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My 12" Newtonian would roughly equal the best available about... when?

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Our astronomy club has a tentative 'field trip' planned for Lowell observatory, so I started reading up a bit. I read about the 24" refractor at Lowell (circa 1896) and was wondering if anyone had any thoughts on the capability of scopes built "back then" to today's better hobby scopes. I have no idea what type of glass those historic scopes were made of or how well such huge lenses could be manufactured given the technology of the day.

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My 12" Newtonian would roughly equal the best available about... when?

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A number of reflectors were built in the 18th century that could probably outperform a modern 12-inch reflector in most regards. Of those, William Herschel's 18-incher is the most important by far. The list of deep-sky objects that he discovered with it speaks for itself.

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Herschel lived before the invention of the silvered glass mirror, he used Speculum metal mirrors. Speculum reflects 66% of the light at best and tarnishes quickly. His 18 inch scope probably had the light thru-put similar to a modern 12 inch. His discoveries are impressive.

Jon

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Herschel lived before the invention of the silvered glass mirror, he used Speculum metal mirrors. Speculum reflects 66% of the light at best and tarnishes quickly. His 18 inch scope probably had the light thru-put similar to a modern 12 inch.

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William Hershel also did all of his oberservations before the invention of the LIGHT BULB.
He had the darkest skies on Earth, compared to most modern observers.
I have not been to a blue, grey or black zone yet, but the difference between my orange home skies and a green site 40 miles away is tremendous. At that green site I have stumbled on NGC's by accident with a 10', they seem to jump right out at me, probably because I spend so much time trying to tease objects out of the light pollution at home.
With a mediocre 18" at a black site, I'm sure by eye would bleed from pure pleasure.

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It is my understanding that the skies were not all that black, there were still lights, fires and the like. The skies were smoky though since wood and coal were the only forms of heat and power. Add in the English weather, I don't think he had it all that great...

Don't know though, that was before my time.

Herschel's scopes were built before the invention of the Foucault test which is really one of the two things that made reflectors practical. Without bench tests to determine the progress of the mirror, figuring a quality mirror was hit and miss, my guess is that mirrors were not very good by today's standards.

Silvering of the mirrors allowed for the use of glass as a material, it tarnished more quickly but had a much higher reflectivity and recoating was relatively easy. The silvering of mirrors and the Foucault test were both develeped around 1860 and marked the end of the speculum mirrors and the rise of the modern reflector both in amateur astronomy and research. I believe aluminization was developed in the 1930's.

It's difficult to say exactly how effective Herschel's scopes were but if you add it all up, the poor reflectivity of the mirror, the heavy metal mirror, the crude eyepieces (probably 2 element), the lack of optical coatings, whether his 18 inch performed as like a 14-15 inch or a 12 inch, or less, I sure glad I observing today with all the modern conveniences and lightweight portable scopes.

Jon

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