If you don't use binoculars for astronomy you're missing a useful
and joyous tool. Any binoculars will work, but of course some
are better than others.
I've enjoyed a great fascination with binoculars for many years.
During this time I've evaluated many binoculars for both
astronomy and daytime use.
In astronomy, binoculars are primarily used either for finding
objects or viewing them. Each application sets different
priorities on binocular specifications.
Object-Finding:
The most basic use is simply finding objects in the sky. FIELD
OF VIEW is the single most important criterion for this use.
Remember that the amount of sky area visible is proportional to
field of view squared! Binocs with an 8 degree field reveal four
times as much sky as binocs with a 4 degree field. Also, most
finding is done in relation to other stars and constellations so
it's much easier to see the positional relationships with a wide
field of view.
My favorites for a quick-find are my Zeiss 7x45's which have an
exceptionally large 8.6 degree field. But I can also find
objects easily with my Orion Ultraview 10x50's (6.5 degree
field). To me any field smaller than 6.5 degrees is noticeably
less convenient for finding. If you're searching for a small
object (like a small galaxy or globular cluster) it's worth
giving up some field to gain magnification. You might pass over
the object at only 7x.
Wide field binoculars are also a joy for scanning the Milky Way
from a dark viewing site. My favorite regions are Sagittarius,
Cygnus and Cassiopeia.
Object-Viewing:
There are many big beautiful objects in the sky which look better
in binoculars than in any telescope. Such objects include big
comets (Hyakutake and Hale-Bopp), large galaxies (M31 and M33)
and large star fields or clusters (M7, M24, M45).
For object-viewing MAGNIFICATION is the most important single
criterion. But you will need some means of steadying high
magnification binocs. I find that 15x binocs simply "blow away"
any 10x binocs, even my 10x70's. In particular, nebulous objects
(comets, M31, M42 etc.) have much greater contrast and impact at
15x than at 10x.
For some time I've greatly enjoyed my Optolyth 15x63's (3.7
degree field) mounted on my T-pod (a monopod with a foot). Just
recently I've obtained Canon 15x45 image-stabilized binocs.
These are my new favorites for object-viewing due to their
slightly larger (4.5 degree) field and the elimination of the
need for a mount. I find that their smaller aperture (45mm vs
63mm) makes little difference. More on aperture later.
There are also many medium sized objects which I believe look as
good in 15x binoculars as in a telescope at higher magnification.
While they fit easily into a telescopic field, they take on a
more distinctive appearance when viewed with the wider field of
binoculars. Such objects include M16, M18, M44, The Double
Cluster, and perhaps even M42. Of course M42 fits any telescope
but the surrounding rich field of star clusters makes the
binocular view very rewarding. M18 (Eagle Nebula) almost always
shows more nebulosity in binoculars than a telescope. Another
favorite of mine is the combined view of M46 and M47. I've come
to enjoy binocular viewing so much that I left my telescope home
on my last two trips (Hawaii and Curacao), despite my owning a
nice portable refractor.
Viewing objects above about twenty degrees altitude is far better
from a chair or chaise lounge than standing. It's more
comfortable for your neck and you'll enjoy a steadier view. Arm
chairs are best to help brace your elbows.
Binocular astronomy is a also great way to increase your
knowledge of the night sky.
Binocular Specifications:
The four most important binocular specifications are:
Field of View
Magnification
Objective Diameter (and exit pupil)
Eye Relief
I've already discussed the first two, but let me summarize that
for object-finding FIELD is the most important criterion and
you'll be happiest with 6.5 degrees or more. Keep in mind that
manufacturers often overstate their field of view. You can test
it by timing a star's passage. Below are the widest fields
available for various magnifications:
Magnification: Widest Available Field (Degrees):
7x 8.6 (Zeiss 7x42 or 7x45)
10x 6.6 (Leica 10x50)
6.5 (Orion Ultraview 10x50)
15x 4.5 (Canon 15x45)
20x 3.5 (Celestron,Meade,Orion 20x80)
For object-viewing the priority shifts to MAGNIFICATION. For
this I consider 10x a minimum and 15x is MUCH better.
Exit Pupil Diameter:
Specifications interact, for example exit pupil diameter equals
objective diameter divided by magnification. As magnification
increases objective diameter becomes somewhat more important.
Many astronomers recommend 7mm exit pupil diameter however I have
found that as little as 3mm works surprisingly well.
Incidentally, I have photographically measured my own dark
adapted pupils at 6.5mm diameter so my pupils can detect the
difference between binoculars having 7mm vs 3mm exit pupils.
Large (7mm) exit pupils are impressive for what I call their
"instant image" factor. You put them to your eyes and instantly
see the entire image with a minimum of positioning or adjusting.
