J. Brit. Astron. Assoc., 109, 4, 1999, p.219-220


(Note: The Association is not responsible for individual opinions expressed in articles, reviews, letters or reports of any kind.)

SOHO - some corrections

From Mr D. M. Simpson

I write to correct some of the statements concerning the SOHO spacecraft, reported at the Ordinary Meeting of 1998 November 25 (Journal 109(2), April 1999). I was the SOHO payload module (PLM) thermo-mechanical systems engineer, with responsibility for achieving and retaining the stringent alignment and co-alignment of all SOHO's instruments, and therefore feel close to the issues discussed.

I assume that the statement concerning the inability of the European Space Agency (ESA) to afford the use of 'copious gold plating' for the thermal control of the spacecraft was offered in jest. It is true that SOHO's appearance is relatively drab when compared with other flight hardware, but this is for sound engineering reasons. The gold colour often seen on spacecraft is the metallised finish of kapton. The thermo-optical properties of this material did not meet the SOHO requirements, necessitating the design team at Matra Marconi Space UK Ltd, Portsmouth to use an alternative approach. The Principal Investigators of the instruments carried by SOHO would not have welcomed specular reflection of sunlight into the fields of view of their optics, so multi-layer insulation blankets (MLI) were given a matt black outer layer, this being carbon-loaded, black-painted kapton. This material provides low specularity at optical wavelengths. It was also used because it provided the required solar absorptance to infrared emissivity (a/e) ratio needed to guarantee the required isothermal performance for the PLM. It was essential to provide a stable optical bench for the astronomical instrumentation.

Long-term thermo-elastic stability is required for the SOHO PLM, which had to be at least equal to the best performance of any space structure produced for any spacecraft to date. The PLM structure has to cater for the short-term transients caused by switched electrical loads, the effects of mid-term environmental loading resulting from seasonal effects around the orbit, and long-term changes caused by ageing, resulting in increased power demands, and degradation of materials, which in turn affects thermal loading. The black-painted MLI prevents any significant electrical discharge occurring, as the carbon loading provides a conductive path, and the blankets are grounded to the PLM structure.

No spacecraft surface has had to endure so much exposure to sunlight (equivalent Sun hours = solar intensity x time), and the consequent UV degradation, as has the SOHO sunshield, necessitating the use of rigid optical solar reflectors. This surface was always protected whilst on-ground, and consequently it did not appear on publicity photographs.

The meeting report also stated that '.. the hydroxine fuel froze.' The fuel is actually hydrazine. SOHO was required to survive for 2 years on-station at the L1 Lagrangian point, with a 6-month cruise to reach that position. It was designed to have consumables to last for 6 years, assuming worst-case orbit insertion errors and mid-course correction manoeuvres. The orbit insertion was so precise, and the need for mid correction so minimal that we do have spare fuel, but not for 217 years! The figure is in error by about an order of magnitude.

The temporary loss of SOHO in June 1998 was not due to any fault on the satellite. The spacecraft has now exceeded its design requirements and despite the recent loss of its gyroscopes, the Matra Marconi Space engineers have allowed operations to continue, hopefully through the impending solar maximum period and beyond. The fuel margin will be put to good use.

David M. Simpson
'Westway', High Street, Soberton, Hampshire SO32 3PN.

National Astronomy Week and the Campaign for Dark Skies

From Mr Nigel Henbest

Congratulations to Bob Mizon and all involved in the BAA Campaign for Dark Skies over the past decade (J. Brit. Astron. Assoc., 109(3), 114). I'm somewhat surprised, though, that Bob hasn't mentioned the role of National Astronomy Week 1990 when ascribing credit for the initial 'whistle-blowing'. It wasn't just coincidence that the change in public attitude, as Bob notes, came in 1990. On behalf of the committee members, especially on the Dark Skies 2000 subcommittee, the coordinators and all local society members that took part, I'd appreciate the opportunity to set the record straight.

In 1989, Robin Gorman of the National Astronomy Week committee identified light pollution as the major campaign for the 1990 event. The Week itself, in November 1990 featured over 100 star parties up and down the country, enabling local societies to inform some 50,000 members of the public directly about the problem of light pollution. Perhaps more important, it provided a focus for extensive media coverage. National Astronomy Week's Dark Skies 2000 campaign was highlighted in radio and television programmes and numerous publications, including influential articles in New Scientist (1990 December 8), Sunday Telegraph (1990 August 5), The Independent (1990 October 29), the Daily Telegraph (1990 September 14), the Guardian (1990 November 9) and The Times (1990 November 10).

