed can be turned to immediate use in making
adjustment 3. To this end slip into the plane of the finder cross wires
a circular stop of metal or paper having a radius of approximately 1°
15′ which means a diameter of 0.52 inch per foot of focal length.
Then, leaving the telescope clamped in declination as it was after
adjustment 1, turn it in azimuth across the pole until the pole star
enters the field which, if the finder inverts it will do on the other
side of the center; i.e. if it stands at IV to the naked eye it will
enter the field apparently from the XVI o’clock quarter. When just
comfortably inside the field, the axis of the telescope is pointing
substantially at the pole.
It is better to get Polaris in view before slipping in the stop and if
it is clearly coming in too high or too low shift the altitude of the
polar axis a trifle to correct the error. This approximate setting can
be made even with the smallest finder and on any night worth an attempt
at observation.
With a finder of an inch or more aperture a very quick and quite
accurate setting to the meridian can be made by the use of Fig. 170,
which is a chart of all stars of 8 mag. or brighter within 1° 30′ of
the pole. There are only three stars besides Polaris at all conspicuous
in this region, one quite close to Polaris, the other two forming with
it the triangle marked on the chart. These two are, to the left, a
star of magnitude 6.4 designated B. D. 88 112, and to the right one of
magnitude 7.0, B. D. 89 13.
The position of the pole for the rest of the century is marked on the
vertical arrow and with the stars in the field of the finder one can
set the cross wires on the pole, the instrument remaining clamped in
declination, within a very few minutes of arc, quite closely enough
for any ordinary use of a portable mount. All this could be done even
better with the telescope itself, but it is very rare to find an
eyepiece with sufficient field.
[Illustration: FIG. 170.—The Pole among the Stars.]
At all events the effect of any error likely to be made in these
adjustments is not serious for the purpose in hand, since if one makes
an error of a minute of arc in the setting the resulting displacement
of a star in the field will even in the most unfavorable case reach
this full amount only after 6 hours following. I.e. with any given
eyepiece an error of adjustment equal to the radius of the field will
still permit following a star for an hour or two before it drifts
inconveniently wide of the center.
Considerable space has been devoted to these easy approximations in
setting up, since the directions commonly given require circles and
often a clock drive.
In some cases one has to set up a portable equatorial where from
necessity for clear sky space, Polaris is not visible. The best plan
then is to set up with great care where Polaris can be seen, paying
especial attention to the levelling. Then establish two meridian marks
on stakes at a convenient distance by turning the telescope 180° on its
declination axis and sighting through it in both directions. Now with
a surveyor’s tape transfer the meridian line East or West as the case
may be until it can be used where there is clear sky room.
Few observers near a city can get good sky room, from the interference
of houses, trees or blazing street lamps, and the telescope must often
be moved from one site to another to reach different fields. In such
case it is wise to take the very first step toward giving the telescope
a local habitation by establishing a definite placement for the tripod.
To this end the three legs should be firmly linked together by chains
that will not stretch—leg directly to leg, and not to a common
junction. Then see to it that each leg has a strong and moderately
sharp metal point, and, the three points of support being thus
definitely fixed, establish the old reliable point-slot-plane bearing
as follows:
Lay out at the site (or sites) giving the desired clear view, a circle
scratched on the ground of such size that the three legs of your tripod
may rest approximately on its periphery. Then lay out on the circle
three points 120° apart. At each point sink a short post 12 to 18
inches long and of any convenient diameter, well tarred, and firmly set
with the top levelled off quite closely horizontal.
To the top of each bolt a square or round of brass or iron about half
an inch thick. The whole arrangement is indicated in diagram in Fig.
171. In _a_ sink a conical depression such as is made by drilling
nearly through with a 1 inch twist drill. The angle here should be a
little broader than the point on the tripod leg. In _b_ have planed a V
shaped groove of equally broad angle set with its axis pointing to the
conical hole in _a_. Leave the surface of _c_ a horizontal plane.
