The following article describes what I think binocular owners
should know before they attempt simple collimation using the prism tilt adjustment
screws. I have been using astronomical equipment including several binoculars
for over 30 years. I now own about a dozen different pieces of equipment,
including 6 pair of binoculars. For the last three years I have pursued learning
what I can and teaching others what I learn.
Cloudy Nights has been my primary conduit for sharing astronomical
information. I am pleased to be a member of the Cloudy Nights community in
that they have encouraged and supported my endeavors to share my knowledge
of astronomy with kids and the astronomical community at large. I encourage
you all to join with Cloudy Nights in its various attempts to reach out to
children and spread some astronomical knowledge. And if you intend to do it
with binoculars, then by all means, read this article and get adjusted. Thank
Clear skies, and if not, Cloudy Nights.
Is Collimation simple?
How many of you have pried loose the rubber armor grip cover
of your Oberwerk (or similar brand) binoculars to gain access to the prism
tilt “collimation” screw. A quick turn of the screws and the alignment
appears much better than it was before. The star images have been brought
together and you consider your binoculars collimated. Well, maybe the images
appear merged, but are they aligned on the centerline, or the optical axis
of the binocular? I am guilty of this simple procedure. So please allow me
to share my lessons learned with you.
Merged and Collimated
I just recently bought a used pair of Oberwerk 20x80’s
on Astromart. The seller confirmed that he had to collimate the binoculars
when he bought them. The binoculars were observed to have the little protective
glue spot still in place over both screws on the left prism housing, but the
screws on the right housing had been accessed. These binoculars may have been
adjusted to merge the images, but they where by no means collimated. Properly
collimated binoculars will show merged star images AND will show nice round
exit pupils. These binoculars had nearly merged images, but the light in the
exit pupil of the right eyepiece was so cut off it looked like a cat’s
eye. I would estimate by the shape of the exit pupil that there was a 30%
to 40% image loss.
My Exit Pupils Look Like Cat’s Eyes
What I want you to do is step back from your newly collimated
binoculars and take a good hard look at the exit pupils. Are they still perfectly
round? Or does one or both of your exit pupils now look something like a cat’s
eye in daytime, sort of elliptical but with points at both ends. Improperly
adjusting the prism tilt “collimation” screws can tilt the axis
of one prism so badly, in relation to the other prism of that eyepiece, that
the end result is a vignetted light path in the eyepiece. This can get to
the point where you may be able to visually see the cut off in the exit pupil
image. If you see this pointed elliptical image, you should start all over.
What Does this Screw Do?
To see what I’m talking about, do the following test in
daylight. Caution! This test will modify the collimation of your binoculars.
But, if they are already out of collimation, you have nothing to lose by proceeding.
Set your binoculars on a table or mount them on a tripod facing out the window
towards bright daylight. This will allow you to easily see the exit pupils.
Now, with the jeweler’s screwdriver set in the prism tilt screw and ready
to turn, back your eyes away from the eyepieces maybe about a foot or so.
Turn the screw back and forth slightly and watch what happens to the shape
of the exit pupil. Not only does the exit pupil move off center in your eyepiece,
but it also changes shape. You will see that over-adjustment of the prism
tilt screw results in an unacceptable change in the shape of the exit pupil
image. Round images indicates the full light path is passing thru both prisms
and out thru the eyepiece. Cat’s eye shaped images indicates the prisms
are tilted out of alignment and you are losing light that is not passing through
the prisms to the eyepiece. If you do not maintain near perfectly round exit
pupil images, you have gone to far!
What Effect Does All of This Have?
Try this. Do you have a circle template handy? Use it to draw
a perfect circle on a piece of paper. Now slid the template over just a little,
say one quarter of the distance across the first circle you’ve drawn.
Draw another circle. You now have drawn two circles that partly overlap. Color
in the two crescent moon sections that do not overlap the other circle. The
area of the central overlap now stands out. You can see you drew a picture
that represents an exit pupil image that is cutoff as if two circles of light
do not completely overlap. This is the same as the image produced when the
light passing thru the two binocular prisms is not completely passed from
one prism thru the other. This is what happens when you tilt the prisms in
How Much Light Is Lost?
