ASTRONOMY - MEASURING INSTRUMENTS

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP80-00809A000600230733-3
• Release Decision: RIPPUB
• Original Classification: C
• Document Page Count: 8
• Document Creation Date: December 22, 2016
• Document Release Date: July 15, 2011
• Sequence Number: 733
• Publication Date: June 23, 1949
• Content Type: REPORT

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r LASSIFIrAT.ONK 1
COUNTRY UUSH
SUBJECT Astronomy - Measuring inatrumentb
HOW
PUBLISHED Bimonthly periodical
WHERE
PUBLISHED Moscow
DATE
PUBLISHED Jan - Feb 1948
LANGUAGE
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DATE OF
INFORMATION 1948
DATE DIST. 73 Jun 19149
NO. OF PAGES
SUPPLEMENT TO
REPORT NO.
THIS IS UNEVALUATED INFORMATION
SOURCE
PRINCIPAL AAVANTAG . AND SPRUC1tURAL FICATURFI
OF A HORIZONTAL MERIDIAN INSTRUMENT
Figures referred to herein are appended]
A review of aatrometrice in the last 50 - 70 years shows that, in spite
of great progress in the theoretical field, improved methods, and great
accumulation of observational data, the systematic errors of observation re-
main practically at the ease level, namely in the range 0.2" to 0.4", The
reasons for eu^n a long ss.i.vtill in the field of improving accuracy can-
:at ??e accidental; and so long as they are not made public and exhaustively
ars.lyzcc, it is aselese to expect further progress in this d`.rection.
As is well known, the precision of ae'roac rical observations guve:oHa
by three principal fantore: the czad.tiou of the earthli atmosphere, the
Quality of the aacrometric instrument, anh the characteristics of the re-
cording apparatus (eyepieces and photo.;rLpaic plates or photnelemeute). In
this article we shall c:'itloaldy examine the weak points of present-day
aetroc,etrical instruments, not dwelling on their specific role in specialized
operr-tinne, so in the ness of vertica.L and meridian circlet, transit Innt"'. -
mente, etc.
Errors Asso%latod with Graduating a ,role
Normally, a circle has 10,800 or 21,600 divisions. In the most favor-
able instance, individual corrections are determined for only a few handre,i
of them. In the case of the very large number of remaining marks, from which
readings, are actually taken, corrections are averaged according to a curve
ueeoribing the behavior of errors-in the ilvestigated aro of the circle.
Thus, the calculation of errors in the position of the marks is basically
statistical, and even then errors can Nnly partially cancel by means of a
very large number of observations. Bat even numerous observations cannot coa-
p:etely eliminate error, since in ietormining the decllnati~n (altitude)
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all~Ai
of a star n::: aora ti an n ere -u;: among which there may not be a
siog1r ::chic r,'tls ::a inul rec:`: It may be sold vtt'iout exaggeration
that ea-h instrument must R'.:-_, L:.; own. "Syutam" of star peeitions.
The quality of the marks drawn upon silver does not permit trading lines
for microsncpio reading with a linear error less than 0.5 - 1 micron; in case
of a glass circle, it gives an error not ices than 0.2 - 0.5 micron. These
errors are approximately equal in effect since metal circles are about one
meter in diameter and glass circles are about 40 centimeters. (Here we are
not taking, into consideration the difficulty of uniting glass and metal in
high-temperature intervals.) In terms of angular measure, this linear error
gives an error of 0.2" to 0.4" for both cases. In astronomical practice the
error is closer to the latter figure, 0.4".
The problem of tube curvature is one of the most difficult in astrometric
practice. This is completely natural, inasmuch as "curvature" depends; In one
way or. another, on the following factors:
1. The form and elastic properties of the matertal of the tube and the
tube's position r. Letive to the direction of. gravitational force;
2. Temperature, influencing the dimensions and elastic properties of
the tube;
3. Deviation of the optical axis of the objective relative t.n the geo-
metrical axis of the tube, and the relative dis?l*_cement cf the object glass
due to a difference in coefficients of thermal expansion and to the different
thermal conductivity of the materials. Errors of thi6 sort are not compensated
by displacement of the center ocular wire grid, where, during temperature
change, the symmetry relative to the tube axis should not be disturbed. (Pre-
liminary calculations have indicated that errors due to relative displacement
of the object glass in the course of a day during unfavorable conditions may
amount to a tenth or more of an angular second.)
4. local heating of r.:-ts of the instrument, due to the heat radiated by
the observer. This influence would be especially strong during cold weather
and also during observation of stars at the zenith, when the observer is lo-
cated beneath the Instrument and a current of warm air flows up around it; this
neat may also affect refractic-i.
At present, the appearance of "curvature" is one of the chief obstacles
to increasing, the optical strength of meridian instruments.
