THE CARE AND TESTING OF TELESCOPES
A word at the start concerning the choice and purchase of telescopes.
The question of refractors vs. reflectors has been already considered.
The outcome of the case depends on how much and how often you are
likely to use the instrument, and just what you want it for. For casual
observations and occasional use—all that most busy buyers of telescopes
can expect—the refractor has a decided advantage in convenience. If
one has leisure for frequent observations, and particularly if he can
give his telescope a permanent mount, and is going in for serious work,
he will do well not to dismiss the idea of a reflector without due
deliberation.
In any case it is good policy to procure an instrument from one of
the best makers. And if you do not buy directly of the actual maker
it is best to deal with his accredited agents. In other words avoid
telescopes casually picked up in the optical trade unless you chance to
have facilities for thorough testing under competent guidance before
purchase. No better telescopes are made than can be had from the best
American makers. A few British and German makers are quite in the same
class. So few high grade French telescopes reach this country as to
cause a rather common, but actually unjust,[28] belief that there are
none.
[28] E. g., the beautiful astrographic and other objectives turned out
by the brothers Henry.
If economy must be enforced it is much wiser to try to pick up a used
instrument of first class manufacture than to chance a new one at a
low price. Now and then a maker of very ordinary repute may turn out
a good instrument, but the fact is one to be proved—not assumed. Age
and use do not seriously deteriorate a telescope if it has been given
proper care. Some of Fraunhofer’s are still doing good service after a
century, and occasionally an instrument from one of the great makers
comes into the market at a real bargain. It may drift back to the maker
for resale, or turn up at any optician’s shop, and in any case is
better worth looking at than an equally cheap new telescope.
The condition of the tube and stand cuts little figure if they are
mechanically in good shape. Most of the older high grade instruments
were of brass, beautifully finished and lacquered, and nothing looks
worse after hard usage. It is essential that the fitting of the parts
should be accurate and that the focussing rack should work with the
utmost smoothness. A fault just here, however, can be remedied at
small cost. The mount, whatever its character, should be likewise
smooth working and without a trace of shakiness, unless one figures on
throwing it away.
As to the objective, it demands very careful examination before a
real test of its optical qualities. The objective with its cell
should be taken out and closely scrutinized in a strong light after
the superficial dust has been removed with a camel’s hair brush or
by wiping very gently with the soft Japanese “lens paper” used by
opticians.
One is likely to find plenty to look at; spots, finger marks, obvious
scratches, and what is worse a network of superficial scratches, or a
surface with patches looking like very fine pitting. These last two
defects imply the need of repolishing the affected surface, which means
also more or less refiguring. Ordinary brownish spots and finger marks
can usually be removed with little trouble.
The layman, so to speak, is often warned never to remove the cell from
a telescope but he might as well learn the simpler adjustments first as
last. In taking off a cell the main thing is to see what one is about
and to proceed in an orderly manner. If the whole cell unscrews, as
often is the case in small instruments, the only precaution required is
to put a pencil mark on the cell and its seat so that it can be screwed
back to where it started.
If as is more usual the cell fits on with three pairs of screws, one of
each pair will form an abutment against which its mate pulls the cell.
A pencil mark locating the position of the head of each of the pulling
screws enables one to back them out and replace them without shifting
the cell.
The first inspection will generally tell whether the objective is
worth further trouble or not. If all surfaces save the front are in
good condition it may pay to send the objective to the maker for
repolishing. If more than one surface is in bad shape reworking hardly
pays unless the lens can be had for a nominal figure. In buying a used
instrument from its original source these precautions are needless
as the maker can be trusted to stand back of his own and to put it in
first class condition.
However, granted that the objective stands well the inspection for
superficial defects, it should then be given a real test for figure
and color correction, bearing in mind that objectives, even from first
class makers, may now and then show slightly faulty corrections, while
those from comparatively unknown sources may now and then turn out
well. In this matter of necessary testing old and new glasses are quite
on all fours save that one may safely trust the maker with a well
earned reputation to make right any imperfections. Cleansing other than
dusting off and cautiously wiping with damp and then dry lens paper
requires removal of the lenses from their cell which demands real care.
With a promising looking objective, old or new, the first test to be
applied is the artificial star—artificial rather than natural since
the former stays put and can be used by day or by night. For day use
the “star” is merely the bright reflection of the sun from a sharply
curved surface—the shoulder of a small round bottle, a spherical flask
silvered on the inside, a small silvered ball such as is used for
Christmas tree decoration, a bicycle ball, or a glass “alley” dear to
the heart of the small boy.
The object, whatever it is, should be set up in the sun against a dark
background distant say 40 or 50 times the focal length of the objective
to be tested. The writer rather likes a silvered ball cemented to a
big sheet of black cardboard. At night a pin hole say 1/32 inch or
less in diameter through cardboard or better, tinfoil, with a flame,
or better a gas filled incandescent lamp behind it, answers well. The
latter requires rather careful adjustment that the projected area of
the closely coiled little filament may properly fill the pinhole just
in front of it.
Now if one sets up the telescope and focusses it approximately with a
low power the star can be accurately centered in the field. Then if
the eyepiece is removed, the tube racked in a bit, and the eye brought
into the focus of the objective, one can inspect the objective for
striæ. If these are absent the field will be uniformly bright all
over. Not infrequently however one will see a field like Fig. 152 or
Fig. 153. The former is the appearance of a 4 inch objective that the
author recently got his eye upon. The latter shows typical striæ of
the ordinary sort. An objective of glass as bad as shown in Fig. 152
gives no hope of astronomical usefulness, and should be relegated to
the porch of a seashore cottage. Figure 153 may represent a condition
practically harmless though undesirable.
The next step is a really critical examination of the focal image.
