Double refractor

Analysis of Starlight
150 years of Astronomical spectroscopy
Hearnshaw

Page 82, talking about Hermann Carl Vogel and the - newly established
Astrophysical Observatory at Potsdam near Berlin...

"The building was completed in 1879 and was originally to be only a
Solar Observatory. However, the extension of activities to the wider field
of stallar astrophysics was soon approved, and Vogel then became the
director in 1882. Celestial spectroscopy was from the start one of the major
activities. The Principal telescope was a 30cm Schroder refractor, and with
this instrument Vogel was able to continue his spectroscopic work. Two
larger telescopes were however installed during Vogel's directorship. The
first was a double refractor (on a single mount) with twin 32cm photographic
and 24cm visual objectives, which was erected in 1889. Ten years latter
there followed a second larger double refractor with 80cm (photographic) and
50cm (visual) apertures."

What is a "double refractor"?!!? All I can think of is a pair of
refractors set up like binoculars but yet one of the above scopes has three
objectives and the other scope suggests it has two objectives. I have a
picture of the 30cm scope with Vogel's spectrograph but no pictures of these
three objective's double refractors. 1,2,3,4, single, Bi-, Tri-, Quad-,
err.. bitri- err... Arrg I'm so confused!
--
Michael A. Barlow

"Michael Barlow" wrote in message
...
Analysis of Starlight
150 years of Astronomical spectroscopy
Hearnshaw

Page 82, talking about Hermann Carl Vogel and the - newly established
Astrophysical Observatory at Potsdam near Berlin...

"The building was completed in 1879 and was originally to be only a
Solar Observatory. However, the extension of activities to the wider
field
of stallar astrophysics was soon approved, and Vogel then became the
director in 1882. Celestial spectroscopy was from the start one of the
major
activities. The Principal telescope was a 30cm Schroder refractor, and
with
this instrument Vogel was able to continue his spectroscopic work. Two
larger telescopes were however installed during Vogel's directorship. The
first was a double refractor (on a single mount) with twin 32cm
photographic
and 24cm visual objectives, which was erected in 1889. Ten years latter
there followed a second larger double refractor with 80cm (photographic)
and
50cm (visual) apertures."

What is a "double refractor"?!!? All I can think of is a pair of
refractors set up like binoculars but yet one of the above scopes has
three
objectives and the other scope suggests it has two objectives. I have a
picture of the 30cm scope with Vogel's spectrograph but no pictures of
these
three objective's double refractors. 1,2,3,4, single, Bi-, Tri-, Quad-,
err.. bitri- err... Arrg I'm so confused!
--

http://bdaugherty.tripod.com/astronomy/berlin.html

Look about 2/3 of the way down the page.

I'm afraid you were misled by the word "twin". A double refractor is a
method of overcoming the very different focal points of different colors of
light in achromats. The "photographic" objective was optimized for light
where the early photographic plates were most sensitive (in the blue) while
the "visual" objective was optimized for visual use. They were placed on
the same mount so that the visual objective could also be used as a guide
telescope.

In the case of the instrument you mentioned, "twin" really refers to two
telescopes not two 32cm objectives and 1 24cm objective.

Ah, the joys of the English language!

Regards.

Bruce Schupler

"Michael Barlow" wrote in message
...
Analysis of Starlight
150 years of Astronomical spectroscopy
Hearnshaw

Page 82, talking about Hermann Carl Vogel and the - newly established
Astrophysical Observatory at Potsdam near Berlin...

"The building was completed in 1879 and was originally to be only a
Solar Observatory. However, the extension of activities to the wider
field
of stallar astrophysics was soon approved, and Vogel then became the
director in 1882. Celestial spectroscopy was from the start one of the
major
activities. The Principal telescope was a 30cm Schroder refractor, and
with
this instrument Vogel was able to continue his spectroscopic work. Two
larger telescopes were however installed during Vogel's directorship. The
first was a double refractor (on a single mount) with twin 32cm
photographic
and 24cm visual objectives, which was erected in 1889. Ten years latter
there followed a second larger double refractor with 80cm (photographic)
and
50cm (visual) apertures."

What is a "double refractor"?!!? All I can think of is a pair of
refractors set up like binoculars but yet one of the above scopes has
three
objectives and the other scope suggests it has two objectives. I have a
picture of the 30cm scope with Vogel's spectrograph but no pictures of
these
three objective's double refractors. 1,2,3,4, single, Bi-, Tri-, Quad-,
err.. bitri- err... Arrg I'm so confused!
--
Michael A. Barlow

Thanks Mr. Taylor for the link to the picture and more importantly the whole
site. And Thanks Mr. Schupler for the description, all makes sense now:-)
And the "English Language" bit.. There were a bunch of words spelled wrong
in the book, Damn British and Canadians screwed it all up. Color -vs-
Colour? ;-)
--
Michael A. Barlow

"Bruce Schupler" wrote in message
...
I'm afraid you were misled by the word "twin". A double refractor is a
method of overcoming the very different focal points of different colors
of
light in achromats. The "photographic" objective was optimized for light
where the early photographic plates were most sensitive (in the blue)
while
the "visual" objective was optimized for visual use. They were placed on
the same mount so that the visual objective could also be used as a guide
telescope.

In the case of the instrument you mentioned, "twin" really refers to two
telescopes not two 32cm objectives and 1 24cm objective.

Ah, the joys of the English language!

Regards.

Bruce Schupler

"Michael Barlow" wrote in message
...
Analysis of Starlight
150 years of Astronomical spectroscopy
Hearnshaw

Page 82, talking about Hermann Carl Vogel and the - newly established
Astrophysical Observatory at Potsdam near Berlin...

"The building was completed in 1879 and was originally to be only a
Solar Observatory. However, the extension of activities to the wider
field
of stallar astrophysics was soon approved, and Vogel then became the
director in 1882. Celestial spectroscopy was from the start one of the
major
activities. The Principal telescope was a 30cm Schroder refractor, and
with
this instrument Vogel was able to continue his spectroscopic work. Two
larger telescopes were however installed during Vogel's directorship.
The
first was a double refractor (on a single mount) with twin 32cm
photographic
and 24cm visual objectives, which was erected in 1889. Ten years latter
there followed a second larger double refractor with 80cm (photographic)
and
50cm (visual) apertures."

What is a "double refractor"?!!? All I can think of is a pair of
refractors set up like binoculars but yet one of the above scopes has
three
objectives and the other scope suggests it has two objectives. I have a
picture of the 30cm scope with Vogel's spectrograph but no pictures of
these
three objective's double refractors. 1,2,3,4, single, Bi-, Tri-, Quad-,
err.. bitri- err... Arrg I'm so confused!
--
Michael A. Barlow