Maintenance Of The Tomline Refractor Object Glass
The object glass (OG) of the Tomline Refractor is an achromatic doublet, two lenses mounted in close proximity. A crown glass lens is positioned at the end of the telescope (closest to the object under observation), in close proximity (a small gap of circa 1 mm) from a flint glass lens closest to the eyepiece end of the instrument. The compound lens is designed to generate minimal spherical and chromatic aberration. Unfortunately, although the the OG is of generally good quality, it was not manufactured to the very highest standard of its era and, in the 20th Century, has benefitted from refiguring. There is no seal between the glass elements of the OG and therefore moisture and dust can, over the years, settle between them, necessitating occasional cleaning. The main episodes of maintenance of the OG are described below.
During the first decade of the 21st Century, there was occasional discussion among members of OASI about the possibility of having the OG professionally tested and its quality assessed. (The ready availability of high quality, low cost modern lenses made it possible to contemplate changing the original OG for a better modern equivalent based on the latest manufacturing technology.)
Society Chairman Neil Morley obtained a quote for the work from Es Reid of Cambridge, and arrangements were made to remove the OG in late July 2012, during the summer "maintenance period", when the telescope was out of action as the floor of the equatorial room was to be sanded and re-varnished. The lens was removed from the refractor on 25 July and, on 28 July, Neil duly it to Es by car. Es began work on the OG on 04 August and in the following days undertook a variety of tests, thoroughly cleaned the lenses, re-polished the flat rear surface which seemed to carry most of the zonal aberrations, and refigured the surfaces to a much smoother and flatter wavefront than before. Extracts from Es' reports on the work:
Put the lens on an autocollimation test about an hour ago and first looks suggest a somewhat lumpy, zoney wavefront with the expected level of colour and quite a lot of scatter.
Tested [the OG] as supplied. The three large spacers lined up with the small amount of astigmatism so replacing those with six will reduce that effect.
It doesn't look mouldy, just a fine film of dust over each surface but I'll take a closer look to see if I can work out what it might be.
I've been really active this weekend and am re-polishing the flat rear surface [the surface of the flint glass facing inside the telescope tube] which seemed to carry most of the zonal aberrations. The cell is very well made of bronze and brass and fits together very smoothly - I shall give each part a hot water wash to degunge the surfaces. The lead spacers are fine in thickness but I will cut each in two and redistribute them.
I’ve worked for four days blasting away at the OG and it is back together, cleaner and refigured to a much smoother and flatter wavefront than before. There are some small, deep erosion pits on the inside surface of the flint plus some old scratches but there seemed little point in risking the objective to grind and polish them out. I hope that the 6 spacers keep the airgap more stable. There is some residual multiple astigmatism that may be from any of the surfaces and might be due to glass inhomogeneities but it’s not severe.
This OG is the best I've seen of its size from so long ago when glass technology was trial and error.
Es also reported a patch of deterioration, due to mould inside the air gap, affecting the front (sky-facing) surface of the flint glass, but didn't believe it warranted repolishing the entire surface. He reported that the quality of the flint and of the cell assembly were among the best that he'd seen from the period. The OG was a perfect fit inside the inner cell (stamped Merz); the outer cell looked like a naval phosphor bronze bearing and was extremely well machined - he wondered if this could have been the work of Ransomes.
While the lens was removed from the telescope tube, members of OASI took the opportunity to spray-paint the inside of the tube matt black, to minimise internal reflections.
Photographs illustrating aspects of the work:
Removing the OG cell from the body of the telescope. (Neil Morley.)
The OG cell. (Matthew Leeks.)
Martin Cook peers through the OG; the dirt is clearly visible. (Matthew Leeks.)
Packing the lens for transportation to Es Reid. (Neil Morley.)
The OG after completion of cleaning and refiguring. (Es Reid.)
Removing the eyepiece holder from the body of the telescope. (James Appleton.)
Painting the inside of the telescope tube. (James Appleton.)
Refitting the eyepiece holder. (James Appleton.)
On 12 September, the OG was refitted to the telescope tube. Unfortunately, observers subsequently noticed a lack of clarity of the image which, on investigation, appeared to be associated with a slight mist on the inner surfaces of the two lenses. The appearance of mist so soon after the lens had been cleaned was disappointing and surprising. Atmospheric moisture was suspected to be responsible, water vapour condensing against the cold of the mirror cell and being drawn into the gap between the crown and flint glass by capillary action. On 05 April 2013, Martin Cook and Alan Smith fitted a home-constructed de-humidifier to the telescope in an effort to ensure that the air around the OG remains dry. They constructed the de-humidifier from an empty mayonnaise tub, two flexible hoses and a fan salvaged from a surplus PC! The fan circulates air drawn from the telescope tube via a side vent, into the tub and back to the telescope via the the eyepiece tube. The tub is filled with silica gel, removed and baked weekly, to absorb moisture. See the photograph below. After running the de-humidifier for several weeks whenever the telescope was not in use, the mist inside the OG appeared to diminish and it is hoped, as of the time of writing (early 2014), that the problem has been resolved.