Using a moderately high power ocular, magnifying say 50 to the inch of
aperture, the star should be brought to the sharpest focus possible
and the image closely examined. If the objective is good and in
adjustment this image should be a very small spot of light, perfectly
round, softening very slightly in its brilliancy toward the edge,
and surrounded by two or three thin, sharp, rings of light, exactly
circular and with well defined dark spaces separating them.
[Illustration: FIG. 152.—A Bad Case of Striæ.]
[Illustration: FIG. 153.—Ordinary Striæ.]
Often from the trembling of the air the rings will seem shaky and
broken, but still well centered on the star-disc. The general
appearance is that shown in Fig. 154.[29]
[29] This and several of the subsequent figures are taken from quite
the best account of testing objectives: “On the Adjustment and Testing
of Telescope Objectives.” T. Cooke & Sons, York, 1891, a little
brochure unhappily long since out of print. A new edition is just now,
1922, announced.
[Illustration: FIG. 154.—A First Class Star Image.]
Instead, several very different appearances may turn up. First, the
bright diffraction rings may be visible only on one side of the central
disc, which may itself be drawn out in the same direction. Second, the
best image obtainable may be fairly sharp but angular or irregular
instead of round or oval and perhaps with a hazy flare on one side.
Third, it may be impossible to get a really sharp focus anywhere, the
image being a mere blob of light with nothing definite about it.
One should be very sure that the eyepiece is clean and without fault
before proceeding further. As to the first point a bit of lens paper
made into a tiny swab on a sliver of soft wood will be of service, and
the surfaces should be inspected with a pocket lens in a good light to
make sure that the cleaning has been thorough. Turning the ocular round
will show whether any apparent defects of the image turn with it.
In the first case mentioned the next step is to rack the ocular gently
out when the star image will expand into a more or less concentric
series of bright interference rings separated by dark spaces, half a
dozen or so resulting from a rather small movement out of focus. If
these rings are out of round and eccentric like Fig. 155 one has a
clear case of failure of the objective to be square with the tube, so
that the ocular looks at the image askew.
[Illustration: FIG. 155.—Effect of Objective Askew.]
In the ordinary forms of objective this means that the side of the
objective toward the brighter and less expanded part of the ring system
is too near the ocular. This can be remedied by pushing that side
of the objective outwards a trifle. Easing off the pulling screw on
that side and slightly tightening the abutment screw makes the needed
correction, which can be lessened if over done at the first trial,
until the ring system is accurately centered. It is a rather fussy job
but not at all difficult if one remembers to proceed cautiously and to
use the screw driver gently.
[Illustration: FIG. 156.—Effect of Flaws in Objective.]
In the second case, racking out the ocular a little gives a ring
system which exaggerates just the defects of the image. The faults may
be due to mechanical strain of the objective in its cell, which is
easily cured, or to strains or flaws in the glass itself, which are
irremediable. Therefore one should, with the plane of the objective
horizontal, loosen the retaining ring that holds the lenses, without
disturbing them, and then set it back in gentle contact and try the out
of focus rings once more. If there is no marked improvement the fault
lies in the glass and no more time should be wasted on that particular
objective. Fig. 156 is a typical example of this fault.
In dealing with case three it is well to give the lens a chance by
relieving it of any such mechanical strains, for now and then they will
apparently utterly ruin the definition, but the prognosis is very bad
unless the objective has been most brutally mishandled.
In any case failure to give a sharply defined focus in a very definite
plane is a warning that the lens (or mirror) is rather bad. In testing
a reflector some pains must be taken at the start with both the main
and the secondary mirror. Using an artificial star as before, one
should focus and look sharply to the symmetry of the image, taking
care to leave the instrument in observing position and screened from
the sun for an hour or two before testing. Reflectors are much more
sensitive to temperature than refractors and take longer to settle
down to stability of figure. With a well mounted telescope of either
sort a star at fair altitude on a fine night gives even better testing
conditions than an artificial star, (Polaris is good in northern
latitudes) but one may have a long wait.
If the reflector is of good figure and well adjusted, the star image,
in focus or out, has quite the same appearance as in a refractor except
that with a bright star in focus one sees a thin sharp cross of light
centered on the image, rather faint but perfectly distinct. This is
due to the diffraction effect of the four thin strips that support the
small mirror, and fades as the star is put out of focus.
The rings then appear as usual, but also a black disc due to the
shadowing of the small mirror. Fig. 157 shows the extra-focal image of
a real or artificial star when the mirror is well centered, and the
star in the middle of the field. There only are the rings round and
concentric with the mirror spot. The rings go out of round and the spot
out of center for very small departure from the middle of the field
when the mirror is of large relative aperture—F/5 or F/6.
[Illustration: FIG. 157.—Extra-focal Image from Reflector.]
If the star image shows flare or oval out-of-focus rings when central
of the field, one or both mirrors probably need adjustment. Before
laying the trouble to imperfect figure, the mirrors should be adjusted,
the small one first as the most likely source of trouble. The side of
the mirror toward which the flare or the expanded side of the ring
system projects should be slightly pushed away from the ocular. (Note
that owing to the reflection this movement is the reverse of that
required with a refractor.)
If the lack of symmetry persists one may as well get down to first
principles and center the mirrors at once. Perhaps the easiest plan is
to prepare a disc of white cardboard exactly the size of the mirror
with concentric circles laid out upon it and an eighth inch hole in the
center. Taking out the ocular and putting a half inch stop in its place
one can stand back, lining up the stop with the draw tube, and see
whether the small mirror looks perfectly round and is concentric with
the reflected circles. If not, a touch of the adjusting screws will be
needed.
Then with a fine pointed brush dot the center of the mirror itself
through the hole, with white paint. Then, removing the card, one will
see this dot accurately centered in the small mirror if the large one
is in adjustment, and it remains as a permanent reference point. If
the dot be eccentric it can be treated as before, but by the adjusting
screws of the large mirror.