Now if you set a tripod leg in _a_, another in the slot at _b_ and
the third on _c_, the tripod will come in every instance to the same
level and orientation. So, if you set up your equatorial carefully in
the first place and leave the head clamped in azimuth, you can take it
in and replace it at any time still in adjustment as exact as at the
start. And if it is necessary to shift from one location to another you
can do it without delay still holding accurate adjustment of the polar
axis to the pole, and avoiding the need of readjustment.
In case the instrument has a declination circle the original set-up
becomes even simpler. One has only to level the tripod, either with
or without the equatorial head in place, and then to set the polar
axis either vertical or horizontal, levelling the tube with it either
by placing the level across the objective cell perpendicular to the
declination axis, or laying it along the tube when horizontal.
[Illustration: FIG. 171.—A Permanent Foothold for the Tripod.]
Then, reading the declination circle, one can set off the co-atitude
or latitude as the case may be and, leaving the telescope clamped in
declination, lower or raise the polar axis until the tube levels to
the horizontal. When the mount does not permit wide adjustment and has
no latitude scale one is driven to laying out a latitude templet and,
placing a straight edge under the equatorial head, or suspending a
plumb line from the axis itself, setting it mechanically to latitude.
Now suppose we are dealing with the same instrument, but are planning
to plant it permanently in position on its pillar mount. It is now
worth while to make the adjustments quite exactly, and to spend some
time about it. The pillar is commonly assembled by well set bolts on a
brick or concrete pier. The preliminary steps are as already described.
The pillar is levelled across the top, the equatorial head, which turns
upon it in azimuth, is levelled as before, the adjustment being made
by metal wedges under the pillar or by levelling screws in the mount
if there are any. Then the latitude is set off by the scale, or by
the declination circle, and the polar axis turned to the approximate
meridian as already described.
There is likely to be an outstanding error of a few minutes of arc
which should in a permanent mount be reduced as far as practicable. At
the start adjust the declination of the optical axis of the telescope
to that of the polar axis. This is done in the manner suggested by Fig.
172.
Here _p_ is the polar axis and _d_ the declination axis. Now if one
sights, using the cross wires, through the telescope a star near the
meridian, i.e., one that is changing in declination quite slowly,
starting from the position _A_ with the telescope _E_. of the polar
axes, and turns it over 180° into the position _B_, _W_. of the polar
axis, the prolongation of the line of sight, _b_, will fall below _a_,
when as here the telescope points too high in the _A_ position.
[Illustration: FIG. 172.—Aligning the Optical Axis.]
In other words the apparent altitude of the star will change by twice
the angle between _A_ and _p_. Read both altitudes on the declination
circle and split the difference with the slow motion as precisely as
the graduation of the declination circle permits.
The telescope will probably not now point exactly at the star, but as
the tube is swung from the _A_ to the _B_ position and back the visible
stars will describe arcs of circles which should be nearly concentric
with the field as defined by the stop in the eyepiece. If not, a very
slight touch on the declination slow motion one way or the other will
make them do so to a sufficient exactness, especially if a rather high
power eyepiece is used.
The optical axis of the telescope is now parallel to the polar axis,
but the latter may be slightly out of position in spite of the
preliminary adjustment. Now reverting to the polar field of Fig. 170,
swing from position _A_ to _B_ and back again, correcting any remaining
eccentricity of the star arcs around the pole by cautious shifting
of the polar axis, leaving the telescope clamped in declination. The
first centering is around the pole of the instrument, the second around
the celestial pole by help of a half dozen small stars within a half
degree on both sides of it, magnitudes 9 and 10, easily visible in a 3”
or 4” telescope, using the larger field of the finder for the coarse
adjustment.
If the divided circles read to single minutes or closer, which they
generally do not on instruments of moderate size, one can use the
readings to set the polar axis and the declination circle, and to make
the other adjustments as well.