Alan proudly displays the home-constructed de-humidifier (05 April 2013).
By 1994, once more there was evidence of the lens becoming "cloudy" through a combination of moisture, dust and a fungal growth between the inner glass surfaces. Members of OASI removed the lens, dismantled the elements of the doublet from the cell, and cleaned all the glass surfaces. The operation was captured in the photographs below (by Martin Cook).
Alan Smith removes the OG cell from the body of the telescope.
Separating the two lenses from the cell.
Washing a lens in iso-propyl alcohol.
Rinsing a lens in water prior to drying and refitting in the cell.
On 11 June 1986, members of OASI removed the lens cell from the Tomline Refractor and clamped the assembly between 25 mm sheets of plywood for safety during transportation to David Payne, who cleaned it. The lens thereafter remained undisturbed for almost a decade.
In late 1978, a snowflake-like fungus developed on the lens. The Committee of OASI estimated that the fungus reduced the limiting magnitude of the telescope by three or four and also diminished its resolving power. Therefore, once more the Committee arranged for Dall to clean the OG. The then Secretary of OASI, Mike Barriskill, transported the lens to Dall on 22 January 1979 and returned it to Orwell Park Observatory on 25 January for refitting in the telescope tube.
By 1977, once more moisture and dust had settled between the two inner surfaces of the compound lens and a noticeably dirty area was visible. The Committee of OASI therefore arranged again for Dall to attend to the lens, this time to dismantle the doublet and clean and refigure the inner surfaces. The Committee hoped that the cleaning and refiguring would extend the visual magnitude range of the instrument to 14.2 and improve image quality.
Members of OASI removed the lens from the telescope on 03 March 1977 and David Bearcroft subsequently collected it from the observatory and delivered it to Dall in Luton. David returned to Luton to collect the lens on 18 March and members of OASI re-fitted it to the telescope tube on the following evening. The following photograph by Martin Cook shows the lens cell on 19 March 1977 on re-fitting. (Contrary to what appears in the photograph, written on the lens cell, 19 March 1977 was the date of refitting the lens, refiguring having been completed some days previously.)
The night of 19 March was perfectly clear and, after refitting the lens, the telescope was turned to the sky. The results far exceeded the wildest expectations of those present: the bands on Jupiter's disk and Saturn's rings were crystal clear and it was possible to resolve double stars with considerably more definition than previously. The results were so good that the members of OASI present found it hard to leave the telescope!
In January 1973, the Committee of OASI investigated having the OG removed for cleaning, refiguring and general adjustment by Horace Dall of Luton, one of the leading optical experts in the UK and vice-president of the BAA. Initial investigations suggested that the work was within the modest means of the Society and the Committee therefore decided to proceed with work and also to overhaul the other main optical components at the observatory. Accordingly, members of OASI removed the OG from the Tomline Refractor on 20 May 1973 and, on 22 May, Geoffrey Collier, OASI Treasurer, transported to Dall the OG of the Tomline Refractor, the OG of the transit telescope, plus eyepieces for both instruments.
On initial examination of the OG of the Tomline Refractor, Dall found the following problems:
- Bad over-correction for spherical aberration. Rays travelling through the centre of the lens arrived at a focal point separated by approximately one centimetre from the focal point of rays travelling through the edge of the lens. This discrepancy is about the same as the focal length of a high power eyepiece, and may have accounted for the poor performance of the telescope in splitting close double stars. Dall planned to correct this error by refiguring the lens, and to address some localised zones of poor figure (zonal aberrations) as part of the same correction.
- Achromatic aberration was very slightly over-corrected, but not sufficient to require corrective action.
Dall estimated that he would finish work on the OG on 20 August, and the Committee made plans to re-install it on 29 August.
While Dall was working on the OG, members of OASI attempted to adjust the declination circle of the Tomline Refractor, which had been 45° out of alignment for as long as anyone could remember (it was unclear whether the circle had been fitted incorrectly when the telescope was installed, or subsequent alteration had resulted in the error). However, with the OG removed, the telescope was badly out of balance along the length of its tube and, in order to adjust the declination circle, it proved necessary to remove the massive lead counterweight on the other end of the declination axis from the telescope. When the counterweight was finally removed, there was a "tense moment" as the telescope, now severely unbalanced in both RA and dec, started to slew rapidly and threatened to collide with other equipment or occupants in the dome! Everyone present scrambled to help control the rampaging instrument and fasten a system of ropes to hold it steady, enabling work to continue safely. Eventually, the declination circle was removed and refitted to show the correct reading.
Dall stuck to his estimated schedule and members of OASI collected the OG from him on 26 August and duly re-installed it in the Tomline Refractor on 29 August. Here are his reports on the micrometer eyepiece (June 1973) and the OG and various eyepieces (25 August 1973):
Remains of Merz Micrometer Eyepiece
Whilst I have tidied this wrecked/damaged instrument and replaced a few missing screws, there is so much damage & vital parts missing that I cannot consider restoring it.