The final adjustment can then be made by getting a slightly extra-focal
star image fairly in the center of the field with a rather high power
and making the system concentric as before described. This sounds a
bit complicated but it really is not. If the large mirror is not in
place, its counter cell may well be centered and levelled by help of a
plumb line from the center of the small mirror and a steel square, as a
starting point, the small mirror having been centered as nearly as may
be by measurement.[30]
[30] Sometimes with ever so careful centering the ring system in the
middle of the field is still eccentric with respect to the small
mirror, showing that the axis of the parabola is not perpendicular to
the general face of the mirror. This can usually be remedied by the
adjusting screws of the main mirror as described, but now and then
it is necessary actually to move over the small mirror into the real
optical axis. Draper (loc. cit.) gives some experiences of this sort.
So much for the general adjustment of the objective or mirror. Its
actual quality is shown only on careful examination.
As a starting point one may take the extra-focal system of rings given
by an objective or mirror after proper centering. If the spherical
aberration has thoroughly removed the appearance of the rings when
expanded so that six or eight are visible should be like Fig. 158. The
center should be a sharply defined bright point and surrounding it, and
exactly concentric, should be the interference rings, truly circular
and gradually increasing in intensity outwards, the last being very
noticeably the strongest.
One can best make the test when looking through a yellow glass screen
which removes the somewhat confusing flare due to imperfect achromatism
and makes the appearances inside and outside focus closely similar.
Just inside or outside of focus the appearance should be that of Fig.
159 for a perfectly corrected objective or mirror.
[Illustration: FIG. 158.—Correct Extra-focal Image.]
Sometimes an objective will be found in which one edge of the focussed
star image is notably red and the opposite one tinted with greenish or
bluish, showing unsymmetrical coloring, still more obvious when the
image is put a little out of focus. This means that the optical centers
of crown and flint are out of line from careless edging of the lenses
or very bad fitting. The case is bad enough to justify trying the only
remedy available outside the optician’s workshop—rotating one lens upon
the other and thus trying the pair in different relative azimuths.
The initial positions of the pair must be marked plainly, care must be
taken not to displace the spacers 120° apart often found at the edges
of the lenses, and the various positions must be tried in an orderly
manner. One not infrequently finds a position in which the fault is
negligible or disappears altogether, which point should be at once
marked for reference.
[Illustration: FIG. 159.—Correct Image Just Out of Focus.]
In case there is uncorrected spherical aberration there is departure
from regular gradation of brightness in the rings. If there is a “short
edge,” _i.e._, + spherical aberration, so that rays from the outer
zone come to a focus too short, the edge ring will look too strong
within focus, and the inner rings relatively weak; with this appearance
reversed outside focus. A “long edge” _i.e._, - spherical aberration,
shows the opposite condition, edge rings too strong outside focus and
too weak within. Both are rather common faults. The “long edge” effect
is shown in Figs. 160 and 161, as taken quite close to focus.
It takes a rather sharp eye and considerable experience to detect small
amounts of spherical aberration; perhaps the best way of judging is
in quickly passing from just inside to just outside focus and back
again, using a yellow screen and watching very closely for variations
in brightness. Truth to tell a small amount of residual aberration,
like that of Fig. 160, is not a serious matter as regards actual
performance—it hurts the telescopist’s feelings much more than the
quality of his images.
[Illustration: FIG. 160.—Spherical Aberration Just Inside Focus.]
[Illustration: FIG. 161.—Spherical Aberration Just Outside Focus.]
A much graver fault is zonal aberration, where some intermediate
zone of objective or mirror comes to a focus too long or too short,
generally damaging the definition rather seriously, depending on the
amount of variation in focus of the faulty zone. A typical case is
shown in Fig. 162 taken within focus. Here two zones are abnormally
strong showing, just as in the case of simple spherical aberration,
too short focus. Outside of focus the intensities would change places,
the outer and midway zones and center being heavy, and the strong
zones of Fig. 162 weak. These zonal aberrations are easily detected
and are rather common both in objectives and mirrors, though rarely as
conspicuous as in Fig. 162.
Another failing is the appearance of astigmatism, which, broadly, is
due to a refracting or reflecting surface which is not a surface of
revolution and therefore behaves differently for rays incident in
different planes around its optical axis. In its commonest form the
surface reflects or refracts more strongly along one plane than along
another at right angles to it. Hence the two have different foci and
there is no point focus at all, but two line foci at right angles.
Figs. 163 and 164 illustrate this fault, the former being taken inside
and the latter outside focus, under fairly high power. If a star image
is oval and the major axis of this oval has turned through 90° when one
passes to the other side of focus, astigmatism is somewhere present.
As more than half of humanity is astigmatic, through fault of the eye,
one should twist the axis of the eyes some 90° around the axis of the
telescope and look again. If the axis of the oval has turned with the
eyes a visit to the oculist is in order. If not, it is worth while
rotating the ocular. If the oval does not turn with it that particular
telescope requires reworking before it can be of much use.
This astigmatism due to fault of figure must not be confused with the
astigmatic difference of the image surfaces referred to in Chapter IV
which is zero on the axis and not of material importance in ordinary
telescopes. Astigmatism of figure on the contrary is bad everywhere and
always. It should be especially looked out for in reflecting surfaces,
curved or plane, since it is a common result of flexure.
Passing on now from these simple tests for figure, chromatic aberration
has to be examined. Nothing is better than an artificial star formed
by the sun in daylight, for the preliminary investigation. At night
Polaris is advantageous for this as for other tests.
[Illustration: FIG. 162.—A Case of Zonal Aberration.]
[Illustration: FIG. 163.—Astigmatism Inside Focus.]
[Illustration: FIG. 164.—Astigmatism Outside Focus.]
The achromatization curves, Fig. 163, really tell the whole story
of what is to be seen. When the telescope is carefully focussed for
the bright part of the spectrum, getting the sharpest star image
attainable, the central disc, small and clean, should be yellowish
white, seen under a power of 60 or 70 per inch of aperture.
