Extracts from the Reports of the Astronomer Royal relating to the Horizontal Transit Instrument & Mirror Transit Circle

The information below has been extracted from the Reports of the Astronomer Royal to the Board of Visitors. The Horizontal Transit Instrument was mentioned in the reports for: 1945, 1946, 1947, 1948, 1950, 1951, 1953. The Mirror Transit Circle (also referred to as the Mirror Transit Instrument and the Mirror Transit-Circle) was mentioned in the reports for: 1947, 1948, 1949, 1959, 1961

The extracts are arranged in chronological order. Between 1945 and 1954, the reporting year ended on 30 April. In the period 1955 to 1961, it ended on 31 March.





Horizontal Transit Instrument

It has been realised for some time that, with the introduction of quartz controlled clocks as time standards at the Observatory, a limiting factor to the precision of the time-service provided by the Observatory would be imposed by the errors to which the determinations of time, with the conventional type of small transit instrument, are liable. The photographic zenith tube will reduce considerably the errors of the time determinations. The stars observed with this instrument are restricted to a narrow belt around the zenith. These stars are for the most part faint and their positions will require to be determined on the system of the FK3 fundamental catalogue by special meridian observations; these positions will be gradually smoothed out and their accuracy improved by the observations with the photographic zenith tube. The selection of the star programme will depend upon the precise latitude in which the instrument is located. With the removal of the Observatory to a new site under consideration, the selection of the star programme must be deferred until the new site is selected. Some while must therefore necessarily elapse before full advantage can be obtained of the high degree of accuracy in time determinations of which the photographic zenith tube is capable.

As mentioned in the previous report, consideration was being given to the possibility of improving the accuracy of time determinations with the small transit instrument. Attention was directed at the outset to reducing the uncertainty of the most troublesome and probably the largest source of error, namely the level of the instrument. A method was devised and some preliminary experiments were carried out to measure the level by purely optical means, instead of with the striding level. One important advantage of this method is that the level is determined for each star observed, with the telescope in the position in which the observation is made; the effect of pivot errors are thereby completely allowed for. This advantage is gained, however, at the expense of an appreciable increase in labour.

Arising from a more general consideration of the problem, a new type of transit instrument has been designed, the important feature of which is that the telescope system remains fixed (though adjustable), with its axis horizontal and in an east-west direction. The light from a star of any declination, near the position of meridian transit, is directed along the optic axis by a subsidiary optical system, which can be rotated about an east-west axis and can be set to the appropriate declination. The effect on time determination of its positional errors (whether due to maladjustment of the axis of rotation or to pivotal errors) is reduced to the second order. Level and azimuth errors of the telescopic system have the same effect on the observed time of transit as they do with the ordinary transit instrument; but since the telescopic system is not deliberately subjected to gross mechanical disturbances and suffers from no pivotal errors, these level and azimuth errors should be far more stable than in the reversible instrument. Small pier movements and temperature effects are the most likely source of residual disturbance. The collimation error is dealt with by duplication of the telescopic system and reversal of the subsidiary system, so that the essential advantage of the reversible instrument is not sacrificed. Observation is made at the mid-point of the duplex telescopic system from the two sides successively. The fixity of the telescopic system removes errors due to flexure, and thus permits the advantageous use of a focal length considerably greater than can profitably be used in the ordinary reversible instrument. The level is determined with reference to two mercury surfaces, one at each end of the instrument, by means of an autocollimation method.

In addition to the subsidiary optical system referred to above a variable-deviation system is used, by means of which the light in the telescopic portion of the instrument is kept always axial as the direction of the incident starlight rotates. In this way the tolerances of certain essential adjustments are greatly increased. Further, this variable-deviation system acts also as a micrometer and as the means by which signals are sent to the chronograph. An additional advantage of this axial method is that the fiducial line that bisects the star image is not required to move in order to follow the star's image or to be linked to the signalling system as at present. Thus no mechanical errors are introduced at this point, and an optical method is contemplated for defining the position of the variable-deviation system in such a way that in its performance as a micrometer or signal emitter the system will be effectively free from the effects of mechanical errors.

After an investigation of the theoretical aspects of the system had been made, it was decided that development on these lines was more promising than the original plan of trying to allow for inherent defects of the present reversible instrument. In order to try the system out in practice the construction of an experimental instrument is at present in progress.