Keep in mind too, that if the exit pupils are too small (less
than 3mm) your eyes can scan right out of the illuminated zone as
you look to the edge of the field. This occurs with pocket sized
binoculars such as 8x20 even in daylight!
Objective Diameter:
I've found through many comparative tests that objective diameter
is far less important than most astronomers think. I've made
extensive comparisons of my Orion 10x50's and Fujinon 10x70's on
a wide variety of objects. I see little or no difference between
them on nebulous objects such as comets, M42 or M31 and no
difference on bright star clusters such as M44 or M45. It's only
on dim stars or clusters that the larger objectives are better.
But even here the difference is small.
I've also made numerous comparison between my 15x45 Canon and
15x63 Optolyth. These comparison yielded very similar results.
In both the 10x and the 15x comparisons, the smaller binocs
gathered only half as many photons as their larger brothers. Yet
their images were very nearly equal. I believe that the reason
for this surprising result is that the eye quickly adjusts to
light level. With smaller objectives the eyes simply increase
their sensitivity, after only a few seconds, to compensate for
the reduced brightness.
Prior to these tests, I used Canadian Roy Bishop's binocular
index of "magnification times objective diameter". Now, after my
tests, I find that a more accurate index is "magnification times
the square root of objective diameter". And even this square
root factor may give slightly too much credit to objective
diameter.
I list below my "Astro Index" (magnification time square root of
objective diameter) for some binoculars. I find this rating to
be proportional to the amount of detail binoculars reveal in star
clusters and nebulae.
Binocular: Astro Index:
7x45 47
7x50 49
8x40 51
9x63 71
10x40 63
10x50 71
10x70 84
15x45 101*
15x63 119*
20x80 179
* The 15x binocs tested out closer to each other than these
numbers imply.
Eye Relief:
I wear glasses with three diopter astigmatic correction, so to me
long eye relief is absolutely essential. But even if you don't
wear corrective glasses, you may still enjoy long eye relief
while using binoculars in daylight with sunglasses.
My test for eye relief is simply to look through the binocs and
see if the entire field of view is visible. If I can't easily
see the full field I won't even consider buying them.
In terms of numbers, the minimum amount of acceptable eye relief
for use with glasses ranges from about 15 to 20mm. Many
binoculars are grossly overrated. For example Celestron 20x80's
are rated at 18mm but they are actually about 6mm.
You can easily measure eye relief as follows. Point the
binoculars at some light (a daylit window or a lamp is fine) and
hold a piece of white paper near one eyepiece. Move the paper in
and out until the disc of light is sharply defined. The distance
from the paper to the eyecup is the eye relief.
Roof Prisms or Porro Prisms:
Many people have written that porro prism are capable of sharper
images than roof prisms but I've found no correlation between
sharpness and prism type. However I can hold the narrower roof
prism binoculars steadier than porros. That's probably because
my two hands are closer and thus impart less twist to the
binoculars. I also prefer roof prism binoculars when viewing
nearby targets such as birds. The wider spacing of porro prism
binoculars tends to show double images for nearby objects.
Center or Individual Focus:
Despite the fact that all astronomical objects are at "infinity"
it's often necessary to adjust focus as you look at objects of
varying altitude. Center focus is a definite plus.
Coatings:
Zeiss and Fujinon each claim that their coatings yield the
highest light transmission. That might be so, but transmission
differences between these and other makes are only a few percent
and usually not detectable.
I do find significantly visible differences in contrast when I
compare binoculars in daylight. To me that's quite important.
(Coatings are just one of several factors which determine
contrast).
Weight:
I've found holding binoculars steady to be more difficult at
night (when my dark adapted eye exhibits long persistence) than
in daylight. I can easily hold 15x steady in daylight, but I
need a mount (or image stabilization) at night.
Heavier binoculars (40-50 oz) are more tiring on the arms but
steadier than lighter binocs (20-35 oz). For birding (which
often involves carrying the binocs a substantial distance) light
weight, preferably less than 35 ounces is greatly appreciated.
Buying Binoculars:
First decide what type of use is most important. (Finding,
viewing, or compromise). Then visit a store that has a large
selection. I've found only three dealers which have a wide
selection on hand. They are Orion (Cupertino and San Francisco)
and "Out of This World" (Mendocino).
As already mentioned, I first check eye relief. Next I compare
contrast and sharpness on distant objects. I find that contrast
varies more between brands than sharpness. And of course I
always want the best available field of view. Why settle for 5
degrees in a 10x binoc when you can have 6.5 degrees and see 69%
greater sky area? I also pay attention to ergonomics. At high
power it's easiest to hold binoculars steady when you grip them
out towards the objective end. But some binocs place the focus
knob too far back to reach with this grip.