But the red and blue rays have a longer focus and hence rim the image
with a narrow purplish circle varying slightly in hue according to the
character of the achromatization. Pushing the ocular a little inside,
focus, the red somewhat overbalances the blue and the purple shades
toward the red. Pulling out the ocular very slightly one brings the
deep red into focus as a minute central red point, just as the image
begins to expand a little. Further outside focus a bluish or purplish
flare fills the center of the field, while around it lies a greenish
circle due to the rays from the middle of the secondary spectrum
expanding from their shorter focus.
In an under-corrected objective this red point is brighter and the
fringe about the image, focussed or within focus, is conspicuously
reddish. Heavy overcorrection gives a strong bluish fringe and the red
point is dull or absent. With a low power ocular, unless it be given a
color correction of its own, any properly corrected objective will seem
under-corrected as already explained.
The color correction can also be well examined by using an ocular
spectroscope like Fig. 140, with the cylindrical lens removed.
Examining the focussed star image thus, the spectrum is a narrow line
for the middle color of the secondary spectrum, widening equally
at F and B, and expanding into a sort of brush at the violet end.
Conversely, when moved outside focus until the width is reduced to a
narrow line at F and B, the widening toward the yellow and green shows
very clearly the nature and extent of the secondary spectrum. In this
way too, the actual foci for the several colors can easily be measured.
The exact nature of the color correction is somewhat a matter of taste
and of the uses for which the telescope is designed, but most observers
agree in the desirability of the B-F correction commonly used as best
balancing the errors of eye and ocular. With reflectors, achromatic or
even over-corrected oculars are desirable. The phenomena in testing a
telescope for color vary with the class of star observed—the solar type
is a good average. Trying a telescope on α Lyræ emphasizes unduly
the blue phases, while α Orionis would overdo the red.
The simple tests on star discs in and out of focus here described are
ample for all ordinary purposes, and a glass which passes them well is
beyond question an admirably figured one. The tests are not however
quantitative, and it takes an experienced eye to pick out quickly minor
errors, which are somewhat irregular. One sometimes finds the ring
system excellent but a sort of haze in the field, making the contrasts
poor—bad polish or dirt, but figure good.
A test found very useful by constructors or those with laboratory
facilities is the knife edge test, worked out chiefly by Foucault and
widely used in examining specula. It consists in principle of setting
up the mirror so as to bring the rays to the sharpest possible focus.
For instance in a spherical mirror a lamp shining through a pin hole is
placed in the centre of curvature, and the reflected image is brought
just alongside it where it can be inspected by eye or eyepiece. In
Fig. 165 all the rays which emanate from the pinhole _b_ and fall on
the mirror a are brought quite exactly to focus at _c_. The eye placed
close to _c_ will see, if the mirror surface is perfect, a uniform disc
of light from the mirror.
[Illustration: FIG. 165.—The Principle of the Foucault Test.]
If now a knife edge like _d_, say a safety razor blade, be very
gradually pushed through the focus the light will be cut off in a
perfectly uniform manner—no zone or local spot going first. If some
error in the surface at any point causes the reflected ray to miss the
focus and cross ahead of or behind it as in the ray _bef_, then the
knife edge will catch it first or last as the case may be, and the spot
_e_ will be first darkened or remain bright after the light elsewhere
is extinguished.
[Illustration: FIG. 166.—Foucault Test of Parabolic Mirror.]
One may thus explore the surface piecemeal and detect not only zones
but slight variations in the same zone with great precision. In case of
a parabolic mirror as in Fig. 166 the test is made at the focus by aid
of the auxiliary plane mirror, and a diagonal as shown, the pinhole and
knife edge being arranged quite as before. A very good description of
the practical use of the knife edge test may be found in the papers of
Dr. Draper and Mr. Ritchey already cited.
It is also applied to refractors, in which case monochromatic light had
better be used, and enables the experimenter to detect even the almost
infinitesimal markings sometimes left by the polishing tool, to say
nothing of slight variations in local figure which are continually lost
in the general illumination about the field when one uses the star test
in the ordinary manner.
The set-up for the knife edge experiments should be very steady and
smooth working to secure precise results, and it therefore is not
generally used save in the technique of figuring mirrors, where it is
invaluable. With micrometer motions on the knife edge, crosswise and
longitudinally, one can make a very exact diagnosis of errors of figure
or flexure.
A still more delicate method of examining the perfection of figuring
is found in the Hartmann test. (Zeit. fur Instk., 1904, 1909). This is
essentially a photographic test, comparing the effect of the individual
zones of the objective inside and outside of focus. Not only are the
effects of the zones compared but also the effects of different parts
of the same zone, so that any lack of symmetry in performance can be at
once found and measured.
The Hartmann test is shown diagrammatically in Fig. 167. The objective
is set up for observing a natural or artificial star. Just in front
of it is placed an opaque screen perforated with holes, as shown in
section by Fig. 167, where A is the perforated screen. The diameters
of the holes are about 1/20 the diameter of the objective as the test
is generally applied, and there are usually four holes 90° apart for
each zone. And such holes are not all in one line, but are distributed
symmetrically about the screen, care being taken that each zone shall
be represented by holes separated radially and also tangentially,
corresponding to the pairs of elements in the two astigmatic image
surfaces, an arrangement which enables the astigmatism as well as
figure to be investigated.
[Illustration: FIG. 167.—The Principle of the Hartmann Test.]
The arrangement of holes actually found useful is shown in Hartmann’s
original papers, and also in a very important paper by Plaskett (Ap.
J. _25_ 195) which contains the best account in English of Hartmann’s
methods and their application. Now each hole in the screen transmits
a pencil of light through the objective at the corresponding point,
and each pencil comes to a focus and then diverges, the foci being
distributed somewhere in the vicinity of what one may regard as the
principal focus, _B_. For instance in Fig. 167 are shown five pairs
of apertures _a_, _a′_, _b_, _b′_, etc., in five different zones.
