Where
a normal lens for a 35-mm camera has a focal length of 50 mm, a medium
format camera uses an 80-mm focal length lens to equal the field
of view of the naked eye. This means higher resolution photographing
the same field of view. The larger the aperture of a lens, the
higher the limiting magnitude. So at
the same f-number, more stars will be recorded in the same time with a
medium format lens. This results in richer star fields photographing
the
same area of the sky.
Not every telescope is suitable for medium format astrophotography.
Indeed, a large number of telescopes produce a too small image circle.
It is already difficult to create coma-free 35-mm images with fast
Newtonians. Using a Newtonian reflector for medium format
astrophotography would mean a large secondary mirror and a special
designed coma corrector. The Takahashi Epsilon hyperbolic Newtonians
are, as far as I know, the only examples on the market that nearly fit
these requirements, although they still produce a too small image
circle to fully illuminate the film area. So
cancel the Newtonian and bring in the refractor. There are several
apochromatic refractors that are specially designed for medium format
astrophotography. Pentax and Takahashi offer a wide choice, but they
are very expensive compared to other brands. In the 4" category both
offer 4-element refractors corrected for 67 format: the 100 SDUF
II and FSQ-106 respectively. The latter one uses two fluorite elements
and probably has the best color correction. A while ago, Vixen also
offered such a design, called the DED-108, but it went out of
production.
In my quest for a not too expensive apochromatic refractor designed for
medium format photography, I got interested in an other manufacturer
from Japan that goes by the name BORG Oasis Studio. According to
their catalog, BORG is derived from the Japanese words "Boenkyo" and
"Dogu", meaning telescope and equipment. Hutech Astronomical Products
is the worldwide distributor of BORG telescopes and parts. They offer a
4" apochromatic refractor that immediately took my attention. The
2-element ED-objective of this refractor has a focal length of 640 mm
(f/6.4) and is placed on front of a 115-mm diameter tube. On the other
end of the tube a helical focuser with an inner diameter of 89 mm makes
it suitable for medium format astrophotography. Hutech also offers an
0.625x super reducer for this telescopes, turning it into a fast 400-mm
(f/4) astrograph that produces an image circle of a whopping 110 mm.
Making the whole package cheaper than either the Pentax or Takahashi
scopes, my choice was easily made.
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In the image above, I
have rendered the fields of view that are within my reach with my
current equipment. The picture itself equals the field of view of the
Mamiya Sekor-C 80-mm f/2.8 lens and is, of course, made with this lens.
The biggest yellow frame corresponds to the field of view of the Mamiya
Sekor-C 210-mm f/4 lens. Going deeper with a focal length of 400 mm,
equals the image area of the BORG 4" ED refractor operating at f/4,
followed
up by the 640 mm true focal length of this telescope without using the
super reducer.
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