In default of this help, the declination circle adjustment may be set
to read 90° when the optical axis is brought parallel to the polar
axis, and after the adjustment of the latter is complete, the R. A.
circle can be set by swinging up the telescope in the meridian and
watching for the transit of any star of known R. A. over the central
cross wire, at which moment the circle should be clamped to the R. A.
thus defined.
Two possible adjustments are left, the perpendicularity of the polar
and declination axes, and that of the optical axis to the declination
axis. As a rule there is no provision for either of these, which are
supposed to have been carried out by the maker. The latter adjustment
if of any moment will disclose itself as a lateral wobble in trying
to complete the adjustment of optical axis to polar axis. It can be
remedied by a liner of tinfoil or even paper under one end of the
tube’s bearing on its cradle. Adjustment of the former is strictly a
job for the maker.
For details of the rigorous adjustments on the larger instruments the
reader will do well to consult Loomis’ _Practical Astronomy_ page 28
and following.[31] The adjustments here considered are those which can
be effectively made without driving clock, finely divided circles,
or exact knowledge of sidereal time. The first and last of these
auxiliaries, however, properly belong with an instrument as large as
Fig. 168, on a fixed mount.
[31] See also two valuable papers by Sir Howard Grubb, _The
Observatory_, Vol. VII, pp. 9, 43. Also in Jour. Roy. Ast. Soc.
Canada, Dec., 1921, Jan. 1922.
There are several rather elegant methods of adjusting the polar axis
to the pole which depend on the use of special graticules in the
eyepiece, or on auxiliary devices applied to the telescope, the general
principle being automatically to provide for setting off the distance
between Polaris and the pole at the proper hour angle. A beautifully
simple one is that of Gerrish (_Pop. Ast._ =29=, 283).
The simple plan here outlined will generally, however, prove sufficient
for ordinary purposes and where high precision is necessary one has to
turn to the more conventional astronomical methods.
If one gives his telescope a permanent footing such as is shown in Fig.
171 adjustment has rarely to be repeated. With a pillar mount such as
we have just now been considering the instrument itself can be taken in
doors and replaced with very slight risk of disturbing its setting, but
some provision must be made for sheltering the mount.
A tarpaulin is sometimes recommended and indeed answers well,
particularly if a bag of rubber sheeting is drawn loosely over the
mount first. Better still is a box cover of copper or galvanized iron
set over the mount and closely fitting well down over a base clamped to
the pillar with a gasket to close the joint.
But the fact is when one is dealing with a fine instrument like Fig.
168 of as much as 5 inches aperture, the question of a permanent
housing (call it observatory if you like) at once comes up and will not
down.
It is of course always more convenient to have the telescope
permanently in place and ready for action. Some observers feel that
working conditions are better with the telescope in the open, but most
prefer a shelter from the wind, even if but partial, and the protection
of a covering, however slight, in severe weather.
In the last resort the question is mainly one of climate. Where nights,
otherwise of the best seeing quality, are generally windless or with
breezes so slight that the tube does not quiver a telescope in the
open, however protected between times, works perfectly well.
In other regions the clearest nights are apt to be those of a steady
gentle wind producing great uniformity of conditions at the expense
of occasional vibration of the instrument and of discomfort to the
observer. Hence one finds all sorts of practice, varied too, by the
inevitable question of expense.
The simplest possible housing is to provide for the fixed instrument
a moveable cover which can be lifted or slid quite out of the way
leaving the telescope in the open air, exposed to wind, but free from
the disturbing air currents that play around the opening of a dome.
Shelters of this cheap and simple sort have been long in use both for
small and large instruments.
[Illustration: FIG. 173.—The Simplest of Telescope Housings.]
