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Horace Dall (1901-1986): an optical genius
by Martin Mobberley
Well before the webcam era (and before the birth of Damian Peach!), Horace Edward Stafford Dall was the King of UK planetary photography; from the 1940s to
he dominated both lunar and planetary work, in an era where the visual observers and sketchers ruled the amateur world. After he died,
Terry Platt (founder of Starlight Xpress) became the UK's top planetary imager. After him, from the late 1990s, Damian Peach took over that
role. But Horace was just as famous as an innovative optician, mirror-maker and lens grinder.
Back in February 1984 I received an offer I could not refuse: an invitation to visit the optical genius and planetary photographer, at his
home in Luton. I had met Horace at a BAA meeting in Norwich in 1973, some eleven years earlier when, as a teenager, I was grinding and
polishing my own 22 cm mirror. He gave me some sage advice and I exchanged many letters with him from then on, and up to his death in 1986.
A correspondence period of 13 years.
In 1983 I had become the BAA's lunar section photographic co-ordinator and, 2 years earlier, I had won a BAA prize for the best
lunar photograph at the 1981 Exhibition meeting. So, I was starting to make a name for myself in UK astrophotography and
Horace had spotted this, which is why he wanted me to visit his home. He seemed immortal at that time, but died only 2 years later.
I was so glad I found the time to visit him when I did.
It was certainly an honour to be invited to the great Horace Dall’s home. Looking back on that visit, there are only two other
amateur's homes where the memory of a visit have stuck with me, so vividly, for such a long period. Firstly, George Alcock's home at
Peterborough (George knew Horace well) which I visited in 1991 and, secondly, Patrick Moore's famous home 'Farthings' at Selsey, which I
have visited many times. While George knew the sky better than anyone else and Patrick knows how to write books better than anyone else,
Horace knew more about optics than anyone else. Plus, with him, it was not just theoretical bullshit like so many armchair 'experts' spout
on astro forums today, he built telescopes too, and cleaned, restored, ground and polished many mirrors and lenses as a part-time
hobby/business. He had little interest in verbal debate at all. He just continually proved he was an expert in optics and solar
system photography. He was one of the first amateurs to monitor the Sun in H-alpha too, decades before Coronado's PST.
On the famous solar eclipse 'Monte Umbe' cruise of 1973 various other travellers had brought incredible gadgets along
to enable photography from the deck of a ship. Horace could have made any gadget he liked, but he decided that balancing
his camera on his nose, while lying flat on the deck, was the best solution. His eclipse photographs were not bettered by
anyone else on that trip!
The Early Years
Horace Dall was born in 1901 and his father had worked for the inventor of radio, Marconi, as an instrument maker. In his early twenties
(1925) Horace joined the British Astronomical Association and started making his own telescope mirrors. For the next sixty years his
passion for optics, astronomy and inventing would never falter. In 1929 Horace invented a Cassegrain reflector in which the parabolic
primary mirror was under-corrected (elliptical) so that the secondary mirror could be spherical (instead of hyperbolic). This drastically
reduced the difficulty of making Cassegrain telescopes. A few years later Alan Kirkham, in the USA, experimented with this design too. The
resultant Cassegrains became known as Dall-Kirkhams and the design is now used throughout the world by professionals and amateurs alike.
Commercially, the best examples are the Takahashi Mewlon series of instruments.
In 1934 Dall had a custom house built on the highest hill in Luton, at 166 Stockingstone Road. The house was made especially tall because
of Horace’ camera obscura in the roof above his attic workshop. A camera obscura is simply a refractor whose light is reflected and
projected down onto a working surface below. Thus images are viewed by examining a projected image in a darkened room. Of course, this is
ideal for solar observing, but was good for spying down on the inhabitants of Luton too! During my visit to Horace, in 1984, we watched a
cricket match half a mile away on his specially smooth, white table onto which the image of the town could be projected and enlarged:
remarkable! A similar system was recently built into the Obscura Café in the centre of Bournemouth.
In 1937 Horace completed a 15.5 inch (39 centimetre) Dall-Kirkham Cassegrain which he used primarily for lunar and planetary photography. This was a
monstrous aperture for those times: like having a 60 centimetre Cassegrain today. The telescope was completely tubeless and featured a thin
mirror and transfer lens (like a built in Barlow lens) which could project planetary images to f/200 to overcome film grain.The 15.5 inch Dall-
Kirkham was mounted on the equatorial head of a former telescope he had acquired: an 8.5 inch (22cm)Newtonian made by the East
Anglian telescope maker Calver. Where the German equatorial mount's declination axis had been he fitted a fork, but, as the tines were
not wide enough to fit a 15.5 inch tube in-between he simply mounted a tripod-like/wigwam 'truss' structure to support the weight of the
mirror and the secondary. As he only wanted to look at the moon and planets near the meridian, with the telescope, such a flimsy-looking
structure was perfectly adequate: he had no interest in Deep Sky objects. The 15.5 inch/39cm mirror was very light, but very thin too, however Horace'
mirror cell was specially designed to prevent any flexure and preserve a good mirror shape. Because Horace hated trying to work out
where the edge of the eyepiece field was, he ground a tertiary mirror surface in the dead centre of the DK's secondary mirror.
