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home page: http://home.europa.com/~telscope/binotele.htm 251-300
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Binocular List #251: 31 March 2003
============================================================ Subject: Hastings Brashear Binocular
From: "Steve Stayton"
The Hastings-Brashear Roof Prism binocular that was acquired on ebay
recently is really fascinating and was new to binocular historians and
collectors as far as I know until this one turned up. I will be posting a
detailed description and pictures of it as found and disassembled on my
website (under construction as they say) in the future as my time allows.
It is a very early example of a roof prism binocular and the first one that
I know of using this particular type of prism arrangement. Here are some
links to some low resolution pictures:
Hastings-Brashear Binocular
http://www.lightmechanics.com/HB1/HB_Binoc2.jpg
Engraving: http://www.lightmechanics.com/HB1/HB_Engraving1.jpg
Prism Assembly: http://www.lightmechanics.com/HB1/HB_Prism2.jpg
The build and finish of the binocular is consistent with quality
commercial binoculars of the time but it would seem to me that this
binocular is a prototype built to evaluate the usefulness of the Hastings
erecting prism. The binocular is constructed in a form more typical of 19th
century non-prismatic twin telescopes than that of the new Zeiss type prism
binoculars of the day. It may well have been one of a kind or one of a
limited production prototype build. Maybe it was a modified twin telescope
body with the prism and new imaging optics replacing the original erecting
lens system but I don't recognize the particular model as one I have seen
before. If anyone knows of a similar binocular body without prisms please
let us know.
John Brashear (1840-1920) was a famous American telescope maker and
optician known for his high quality of workmanship. For some info on
Brashear see: http://johnbrashear.tripod.com/
The ebay seller told me that the binocular was purchased at a flea
market in Pittsburgh so it is possible that it spent its first 100 or so
years close to its presumed origin at the John A. Brashear Co. works in
Pittsburgh.
It is well known that John A. Brashear Co. built production quantities
of lenses and porro prisms for the Warner and Swasey binoculars starting in
1899. (ref: W.R. Warner, The How and Why of the Porro Prism Field-Glass,
Transactions of the ASME No.926, December 1901) The porro prisms used in
the W&S required much less accurate face angles than the roof prism type of
the Hastings. The level of precision required to fabricate the roof prism
in the Hastings prism would be an obstacle in making the design cost
effective for the commercial market.
The Hastings-Brashear binocular is engraved with a patent date of July
20, 1897. The patent (US Patent 586,708 titled "Erecting Prism" dated July
20, 1897) is awarded to Charles S. Hastings (well known American optical
scientist of Hastings Triplet fame) and no mention is made of Brashear in
the Patent. Charles Hastings was employeed by Brashear to "calculate the
curves of lenses" (as they called optical design back in those days)
starting in 1887.
Hastings 1897 Roof Prism Patent:
www.lightmechanics.com/Patents/Hastings-586708-1.tif
www.lightmechanics.com/Patents/Hastings-586708-2.tif
www.lightmechanics.com/Patents/Hastings-586708-3.tif
www.lightmechanics.com/Patents/Hastings-586708-4.tif
The invention is for an erecting prism of the type commonly known today
as the Abbe-Koenig (or Abbe-Konig) roof prism and was used extensively
since 1905 by the German firm of Hensoldt in many of their binocular
models. The Hastings prism type (aka Abbe-Koenig) is currently used in the
Zeiss Victory series of binoculars. It is not clear to me why Zeiss and
others refer to this prism type as the Abbe-Koenig prism and whether or not
there was a description of this prism in Germany prior to the 1897 US
patent. The "Abbe" refers of course to Ernst Abbe at Zeiss and Albert
Koenig was an optical scientist at Zeiss. If any list member knows more
about the German history of this prism please add to our knowledge. There
is reference to a German Patent by Hensoldt: D.R.P. Nr. 180644, dated April
14,1905 in a recent Zeiss binocular brochure (What You Should Know About
Binoculars, p.4) and in Seeger's Red Book (p114).