For a binocular with a perfect round 5mm exit pupil, if the
image is cutoff to represent a normally 5mm high but now only 4mm wide pointed
ellipse, instead of a perfect 5mm circle, a mathematical computation will
show the resultant area of the image has been reduced by approximately 30%.
That’s the equivalent of taking one side of a 80mm binocular and masking
the objective lens down to 66mm, or for 70mm binoculars, reducing the objective
to 58mm. Remember we are dealing with the area of circles here. A 4mm circle,
only 20% smaller in diameter than a 5mm circle, has only 64% of the area.
For those of you who have already created the cat’s eye
exit pupil, resetting the prism tilt screw to the original position may be
as simple as remembering which way you turned the screws before and turning
them back to where they were. You need to get the exit pupil images back to
round. This will probably result in your binoculars once again being out of
collimation, however you will at least have a fully illuminated optical light
path thru each side of your binocular.
The first, and probably most important thing you need to know
is that proper collimation of binoculars is achieved primarily by adjustment
of the objective lens, not by tilting the prisms. TURNING THE PRISM TILT SCREWS
IS THE LAST RESORT IN COLLIMATING BINOCULARS. But to get down to real life,
many of us are not going to remove the objective lenses from our binoculars
to re-center the object lens with the optical axis. And in many cases, the
binoculars you own may not even allow for removal of the objective lens. For
those of you who will attempt lens removal, for a thorough discussion of this
procedure I refer you to “Choosing, Using and Repairing Binoculars”,
by J.W.Seyfried, University Optics, Inc., 1995. Seyfried is founder and President
of University Optics. This book is available through Edmund Scientifics Online
at www.scientificsonline.com, item # cr30075-24, cost $19.95. Odd, but I did
not see this book offered for sale on UO’s website.
There’s More Than One Screw
The second thing you need to know is there are two prisms in
each eyepiece, and THERE ARE TWO PRISM TILT SCREWS ON THE OUTSIDE OF EACH
EYEPIECE PRISM HOUSING. By this time, you have already found one of the screws,
near the back end of the binoculars, on top of the prism housing, just under
the rubber edge. The other screw is on the prism housing towards the front
very nearly behind the binocular pivot bar. With the binoculars setting on
a table, consider up to be 12 o’clock. Facing as you would if you were
viewing thru the binoculars, for the left eyepiece, if the first screw you
found is located at 11 o’clock near the back of the housing, then the
other screw is exactly 90 degrees or at 2 o’clock and towards the front.
Likewise, for the right eyepiece, if the first screw you found is located
at 1 o’clock near the back then the other screw is exactly 90 degrees
or at 10 o’clock and towards the front.
Do I Need Adjustment?
Back to the world of people who need a small adjustment, (that
would be most of us!). No, no, no. I mean, people whose binoculars need a
small adjustment. If your binocular collimation is out of adjustment by only
the slightest little bit, you could probably get away with adjusting only
one screw on each eyepiece with no drastically noticeable light cutoff. And
if you can achieve merged images without noticeably altering the exit pupil
image, then you can stop and consider yourself lucky. But if your collimation
needs quite a bit of adjustment, the process should involve turning the screws
on one eyepiece a little bit and then turning the screws on the other eyepiece
a little bit. Screws should be turned in baby steps, sort of like collimating
a Schmidt Casegrain Telescope.
Are My Images Merged?
So how do you determine if the images are not merged, indicating
binoculars in need of collimation? If they are off by a lot, it’s easy.
Everything you look at will look screwy. You will not see the same image in
each side of the binoculars. If they are off by a moderate amount, your eyes
may be working hard at pulling the images together, causing undue eyestrain.
If they are off by a little, your eyes will do the work for you of pulling
the images together and you may never notice it.
