APO refractor apeture

(CHASLX200) wrote:

I was using powers from 850 to 1100x to see details on Gannymede with a 15"
Obsession - OMI Torus mirror last year!

William Herschel used a reflector 7 foot, with a 7 inch mirror of
speculum metal. He claimed he could use magnifications up to 6000x
and was disbelieved. He took his scope to Maskelyne, then astronomer
Royal at the RGO and Maskelyne immediately wanted to throw away all
the RGO scopes and pronounced Herschels the best he had ever looked
through and probably the best in the world.

The original papers are in the RAS archives and Herschel certainly
knew his optics. He made his eyepieces by melting glass and using the
near spherical droplets.

Herschel seemed able to use 800x per inch and some to quite good
effect from dank Slough, west of London.

Any ideas how he did it?

On Fri, 24 Sep 2004 13:26:56 +0100, Martin Frey
wrote:


The original papers are in the RAS archives and Herschel certainly
knew his optics. He made his eyepieces by melting glass and using the
near spherical droplets.

AKA ball lenses, which are typically for very high magnicfications
and horribly aberrated for anything more than 10 or 20 arcsec from the

center, even for a slow focal ratio. Due to the single lens and two
air-glass surfaces, it does offer high throughput and low scatter;
especially compared to other eyepieces of the time that did not
benefit from modern AR coatings.

Herschel seemed able to use 800x per inch and some to quite good
effect from dank Slough, west of London.

Better question is WHAT he used 800x/in on. I bet it wasn't for
pulling out detail in the GRS. And I also find it hard to believe
he saw anything at that magnifaction that he couldn't see at a power
10x lower.

Any ideas how he did it?

I see no reason to believe his optics were superior to many fine
examples of scopes today. His speculum mirrors guarenteed
a view not much more than half the illumination of a modern newtonian
of the same aperture and I suspect the scatter from polished metal
would be substantially worse than even the cheapest aluminum coating.
Since most telescopes of the day were probably so bad, one of truely
good quality must have been such a revelation that it could easily
have attained miraculous legendary status. None of this diminishes
his contributions, of course, but I'd like to keep it in
perspective...

DC