The other work on the transit telescope optics is now complete.
Signed: H Dall, June 1973
Luton, August 25th 1973. OASI Merz Refractor (1872).
f/15.1. Clear aperture 258 mm or 10.16". Focal length 3894 mm or 153.3".
Refigured by H Dall August 1973.
The zonal errors of spherical aberration as reported by Dr Steavenson in 1936 and confirmed by H E Dall in 1973 are now mostly removed. The two deep zones set in the central 4½" area are gone and the general over-correction of 0.243" is now removed. Astigmatism at the present orientation of the elements (not the same as found on receipt) is undetectable at steady temperatures. With good seeing conditions the instrument should now take high power eyepieces if desired.
6 eyepieces in wooden box have been cleaned and power measured as follows:
- Large Ramsden, f = 2.05" = x75
- Large Huygenian, f = 2.00" = x77
- Huygenian, f = 0.90" = x170
- Huygenian, f = 0.60" = x256
- Huygenian, f = 0.374" = x410
- Huygenian, f = 0.300" = x512
Signed: H Dall
Dall's failure to repair the micrometer eyepiece was not unexpected - members of OASI had found the eyepiece abandoned in a cupboard with a toffee paper in it when the Society first arrived at Orwell Park!
In a letter to OASI committee member Charles Radley, on 20 August 1973, Dall explained why Steavenson had not noticed the over-correction of the OG for spherical aberration:
Dr Steavenson did not notice the spherical aberration over-correction because of two deep zones in a region which masked the error. The only aberrations left now are on a surface I have not worked on, and they are feeble, having no effect on performance. The zonal spherical errors originated in the Merz workshops, and have been there for 100 years or more.
Following the formation of OASI in 1967, the first recorded removal and cleaning of the OG was in circa 1970 by a Mr Wilton. (By 1973, he had moved away from Ipswich.)
The first recorded inspection of the quality of the OG of the Tomline Refractor was in 1936 by Dr D W H Steavenson1 (1894–1975), commissioned by Messrs C Baker, scientific instrument manufacturers of 244 High Holborn, London, on behalf of the Orwell Park Estate office which, at the time, was attempting to dispose of the telescope. (Steavenson's report mentions an earlier investigation of astigmatism of the OG in 1930 but, unfortunately, no record of this survives.) His report is reproduced below:
Report on 10" Merz Objective, Tested 3rd July 1936
The material of the components is of good quality having regard to their age. They are free from chips, scratches, and harmful striae, and are of good polish, apart from some deterioration of the polish on the second surface (near surface of the crown lens) due to long exposure to damp. This slight cloudiness, which cannot be removed by cleaning, has no appreciable effect on performance. I have cleaned all four surfaces.
The three tin-foil spacers were found to be too thin, allowing of central contact of the components. They have been replaced by three spacers of adequate thickness. The cell is satisfactory except for the arrangement of double springs for maintaining centration of the components. This is a harmful system, no longer employed by the best makers, as it performs its function only by keeping the lenses under constant pressure. The springs have therefore been loosened, and the centration maintained by the insertion of paste-board packing.
Results of tests on artificial star
Colour Correction: This is satisfactory, but has been carried further than is usual in more modern objectives. It could be improved by an increase in the separation of the components.
Spherical Correction: This is satisfactory in general, but see under "Zonal Abberation".
Zonal Abberation: This is present in rather a marked degree. There are zones of short focus at radii of 1", 2" (the worst), 3" and 4". Also a central point of short focus.
Astigmatism: When the OG was first tested, as received, this was present in a marked degree, as had been noted by Orwell Park in 1930. On removing the lenses from their cells, they were found to be properly spaced, according to makers' own marks, and I think it almost certain that this astigmatism arose from a permanent distortion of the lenses, produced by long pressure of the spring side-support. Unfortunately one lens (the flint), is much more affected than the other so that complete balancing-out by rotation is impossible. But a considerable improvement has been obtained by rotating the flint through 40°, and new marks (signed W.H.S.) have been made at the edges. The old marks have been labelled but not removed. Some slight astigmatism still remains, but it is not serious enough to impair performance for general purposes.
This is a serviceable old objective, though not one of the highest excellence. It gives excellent results with powers up to 150, and satisfactory with medium powers (up to 300). But it is not capable of the most delicate work that a first class objective of its size should do. The aberrant zones throw too much light into the diffraction rings, and this would affect work on close unequal stars and would destroy contrast in fine planetary details. The glass is therefore unsuited to the more exacting work (research), but would do admirably for general demonstrations and instructional work, in some school or institution, where the extreme limits of performance under high powers were not called for.
Signed: W H Steavenson
||Steavensen was to become president of the Royal Astronomical Society 1957-59.