Review: Orion 25x100 Binoculars (Part 1)

Review: Orion 25x100 Binoculars (Part 1 of 3)

This updated version of Part 1 incorporates a few minor changes from
the version that was posted earlier on saa. Parts 2 and 3 are new.
The measurements provided below are not necessarily accurate; but
they're the best I can provide.

My Orion 25x100 Explorer binoculars arrived well packed. The
attractive, black, metal case with bare metal trim arrived in a
cardboard box that was packed inside another cardboard box. The
binoculars were well protected in the foam lined, metal case.

The binoculars are mostly rubber coated and have individually focusing
eyepieces. All optical surfaces appeared to be very clean. The
exterior and interior of the binoculars appeared to be clean. The
interiors of the tube barrels were blackened and had some obvious
scratch marks in them. I don't believe these interior blemishes cause
any visible effect in the final image when one's looking through the
eyepieces.

A 1/4 20 screw screwed into the mounting socket seemed to fit a bit
looser than I would have expected. I would want to make sure that any
screw used in mounting these binoculars would screw into the mounting
socket most of the full 12mm that's allowed.

The (moveable) mounting plate needs to be moved back, behind the
center of gravity before one can 'bend' the two barrels together for
narrower inter-pupillary distances. When the barrels are close
together the bottom of the mounting plate ends up higher than the
bottoms of the barrels. This *might* result in problems with some
commercial mounts, but can be easily handled by those who prefer to
build their own mounts

The underside of one of the bare metal strips that encircle the
objective barrels had what was most likely a scratch (or series of
lesser scratches) on it. It looks as if someone had tried to polished
a 2 by 12mm area. This is a cosmetic thing that in no way effects
performance.

The thick, soft rubber, objective lens caps were difficult to remove.
Partial vacuums had developed in the gaps between the caps and
objectives. Once the caps were pried enough to allow air to enter
they became much easier to remove. After subsequent use one could
press in the cap centers if it was desirable to re-create the partial
vacuum; or one could simply press each cap (around its perimeter only)
in place for easier removal.

The two, soft rubber, eyepiece caps are connected to each other in a
sort of accordion fashion. They can be easily removed or replaced
regardless of the inter-pupillary adjustment.

Optical coatings produced green or blue-green reflections throughout.
The optics appeared to be "fully multi-coated" -- as advertised.

The minimum inter-pupillary distance (measured between the center of
one exit pupil to the center of the other) was measured as being about
62.75mm. After carving some aluminum away from the hinges (a rather
drastic measure) it was possible to go down to 60mm -- about right for
my eyes. No amount of hinge carving would have reduced this number
any further. With a 60mm IPD the two objective barrels made contact
with one another. The maximum inter-pupillary distance was measured
at 74mm.

The objectives are 100mm in diameter, 87 to 94mm of which is imaged in
the exit pupils (see below). The left exit pupil appeared to be
circular, the right was a fat "cat's pupil" (that is, similar in
appearance to what is produced by the mathematical intersection of two
mostly overlapping disks). On closer inspection the left exit pupil
was "almost" circular. In reality it too had a (not too obvious)
cat's pupil shape.

I measured exit pupils as well as effective aperture on two
perpendicular diameters for each barrel -- once on the longer diameter
and once on the shorter diameter. The center of the exit pupils did
not correspond to the center of the objectives. The left exit pupil
measured 3.77 by 3.88mm. The right exit pupil measured 3.72 by
3.97mm. I don't claim 1/100mm accuracy, but I believe these
measurements to be accurate to within about 1/10mm

The effective apertures, as imaged in the exit pupils, measured
91.25mm by 94mm on the left side and 87mm by 93mm on the right side.

From the above exit pupil and effective aperture measurements I
computed the magnification on the left side to be 24.2x and for the
right side the computation yielded 23.4x. This translates into a
difference of just over 3%; but the actual difference may be less.
(Comparing the true fields of view reveals no discernable difference
between the left and right sides.)

Eye-relief was about 18mm as measured from the 22mm diameter eye
lenses. The exit pupils were located about 3mm beyond the outer edge
of the fully extended rubber eye shields.

With the eye shields folded back it was possible for me to just barely
take in the full field of view (no vignetting) with my eyeglasses on
-- as long as I concentrated my gaze on the field center.

With my eyeglasses off and the rubber eye shields fully extended, the
full field of view could be viewed. Yet, I suspect comfort might be
improved just a little by removing 1 or 2mm from the length of the eye
shields. In other words, it was necessary to press the eye shields
close to my face in order for me to see the full fields of view. I'm
undecided if the minor inconvenience would justify cutting and sanding
the eye shields. I need more experience with these binoculars before
deciding: some (more) minor surgery or not . . . (After more time
passed, I decided to not modify the eye cups.)

True field of view was estimated by observing the Pleiades and noting
a star at the top edge of the field and another at the bottom edge of
the field. The stars were marked on a computer generated printout.
The angular separation of the two stars were later measured via a
piece of astronomical software. The true field of view was determined
to be about 2 degrees 20 minutes, or 2 1/3 degrees -- a little less
than the 2.5 degrees advertised; but this measurement wasn't made with
the binoculars firmly mounted. Instead it was made with the
binoculars resting on a blanket draped over a concrete block, open
air, observatory wall. I believe the above field diameter to be quite
close to the true value, but a new measurement will be made once the
binoculars are mounted. (A later determination made with the
binoculars firmly mounted resulted in precisely the same measurement.)

Minimum focus distance was estimated as about 60 meters with my
eyeglasses on.

I haven't yet built a mount for these binoculars. For a while I was
considering salvaging the Orion 25x100s for parts. I have no doubt
that I could use some of the components, along with parts from other
sources, to build a more efficient (100mm instead of 87 to 94mm
effective aperture) pair of re-modeled, right-angle binoculars.

A note on hand holding 25x100 binoculars: All I've read states that
hand holding is basically impossible. I'm no Arnold Schwarzenegger;
but I was able to hand hold these binoculars, successfully point them,
and view a number of deep sky objects before feeling the need to lay
them down. For example, I picked up the binoculars, looked at Comet
Machholz, M42/43, M78, M45, M1, M31/32/110 (an impressive sight!) and
M97 before setting them down. On the other hand, I've had some
practice using 20x80s hand held even though I usually used them firmly
mounted. A considerable amount of fine detail is lost when large
binoculars are used hand held. It would, in my opinion, be foolish to
purchase such binoculars with the expectation of getting by without a
*solid* tripod and mount.

Sketcher
To sketch is to see.