Conservation report of a refracting telescope on a universal equatorial mount signed Hindley, YORK
Updated: May 14
Timothy M. Hughes and Matthew Read
The instrument (Fig.1) described in this report comprises a brass refracting telescope with a micrometer box intersecting the telescope tube. The micrometer box contains adjustable embracing reticles. The telescope is on a universal equatorial mount with tripod base. The instrument is of particular interest because of the use of worm gears of enveloping or globoidal form used in setting the position of the instrument via four circles: the Declination Circle, the Equatorial Circle, the Meridian Circle, and the Setting Circle. In engineering, this particular design of gear is widely referred to as the Hindley Worm.
This conservation report is based on a conservation project carried out at West Dean College. The instrument, inventory number BCF 56, property of the Burton Constable Foundation, is now on permanent display at Burton Constable House, East Yorkshire.
As part of the conservation process, all components of the instrument were disassembled for cleaning and recording in both day-book and photographic formats. Component parts were allocated individual part numbers.
The instrument was described in 1785 by John Smeaton in a paper read at The Royal Society on 17 November, entitled, ‘Observations on the graduation of astronomical instruments, with an explanation of the method invented by the late Mr Henry Hindley of York, clockmaker, to divide circles into any given number of parts’.
The transcription of the lecture describes the instrument as follows:
This instrument was of the equatorial kind, the wheel parallel to the equator, the quadrant of latitude, and semi-circle of declination, being all furnished with screws containing fifteen threads each, framed and moved in the same manner as that of the engine4 […] and the telescope tube in its place, which was intended to be of the inverting refracting kind, and to be furnished with a micrometer.
Fig. 1 The instrument photographedbefore treatment
The mount comprises an upper, Declination Circle over an Equatorial Circle above a Meridian Circle. The whole rotating about a vertical axis via a Setting Circle mounted between the tripod legs. The positioning of all four circles is via ground worm wheels of enveloping hour-glass shape, engaging with throated gears cut into the edges of the circles. All four worm gears are set in pivoted sprung frames that can be latched out of engagement for rapid setting. The sprung nature of the worms enables them to effectively have zero back-lash. The flanks of the threads are tapered, and worm and worm wheel are pushed together in use by leaf springs. The equatorial worm is fitted with a universal joint and square female socket, presumably for a hand crank or clock drive.
The 825mm long brass-bodied refracting telescope has a micrometer reticle box towards the eyepiece end (Fig. 2). The objective end has a dew shade and a push fit dust cap.
Fig. 2 The telescope tube with micrometer box. Overall length approximately 825mm.
The telescope optics comprise four lenses. The doublet objective lens is 46mm diameter (Fig. 3). Focusing is achieved by turning the winding wheel on the micrometer box. The eyepiece has a sliding dust cover (Fig. 4).
Fig. 3 Internal lens holding tubes within the body.
Fig. 4 Objective lens and dewshade
The cuboid micrometer box is approximately 80mm square with two hand-operated knurled wheels; one wheel, set on one of the box edges (Fig. 5), for focusing the telescope. The other set in the centre of the face, opposite the engraved scale and signature.
Fig. 5 Micrometer box with two setting knobs, one for adjusting the reticle frame (centre) size
and one for focussing the telescope (box edge).
The wheel in the centre of the face is for adjusting the four reticle frames within the box and is geared to a blued steel indicator hand on one face of the box (Fig. 6), with an engraved graduated scale. The dial plate is signed across the centre, Hindley, YORK.
Fig. 6 Micrometer box scale and blued steel indicator.
The micrometer box scale is engraved from zero to 120 in increments of one, with every ten marked with Arabic numerals. The box has a bevelled square aperture to one corner of the dial plate with a revolution counter graduated from 1 to 12, indexing one station for every full revolution of the hand (Fig. 7).
Fig. 7 Micrometer box with engraved scale and signature.