For example several small astrographic instruments in the Harvard
equipment are mounted as shown in Fig. 173. Here are two fork mounts,
each on a short pier, and covered in by galvanized iron hoods made in
two parts, a vertical door which swings down, as in the camera of the
foreground, and the hood proper, hinged to the base plate and free to
swing down when the rear door is unlocked and opened. A little to the
rear is a similar astrographic camera with the hood closed. It is all
very simple, cheap, and effective for an instrument not exceeding say
two or three feet in focal length.
A very similar scheme has been successfully tried on reflectors as
shown in Fig. 174. The instrument shown is a Browning equatorial of
8½ inches aperture. The cover is arranged to open after the manner
of Fig. 173 and the plan proved very effective, preserving much greater
uniformity of conditions and hence permitting better definition than in
case of a similar instrument peering through the open shutter of a dome.
Such a contrivance gets unwieldly in case of a refractor on account of
the more considerable height of the pier and the length of the tube
itself. But a modification of it may be made to serve exceedingly well
in climates where working in the open is advantageous. A good example
is the equatorial of the Harvard Observatory station at Mandeville,
Jamaica, which has been thus housed for some twenty years, as shown in
Fig. 175.
This 11 inch refractor, used mainly on planetary detail, is located
alongside the polar telescope of 12 inches aperture and 135 feet 4
inches focal length used for making a photographic atlas of the moon
and on other special problems. The housing, just big enough to take in
the equatorial with the tube turned low, opens on the south side and
then can be rolled northward on its track, into the position shown,
where it is well clear of the instrument, which is then ready for use.
[Illustration: FIG. 174.—Cover for Small Reflector.]
The climate of Jamaica, albeit extremely damp, affords remarkably
good seeing during a large part of the year, and permits use of the
telescope quite in the open without inconvenience to the observer. The
success of this and all similar housing plans depends on the local
climate more than on anything else—chiefly on wind during the hours
of good seeing. An instrument quite uncovered suffers from gusts far
more than one housed under a dome, which is really the sum of the whole
matter, save that a dome to a slight extent does shelter the observer
in extremely cold weather.
Even very large reflectors can be housed in similar fashion if suitably
mounted. For example in Fig. 176 is shown the 36 inch aperture
reflector of the late Dr. Common, which was fitted with an open fork
equatorial mounting. Here the telescope itself, with its short pier and
forked polar axis, is shown in dotted lines.
[Illustration: FIG. 175.—Sliding Housing for 11-inch Refractor.]
Built about it is a combined housing and observing stand rotatable on
wheels _T_ about a circular track _R_. The housing consists of low
corrugated metal sides and ends, here shown partly broken away, of
dimensions just comfortably sufficient to take in the telescope when
the housing is rotated to the north and south position, and the tube
turned down nearly flat southward. A well braced track _WW_ extends
back along the top of the side housing and well to the rear. On this
track rolls the roof of the housing _X,X,X_, with a shelter door at the
front end.
[Illustration: FIG. 176.—Sliding Housing for a Big Reflector.]
The members _U_ constitute a framing which supports at once the housing
and the observing platform, to which access is had by a ladder, _Z_,
provided with a counterbalanced observing seat. The instrument is
put into action by clearing the door at the end of the roof, running
the roof back to the position shown in the dotted lines, raising the
tube, and then revolving the whole housing into whatever position is
necessary to permit the proper setting of the tube.
[Illustration: FIG. 177.—Sliding Roof Observatory.]
This arrangement worked well but was found a bit troublesome owing to
wind and weather. With a skeleton tube and in a favorable climate the
plan would succeed admirably providing an excellent shelter for a large
telescope at very low cost.
Since a fork mount allows the tube to lie flat, such an instrument, up
to say 8 or 10 inches aperture can be excellently protected by covers
fitting snugly upon a base and light enough to lift off as a whole.
The successful use of all these shelters however depends on climatic
conditions. They require circumstances allowing observation in the
open, as with tripod mounts, and afford no protection from wind or
cold. Complete protection for the observer cannot be had, except by
some of the devices shown in Chapter V, but conditions can be improved
by permanent placement in an observatory, simple or elaborate, as the
builder may wish.