:ionuimnlsaneity in Determining the Collimation Errur and the Inclination
of the Axis Daring Observations
Corrections, Introduced during observation, for collimation errors and
inclination of the axis are chiefly based upon the preliminary study of pivot
forms. The form of the pivot is necessarily adopted once and for all, and is
independent of consideretione of time, temperature, and other conditions
accompanying observation. Chance errors due to dust in the bearings, con-
gealing of lubricant, etc., are not oonsidered. It can be assumed that, for
example, the congealing of the lubricant due to .temperature decrease may
Introduce considerable error. For small, specific pressures between the
pivot and bearings customarily used in astrometric instruments, the eongealee
lubricant may not flow sufficiently from the space between the bearing sur-
faces, and the instrument will "swim." This floating Introduces a certain
indeterminateness in its position. In instruments of ordinary construction
it is difficult to detect and correct these errors.
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Such, in a ehrrt outline, are the basic shortoominge in astronomical
instruments of the present timo. These ehortoosings hinder the wide appli-
cation of obyeotive methods of rezording phenomena, inasmuch as they
effectively lis?it the weight and dimensions of any recording equipment set
on the savable part of an instrument.
Consideration of Basically Vow Nsims
Now that we have established the causes that hinder farther accuracy and
refinement in determining stellar coordinates by aeridian instrnaante, we
can contemplate various schemes for iastruasnta which are to a oonaidsrable
extent free of the above-mentioned shortcomings. This article will describe
one such system.
Basically, the system, consists of a plane airier rotating about a
horizontal axis, in conjunction with a rigid tube. From the history of
astronomical instruments it to known that such an idea was first conceived
by Peaffart in 1682; but a practical statement of the problem concerning
this combination for accurate astraastrio measurements became possible only
snob later, thanks to experience gained during work an t'raneit instruments
with the Hamberg..type articulated tube, in which a prisi, inserted between
the objective and the filar grid, acts as a reflector. Practical application
also had to await improvement in the metallurgy of stainless stools and for
other technical Improvements.
Professor 1. N. Pavlov of the Pelkovskiy Observatory becana oouvlaoed
of the expediency of operating transit instruments possessing a rotating re-
floctor and rigid tube, and proposed his own design for such an Luetramat.
(T. N. Pavlov, ?1Photoelsotrio Recording of Star Passage," B
1937, p 662. A. A. Ilinich raised the question of applying this principle
to a meridian circle.
The author of this article studied this question in 1924; only after a
lapse cl 20 years, however, did he find it possible Zo begin work, wtiob
included the following Investigations: determination of right ucensirn,
time correction, and the deviction of stars. Heeevar, the possibility rf
determining separately the coordinates and partially the zenith observations
was not excluded. Further discussion will be devoted to one of the attuepts
to wake a rational solution of thio problem.
Figure 1 shoes the basic scheme of a horizontal m ridtan lastrument
with the following designations:
0 is the plane reflector aith a central aperture, rotating about the
horizontal axis ZZ';
T(S) and T(N) are the main tubes, south and north (corresponding to
the indices within the r_nthaoss);
U(S) and 0(11) are the obleotives of the min tubes;
F(8) and F(1) are the focal planes '.f the objectives 0(8) eai 0(1:);
1((8) and 3(11) a`^e the month and north illumination sources;
01((8) and O((1) are the collimator lenses;
C and C' are the rings of the instrument;
0'(8) and O'(A) are the objectives of the eutooollismtlon tubed;
0'i(S) and 0'F:() are the eyep!.soas of the autooollisation tubas;
-3-
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PanT P' er=..
L and L' are hearings; and
H is the floating horl.zcn.
We will, begin the review of the diagram with a discussion of the optical
paths.
The light rays from the at-r under study are incident upon the mirror G
and are refracted by either lens 0(S) or 0(N) as one chooses. let us assume
that they are reflected in 0(S), as shown in Figure 1. Refracted by the lens,
these rays form a real image of the star on the focal plane F(S), where the
star is actually observed. The zenith interval is read an circlet 0 and C',
to which we shall return.
Let tie consider for a moment the probiems concerning the azimuth control
of the main tubes, the calculation of the axis movement of the main reflector
G, and the determination of the nadir point on the circles.
The main tube's change in azimuth car, be determined at the moment
directly preceding or following observation F6r this purpose, a collimation
lens Cl4(ld) is placed in front of the illumination source M(N). This lane
reflects the illumination source on the surface F(N) at the intersection caf
the wires of the ocular grid (during this time, the eyepiece is moved aside).
The objectives 0(N) and C(S) transfer this image (through an aperture in the
main mirror) to the focal surface F(S) along the optical axis or close to
it. Measurement of the amount of fluctuations in azimuth of tube T(S) can
be made with the same ocular micrometer which is employed for stellar
observation. The illumination source at the time when the star is being
observed can be extinguished from the eyepiece with the help of a key which
switches off the illumination lamp.