Nigel Henbest

Telescope limiting magnitudes and the exit pupil

From Mr Richard Plasencia

David Frydman is absolutely correct in his calculations and the basis for them (Journal, 109(2), April 1999). A 7mm exit pupil is valid only for fully dark-adapted young eyes and is also a theoretical maximum which does not in practice encompass the whole population regardless of age. Most old duffers, that is people who can afford serious telescopes, are probably starting to use bifocals and definitely have a hard time opening up to much more than a 5mm or 6mm pupil. Also to be considered is that very few are going to be operating in a completely dark environment; that tends to close up the pupil even further.

But the real problem is that the moment that dark-adapted eye is put to the eyepiece it is no longer totally dark-adapted due to the starlight gathered by that 'serious' telescope. A one millimeter pupil may be extreme. I would go along with something in the range of one to five millimeters as representing a suitable number for calculation purposes.

One issue that should not be glossed over is visual acuity, that physiological variable in individuals often referred to as 'a good eye'. Eliminating medical conditions detrimental to it, acuity of perception is an acquired characteristic devolving from long periods of training the eye, as it were. Most individuals can gain an additional magnitude in this manner. Alas, I know some people who can never master this ability.

I believe with some certainty that the optical engineering formulae do not take into account these physiological considerations. There is good reason I believe, being an engineer myself. A good design formula is predicated on minima to be achieved. If the user can obtain some higher performance that is to their credit. The maker has only to account for maintaining a certifiable quality in the product.

Richard Plasencia
3632 Lynbrook Dr. N.E., Cedar Rapids, IA 52402, USA. [rzp@juno.com]

Early photography

From Mr H. J. P. Arnold, Space Frontiers Limited

May I comment on two statements reportedly made by Bob Marriott in his account of early photography given at the Ordinary Meeting on 1999 January 27 (Journal 109(3), 1999 June). While Sir John Herschel has an honoured place in the early history of photography he did not 'devise the negative/positive process' nor did he 'invent the term "photography"'.

It was William Henry Fox Talbot who from the first realised how the negative photographic image could be reversed to a positive (although he did not use the terms negative and positive at the time of the early experiments). Thus on 28 February 1835 he wrote in one of his notebooks: 'In the Photogenic or Sciagraphic [Greek - skia: a shadow] process, if the paper is transparent, the first drawing may serve as an object, to produce a second drawing, in which the lights and shadows would be reversed.'[1] The world's earliest surviving photographic negative - of a window in one of the galleries at Lacock Abbey in Wiltshire - was taken by Talbot using a small camera obscura in August of that same year.

It is true that from the beginning Herschel strongly advocated the use of the term 'photography' [from the Greek 'phos' - light and 'graphein' - to write] and it was certainly preferable to Talbot's somewhat laboured 'photogenic drawing'. However, while Herschel first used the word 'photographic' in a letter to Talbot dated February 10 1839, a letter of February 2 1839 written to Talbot by Charles Wheatstone has the honour of containing the earliest recorded usage. Herschel's reference to 'photography' in his famous paper delivered to the Royal Society on March 14 1839 was for many years thought to have been the first public appearance of the word, but subsequent research revealed it being used in a review of Talbot's invention of photogenic drawing that appeared in the Berlin Vossische Zeitung of February 25 1839.

Bob Marriott mentions the announcement of Daguerre's process in 1839 but seems to have made no comment about the announcement of Talbot's process in the same year. Louis Daguerre, with his claims being energetically supported by distinguished French national figures like François Arago, and his system being an undoubted subsequent commercial success, has traditionally had a better press than Talbot. I try to redress the balance by making the point that the Daguerreotype was a beautiful and (for a time) commercially successful road to nowhere: Talbot's concept of negative-positive photography was the road to the future - and is still with us.

H. J. P. Arnold
30 Fifth Avenue, Havant, Hampshire PO9 2PL [100411.1740@compuserve.com]
[1] - Arnold H. J. P., William Henry Fox Talbot: Pioneer of Photography and Man of Science, Hutchinson Benham, 1977

Mallard's record

From Mr John Farquharson

With reference to R. A. Marriott's excellent paper, '1927: a British eclipse' in the June Journal (vol. 109(3), p.128), the world steam speed record held by the Gresley A4 Pacific locomotive Mallard should be 126 mph and not 112 as stated.

This record was attained on 3rd July 1938 and the 'wild duck' has never been beaten.

John Farquharson
21 Woodlinn Avenue, Cathcart, Glasgow G44 5TY

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