Horizontal Transit Instrument

Work on this instrument has been continued, but has been somewhat interrupted by the prior demands of the P.Z.T. The construction of the mountings and adjustments of the telescopic system have been completed. The mechanical design for the constraint and driving of the variable-deviation system has also been completed and the construction of the essential optical components is in an advanced stage. The application to this instrument of the balanced photoelectric system, which had been proposed for giving the timing-signals of the P.Z.T., was found to present considerable difficulty owing to the rotation of the system, required to adjust it to correspond to a star of any declination. The alternative photoelectric method, similar to that which has been used on the pendulum-clock, suffers from the serious defect that slit-width has an effect, equivalent to backlash, which, apart from the difficulty of determining it at any given time, is liable to variation due to instability of the photocell and its allied circuits. It was in the attempt to escape from this dilemma that the photographic method referred to in the section of this report dealing with the P.Z.T. was devised. As in the case of the P.Z.T. this method eliminates the use of a chronograph. Its application to the horizontal transit instrument is however rather more difficult in detail owing to the fact that, whereas the speed of the P.Z.T. carriage is constant, in this case the speed of the mechanism is subject to variations imparted by the manual adjustment required to satisfy the visual criterion of image-bisection. In consequence the recorded time-scale is not uniform, and in order to permit a sufficiently accurate interpolation to be made, the spacing of the known points of the scale will probably have to be 10 times less than in the case of the P.Z.T. This narrower spacing can, however, easily be achieved by suitable modification of the electronic-circuits.

A further point to which some consideration has recently been given is the choice of the electrical system best adapted to actuate the drive for the variable-deviation system, since this affects the design of other parts of the instrument. This question is also of wider importance, as it relates to the more general application of drives for impersonal micrometers.





Horizontal Transit Instrument

Direct work on the construction of this instrument has been considerably delayed by the prior claims of other work. The construction of the glass optical components of the variable-deviation system, and also of a special set of polished stainless steel blocks for the timing-system has been completed. Some of the experimental work referred to under the heading of the Photographic Zenith Tube is applicable to this instrument also; for example, the work on the Arditron Lamp, on the selection of photographic plates, and on the mercury basin.

A considerable amount of experimental work on the development of an electrical drive of the D.C. motor-generator type suitable for operating the variable-deviation system has been brought to a satisfactory conclusion: adequate rapidity of response to hand control and stability of motor-speed over a wide range have been achieved. As was pointed out in the previous report, this work is of wider importance; and in point of fact the development of a similar electrical system, intended for incorporation in the Bamberg Transit Instrument, has been nearly completed. We are indebted to Dr. Uttley of Telecommunications Research Establishment for his suggestions and help in the early stages of this work.

In designing a system by means of which the maintenance of bisection of a stellar image is to be secured it is clearly important to have quantitative information concerning the erratics to which the position of the image is subject on account of atmospheric irregularities. Investigation of such erratics has been carried out in America, for example by Schlesinger; but it was considered that direct information of the conditions pertaining to this country would have greater relevance. Consequently an investigation has been in progress of which the essential feature is a micrometric examination of the stellar trails provided on a number of plates taken with the Cookson floating zenith telescope. By this means information on the short period stability required from the motor drive has been obtained and a rough estimate has been made of the probable error in time-determination due to atmospheric erratics.


Mirror Transit Instrument

Some preliminary consideration has been given to the design of a transit circle in which the moving parts are reduced to a minimum, and in which an accurate control of the instrumental errors is possible. The moving portion consists of a mirror supported by trunnions to the east and west. Two horizontal viewing telescopes in the meridian, to the north and South of the mirror, are used for Star observations and also as collimators. A general discussion of the principles of the instrument is in course of publication by the Royal Astronomical Society.





Horizontal Transit Instrument

Work on this instrument has been postponed, for the most part, owing to pressure of other needs and loss of staff. Preliminary measurements have been made to test the method of determining the deviations of two types of optical square proposed for the instrument. A paper has been submitted for publication on the effect of atmospheric disturbances on apparent star-places; this investigation was undertaken in connection with the proposed motor-drive for this and other transit-instruments.


Mirror Transit-Circle

Progress has been made with the design of the proposed Mirror Transit-Circle mentioned in the last Report. A model has been constructed in which the usual counterpoise-weights are replaced by disk-shaped floats running in mercury troughs. The pivots are inverted, so that the axis lies below the Y's, and the instrument is floated up into the Y's by operating small plungers in auxiliary cylinders. The pivot-load can be very delicately controlled and very smoothly applied by this method, and in principle it can readily be reduced to zero every time the circle reading is to be changed; whether this is in fact desirable can probably be decided only in actual use, but it seems probable that a very low loading will be found practicable, and possible even that oil can be eliminated. It is proposed to use the pivot-testing telescope also as an azimuth reader. Word has been received from Dr. de Barros of Oporto, who visited the Observatory last year, that he is now proposing to construct a Mirror Transit Circle, and these proposals form at present the basis of his plans also. Close contact will be maintained on the subject.