Now if a photographic plate be exposed a few inches inside focus as
at C each pencil from an aperture in the screen will be represented
by a dot on the photograph, at such distance from the axis and from
the corresponding dot on the other side of the axis as the respective
inclinations of the pencils of light may determine.
Similarly a plate exposed at approximately equal distance on the other
side of the general focus, as at _D_, will show a pattern of dots due
to the distribution of the several rays at a point beyond focus. Now
if all the pencils from the several apertures met at a common focus in
_B_, the two patterns on the plates _C_ and _D_ would be exactly alike
and for equal distance away from focus of exactly the same size. In
general the patterns will not exactly correspond, and the differences
measured with the micrometer show just how much any ray in question has
departed from meeting at an exact common focus with its fellows.
For instance in the cut it will be observed that the rays _e_ and _a′_
focus barely beyond _C_ and by the time they reach _D_ are well spread
apart. The relative distance of the dots upon these corresponding
plates, with the distance between the plates, shows exactly at what
point between _C_ and _D_ these particular rays actually did cross and
come to a focus.
Determining this is merely a matter of measuring up similar triangles,
for the path of the rays is straight. Similarly inspection will show
that the rays _d_ and _d′_ meet a little short of _B_, and measurement
of their respective records on the plates _C_ and _D_ would show the
existence of a zone intermediate in focus between the focus of _e,e′_
and the general focus at _B_. The exact departure of this zone from
correct focus can therefore be at once measured.
A little further examination discloses the fact that the outer zone
represented by the rays _a,b_, and _a′,b′_ has not quite the same focus
at the two extremities of the same diameter of the objective. In other
words the lens is a little bit flatter at one end of this diameter
than it is at the other, so that the rays here have considerably
longer focus than they should, a fault by no means unknown although
fortunately not very common.
It will be seen that the variations between the two screen patterns on
_C_ and _D_, together with the difference between them, give accurately
the performance of each point of the objective represented by an
aperture in the screen. And similar investigations by substantially
the same method may be extended to the astigmatic variations, to the
general color correction, and to the difference in the aberrations for
the several colors. The original papers cited should be consulted for
the details of applying this very precise and interesting test.
It gives an invaluable record of the detailed corrections of an
objective, and while it is one with which the ordinary observer has
little concern there are times when nothing else can give with equal
precision the necessary record of performance. There are divers other
tests used for one purpose or another in examining objectives and
mirrors, but those here described are ample for nearly all practical
purposes, and indeed the first two commonly disclose all that it is
necessary to know.
Now and then one has to deal with an objective which is unmitigatedly
dirty. It can be given a casual preliminary cleaning in the way already
mentioned, but sometimes even this will not leave it in condition for
testing. Then one must get down to the bottom of things and make a
thorough job of it.
The chief point to remember in undertaking this is that the thing which
one is cleaning is glass, and very easy to scratch if one rubs dust
into it, but quite easy to clean if one is careful. The second thing to
be remembered is that once cleaned it must be replaced as it was before
and not in some other manner.
The possessor of a dirty objective is generally advised to take it
to the maker or some reliable optician. If the maker is handy, or an
optician of large experience in dealing with telescope objectives is
available, the advice is good, but there is no difficulty whatever in
cleaning an objective with the exercise of that ordinary care which the
user of a telescope may be reasonably expected to possess.
It is a fussy job, but not difficult, and the best advice as to how to
clean a telescope objective is to “tub” it, literally, if beyond the
stage where the superficial wiping described is sufficient.
To go about the task one first sets down the objective in its cell on
a horizontal surface and removes the screws that hold in the retaining
ring, or unscrews the ring itself as the case may be. This leaves the
cell and objective with the latter uppermost and free to be taken
out. Prepare on a table a pad of anything soft, a little smaller than
the objective, topping the pad with soft and clean old cloth; then,
raising up the cell at an edge, slip the two thumbs under it and lay
the fingers lightly on the outer lens of the objective, then invert the
whole affair upon the pad and lift off the cell, leaving the objective
on its soft bed.
Before anything else is done the edge of the objective should be marked
with a hard lead pencil on the edge of both the component lenses,
making two well defined v’s with their points touching. Also, if, as
usual, there are three small separators between the edges of the flint
and crown lenses, mark the position of each of these 1, 2, 3, with the
same pencil.
Forming another convenient pad of something soft, lift off the upper
lens, take out the three separators and lay them in order on a sheet of
paper without turning them upside down. Mark alongside each, the serial
number denoting its position. Then when these spacers, if in good
condition, are put back, they will go back in the same place rightside
up, and the objective itself will go back into place unchanged.
Now have at hand a wooden or fibre tub or basin which has been
thoroughly washed out with soap and water and wiped dry. Half fill it
with water slightly lukewarm and with a good mild toilet soap, shaving
soap for example, with clean hands and very soft clean cloth, go at
one of the lenses and give it a thorough washing. After this it should
be rinsed very thoroughly and wiped dry. As to material for wiping,
the main thing is that it must be soft and free from dust that will
scratch. Old handkerchiefs serve a good turn.
Dr. Brashear years ago in describing this process recommended cheese
cloth. The present day material that goes under this name is far from
being as soft at the start as it ought to be, and only the best quality
of it should be used, and then only after very thorough soaking,
rinsing and drying. The very soft towels used for cleaning cut glass,
if washed thoroughly clean and kept free from dust, answer perfectly
well. The cheese cloth has the advantage of being comparatively cheap
so that it can be thrown away after use. Whatever the cloth, it should
be kept, after thorough washing and drying, in a closed jar.
Rinsing the lens thoroughly and wiping it clean and dry is the main
second stage of cleansing. It sometimes will be found to be badly
spotted in a way which this washing will not remove. Sometimes the
spotting will yield to alcohol carefully rubbed on with soft absorbent
cotton or a bunch of lens paper.
