EMS Product Line | English

EMS is the Erecting Mirror System invented by me, Tatsuro Matsumoto, Japan.
EMS offers a non-reversed upright image with only two reflections.

There have been a lot of erecting prisms as the attachment of the astronomical telescopes, but EMS is the only one that offers the right image with only two reflections and no edge line in the light path.
Another outstanding advantage is that it is used in the right-angled viewing position and its unique lateral shift of the light path that can be symmetrically set up for binocular use.

It is my pleasure to share my thrilled experience of EMS with Astro-friends.


Direct image of Astronomical Telescope

Reversed image by diagonal prism

Non-reversed upright image by EMS

The Sectional Sample of the Component of EMS

(click to enlarge)
The tapered flange(2″ diameter) on top of the EMS enables the adapters to be replaced by each other in a second with a set screw.
31.7AD is included in the EMS-UM/US, and 50.8AD with 2mm-profile 31.7AD in the EMS-UL.

More than 33years have passed since the article about my homemade binoculars appeared in the November 1982 issue of the Sky & Telescope magazine. Adding the period of my study for the optical system before the article, I have been studying the optical system for more than 36years.
I succeeded in developing the EMS which offers non-reversed upright image at the right-angle viewing about 26 years ago, and have come to accept the order from amateurs.
The EMS has never betrayed users’ expectations, and every one of them admires the performance that never degrades the image quality.


Evolution from aluminum to Silver

from left, EMS-UMA, UMA, and UL

EMS-ULTIMA, equipped with enhanced silver coated mirrors, is now on sale. Now that EMS has achieved not only the diffraction-limited resolution but also the practically total reflection of the incoming light, there will be no reason even for the most conservative users to dispense with EMS in their visual observation.
I believe it declares the advent of the new age of visual Telescopes.

The coating maker guarantees the durability of the new silver coating by passing the tests of high humidity and high temperature, over 90 degrees centigrade with 90 percent humidity, soaking in salt water, etc.

More details

EMS for the standard single type;

Shown below are the prices for the EMS series of the single use, as a diagonal mirror with the non-reversed upright image at right angle viewing.  The binocular set is not a mere set of right and left handed EMS, but a special set that includes X-Y collimation device. For the prices of the binocular set, browse the following lists of “EMS for Binoculars”.

(Those who can make the efficient collimation devices by themselves can choose the left and right handed single-use EMS as the material of a bioscope, as the cheapest choice; but I do not recommend it, because the following specialized binocular set will offer a far better result.)

EMS-UMA 50,000 yen

Eyepiece: 31.7mm
Light path: 122mm
Joint to scope: 50.8mm
The image circle (modest number with much margin): 30mm
lateral shift of the light axis: 26.9mm
Weight(with adapter): 395g

EMS-UMB 51,500 yen

Eyepiece: 31.7mm and 50.8mm
Light path: 136mm
Joint to scope: 50.8mm
48mm dust filter
The image circle (modest number with much margin): 30mm
lateral shift of the light axis: 26.9mm
Weight(with adapter): 470g

EMS-UL 65,000 yen

Eyepiece: 50.8mm and 31.7mm
Light path: 148mm
Joint to scope: 50.8mm
48mm dust filter
The image circle (modest number with much margin):38mm
lateral shift of the light axis: 35.4mm
Weight(with 50.8 sleeve): 500g
42mm or larger field stop of ocular will give no visible trouble. I am using 32mm by 80degrees ocular without any visible vignetting.

EMS for Binoculars

The EMS-SET for binocular use is equipped with the innovative X-Y image shifter on the right unit, that promises you the easiest collimation to merge the two images into one even at the highest magnifications.
So, “Binocular Set” is far from mere “a Right and Left handed pair” of the EMS for single use.

X-Y knobs and the image shift

In planning to build your own Binoscope, your first task is to decide the “D” of your binoscope.
And then, you will have to choose the way of the IPD adjustment among the three choices below.

1: Fix the IPD to your own. (easiest and cheapest plan for the novice)
2: Introduce a sliding mechanism base for IPD adjustment. (DIY or commercial…)
3: Helicoid extension tubes in the EMS. (It will save your effort quite a lot.)

When you decide on the above factors, I will advise you of the best choice of the EMS-SET for your plan.

You can calculate the required back-focus of the OTA to accept the light path of
the EMS by the “D”. How to calculate the approximate light path of the EMS is shown in the linked file.

You will have to choose the OTA of enough back-focus or cut short your OTA to extend the back-focus for your need.
(Back-focus is the tail of the light cone out of the OTA.)


EMS is the ultimate solution to form a binoscope in combination with the refractive telescopes.

EMS offers;

  • Non-reversed upright image;
  • Symmetrical lateral shift of the light axis;
  • Normal stereoscopic vision;
  • Right angled viewing position;

With the least possible number, two reflections by the high quality silver mirrors 3-dimensionally placed in the back-focus of a telescope.

X-Y image shifter

IPD Helicoid Option

There are two methods for adjusting the IPD

1: To slide one of the OTAs.
2: To set Helicoid extension tubes in each of the EMS Units.

Method-1 offers the IPD change with no focus shift, which is a merit. But it requires precision mechanics for the sliding mount, which means not only the weight gain but also the complicated mechanism.

Method-2 has the focus shift while adjusting the IPD, but actually, it is a minor issue, because the change of the user means the diopter readjustment regardless of the IPD change. The great merit of the Method-2 is the elimination of the additional mechanism such as the sliding mount, and the OTAs can be rigidly fixed with each other.

Generally speaking, the IPD-Helicoid is more suited to the larger Binoscope, and the sliding mount (Bino Platform) is more suited to the smaller Binoscope.

IPD can be adjusted with the OTAs perfectly fixed, by using the IPD Helicoid.

Small Helicoid A pair 40,000 yen

Compatible with EMS-UMA, UMB, UL
Stroke: 16mm
Default IPD range: 60 – 78mm
Helicoid light path: 22 – 38mm
Weight: 110g

Large Helicoid A pair 60,000 yen

Compatible with EMS-UXL, UXL Premium
Stroke: 16mm
Default IPD range: 60 – 78mm
Helicoid light path: 27 – 43mm
Weight: 138g

The screw of the right or the left Helicoid is made reversed to the counterpart, which enables the comfortable and ergonomic operation.

EMS for BINO Lineup

EMS-UMA (R+L pair) 120,000 yen

Eyepiece: 31.7mm
Joint to scope: 50.8mm
Equipped X-Y image shifter

EMS-UMB (R+L pair) 123,000 yen

Eyepiece: 31.7mm and 50.8mm
Joint to scope: 50.8mm
48mm dust filter
Equipped X-Y image shifter

EMS-UL (R+L pair) 150,000 yen

Eyepiece: 50.8mm
Joint to scope: 50.8mm
48mm dust filter
Equipped X-Y image shifter

Small Helicoid for IPD adjusting option: 20,000 yen each (A pair 40,000yen)


EMS-UXL (R+L pair) 250,000 yen

Eyepiece: 50.8mm
Joint to scope: 90mmφ
48mm dust filter
Equipped X-Y image shifter
Weight(with 50.8 sleeve): 810g X 2

Equipped Large Helicoid for IPD adjusting

EMS-UXL-Premium (R+L pair) 450,000 yen

(This is one example. I will quote each special order.)

Eyepiece: 50.8mm
Joint to scope: 150mmφ
48mm dust filter
Equipped X-Y image shifter
Equipped Large Helicoid for IPD adjusting
Largest Silver mirror 100X141mm






EMS first mirror size


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