The telescope tube was rebuilt and the equatorial refurbished by Ronald N. Irving,
proprietor of the famous Teddington based instrument company,
H.N. Irving & Son over a six-year period. The tube is 12-inch
IDx16 SWG (0".028) rolled steel, seam welded, lined with
1/4
-inch dense cork obtained from Augen Optics, and supported in a rotating cradle.
The primarycell is a simple closed three point support with push-pull collimation
screws. The four-vaned spider and diagonal holder are constructed
to Hargreaves' design. There are two finders, the original 2-inch
Ottway, which has a 5º field, and a 2-inch Cooke, Troughton
& Simms with rack-objective focusing, a drawplate, helical
focusing pancratic eyepiece, field illuminator and slip rings. The 3
1/2
-inch aperture guide telescope has a swivel elbow that affords a comfortable
viewing position at the projection reticle eyepiece. To minimise differential flexure
it was constructed of 22SWG (0".028) titanium alloy and weighs
only 7lb:8oz. The rackmount has a worm and wheel driven PA circle
in German silver reading to 30 arcsecs by means of twin verniers,
and the eyepiece adaptors have breach lock bayonet couplings.
The tube is fitted with adjustable radial weights. The forward weight
can be adjusted by means of a handle to a scale calibrated in ounce-inches.
The tube accessories and rackmount are so disposed with respect to
the radial counterweights as to maintain the centre of gravity
on the mechanical axis, ragardless of the rotational orientation
of the tube within the cradle.
The hour/right ascension circle can be locked and driven. It has an internal planet gear and pinion for setting. It reads by means of verniers to 5s time. The declination circle was re-engraved by O.H. Kampf's of Wimbourne Minster, Hampshire; it reads by a repeating vernier to 1'arc. There is a tangent arm half way along the hour axis that engages with a clutched wormwheel. It was originally the driving worm and wheel system, coupled to a falling weight clock drive, which was missing. I utilize it to set the telescope in hour angle before engaging the synchronous drive, which is carried on an eccentric.
The primary mirror is of some interest. It is 10
1/4
-inches diameter by 1
1/2
-inches thick plate glass and
was originally ground and polished by the Rev. Chas Tweedale.
It was subsequently refigured by Calver in 1903, and then by an
unknown hand in the early 1980's as evinced by cerium oxide embedded
around the edge. I had it reground and refigured to f/10@1/20wave
[P-V] by Ez Reid of A.E. Optics.
The interesting feature of the mirror is the engraving on the back, executed by Tweedale. There is an inscription around the circumference from the Epistle to the Hebrews I,vv10-12, cited from Psalms 102,vv25-27, King James Authorized Bible, which reads: "Thou Lord in the beginning hast created the earth and the heavens are the work of thine hands. They shall perish but thou remainest. They shall all wax old as doth a garment but thou art the same and thy years shall not fail." Circumscribing the quotation are engraved astrological zodiacal symbols, clockwise from Pisces uppermost. Within the quotation and quartering the enclosure are engravings of the waxing crescent Moon, the Earth, the Sun, and a comet. At the centre are the signatures of the maker, 'Charles L. Tweedale Fecit Anno 1894' and 'G. Calver /03'.
Tweedale refers to the production of the mirror in his book, 'Reflecting Telescope Making: The Daylight Comet 1910; The Total Solar Eclipse 1927', published by Werner Laurie in 1942. He describes how he made the 10 1/4 -inch f/8 mirror. It was ground and polished on a manual machine of his own design and construction from Pilkington plate glass 1 1/2 -inches thick. He says, "...on no account must the amateur exceed this size, which gives a proportion of 6.5 between the thickness and diameter" (p.16).
Testing was done using what we would now call the Foucault method, which is described in the book as being invented by H.A. Wassell: " ...to Mr. H.A. Wassell, of Addenbrook Villa, Stourbridge, belongs the great honour of having invented and perfected an apparatus which would literally measure off with definite and reliable exactitude what the beautiful shadow test of Leon Foucault showed to the eye" (p.24).
The final figuring of his mirror was done using the 'Tweedale Triple Stop', which looks very similar to the Couder Screen. To quote: " My triple screen enables one, at a glance, to determine the figure, whether over-corrected, under-corrected, or just the right parabola. Without this check the amateur might wander in the wilderness ad infinitum.....Therefore it is only by vast experience that the parabola can be determined from the Foucault test alone. With the triple screen and the calculated aberration, this can be obtained with certainty" (p.43). As the first published description of a mask or screen was in George Willis Ritchey's paper: 'On Methods of Testing Optical Mirrors During Construction' (Astrophysical Journal 19, 53-69, 1904), and the Couder screen in 1935 (Danjon, A. & Couder, A. 'Lunettes et Telescopes', A. Blanchard, Paris, 1983, p.534), it would appear Tweedale had independently devised a similar test, and may well have done so at a much earlier date!
Using this algorithm would result in an under-corrected figure, which would not be uncommon for mirror makers of this period when using plate glass, the theory being that whilst observing during a late evening in a falling temperature, the contraction of the plate would cause the figure to become over-corrected. The technique was to judge the under-correction necessary to offset this effect to a nicety.
Tweedale also mentions on numerous occasions his friendship with With and Calver, both of whom extended every assistance possible to aspiring mirror makers; silver-on-glass mirror making being very much in its infancy. He also states in one of his digressions that, "Years afterwards when Mr. With had retired from making specula...[he] put about sixty exceptionally choice specimens [termed his 'special reserves'] into Mr. Calver's hands...."(p.46).
The comet engraving also has an intriguing aspect. Tweedale describes his 'independent discovery' (p.54) of a comet on 1886 December 2nd midway between ß Serpentis and Arcturus. The SAO Catalog of Cometary Orbits records only one comet visible at this time, 1886 IX Barnard-Hartwig, discovered in October 1886. The calculated coordinates for December 2 are RA 15h:25m.5 DEC +17º:05', which compares with Tweedale's estimated position i.e. RA 15h:03m DEC +17º:05'. Could the comet Tweedale engraved on his mirror be 1886 IX?
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