If alcohol fails the condition of the surface is such as to justify
trying more strenuous means. Nitric acid of moderate strength rubbed on
with a swab of absorbent cotton will sometimes clear up the spotting.
If this treatment be used it should be followed up with a 10 per cent
solution of pure caustic potash or moderately strong c.p. ammonia and
then by very thorough rinsing. Glass will stand without risk cautious
application of both acid and alkali, but the former better than the
latter.
Then a final rinsing and drying is in order. Many operators use a final
washing with alcohol of at least 90 per cent strength which is allowed
to evaporate with little or no wiping. Alcohol denatured with methyl
alcohol serves well if strong enough but beware denatured alcohol of
unknown composition. Others have used petroleum naphtha and things of
that sort. At the present time these commercial petroleum products are
extremely uncertain in quality, like gasoline, being obtained, Heaven
knows how, from the breaking down of heavier petroleum products.
If pure petroleum ether can be obtained it answers quite as well as
alcohol, but unless the volatile fluid is pure it may leave streaks.
Ordinarily neither has to be used, as after the proper wiping the glass
comes perfectly clean. This done the glass can be replaced on the pad
whence it came and its mate put through the same process.
Flint glass is more liable to spot than the crown, but the crown is
by no means immune against the deterioration of the surface, perhaps
incipient devitrification, and during the war many objectives “went
blind” from unexplained action of this character. As a rule the soap
and water treatment applied with care leaves even a pretty hard looking
specimen of objective in fairly good condition except for the scratches
which previous users have put upon it.
Then if the spacing pieces, usually of tinfoil, are not torn or
corroded they can be put back into place, the one lens superimposed
upon the other, and the pair put back into the cell by dropping it
gently over them and re-inverting the whole, taking care this time to
have soft cloth or lens paper under the fingers. Then the retaining
ring can be put into place again and the objective is ready for testing
or use as the case may be.
If the spacers are corroded or damaged it may be necessary to replace
them with very thin tinfoil cut the same size and shape, leaving
however a little extra length to turn down over the edge of the lower
lens. They are fastened in place on the extreme edge only by the merest
touch of mucilage, shellac or Canada balsam, whichever comes to hand.
The one important thing is that the spacers should be entirely free of
the sticky material where they lap over the edge of the lens to perform
the separation. This lap is generally not over 1/16 of an inch, not
enough to show at the outside of the objective when it is in its cell.
When the upper lens is lightly pressed down into place, after the gum
or shellac is dry, all the projecting portion can be trimmed away with
a sharp pen-knife leaving simply the spacers in the appointed places
from which the original ones were removed.
Some little space has been given to this matter of cleaning objectives,
as in many situations objectives accumulate dirt rather rapidly and it
is highly desirable for the user to learn how to perform the simple but
careful task of cleansing them.
In ordinary use, when dirt beyond the reach of mere dusting with a
camel’s hair brush has stuck itself to the exterior of an objective, a
succession of tufts of absorbent cotton or wads of lens paper at first
dampened with pure water or alcohol and then followed lightly, after
the visible dirt has been gently mopped up, by careful wiping with the
same materials, will keep the exterior surface in good condition, the
process being just that suggested in the beginning of this chapter as
the ordinary cleaning up preparatory to a thorough examination.
The main thing to be avoided in the care of a telescope, aside from
rough usage generally, is getting the objective wet and then letting
it take its chances of drying. In many climates dew is a very serious
enemy and the customary dew cap three or four diameters long, bright
on the outside and blackened within, is of very great service in
lessening the deposit of dew upon the glass. Also the dew cap keeps out
much stray light that might otherwise do mischief by brightening the
general field. In fact its function as a light-trap is very important
especially if it is materially larger in diameter than the objective
and provided with stops.
The finder should be similarly protected, otherwise it will
mysteriously go blind in the middle of an evening’s work due to a heavy
deposit of moisture on the objective. The effect is an onset of dimness
and bad definition which is altogether obnoxious.
As regards the metal parts of a telescope they should be treated like
the metal parts of any other machine, while the moving parts require
from time to time a little touch of sperm or similar oil like every
other surface where friction may occur.
The old fashioned highly polished and lacquered brass tube was
practically impossible to keep looking respectably well provided it
was really used to any considerable extent. About the most that could
be done to it was dusting when dusty, and cautiously and promptly
wiping off any condensed moisture. The more modern lacquered tubes
require very little care and if they get in really bad condition can be
relacquered without much expense or difficulty.
Wooden tubes, occasionally found in old instruments, demand the
treatment which is accorded to other highly finished wooden things,
occasional rubbing with oil or furniture polish according to the
character of the original surface. Painted tubes may occasionally
require a fresh coat, which it does not require great skill to
administer. If the surface of wooden tripods comes to be in bad shape
it needs the oil or polish which would be accorded to other well
finished wooden articles.
Mountings are usually painted or lacquered and either surface can be
renewed eventually at no great trouble. Bright parts may be lightly
touched with oil as an ordinary rust preventive.
Reflecting telescopes are considerably more troublesome to keep in
order than refractors owing to the tender nature of the silvered
surface. It may remain in good condition with fairly steady use for
several years or it may go bad in a few months or a few weeks. The
latter is not an unusual figure in telescopes used about a city where
smoke is plentiful. The main thing is to prevent the deposit of dew on
the mirror, or getting it wet in any other way, for in drying off the
drops almost invariably leave spots.
Many schemes have been proposed for the prevention of injury to the
mirror surface. A close fitting metal cover, employed whenever the
mirror is not in use, has given good results in many places. Where
conditions are extreme this is sometimes lined with a layer of dry
absorbent cotton coming fairly down upon the mirror surface, and if
this muffler is dry, clean, and a little warmer than the mirror when
put on, it seems to be fairly effective. Preferably the mirror should
be kept, when not in use, at a little higher temperature than the
surrounding air so that dew will not tend to deposit upon it.