Let Your Eyes Relax
The easiest way I have found to see the divergence in the images
is to stand back 6” to 12” from tripod mounted binoculars that are
centered and focused on a very bright star. Try to select one that is not
to high in the sky so you don’t need to crane your neck. As you back
away, you need to keep your eyes on the images in the eyepieces. You need
to let your eyes experience that feeling you did purposely when you where
a kid, the one where you were looking at something and then let you eyes go
loose as though you really weren’t looking at anything. What you will
find when you let your eyes “relax” in this manner is that the two
separate images you see before you will start to move together. Keep your
eyes on the images. They will either stop at some point, showing you the divergence
or separation in the images, or they will come completely together. Do this
a few times until you are sure your eyes aren’t working to pull the images
together the last little bit.
OK, So I Need To Collimate My Binoculars.
Once you have determined the need for collimation, you need
to set up to do the collimation. I have found it rather easy to do the collimation
adjustment at night using a bright star. No other object you can view will
give you the pinpoint precision as the in focus image of a bright star. The
same stance is used for the adjustment process, 6” to 12” back away
from the binoculars. Once you acquire the ability to let your eyes relax and
see the split images in the left and right eyepieces, you will easily be able
to watch as you move the images with the adjusting screws.
A Little Bit Here, A Little Bit There.
You should test adjust each of the screws to see which way that
screw moves the image. Move these screws very slowly. I tiny bit of a turn
will move the image a significant amount in the eyepiece. If you try to make
all the adjustment using one screw on one side of the binoculars you will
very quickly distort your exit pupil into the shape of the cat’s eye.
So, I recommend you make a small portion of the corrective adjustment with
each screw on each side of the binocular, a little at a time.
What Kind Of Error Can Be Tolerated?
Convergence is seen when the image in the right eyepiece is
to the left of the image in the left eyepiece. Slight horizontal image convergence
can be tolerated. Divergence is seen when the image in the right eyepiece
is to the right of the image in the left eyepiece. Horizontal image divergence
should not be tolerated. Vertical divergence should not be tolerated. The
eyes have no muscles to accommodate for these two visual errors. For a thorough
discussion of convergence and divergence I once again refer you to Seyfried’s
book noted above.
What Benefits Will Be Gained?
First and foremost, you will see a benefit in the improved images
if you needed a large adjustment to collimate your binoculars. Your eyes will
experience the view thru the binoculars with a lot less work. You will also
note a significant improvement in the light throughput if you successfully
corrected the shape of the exit pupil. I improved my 15x70 binoculars image
to the point that I was able to clearly see a double of 16” and although
not split, correctly identified the position angle in doubles of 13”
and 10”. I improved my 20x80 binocular image shape to the point that
the previously noticeable difference in light throughput between the two eyepieces
became much more closely matched and a double 10” was split.
You Should Know How.
If you own binoculars, you should know how to make minor adjustments
to your binoculars. I view this in the same light as a Schmidt Cassegrain
Telescope or a Newtonian telescope. Anyone who owns a mirror type telescope
must learn the process of collimating the mirror, as these telescopes demand
a regular adjustment. Likewise, you should know how to determine if your binoculars
need adjustment, and if they do, you should know how to proceed.
A Temporary Solution.
I am not advocating collimation of binoculars solely thru the
adjustment of the prism tilt screws. However, I have five pair of binoculars
and two of these seem to need significant adjustment to improve the collimation.
I have not yet discovered how to remove the objective lenses of either of
these two binoculars. In fact one seems to have the objective lenses substantially
sealed in place with a glue-like product. This leaves me with the alternative
of attempting alignment by adjusting the prism tilt screws. I view this above
procedure as a temporary solution to a more involved process that needs to
be fully learned and undertaken to solve the problem completely.
Collimating Binoculars Adendum 11-27-02
Soon after I wrote this article I initiated a discussion on
the Astromart binocular forum relative to collimation. That post ultimately
resulted in an exchange of relevant thoughts and comments between Kevin Busarow
(bigbinoculars.com) and myself. I felt the outcome of that exchange of useful
information was worth reprinting here, since this is the original article
that prompted me to enter into that discussion. Although this reprint is slightly
longer than the final post, (final post was limited to 3000 characters), it
is true to form.