Hensoldt 1905 Roof Prism Patent:
Text: www.lightmechanics.com/Patents/DRP_180644_1.pdf
www.lightmechanics.com/Patents/DRP_180644_2.pdf
Drawings: www.lightmechanics.com/Patents/DRP_180644_3.pdf
(You will need Acrobat Reader to view the Hensoldt patents, it can be
downloaded free from www.Adobe.com )
It is of course possible that the Germans were not aware of the Hastings
Patent since it was not used in any known production binoculars until the
1905 Hensoldt or it is possible that Hastings was not aware of some prior
German invention of the prism by Abbe.
There a construction difference between the Hastings roof prism and the
Abbe-Koenig in that the Hastings is built up from 3 prism pieces bonded
together while the Abbe-K type is made from 2 prism pieces. However the
basic optical form of the reflecting surfaces to accomplish the image
erection is the same. This construction difference is sufficient reason to
justify separate Patents on the two very similar prisms.
The Hensoldt form of the prism allows for a more compact prism cluster
with larger objectives and larger FOV for a given prism size by moving the
roof edge in closer to the optical axis of the objective lens. You can't do
that with the Hastings form as the roof edge will start to vignette the
rays from the objective.
A note from Dick Karlson pointed out that some Hensoldt roof prisms used
an airspace instead of a cemented interface. Indeed, the Hensoldt Dialyt
binoculars that I have disassembled use the roof prism as shown in the 1905
Patent except that they use an airspace between the two prism pieces
instead of a cemented interface. This is a further advantage in making the
prism compact because the roof edge can be moved in even closer and still
maintain total internal reflection (TIR) inside the second prism.
The Hensoldt (aka Abbe-Koenig) prism can be used with bonded prism parts
as indeed is shown in the 1905 Patent. For the on-axis rays shown in the
three Patent figures the cement layer is not in the area of the TIR in the
second prism so there is no problem having cement. With increasing field of
view there would be some vignetting at the cement layer. The addition of
the airspace allows for a larger FOV or smaller F-Number objective without
increasing the prism sizes.
The Hastings prism in three pieces must be bonded or it would suffer
large chromatic aberration problems with the airspaced surfaces not being
perpendicular to the optical axis.
The Hensoldt Patent shows how the prism arrangement can be optimized for
the largest objective lens diameter in Fig.3 vs Fig.1. In Fig. 3 the
telescope tube diameter (dashed lines) is the same as in Fig. 1 but the
objective lens is much larger in diameter.
The Hastings prism and the Hensoldt prism can both be used with or
without displacement of the optical axis. This is shown in the Hensoldt
Patent. Fig. 1 and Fig. 3 have no displacement of the optical axis (labeled
'X') whereas Fig. 2 shows a latereal displacement of the 'X' axis. It all
depends on the relative position of the objective lens axis and the prism
faces.
Just to confuse the issue even more for those who have MIL-HDBK-141 (the
best english language review of optical prisms IMO) see Fig. 13.37 Abbe
Prism, Type A and Fig 13.39 Abbe Prism, Type B. The Abbe Prism, Type A is
the Hensoldt configuration and the Type B is a three piece construction but
not the Hastings arrangement of parts.
It's all done with mirrors as they say. All these prisms have the same
basic optical geometry to perform the image erection as shown in the 1897
Hastings Patent, the variations are for improved manufacturing and reduced
size.
More photos and measured specs on the H-B binocular to come later.
Steve Stayton Tucson
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MIL-HDBK-141 is available on line, each chapter as a separate .pdf,
adding up to about 90 megs:
http://www.azmackes.net/astronomy/mil_hdbk_141/
Or you can buy a reprint for $50. from Surplus Shed at
http://www.surplusshed.com/pages/Miscellaneous#M1514
Hastings & Brashear are two of the most important names in modern
optics. This patent has been known for some time, but the prototype model
is a surprise. A roof prism made in Pittsburgh, establishing precedent
over the Abbe-Koenig prism, is quite interesting. --Peter
================================================================== Subject: Resolution
From: "randle dewees"
>>>sharpness is a function of both resolution and contrast .....If the
Zeiss glass is superior in both of these attributes, how can it be poorer
in sharpness?