Within the box are four, 70mm diameter, intermeshing contrate wheels. Each wheel is mounted on captive steel threaded arbor. When the wheels are rotated via the external setting wheel, four brass reticle frames are drawn in or out (Fig. 8). The (present) reticle wires measure approximately one tenth of a millimetre in thickness and form a square, and are for the purposes of measuring an object in the field of view. (Fig. 9). Stopwork, acting on one of the contrate wheels, ensures travel of the frames is restricted at both ends of their range (Figure 10).
Fig. 8 Micrometer box with cover plates and two of the contrate wheels removed.
Fig. 9 Reticle frame: the individual reticle holders have alternate left-hand and right-hand threads, due to their driving contrate wheels being intermeshed.
Fig.10 Lower plate with stopwork comprising a 13 tooth wheel with two uncut tooth gaps.
This wheel is indexed by a single tooth wheel fixed to the lower contrate wheel.
Telescope mount and frame
The telescope is mounted in an open brass frame, retained by two split clamps that allow the telescope to be rotated within the frame about its axis. The mounting frame incorporates a pivoted spirit bubble mounted parallel with the telescope tube (Fig. 11).
Fig. 11 Telescope tube within the frame clamps with spirit bubble.
The telescope mounting frame, telescope tube, bubble and worm gear, are mounted on a frame pivoting about the central axis of the Declination Circle.
The 6” diameter circle has 180 effective teeth and is engraved plus and minus 90 degrees from perpendicular to the equatorial plane. The corresponding worm gear is single start and of 10 turns. Rotation of the worm is indicated on an engraved circular scale, 0 to 60 arc minutes, in increments of 30 arc seconds (Fig. 12). The telescope mounting frame pivots around the axis of the Declination Circle. The setting worm is pivoted within the frame legs and sprung to engage with the wheel. It can be latched out of engagement for rapid setting. The position of the telescope mount, in relation to the Declination Circle, is indicated by a single pointer screwed to the fixed element of the hub of the mount, indicating against the scale engraved on the side of the Circle.
Fig. 12 he telescope mounting frame pivots around the axis of the Declination Circle. The setting worm is pivoted within the frame legs and sprung to engage with the wheel. It can be latched out of engagement for rapid setting. The levelling bubble can be seen towards the top of the image.
The Declination Circle brass mounting pads are mounted to the upper surface of the Equatorial Circle. The Equatorial Circle has 360 teeth and is engraved on its upper face with an outer track of 0-360 degrees and 360-0 degrees of arc, with an inner track of hours of time. Hours are marked 1 to 12, twice, in Roman numerals. Midday and midnight correspond with the 360-degree and 180-degree marks. Half hours have fleur-de-lis markers at the end of the radial lines and quarter hours are marked by plain radial lines (Fig. 13).
Fig. 13 Equatorial Circle plus detail
The equatorial worm is single start, twelve turns. It is embraced by two graduated micrometer scales; the left scale engraved ‘Min[utes] of Deg[rees]’, 0-60 in minutes of arc. One full revolution of the worm advances the Equatorial Circle by one degree. The right scale engraved in Min[utes] of Time’, 1-4 minutes, each minute sub-divided into 20 40 60 seconds of time (Fig. 14). The Equatorial Worm axis is fitted with a knurled brass thumb wheel via a universal joint.
Fig.14 Equatorial worm and scales before cleaning.
The Equatorial Circle is mounted on a steel spigot, which is 190mm in length and tapers from 25mm to 21mm, rotating within a matching brass tapered tube which is part of a cast brass frame supporting all of the above (Fig. 15).
Fig.15 The Equatorial Circle is mounted on tapered steel spigot.
Equatorial Circle readings are taken from brass pointers mounted on the Meridian Circle Frame (Fig. 16).
Fig.16 Brass pointers on the Meridian Circle frame.