Clear skies, and if not, Cloudy Nights.
Hi Kevin, Thanks again for your response. The number of people
joining the ranks with moderately priced binoculars continues to grow every
day because such products are now made available in the marketplace, and that's
a good thing. People are purchasing binoculars that are adjustable and they
need all the information they can get to understand what they are doing. I
hope this correspondence helps serve that purpose.
I’d like to summarize several of the main points for the
community at large and eliminate wrong assumptions. Since we both agree with
some of what each of us said, I have combined parts of what we both said into
one process flow that I think could be considered a good direction to follow.
Your concurrence would be beneficial. (Kevin did respond positively.)
Collimation is the alignment of all the optical elements along
the binocular optical axis. Originally collimation is set by adjusting the
objective lens and then by making the final critical adjustments with the
prism tilt screws. The most readily available method for the common user to
adjust collimation is by the prism tilt screws. Pause for a second and understand
what is happening when you take this action. Understand the process completely
before you proceed and you will be able to perform this adjustment when it’s
needed.It seems most reasonable to assume that if a binocular arrived in good
collimation and then has been knocked out of alignment, the most likely component
in the optical path that shifted would be the prism as it's the only part
that's not locked into position. If a prism were knocked out of alignment
due to an unfortunate jolt, the user should see not only images that are not
merged, but also the oval appearance of the exit pupil, although possibly
very minor and difficult to see. The oval appearance of the exit pupil would
identify which barrel needs adjustment, but it doesn’t give a clue as
to which prism in that barrel needs adjustment. If the images are not merged
but the exit pupils still appear round, its possible that collimation is only
so slightly off as to need a very minor adjustment. Follow the procedure outlined
trying to maintain round exit pupils.
So a user should attempt to adjust the prisms from the tilted
position back to a true (perpendicular to each other) position using the prism
tilt screws. While adjusting, observe the merging of the images AND observe
the trueness of the exit pupil. It may even help to give a gentle rap with
the knuckles on the binocular body to help "settle" the prism between
the setscrew and spring hold-down, otherwise it often shifts slightly some
time later. If either the images do not merge or the exit pupils do not appear
round the user has failed to collimate the binoculars properly.
Some binoculars provide adjustment screws inside the housing
for the tilting of the prism shelf and do not allow for the tilting of the
front prism in respect to the back prism. This method addresses the fact that
the axis of the prisms must remain true. Any tilting of one prism with respect
to the other prism in a pair effectively reduces the light pass to less than
Once the prism is adjusted properly, the images should appear
merged and the exit pupil should be perfectly round again. If merged images
are achieved and round exit pupils are seen, you have succeeded. If exit pupils
are not round, then it is likely that the prism that shifted is not the one
that was adjusted in the attempt at collimation. The result of two misaligned
prisms is going to be a less-than-round exit pupil. If this occurs, the prism
that was adjusted should be turned back to where it was when you started.
The other prism (on the same side of course) would then be adjusted to attempt
collimation, following the same procedure.
This is not as difficult as it may seem. If the wrong prism
was adjusted, the images can be merged but the shape of the exit pupil will
be less round than before. Setting that prism back to where it was only requires
making a mental note (or a sketch) of where the images were before collimation
was attempted. Turn the screw to reset the prism back to that out-of-collimation
image. Then attempt collimation again, but this time turning the other prism
screw on that same barrel.
So once again, for the majority of collimation problems, adjusting
the collimation screws is the BEST method to achieve perfect collimation AND
round exit pupils. If collimation is still way off and the exit pupils appear
round it may indicate the problem resides within some other component of the
binocular. If this is the case, any further movement to the prisms is causing
a greater misalignment of the optical system. Of course, it's possible that
the binocular was not properly aligned at the factory, and/or may have a flawed
component. In this case, we basically have to start from scratch. But this
is the exception, and not the rule. At this point, most people should probably
send the binocular back for warranty repair or replacement.