The word observatory may sound formidable, but a modest one can be
provided at less expense than a garage for the humblest motor car.
The chief difference in the economic situation is that not even the
most derided car can be picked up and carried into the back hall for
shelter, and it really ought not to be left out in the weather.
The next stage of evolution is the telescope house with a sliding roof
in one or more sections—ordinarily two. In this case the building
itself is a simple square structure large enough to accommodate the
instrument with maneuvering room around it. The side walls are carried
merely high enough to give clearance to the tube when turned nearly
flat and to give head room to the observer. The roof laps with a close
joint in the middle and each half rolls on a track supported beyond the
ends of the building by an out-rigger arranged in any convenient manner.
When the telescope is in use the roof sections are displaced enough
to give an ample clear space for observing, often wide open as shown
in Fig. 177, which is the house of the 16 inch Metcalf photographic
doublet at the Harvard Observatory. This instrument is in an open fork
mount like that shown in Fig. 139.
The sliding roof type is on the whole the simplest structure that can
be regarded as an observatory in the sense of giving some shelter to
the observer as well as the instrument. It gives ample sky room for
practical purposes even to an instrument with a fork mount, since in
most localities the seeing within 30° or so of the horizon is decidedly
bad. If view nearer the horizon is needed it can readily be secured by
building up the pier a bit.
Numberless modifications of the sliding roof type will suggest
themselves on a little study. One rather interesting one is used in
the housing of the 24 inch reflector of the Harvard Observatory, 11
feet 3 inches in focal length, the same of which the drive in its
original dome is shown in Fig. 139. As now arranged the lower part
of the observatory remains while the upper works are quite similar
in principle to the housing of Dr. Common’s 3 foot reflector of Fig.
176. The cover open is shown in Fig. 178. It will be seen that on the
north side of the observatory there is an out-rigger on which the top
housing slides clear of the low revolving turret which gives access to
the ocular fitting used generally to carry the plate holder, and the
eyepiece for following when required.
The tube cannot be brought to the horizontal, but it easily commands
all the sky-space that can advantageously be used in this situation,
and the protection given the telescope when not in use is very
complete. To close the observatory the tube is brought north and south
and turned low and the sliding roof is then run back into its fixed
position. The turret is very easily turned by hand.
[Illustration: FIG. 178.—Turret Housing of the 24-inch Harvard
Reflector.]
Of course for steady work with the maximum shelter for observer
obtainable without turning to highly special types of housing, the
familiar dome is the astronomer’s main reliance. It is in the larger
sizes usually framed in steel and covered with wood, externally
sheathed in copper or steel. Sometimes in smaller domes felt covered
with rubberoid serves a good purpose, and painted canvas is now and
then used, with wooden framing.
But even the smallest dome of conventional construction is heavy
and rather expensive, and for home talent offers many difficulties,
especially with respect to the shutter and shutter opening. A
hemisphere is neither easy to frame nor to cover, and the curved
sliding shutter is especially troublesome.
[Illustration: FIG. 179.—The Original “Romsey” Observatory.]
Hence for small observatories other forms of revolving roof are
desirable, and quite the easiest and cheapest contrivance is that
embodied in the “Romsey” type of observatory, devised half a century
ago by that accomplished amateur the Rev. E. L. Berthon, vicar of
Romsey. The feature of his construction is an unsymmetrical peak in the
revolving roof which permits the ordinary shutter to be replaced by a
hinged shutter like the skylight in a roof, exposing the sky beyond the
zenith when open, and closing down over a coaming to form a water tight
joint.