Meridian Circle and counterpoise
Fixed to the tapered socket that provides the bearing for the Equatorial Circle is the Meridian Circle and counterpoise (Fig. 17). The Meridian Circle has 194 teeth and is engraved plus and minus 90 degrees from the axis of the Equatorial Circle, indicated by a brass pointer. The segment-shaped brass-cased lead weight weighs approximately 3.62kg (Fig. 18).
The Meridian Circle and counterpoise pivots are mounted on four tapering brass columns, which in turn are fixed to a cast brass chassis (Fig. 19). To this chassis, the meridian worm screw is mounted.
The single start worm has 12 turns and a dial reading zero to 60 in increments of ten. The chassis has a tapering brass spigot, the lower end of which which the Setting Circle is mounted (Fig. 20). The spigot within a tapered brass tube is screwed to the tripod frame.
The unengraved Setting Circle is mounted to the bottom of the spigot and is rotated by a worm screw. The screw and its axis are mounted across two legs of the tripod (Fig. 21 and Fig. 22). The tripod has three adjustable screw feet used for levelling.
Fig. 21 Setting circle within the tripod legs.
Fig. 22 Setting Circle worm and pivot blocks screwed to the tripod legs.
The tripod has three adjustable screw feet used for levelling the instrument (Fig. 23).
Fig. 23 Repaired tripod foot
Appraisal of the condition
Overall the instrument appears to be complete, possibly with one knurled knob missing.
There are significant losses to the lacquered finish to the brass work (Fig. 24).
One of the knurled adjustable levelling feet is missing a part of its threaded secion.
Fig. 24 Telescope tube and micrometer box showing losses to the lacquered finish, variegated tarnished brass and finger-printing.
Where loss has occurred to the lacquered finish or where the lacquer has degraded and cracked the brass work has tarnished to a multitude of hues ranging from pale gold to almost black. There is considerable finger-printing to the telescope tube (Fig. 25). Almost all of the exposed steel work shows light surface rusting (Fig. 26).
Many of the screw threads are lightly corroded likely due to the presence of a cleaning/polishing agent residue.
The instrument has a powdery and chalky white residue in almost all of the corners, engraving detail and screw threads etc (Fig. 27), which is restricting the free movement of many components, including the worm screws and rotating components. The Equatorial Circle is locked into its current position. The instrument is superficially dusty and the optic lenses to the telescope are contaminated with organic material including what appears to be fungal growth and insect remains (Fig. 28).
The pointer to the Meridian Circle has broken away from one side of its mount (Fig. 29). One of the adjustable feet is broken and missing a portion of its threaded foot (Fig. 30).\
Fig. 29 Broken foot of the Meridian Circle pointer.
Fig. 30 Three tripod levelling screws R, with fractured and lost threaded section.
1. Completely disassemble the telescope and mount. Wash and rinse all metal components by hand brushing and rinsing in L & R No3 watch rinsing solution.
2. Loose surface corrosion to steel work is to be removed using 0000 grade steel wool soaked in microcrystalline wax.
3. Once cleaned all components are to be sealed with microcrystalline wax.
4. Bearings to be oiled as required using Moebius synthetic lubricant.
5. The cleaning of the optics is to be carried out by a ceramics conservator.
6. All components are to be measured, photographed and recorded throughout the conservation process.
7. Repair the broken tripod levelling screw by letting in a new piece of brass and cutting a new thread to match existing.
Description of the treatment
The instrument was dismantled in sections, cleaned using L & R watch rinsing solution, surface rust corrosion to steel was removed using 0000 grade steel wool, all components were treated with microcrystalline wax. All components were measured, photographed and recorded.
The optics were cleaned by the West Dean College Ceramics Department (Fig. 31) using a diluted solution of industrial methylated spirits. The objective lens, being a doublet (i.e. two lenses pressed together in one mount) was not removed from its mount as this was deemed to be too invasive and could present a potential high risk of damaging the lenses. Hence just the exterior of the two lenses was cleaned and as a result there is still light fogging between the two lenses (Fig. 32).