As to actual protective measures the only thing that seems to be
really efficient is a very thin coating of lacquer, first tried by
Perot at the Paris Observatory. The author some ten years since took
up the problem in protecting some laboratory mirrors against fumes
and moisture and found that the highest grade of white lacquer, such
as is used for the coating of fine silverware in the trade, answered
admirably if diluted with six or eight volumes of the thinner sold
with such commercial lacquers. It is best to thin the lacquer to the
requisite amount and then filter.
If now a liberal amount of the mixture is poured upon the mirror
surface after careful dusting, swished quickly around, and the mirror
is then immediately turned up on edge to drain and dry, a very thin
layer of lacquer will be left upon it, only a fraction of a wave length
thick, so that it shows broad areas of interference colors.
Treated in this way and kept dry the coating will protect the
brilliancy of the silver for a good many months even under rather
unfavorable circumstances. After trying out the scheme rather
thoroughly the treatment was applied to the 24 inch reflector of the
Harvard Observatory and has been in use ever since. The author applied
the first coating in the spring of 1913, and since that time it has
only been necessary to resilver perhaps once in six months as against
about as many weeks previously.
The lacquer used in this case was the so-called “Lastina” lacquer made
by the Egyptian Lacquer Company of New York, but there are doubtless
others of similar grade in the market. It is a collodion lacquer and
in recent years it has proved desirable to use as a thinner straight
commercial amylacetate rather than the thinner usually provided with
the lacquer, perhaps owing to the fact that difficulty of obtaining
materials during the war may have caused, as in so many other cases,
substitutions which, while perfectly good for the original purpose did
not answer so well under the extreme conditions required in preserving
telescope mirrors.
The lacquer coating when thinned to the extent here recommended does
not apparently in any way deteriorate the definition as some years of
regular work at Harvard have shown. Some experimenters have, however,
found difficulty, quite certainly owing to using too thick a lacquer.
The endurance of a lacquer coating where the mirror is kept free from
moisture, and its power to hold the original brilliancy of the surface
is very extraordinary.
The writer took out and tested one laboratory mirror coated seven years
before, and kept in a dry place, and found the reflecting power still
a little above .70, despite the fact that the coating was so dry as
to be almost powdery when touched with a tuft of cotton. At the start
the mirror had seen some little use unprotected and its reflection
coefficient was probably around .80. If the silver coating is thick
as it can be conveniently made, on a well coated mirror, the coat of
lacquer, when tarnish has begun, can be washed off with amylacetate and
tufts of cotton until the surface is practically clear of it, and the
silver itself repolished by the ordinary method and relacquered.
There are many silvering processes in use and which one should be
chosen for re-silvering a mirror, big or little, is quite largely a
matter of individual taste, and more particularly experience. The two
most used in this country are those of Dr. Brashear and Mr. Lundin,
head of the Alvan Clark Corporation, and both have been thoroughly
tried out by these experienced makers of big mirrors.
The two processes differ in several important particulars but both
seem to work very successfully. The fundamental thing in using either
of them is that the glass surface to be silvered should be chemically
clean. The old silver, if a mirror is being resilvered, is removed with
strong nitric acid which is very thoroughly rinsed off after every
trace of silver has been removed. Sometimes a second treatment with
nitric acid may advantageously follow the first with more rinsing.
The acid should be followed by a 10 per cent solution of c.p. caustic
potash (some operators use c.p. ammonia as easier to clear away) rinsed
off with the utmost thoroughness.
On general principles the last rinsing should be with distilled water
and the glass surface should not be allowed to dry between this rinsing
and starting the silvering process, but the whole mirror should be kept
under water until the time for silvering. In Dr. Brashear’s process the
following two solutions are made up; first the reducing solution as
follows:
Rock candy, 20 parts by weight.
Strong nitric acid (spec. gr. 1.22), 1 part.
Alcohol, 20 parts.
Distilled water, 200 parts.
This improves by keeping and if this preparation has to be hurried the
acid, sugar and distilled water should be boiled together and then the
alcohol added after the solution is cooled.
Second, make up the silvering solution in three distinct portions;
first the silver solution proper as follows:
1. 2 parts silver nitrate. 20 parts distilled water.
Second, the alkali solution as follows:
2. 1⅓ parts c.p. caustic potash. 20 parts distilled water.
Third, the reserve silver solution as follows:
3. ¼ part silver nitrate. 16 parts distilled water.
The working solution of silver is then prepared thus: Gradually add
to the silver solution No. 1 the strongest ammonia, slowly and with
constant stirring. At first the solution will turn dark brown and then
it will gradually clear up. Ammonia should be added only just to the
point necessary to clear the solution.
Then add No. 2, the alkali solution. Again the mixture will turn dark
brown and must be cautiously cleared once more with ammonia until it is
straw colored but clear of precipitate. Finally add No. 3, the reserve
solution, very cautiously with stirring until the solution grows
darker and begins to show traces of suspended matter which will not
stir out. Then filter the whole through absorbent cotton to free it of
precipitate and it is ready for use. One is then ready for the actual
silvering.
Now there are two ways of working the process, with the mirror face
up, or face down. The former is advantageous in allowing better
inspection of the surface as it forms, and also it permits the mirror
of a telescope to be silvered without removing it from the cell, as was
in fact done habitually in case of the big reflector of the Alleghany
Observatory where the conditions were such as to demand re-silvering
once a month. The solution was kept in motion during the process by
rocking the telescope as a whole.
When silvering face up the mirror is made to form the bottom of the
silvering vessel, being fitted with a wrapping of strong paraffined
or waxed paper or cloth, wound several times around the rim of the
mirror and carried up perhaps half the thickness of the mirror to
form a retainer for the silvering solution. This band is firmly tied
around the edge of the mirror making a water tight joint. Ritchey uses
a copper band fitted to the edge of the mirror and drawn tight by
screws, and finishes making tight with paraffin and a warm iron.
In silvering face down the mirror is suspended a little distance above
the bottom of a shallow dish, preferably of earthen ware, containing
the solution. Various means are used for supporting it. Thus cleats
across the back cemented on with hard optician’s pitch answer well for
small mirrors, and sometimes special provision is made for holding the
mirror by the extreme edge in clamps.
Silvering face down is in some respects less convenient but does free
the operator from the very serious trouble of the heavy sediment which
is deposited from the rather strong silver solution. This is the
essential difficulty of the Brashear process in silvering face up. The
trouble may be remedied by very gentle swabbing of the surface under
the liquid with absorbent cotton, from the time when the silver coating
begins fairly to form until it is completed.
The Brashear process is most successfully worked at a temperature
between 65° and 70° F. and some experience is required to determine the
exact proportion of the reducing solution to be added to the silvering
solution. Ritchey advises such quantity of the reducing solution as
contains of sugar one-half the total weight of the silver nitrate used.
The total amount of solution after mixing should cover the mirror about
an inch deep. Too much increases the trouble from sediment and fails to
give a clean coating. The requisite quantity of reducing solution is
poured into the silvering solution and then immediately, if the mirror
is face up, fairly upon it, without draining it of the water under
which it has been standing.
If silvering face down the face will have been immersed in a thin
layer of distilled water and the mixed solutions are poured into the
dish. In either case the solution is rocked and kept moving pretty
thoroughly until the process is completed which will take about five
minutes. If silvering is continued too long there is likelihood of an
inferior whitish outer surface which will not polish well, but short of
this point the thicker the coat the better, since a thick coat stands
reburnishing where a thin one does not and moreover the thin one may be
thin enough to transmit some valuable light.
When the silvering is done the solution should be rapidly poured off,
the edging removed or the mirror lifted out of the solution, rinsed
off first with tap water and then with distilled, and swabbed gently
to clear the remaining sediment. Then the mirror can be set up on edge
to dry. A final flowing with alcohol and the use of a fan hastens the
process.
In Lundin’s method the initial cleaning process is the same but after
the nitric acid has been thoroughly rinsed off the surface is gently
but thoroughly rubbed with a saturated solution of tin chloride,
applied with a wad of absorbent cotton. After the careful rubbing the
tin chloride solution must be washed off with the utmost thoroughness,
preferably with moderately warm water. It is just as important to get
off the tin chloride completely, as it is to clean completely the
mirror surface by its use. Otherwise streaks may be left where the
silvering will not take well.
When the job has been properly done one can wet the whole surface with
a film of water and it will stay wet even when the surface is slightly
tilted. As in the Brashear process the mirror must be kept covered with
water. Mr. Lundin always silvers large mirrors face up, and forms the
dish by wrapping around the edge of the mirror a strip of bandage cloth
soaked in melted beeswax and smoothed off by pulling it while still hot
between metal rods to secure even distribution of the wax so as to make
a water tight joint. This rim of cloth is tied firmly around the edge
of the mirror and the strings then wet to draw them still tighter.
Meanwhile the water should cover the mirror by ¾ of an inch or more.
It is to be noted that in the Lundin process ordinary water is usually
found just as efficient as distilled water, but it is hardly safe to
assume that such is the case, without trying it out on a sample of
glass.
There are then prepared two solutions, a silver solution,
2.16 parts silver nitrate (see King, Pop. Ast =30=, 93)
100 parts water.
and a reducing solution,
4 parts Merck’s formaldehyde
20 parts water.
This latter quantity is used for each 100 parts of the above silver
solution, and the whole quantity made up is determined by the amount of
liquid necessary to cover the mirror as just described.
The silver solution is cautiously and completely cleared up by strong
ammonia as in the Brashear process. The silver and reducing solutions
are then mixed, the water covering the mirror poured quickly off, and
the silvering solution immediately poured on. The mirror should then be
gently rocked and the silver coating carefully watched as it forms.
As the operation is completed somewhat coarse black grains of sediment
will form and when these begin to be in evidence the solution should
be poured off, the mirror rinsed in running water, the edging removed
while the mirror is still rinsing and finally the sediment very gently
swabbed off with wet absorbent cotton. Then the mirror can be set up to
dry.
The Lundin process uses a considerably weaker silver solution than the
Brashear process, is a good deal more cleanly while in action, and
is by experienced workers said to perform best at a materially lower
temperature than the Brashear process, with the mirror, however, always
slightly warmer than the solution. Some workers have had good results
by omitting the tin chloride solution and cleaning up the surface
by the more ordinary methods. In the Lundin process the solution is
sufficiently clear for the density acquired by the silver coating to be
roughly judged by holding an incandescent lamp under the mirror. A good
coating should show at most only the faintest possible outline of the
filament, even of a gas filled lamp.
Whichever process of silvering is employed, and both work well, the
final burnishing of the mirror after it is thoroughly dry is performed
in the same way, starting by tying up a very soft ball of absorbent
cotton in the softest of chamois skin.
This burnisher is used at first without any addition, simply to smooth
and condense the film by going over it with quick, short, and gentle
circular strokes until the entire surface has been thoroughly cleaned
and begins to show a tendency to take polish. Then a very little of the
finest optical rouge should be put on to the same, or better another,
rubber, and the mirror gone steadily over in a similar way until it
comes to a brilliant polish.
A good deal of care should be taken in performing this operation to
avoid the settling of dust upon the surface since scratches will
inevitably result. Great pains should also be taken not to take any
chance of breathing on the mirror or in any other way getting the
surface in the slightest degree damp. Otherwise it will not come to a
decent polish.
Numerous other directions for silvering will be found in the
literature, and all of them have been successfully worked at one time
or another. The fundamental basis of the whole process is less in the
particular formula used than in the most scrupulous care in cleaning
the mirror and keeping it clean until the silvering is completed. Also
a good bit of experience is required to enable one to perform the
operation so as to obtain a uniform and dense deposit.