Berthon’s original description of his observatory, which accommodated a
9¼ inch reflector, may be found in Vol. 14 of the _English Mechanic
and World of Science_ whence Fig. 179 is taken. In this plate Fig. 1
shows the complete elevation and Fig. 2 the ground plan, each to a
scale of a eighth of an inch to the foot. In the plan, _A,A_, are the
main joists, _P_ the pier for the telescope, T that for the transit,
and _C_ the clock. Figs. 3, 4, and 5 are of details. In the last named
_A_ is a rafter, _b_ the base ring, _c_ the plate, _d_ one of the sash
rollers carrying the roof, and _e_ a lateral guide roller holding the
roof in place.
The structure can readily be built without the transit shelter, and in
fact now-a-days most observers find it easier to pick up their time by
wireless. The main bearing ring is cut out of ordinary ⅞ inch board,
in ten or a dozen, or more, sections according to convenience, done in
duplicate, joints lapping, and put very firmly together with screws set
up hard. Sometimes 3 layers are thus used.
The roof in the original “Romsey” observatory was of painted canvas,
but rubberoid or galvanized iron lined with roofing paper answers well.
The shutter can be made single or double in width, and counterbalanced
if necessary. The framing may be of posts set in the ground as here
shown, or with sills resting on a foundation, and the walls of any
construction—matched boards of any kind, cement on wire lath, hollow
tile, or concrete blocks.
Chambers’ _Handbook of Astronomy_ Vol. II contains quite complete
details of the “Romsey” type of observatory and is easier to get at
than the original description.
A very neat adaptation of the plan is shown in Fig. 180, of which
a description may be found in _Popular Astronomy_ =28=, 183. This
observatory was about 9 feet in diameter, to house a 4 inch telescope,
and was provided with a rough concrete foundation on which was built a
circular wall 6 feet high of hollow glazed tile, well levelled on top.
To this was secured a ring plate built up in two layers, carrying two
circles of wooden strips with a couple of inches space between them for
a runway. In this ran 6 two-inch truck castors secured to a similar
ring plate on which was built up the frame of the “dome” arranged as
shown. Altogether a very neat and workmanlike affair, in this case
built largely by the owner but permitting construction at very small
expense almost anywhere. Another interesting modification of the same
general plan in the same volume just cited is shown in Fig. 181. This
is also for a 4 inch refractor and the dome proper is but 8 feet 4
inches in diameter. Like the preceding structure the foundation is of
concrete but the walls are framed in spruce and sheathed in matched
boards with a “beaver-board” lining.
[Illustration: FIG. 180.—A More Substantial “Romsey” Type.]
The ring plate is three-ply, 12 sections to the layer, and its mate on
which the dome is assembled is similarly formed, though left with the
figure of a dodecagon to match the dome. The weight is carried on four
rubber tired truck rollers, and there are lateral guide rollers on the
plan of those in Fig. 179.
The dome itself however, is wholly of galvanized iron, in 12 gores
joined with standing seams, turned, riveted, and soldered. There is a
short shutter at the zenith sliding back upon a frame, while the main
shutter is removed from the outside by handles.
[Illustration: FIG. 181.—Detail of Light Metal Dome for Small
Observatory.]
Observatories of the Romsey or allied types can be erected at very
moderate cost, varying considerably from place to place, but running
at present say from $200 to $600, and big enough to shelter refractors
of 4 to 6 inches aperture. The revolving roofs will range from 9 to 12
feet in diameter. If reflectors are in use, those of about double these
apertures can be accommodated since the reflector is ordinarily much
the shorter for equal aperture.
The sliding roof, not to say the sliding shelter, forms of housing cost
somewhat less, depending on the construction adopted. Going to brick
may double the figures quoted, but such solidity is generally quite
needless, though it is highly desirable that the cover of a valuable
instrument should be fire-proof and not easily broken open. The
stealing of objectives and accessories is not unknown, and vandalism
is a risk not to be forgotten. But to even the matter up, housing a
telescope is rather an easy thing to accomplish, and as a matter of
fact for the price of a very modest motor car one can both buy and
house an instrument big enough to be of genuine service.