The micrometer box (Fig. 34) was dismantled, cleaned and recorded, including X-Ray Fluorescence (XRF) readings taken from some of the components.
Fig. 33 Micrometer box disassembled.
The steel spigot mounted to the Equatorial Circle, which was locked into one position, was removed and after soaking overnight in a proprietory releasing agent, then tapping free with a hide mallet. The reason this had seized was that old oil had dried and appears to have turned acidic, corroding the steel (Fig. 34).
Meridian Circle pointer
The brass pointer to the Meridian Circle was repaired by fashioning a piece of cast brass to span the internal 90 degrees of the two broken pieces (Fig. 35 and Fig. 36), which was then glued into position using Araldite 2020 glue epoxy resin (Fig. 37)
Some of the corroded steel components were cleaned with 0000 grade steel wool. Before cleaning: (Fig. 38). After cleaning: (Fig. 39).
Fig. 38 Before cleaning.
Fig. 39 After cleaning and rust removal.
Reinstating lost material and thread cutting for the broken adjustable foot.
It was decided to imitate the historical thread to the adjustable foot and to retain as much of the original component as possible.
A thread cutter was made from 4mm silver steel and turned in the lathe with a graver to the same profile as the historical thread.
A section of 60mm nylon bar was turned to hold the foot with the remaining thread (Fig. 40), the broken end turned flush and a 6mm diameter hole was drilled 20mm into the end (Fig. 41).
Cast brass bar was turned to a diameter of 10.3mm, i.e. the diameter of the crest of the historical thread, and then a plug was turned from cast brass to push fit into the hole in the stock. A small amount of high-strength thread locking fluid was used to make sure the marriage of the components was stable (Fig. 42).
The lathe was turned using the hand crank and the thread was cut in 10mm long sections until the required length was achieved. It took five cuts with the thread cutter for each 10mm length (Fig. 43). Once enough lengths were created a mean average of the two remaining feet was taken. The new stock was parted to length and the foot was created with a graver (Fig. 44).
The thread was then polished with a fine Pumice powder soaked into the string with oil (Fig. 45). The advantage of this method, albeit time consuming, was that a appears to be a near perfect continuation of the thread was achieved and theoretically a perfect match to the pitch of the historical thread (Fig. 46). This means the thread winds successfully through its existing aperture.
Observations and Recommendations
This telescope is believed to be Hindley’s first telescope and also thought to be the first equatorially mounted telescope ever made. As such it may be considered a prototype. There are indications that Hindley may have struggled with some elements of the construction of this instrument. Overall the work demonstrates exceptional design, technical and making skills, such as the worm screw driving mechanism and micrometer box. Some aspects of design such as the telescope mount do not appear to quite equal that design competency, maybe reflecting Hindley’s inexperience in this particular element of instrument making.
Not atypical of eighteenth century clockmaking, two of the worm screw castings have flaws (Fig. 47 and Fig. 48).
Possibly reflecting the early, developmental or ‘prototype’ nature of the work, there were areas where steady pins had been filed back (Fig. 49) after a radius had been created.
The lead filled counter weight had been altered; evidence of re-soldered joints and rivets remain (Fig. 50 and Fig. 51), most probably after realising that the weight was (and still is) undersized and made the instrument unstable at certain angles (Fig. 52). A slot had been cut to the front of the weight to incorporate the spigot tube to the Equatorial Circle (Fig. 53) -which appears to be an afterthought.
It is recommended that once the instrument is installed within its new purpose built display case it is moved on every axis on an annual basis to prevent components seizing.
As always with objects of this nature there is much research still left to do, for example a field test as to its full function and limitations as well as more detailed research into the divisions of the worm screw and circles as to their accuracy and construction.
It is recommended the the instrument is always handled using conservation grade gloves to prevent further finger-printing.