This tertiary mirror reflected and focused the light from a bulb near the primary mirror cell, such that by the flick of a switch, the
eyepiece field was comfortably illuminated. Brilliant!
Lunar & Planetary God!
For over forty years, no-one in the UK could take better lunar and planetary pictures than Horace. He even experimented with stacking film
negatives and photographic unsharp masking: he was decades ahead of his time! Horace hated the way telescopes inverted the image, despite
is up" being the norm amongst amateurs of his generation. Hence his Dall-Kirkham transfer lens system produced an erect image
with north at the top.
The unusual Jupiter photograph in the list at the end of this article was taken by Horace on 1956 April 21st, with his 39cm Cassegrain at
f/120 and it shows a very rare triple shadow transit of Io, Ganymede and Callisto on the Jovian disk. This is from a print given to the
author by Dall. Amazingly, Horace did not know in advance that the event was taking place! Quite an admission.
I recall a story George Alcock told me, of a day in the 1970s, when George Alcock and the famous comet (and supernova) discoverer
Jack Bennet were both in Dall's upstairs attic at Luton, drooling over a 7 inch aperture wide field refractor that Dall had made
for comet sweeping. What a picture that would have made: Dall, Alcock and Bennett in Dall's workshop.
Dall's house was a 2 story building on a high hill but he had it built as high as possible for the best views over Luton. So, with its high ceilings, it was as high as a 3 storey house. His study was on the top floor, overlooking his dome. However, when you asked him where his
optical workshop was, you would see a twinkle in his eye, and, from a seated position, he would grab a long pole, with a hook on
the end, and yank a small ring in the ceiling. Magically, a spring loaded set of stairs would descend from the ceiling, and the
sprightly octogenarian would glide up them beckoning you to follow. Suddenly you were in wonderland, with telescopes, lenses,
eyepieces, and tubes everywhere. Looking up into the roof timbers you would spot his camera obscura lens where a chimney would normally be. Within seconds it
would be projecting an image of the Sun, or a distant landmark, onto the large, white circular table in the attic. He pointed the instrument using long poles that could move the lens/mirror arrangement in the false chimney in azimuth or altitude. It was
like a trip to Prof. Albus Dumbledore's study in that attic!!! Magical stuff everywhere. In the attic you were incredibly high above the ground, like being in a 4 storey building. Horace had a high quality double glazed window built into the attic wall to give him great views visually, and with a telescope, over the town. He could even focus a telescope on the window of the office where he had worked for decades, a mile away. He mainly designed industrial flow meters in his working life and built or restored telescopes in his spare time. The income from the latter enabled him to travel far and wide.
Horace and both his wives (his first wife died in 1964) were also avid explorers. In 1933 Horace became the first man to cross Iceland’s
central region on a bicycle! He met his second wife, Helena, in the Patagonian jungle! As well as designing optics he made folding pocket
telescopes, microscopes, ultra-sensitive barometers and devices that could scratch letters only a few wavelengths of light in height! When I say his barometers were ultra sensitive I mean it. They could measure the difference in atmospheric pressure over 10 centimetres in height! Using optical oil and home made microscopes he could resolve features smaller than the wavelength of light. When he took pictures using UV photography with his microscopes their resolution started to encroach into scanning electron microscope territory.
Horace was a small and very quiet man. He turned down all suggestions that he should be the BAA President simply
because he hated speaking in public, or at the Council table. He was a man of zero bullshit and 100% science and
engineering. A man of action, not words.
Horace also inspired the optician Jim Hysom, who, with his brother Rob (the engineering half - see my telescopes page)
formed Astronomical Equipment limited and made many fine British-made telescopes from the 1960s to the 1990s.
What was Horace like to meet? The best comparison I can make is to look at the earliest Dr Who programs where William Hartnell played the
Doctor. I would not have been surprised if Horace had a Tardis in his attic workshop too!
Pictures of Horace Dall
All pictures by Martin Mobberley, except the Jupiter and Saturn pics by Horace himself.