Control of the axle position of the main mirror during observation is
also possible if this matter is considered during the design of the mirror.
It is very important that the connection between mirrors and axis Z?.' be
reliable. For this purpose, tie mirror may be of stainless steel (or any
other alloy of high reflection capacity and the necessary mechanical
properties) cast as a unit with the axis ZZ'. In this case, the ends of
the axis can be polished and employed as mirrors which may be regarded
firmly bound up with the main mirror.
After we have set up a pentapriem in front of this butt mirror and
set up behind the pentapriem an autaeollimation tube K(S) with an eyepiece
0'K(S) drawn out toward the observer, we look '_!`.o it in the direction of
the ray reflected by the end of the axis 7.Z' and at any k:o ant it is
possible to judge the change in direction of axis Z2.' relative to a certain
reference positior. The presence of two main tubes in conjunction with a
floating horizon makes it. possible, as indicated by theoretical investi-
gations, to solve fully the problem concerning the movement of axis ZZ'
and to determine its position at any moment that the observer desires,
without consideration of pivot form.
Isadir position on circles C and C' can be determined with the help
of a mercury level, which is slightly modified by introducing into the
mercury a flat glass mirror acting as an artificial horizon. This
artificial horizon -- let us call it "floating" -- will give a better
image as compared with the usual mercury one with its constantly moving
surface. In addition, the high position of the centroid of the floating
mirror above the mercury level will contribute to the whole system com-
pebsating features in regard to obviating horizontal diecurbanoee( of a
seismic or some atLer na'ure) which are transmitted to the mirror by the
NHBE IUAt
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mercury. Actually, in th: ocence -!ich disturbing g ^,,rcea, fric'tonal
forces arise betreen the mercury ani he .,.Dating part of the mirror; and
there arises an inertial force that Y, ,t the mirror in the direction
of rotation of the mercury su race, which in its turn tends to rotate the
mirror in the opposite direction. To brim; the upper surface of the mirror
into a parallel position with the mercury surface is difficult to accomplish,
if in this manner one seeks to impart an ideal geometric form and severely
uniform thickness to the mirror. It is much easier to achieve this by
creating the necessary conditions of equilibrium, which is done be re-
moving material from the side of the more protruding part. Any remaining
declivity, if technically impossible to eliminate, can be cancelled during
observations by rotating the mirror ISO degrees.
Above we have only considered the negative aspects of present,-day
rii:lee, the practical impossibility of investigating individual inaccuracies
in graduation marks, and, finally; the insufficient accuracy in placing in
coincidence the micrometer filar wires and the marks. From the standpoint
of principles, circles preferably should have a small number of marks,
which permit making adjustments for each ?,f them, and have a mic-)meter
arrangement for measuring within an interval of one division.
About 12 years ago, while investigating precision-measuring instrp-
menta, the author discovered a way of obtaining and utilizing a physical
straight line, which to undistinguishable in any practical way from a
geometrical straight line and which permits one, un?sr specific conditions,
to transfer from a geometrical rombineti-n ; ' the micrometer filar wire to
photometric "division" fiseitairovani), obtainable on the principle of
Iie'_d-levelling and greatly increasing the accuracy of placing lines in
coincidence. This straight line serves as an edge for the metallic prism,
which is polished with optical precision. If the reading microscope
(Figure 2) to directed on the edge of such a prism (at an angle of about
90 degrees), aligning the axis of the microscope with the bieectrix of the
prism angle, and if one utilizes special vertical illumination, then the
light incident on the prism Is reflected from its face in directions
perpendicular to the original direction, but on the edge of the prism
will 'as seen a bright line which is due to the insignificantly small
curvature of the sage and to diffraction phenomena. If the filar wire
of L:le ocular micrometer is placed in coincidence with this line, then
the line will seem to be completely covered and only the narrow gaps
along the side of the filar wire will be visible (shown in Figure 2, in
the circle). Under these conditions, the e._gutest movement of the filar
wire from the axis abruptly cute out the gap on one side and increases it on
the other.
Tests conducted in )934 - 1936 showed that, the mean square error of
one-lino coincidence is equal to 0.026 micron ')y laboratory me euremenre.
The last measurements of this kind were made in 1945 arid are set forth in
Table 1 where columns 1 to 10 arm independent series cf :aaasurmmente. They
are expressed in the divisions on the cylinder of the vedge-shaped micro-
meter and are combined in three groups (sertee 1 - 5; 6 - 9; 10) depending
upon the circumstances under which they were obtained (each case is described
in .,he note fclluving ?:.e table). A comparison of these data with the data
of placing in. coincidence the mark on the glass inatcct.e too yosot`a lity
sf ieofeaeicg measurements five to ten times.
The practical application of this ider. In meridian instruments involves
the form of a toothed circle resembling a toothed ring. Figure 2 illustrates
the toothed ring in connection with the reading microscope and the special
vertical 'lluminator. The teeth should be one degree apart and the error
in position fo-- each sbruld be determined. The lengths of the degree inter-
val con be measured by a wedge-shaped micrometer. This instrument was
employed successfully by the author for elmiler cork, since the relative
accuracy of the measurements will nct exceed 10-5 if one obtains readings
up to 0.03' - 0,04".
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0 ,:I-
ONFI1!L. DIAL
L
The problem of the ",-thc--d" cis cannot at pri.eent be considered
ac definitely solved,-. ceavt.h::.? ri? -, had to limit himself to a teat
of only one element of the toothed circle. In view of the Importance of
this problem, however, it deserves a very energetic treatment in the
immediate future. The "toothed" circle may be made of stainless steel.
Table I. Errors During Repeated Applications Made by a Wedge-shaped Micro-
meter Under Various Conditions
ROTE: Readings are given in divisions of the cylinder of the micrometer
screw, while the mean square error of one application is given
both in cylinder divisions end in fractions of a micron.
1
2
4
5
6
7
8
9
10
Average reading 6.0
1.2
3.3
3.0
9.7
6.0
3.4
9.4
9.5
9.3
Mean egaure error
of one coinel-
denoe reading
In divieious4.14
0.17
0.12
0.16
0.09
0.39
0.35
0.49
0.38
0.49
In microns 0.025
0,032
0.022
0,029
0.016
0.070
0.063
0.088
0.068
0.017
ROTX: Series No 1 --5. The coincidence readings were made very carefully
but with weak illumination of the prism ridge of the wedge-shaped
micrometer. The value of one division on the micrometer cylinder is
0.18 micron. Series No 6 - 9. The coincidence readings were made
quickly and not especially carefully (attempts to obtain results
close to the probable manufacture's results). The value of one
division on the micrometer cylinder is 0.18 micron. Series No 10.
Measurements were made after the ridge of the prism was polished,
which gave a good result. Series To 10 was made especially care-
fully. The value of one division on the micrometer cylinder is
0.034 micron.
The stumbling blocks in the way of developing present-day meridian :n-
strumente, as we have seen, are the "curve%i" tubes. In our case, the prob-
lem of curvature extends only to the main fle'zi.ble mirror. Preliminary
calculations Indicate that with a ridge design on the mirror and with a 1:15
ratio of width to diameter, the mirror's curvature exerts lees obvious in-
fluence during observations than the tube's curvature. There are ways of de-
creasing it in the future.
The installation of lens and ocular network directly on the foundation
opens new posdibiiities. The lens diameter can be increased at least by
250 - 300 millimeters and later increases can parallel the growth of techni-
cal poesibi Utiee of preparing large s31-metal moveable airr re. The ga??,e
be:?veen the objective lenses can be increased tc a dimension necessary or
creating more advantageous conditions for heat exchange in the intralone
gaps with the outside .ir. An increase in diameter of the objective will
make it possible to try in'practice the single-Iona aspherioal ob~eotives
in ocmbination with an ocular light filter for excluding the influence of
atmospheric dispersion on measurements. Such objectives are free of the
errors possessed by a double-lone objective 'n connection with the relative
shift of the lenses with temperature changes, unavoidable even with a small
difference. in coefficient of thermal exp'sasion and heat-conduction components.
The fixed ocular part opens an unlimited field for aopiying objective methods
of recording phenomena In regard to size and weight of the recording apparatus.
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To sum up, we see tha`- ,he ho,i : ,?1 design of the instrument
Poe cases a number of import nt advanta!.is over instruments with flexible
tubes. The most important of them are. (1) the possibility of calcu-
lating, during observatfon,the- errors due to variati;ne in the position
of the tube's optical axis and the mirror's rotational axis, (2) reduotion
in the error of calculating angles, (3) improvement of the conditions for
applying objective methods of observation, and (4) important reduction of
the influence of survature and the resulting increase in the optio'l power
of the instrument.
An unavoidable peculiarity of the system is the presence of two tubes,
since one cannot perform observations along a meridioaal are greater than
120 130 degrees.
Space does not permit examining the many questions of interest, such
as: further reduction of mirror curvature, strengthening lenses in large
objectives, investigating horizont.1 refraction, etc.
Modern metallurgy and optical technics possess the necessary tools
for st.oceWally solving the problems of preparing .a horizontal meridian
Instrument, but they still must solve great problems of improving pre-
cision, which require skill and technology for solution.
The scheme of the horizontal meridian instrument which we have
examined is not, of course, ideal. The combination of metal and glass
is a complex matter, since both have completely different thermal con-
duotivi:ies and consequently react with different rates to changes in
atmospheric temperature. A present, there is still no completely
definite prospect for solving this problem.
LAppended ft..uree follow]
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