Mirror Transit-Circle

The model of this instrument was shown at the L A.U. meetings in Zurich. Dr. Barros of Oporto is proceeding with his plans to construct such an instrument, even though Sir Howard Grubb, Parsons and Co. are unable at present to undertake any of the work. He has worked out a detailed design and has constructed some of the parts himself. No further work has so far been possible at Greenwich.





Horizontal Transit Instrument

As a result of progress in the design work of the P.Z.T. and of the recent acquisition of staff, it has now become possible to resume the interrupted programme of work on this instrument.





Horizontal Transit Instrument

Preliminary investigational work on this instrument has been in progress. It had previously been decided to use a single optical square, transposed during each observation, instead of the two separate optical squares in the original design. Collimation error is then more readily avoided and, moreover, the wedge micrometer, which it was decided to use, must be transposed, so that the additional transposition of the optical square added no further mechanical complication.

A fundamental requirement of the instrument is constancy of the optical square during the period of observation of a single star. Since this period is only of the order of two minutes the thermal changes, with due precautions, will be small.

It seemed probable that the more important cause of change might be mechanical distortion due to the inevitable change of posture after transposition. The method of transposition, decided upon mainly for mechanical reasons, is rotation through 180° about a vertical axis, combined with rotation through twice the zenith distance about a horizontal axis. One merit of this method is that flexural error vanishes in the zenith, since there is no change of posture, while in the horizon it must be very small since the plates of the built-up pentagonal prism are vertical for each posture. For intermediate zenith distances there is a quasi-symmetry and the effect of flexure will presumably be small over a reasonable range about the zenith. It is, nevertheless, very important to have maximum rigidity of the prism and experiments have been made to determine the relative rigidity of two different types. In one the silica plates are fused to a triangular base by means of two feet; in the other, kindly loaned by Messrs. Cooke, Troughton and Simms, the plates are wrung and cemented to the base over their entire width.

To perform a direct test by measuring the change of deviation with posture, as was originally intended, is very laborious and, for that very reason, unsatisfactory, because the time required for the test is so great that there is no assurance that change of posture is the only variable. An alternative procedure was therefore adopted in which the posture was maintained constant and mechanical force was applied to the plates. As the application and removal of this force is only a matter of seconds, the consequent flexures must be due to it alone.

The results of these measurements show that the wrung and cemented type of construction is much superior in rigidity. The other type has two possible merits: firstly it should be feasible to work on the thickness of the feet until the deflection of both plates is made equal; secondly the more open design at the base permits greater freedom for air circulation and may thus be less liable to trouble from air pockets and stratification.

A marked improvement of rigidity of both types was obtained by cementing a silica bar across the tops of the mirrors. This is a partial approach to the box type, originally suggested for this instrument, which has great rigidity. A decision on the type to be adopted will shortly be made. It is hoped that it will then be possible for the construction of the instrument to be resumed.





Horizontal Transit Instrument

The telescopic system of this instrument has been erected on a concrete pier in the Optical Laboratory and used for preliminary investigation of the photoelectric method of determining transit times which it is proposed to adopt in connection with the Longitude Programme for the International Geophysical Year in 1957-58. In the course of this work, methods of making various auxiliary adjustments such as "squaring-on" and the autocollimation method of level determination have been tested. It had previously been proposed to employ a photographic method for the chronographic part of the instrument. This method, while sound in principle, is liable to be laborious in practice. As a result of the recent photoelectric work an alternative and more direct method of determining the times of parallelism of the "contact" mirrors by photoelectric recording has been proposed. Apparatus for testing this method is under construction.





Mirror Transit Circle

In connection with Dr. Atkinson's investigations of a Mirror Transit Circle, a plane quartz mirror and a steel axis to mount it in have been supplied to specifications; tests will be made to see whether the mirror can be mounted in a sufficiently permanent relation to the axis. The mirror is 12.7 inches in diameter, 2.28 inches thick, with two plane facets worked on its rim, parallel to one another and perpendicular to the main face within a very few tenths of a minute. The axis has 15-inch ball bearings for the weight-relieving gear and 4.73-inch chromium plated pivots at its ends; some lapping of these has been done, but they are still elliptical with a total amplitude of 1 - 2 microns each, besides other errors. Most of the auxiliary gear for counterpoising the mirror, within the axis, has been made.





Dr. Atkinson has continued experimental work in the design and development of a mirror transit.





A design for the proposed Mirror Transit Circle has been worked out in detail with Messrs. Grubb Parsons. A specification and about 45 General Arrangement drawings are now on file.