3D Printed Erecting Mirror System based on the invention of Tatsuro Matsumoto
To make the least expensive and most compact version of a binoscope with objectives up to 80mm in diameter you only need to build up one erecting mirror assembly for one of the objectives and the other uses a standard 90 degree, erecting, Amici prism (see photo below).
The 3D file (screen shot shown below) is available on Tinkercad in my public area. Search for the name "Single Budget EMS" under the people name "petertinkerer". This erecting mirror assembly has an optical path length of 105mm, moves the optical path 36mm to the left or right and has 24mm of open aperture and will enable you to make the 70mm and 80mm refractor binoscopes on this website.
Shown below are the parts you need to make two 70mm F5.7 Celestron Travel scopes or two 80mm F5 Meade Adventure scopes into binoscopes. If you were to buy two 70mm Celestron Travel scopes the 20mm eyepieces are part of the standard kit, but you need to buy a 90 degree erecting amici prism (~$15 to $30). If you were to buy two 80mm Meade Adventure scopes you need to buy two Celestron 20mm eyepieces ($20 each) but the erecting 90 degree amici prism comes as part of the standard kit. The $20, 20mm Celestron eyepieces are actually surprisingly good eyepieces with a good field of view of about 52 degrees and are very light (40gms). To make up the erecting mirror assembly you need to buy two, 90 degree mirror diagonals. The least expensive I have found are from Aliexpress for ~$15 each. The strange looking part on the right is a gluing jig that enables you to rotate the two 60 degree diagonals by exactly 70.53 degrees before they set up and become fixed. After making many EMS sets, of all different sizes, many with variable adjustments of the mirrors and IPD , I believe it is best to make a version that is as simple and practical as possible. This is especially true if you want others to build one with only a limited amount of documentation.
The screen shot below shows how you use the gluing jig to rotate the two 60 degree diagonals to exactly 70.53 degrees. Using the side shown makes the EMS assembly shift the optical axis to the right:
Using the other side will give an EMS assembly that shifts the optical axis to the left:
