Binoscope

Background

I have been searching for an outstanding pair of medium aperture (4-6") binoculars to cruise the galaxy for several years. Thanks to a brilliant designer named Tatsuro Matsumoto in Japan and a great amateur telescope maker named Joe Castoro in New York I believe my search is finally over.

My ideal pair of binoculars would have:

1. User adjustable collimation, essential at high powers
2. Interchangeable eyepieces, preferably 2" standard instead of the proprietary spotting models
3. Ability to use deep sky filters such as the O-III
4. 90 degree diagonal views, preferably non inverted
5. High optical quality, especially chromatic and spherical correction
6. Individual focusing eyepieces
7. Easy to use interpupillary distance (IPD) adjustment, preferably without changing focus
8. Low cost

Most of the high quality giant commercial binoculars have outstanding views but fall short on conditions 1-5. The Erecting Mirror System (EMS) with two refracting telescopes of the user's choice satisfies conditions 1-6 completely, 7 partially, and fails on 8 (what did you expect :-0)

What about a binoviewer with a larger scope? It is possible to get similar views with a 50% larger instrument at some powers, but consider a 130mm f/6 binocular telescope vs. a 180mm f/7 monocular telescope with binoviewer:

	130mm Binoscope	180mm
		(w/ binoviewer/1.25x corrector)
Lowest Power	20x (40mm eyepiece)	49x (32mm eyepiece)
Maximum Field	2.8 deg	1 deg
Exit Pupil at lowest power	6.6mm	3.7mm
Low-Medium Power	49x
Maximum Field	1.4 deg
Exit Pupil	2.8mm

The binoscope setup provides wider views at low powers due to the availability of 2" eyepieces. Also, at a given low to medium magnification the small exit pupil of the binoscope will provide sharper views because of the aberrations of the observer's eye which increase as the pupil dilates. At high powers the binoviewer setup has an advantage because of the increased resolution of the larger scope.

Another nice feature of the EMS units is high transmission and no added chromatic aberration due to the use of two dielectric mirrors (instead of prisms).

EMS Development

The idea of a binocular erecting two way mirror system is not new, according to some sources it is published in a military optics specification handbook published in 1962. In a November 1982 Sky and Telescope article Tatsuro Matsumoto showed how he had used the idea to make 80mm binoculars. The EMS idea is an elegant evolution of an Amici prism that I will try to detail in a future segment of this review. During the next 19 years Mr. Matusmoto developed manufacturing techniques for these units and refined the system to make it easy to use.

The current EMS system is illustrated at:
http://www.page.sannet.ne.jp/mazmoto/intro-e.htm

Binoscope Helicoid Contracted (left) and Extended (right)

Binoscope Close IPD (left) Far IPD (right)

As you can see the system uses two mirrors on each optical tube to provide an erect 90 degree image. Increasing the separation between the two diagonal mirrors with a Pentax helicoid mechanism varies the interpupillary distance.

Collimation is adjusted with two screws on the side of the right EMS unit. The screws tilt the diagonal closest to the objective lens. One screw shifts the image horizontally and the other vertically. There is a set of screws that can be loosened to rotate the image but this adjustment only need to be made once.

Filter Holder

There is a 48mm filter thread that allows the usual 2" filters to be used at the eyepiece ends of the EMS units. Joe included a set of UV blocking camera filters to protect the EMS mirrors from very long barrels (i.e. Barlows) and for dust protection. The filters are used in the opposite orientation from their usual place in the eyepiece but Lumicon assured me that their filters would work equally well from either side.

The construction of these units is absolutely first rate, they look like they are made by one of the major Japanese telescope manufacturers.

Individual focusing is done with the scope's focusers. I will try a special way of individual focusing later using Wolanski 2"-1.25" adapters.

OTA selection

One of the best aspects of these EMS units is that they let the user choose the optical tube assemblies. You select as much chromatic and spherical correction as your wallet will tolerate. However, there are certain considerations that apply to all OTAs:

1. They must have a lot of backfocus, at least 6" in refractors under 6" in aperture and more in larger instruments. Most OTAs will allow 4" of backfocus to accommodate a 2" diagonal but many OTAs need to be shortened by two or more inches. The backfocus requirement precludes the use of some designs, especially Petzvel models with rear assemblies such as the TV 101 or Tak FSQ.
2. You must pay attention to vignetting considerations in some fast scopes with long 2" drawtubes. In general, if you add the 2" drawtube length plus the EMS length (about 6" in smaller scopes) it should not exceed twice the focal ratio and preferably will be an inch or two less. As an example, if a 4" f/5 scope has a 5" drawtube then adding the EMS unit will give 11" path length…which would cut off this steep light cone and produce aperture vignetting.
3. The OTAs should be of nearly equal focal length.
4. The focusers should be solid. Camera angle adjustment is a nice feature and makes setup a lot easier.

In my opinion 4-6" scopes are optimal size for this setup, such as Televue, WYO/Megrez, Astro-Physics, Borg . Takahashi, TMB, Stellarvue, Aries, etc. Here are some pictures of tandem assemblies:

I bought these 80mm f/6 Megrez to demo the EMS system. They are inexpensive but have excellent mechanics. I would not recommend getting the 80's unless you are using the EMS units for other scopes as well. The views are great but for the money you want more aperture. I got these for fantastic 3-4 degree views.

These 125mm f/6.4 ED Borgs have a lot going for them-low weight (9# per tube), good availability, reasonable color correction. A bit expensive, but not outrageous compared to other 5" OTA's.

If you want to kick butt on the Fujinon 150's you're going to have to get these babies, dual AP 155 EDFS's, the ultimate medium aperture setup, requires no OTA cutting but it is big and heavy.

Well, maybe not quite the ultimate, look at Tatsuro Matsumoto's site for his large 6" binoculars with rotating assembly on massive equatorial mount and multiple other setups. http://www.page.sannet.ne.jp/mazmoto/index-e.htm

For those with aperture fever look at this site which uses the EMS theory on 10" scopes…http://homepage2.nifty.com/bigbino/index-e.htm

Tandem assembly construction and Mounting considerations

Joe Castoro, my friend and noted ATM constructed most of the assemblies pictured above. Joe has won many awards for his work at Stellafane and is a pleasure to do business with. You will find more information at his website www.binoscope.com

Joe imports and sells the EMS units and adapts them to scopes. He cuts tubes, adapts focusers, creates mounting plates with Parallax or other tube rings and adjusts the collimation of the units. The quality of his work is excellent.

In addition, Joe makes large alt-az mounts like the one shown in the AP 155 picture. Other mounts to consider include the Lightspeed X-Wagon and the used Unitron alt-az shown in the 80mm pictures.

Eyepieces and Accessories

The image circle formed by the EMS units is approximately 38mm in diameter, so any eyepiece with a larger field stop will vignette.

I am currently testing some large binoviewer eyepieces such as Orion Optilux 32mm, Pentax 40mm XL, and Meade 32 SWA. Unfortunately many eyepieces which have excellent edge sharpness such as TV 35 Panoptic and TV 31 Nagler are too wide to use in this setup.

Watch out on your accessories! I have two 2" -1.25" adapters, one of which is 1/8" shorter than the other. This doesn't work for obvious reasons.

Viewing

After the mandatory clouds dissipated I had two nights to look through the Megrez 80's in the light polluted skies of Los Angeles. There is a learning curve to these units and I got a headache the first night but they are a lot of fun and a pleasure to use.

The steps are:

1. Load the EMS units into the focusers and adjust until they are parallel.
2. Load eyepieces and adjust IPD distance
3. Adjust focus
4. Collimate using two horizontal knobs until image is fused.

After changing eyepieces you may have to adjust focus and slight collimation but this takes no more than a few seconds. Stars were nice pinpoints...(same from here on)

Stars were nice pinpoints, clusters were beautiful, and even in mag 4 skies M42 stood out much better with binocular vision. Panning with 90 degree diagonals was very comfortable. I tried several eyepieces including 32 Optilux, 19 Panoptics and 8mm Plossls. The 8mm eyepieces were easy to fuse but had such a small eye lens the viewing was not as pleasurable. I went to adjustable Olympus 8.3mm eyepieces from APM and the view improved dramatically due to the large lenses. Jupiter was sinking but was still clear and crisp. One interesting thing I noted was that binoscope use allows you to see the scope-generated purple haze around planets more clearly just as nebulae appear brighter.

I can't wait to take these babies to dark skies, will have much better observing report after RTMC…

Spousal Justification

These units cost a lot of money, somewhere about $2600 for the pair (worth every penny in my opinion). Add the cost of two refractors, a mount and double eyepieces and filters and you have a massive hole in your wallet. However, compare these to the Miyauchi 140's and Fujinon 150's-the ability to have interchangeable eyepieces and filters with 90 degree viewing is something to consider.

My wife is incredibly understanding. However, if you need to justify the units further say that you want to teach your spouse or children about the night sky and that reversing the EMS units allows two person viewing …

Conclusion

I can't make a final conclusion until I get more observing time in, but my initial impression is that these units are absolutely first rate. I would recommend looking at these if you can, I think Joe may bring them to some major Eastern star parties soon.

Check back in a couple months for an update…