Specific Binoculars I've Owned Or Extensively Tested:
Zeiss 7x45. 8.64 degree field, excellent eye relief, 41 ounces.
I bought these for their very wide field. They are excellent
binoculars but their magnification is too low for most object-
viewing. I also find them heavy for birding. As I mentioned
earlier, these are my favorite "finder binocs" and great for
scans of the Milky Way. Their wide field made them my first
choice when Hale-Bopp's tail was too long for any optical aid. I
can see all of the constellation Corvus in these binocs. Their
6.4mm exit pupils deliver "instant image".
Zeiss 7x42. 8.6 degree field, excellent eye relief, 28 oz.
Because of their much lighter weight, I sometimes wish I had
purchased these instead of the 7x45's above. I've only tested
them in daylight. But they seem optically equal to the 7x45
despite claims that the 7x45's have more advanced design and
larger prisms.
Orion Ultraview 8x42. 8.2 degree field, excellent eye relief, 27
oz. Excellent finder binocs. They are 95% as good as the 7x
Zeiss for about 15% of the price.
Leitz 8x40. Out of production. 7.3 degree field, excellent eye
relief, incredibly light for their size (21 oz). Ok for finding,
poor for serious astronomical viewing. Their light weight often
makes them my choice for birding.
Leica 8x50. Twice as heavier as the 8x40 above. This weight and
the 6.5 degree field is same as Leica 10x50, I'd go for the 10x.
Celestron 9x63. 5.4 degree field, excellent eye relief, 35 oz.
I compared them extensively to the Orion 10x50's. There was
little or no difference in sharpness but I prefer Orion's wider
field. I did not like Celestron's small eyecup diameter. The
eyecups are so small that when I hold them against my glasses I
can see around the outside of them which is distracting.
Orion 10x50 Ultraview. 6.5 degree field (about as good as it
gets with 10x binocs), great eye relief, very sharp, light at 32
oz (perhaps a bit too light for astronomy). Surely a "best buy".
I use them often, day and night.
Zeiss 10x56. 6.3 degree field, excellent eye relief, 50 oz (too
heavy). Sharp (but not visibly better optically than my Orions),
no tripod socket, astronomically expensive.
Leica 10x50. 6.6 degree field, excellent eye relief, 41 oz.
Other Zeiss comments (above) apply.
Swarovski 10x50, 6.4 degrees, 41 oz. Comments same as Leica and
Zeiss. I found the focusser to be too far back.
Swarovski 10x42. 6.3 degree field, excellent eye relief, 31 oz.
Not as sharp as Orion 10x50's.
Fujinon 10x70's. 5.3 degree field, great 22mm eye relief, very
heavy (76 oz), inconvenient individual focussing. The sharpest
binoculars I've ever used. Stars show as beautiful jewel-like
points. 7mm exit pupils deliver "instant image".
Optolyth 15x63's. 3.7 degree field, barely enough eye relief for
use with glasses, 45 oz (light enough to hand hold), very sharp.
These were my most-used astronomy binocs until I got the Canons
below.
Canon 15x45 IS (Electronically Image Stabilized). 4.5 degree
field, barely enough eye relief, 38 oz with batteries, very sharp
optics. I only wish they had larger objective diameter but as
I've said, my tests revealed objective diameter to be less
important than most astronomers think.
Nikon Zoom 8 to 16 x 40. 5.2 degree field at 8x.
I've often read that zoom binoculars are not as sharp as fixed
power. That's probably generally true, but these Nikons are just
as sharp as any fixed power binocs. They also have good eye
relief over their entire zoom range. The one problem with zoom
eyepieces (on binoculars and telescopes) is their limited field
of view when compared to fixed power binocs. I can hand hold
these at 16x in daylight and about 12x at night (or 14x if I
brace my elbows on an armchair).
Meade 20x80. (No longer in production). 3.5 degree field, 6mm
eye relief (too little for use with glasses), very good optics.
Recommended if you don't wear glasses.
Celestron 20x80. 3.5 degree field, eye relief is advertised as
18mm but is actually 6mm, 74 oz, optics seemed to be slightly
below Meade.
Miyauchi 20x100. 2.5 degree field, amazing 27mm eye relief,
"Battleship style" 10 pounds, for tripod use only. Their 45
degree angled eyepieces are handy for astro use. The narrow
field makes a separate finder desirable. Excellent image
quality. I used these for several years, but eventually found
that I more often preferred a small refractor which offered equal
field of view, lighter weight, and the option of higher
magnification.
All-Around Astronomy Binocs:
If I had to choose only one set of binoculars, it would probably
be my Orion 10x50's. They work for both finding and viewing,
though 10x is a compromise for either use.