STANDARD - Survey Instrument Antique Center!

12
Care of Telescope Lenses.
As dust and moisture, as well as perspiration from the hands, will settle on the
surface of the lenses of a telescope, it becomes necessary that they should be
cleaned at times. A neglect to keep the lenses free from any film, scratches, etc.,
greatly impairs the claar sight through the telescope. To remove the dimness.,
produced by such a film, proceed thus : Brush each lens carefully with a camel's
hair brush, wipe gently with a clean piece of chamois leather moistened with alcohol,
and wipe dry using a clean part of the chamois skin on every portion of the
lens, to avoid grinding and scratching. When perfectly transparent brush again
to remove any fiber that may adhere to the lens. The tubes in which the lenses fit
should be brushed, and if damp should be dried ; this done, restore each lens to
its original place as marked. To remove dampness in the main tube of the telescope,
take out the eye-piece, cover the open end with cloth and leave the instrument
in a dry room for some time.
If an instrument has been exposed to a damp atmosphere, or water has penetrated
the telescope, moisture may settle between the crown and flint glass of
which the object-glass is composed. If such is the case expose the instrument to
the sun for a few hours, but if in the winter, leave it in a warm room some distance
from the stove, the moisture will then generally evaporate. However, if not successful,
unscrew the object-glass from the telescope, and heat it slightly over a
stove or open fire. If a film settles between these glasses nothing can be done except
sending the instrument to the maker. The two glasses form one lens only
and must not be disturbed, as upon their relation to each other the definition and
achromaticity of the telescope depends. Much depends also on the stability, with
which these lenses are mounted in their cell, as any looseness between them or the
cell will affect the adjustment of line of collimation. Of course, if at any time the
object-glass has been unscrewed from the telescope, this latter adjustment must
again be verified before the instrument is used.
Additional Instructions concerning the Care of
Telescope Lenses, etc.
Ever since the introduction of the high power in the telescopes of geodetic instruments,
now used by the best makers, complaints are frequently made of
the loss of light in such telescopes and of the hazy appearance of objects
viewed through the in, the latter in particular when an instrument has seen service
in the field for some time. Now, while the loss of light is wholly due to the greater
power as compared with the low powers formerly in vogue, and to the use of erecting
eye-pieces (see page 31), the "haziness" is produced principally by films
of dirt, settled on or between the lenses of a telescope, and becomes even more
marked as more lenses are used in a telescope.
Perhaps it is proper to say here, that when comparisons are made between low
and high-powered telescopes of geodetic instruments, other things being equal, the
first named, as a rule, will incite favor, because, as in spy-glasses, the image of an
object seen through them has a brilliancy never attained by telescopes of higher
power. But, whenever the results of stadia work, or fine levelling, as obtained with
the more powerful telescope, are compared with those obtained by a lower power,
it will be found that, though less brilliant, the defining power of a high-powered
telescope is superior to the other within the customary range of distances had in
the ordinary engineer's and surveyor's practice,
On the other hand, owing to the less amount of light with high powers, it is
necessary that the fine qualities of the superior lenses required for them should be
preserved, and on this account a more frequent inspection and a more careful
treatment of them is needed than when lower powers are used, inasmuch as the
least impairment of these lenses by films, or dust, etc., will reduce the defining
power accordingly. A little extra care, as consequent upon the use of high-powered
lenses, is, therefore, imperative, but in so doing one is more than compensated by the
satisfaction of having a finer and more penetrating telescope.
To prevent an untimely settling of a film on the lenses of a telescope, and particularly
that apt to form on the inner surfaces of the lenses composing an objectglass
that has not been cemented together such film being so fatal in an objectglass
because it cannot ordinarily be reached and without disarranging the cross-wire
adjustments the treatment of an instrument should be strictly in accordance with
the instructions given under " Prevention better than Cure," page 17. Unless these
conditions are complied with, the greater efficacy of a telescope composed of superior
lenses will be entirely lost.
Upon finding that, after carefully cleaning the object-glass and the lenses of the
eye-piece, the telescope is not as clear as when first received from the maker, then
the cause of it is generally a film between the lenses of the object-glass wo take
for granted that the lenses are not scratched or otherwise impaired but, as a rule,
it takes several years (with carelul use sometimes many years) before such a film