The binewt designer

This software was built in order to ease the design of a newtonian binocular telescope. Is is free, open-source software.
click here to download binewt.jar 1.2 (stable version).
click here to download binewt.jar 1.3 beta9 (development version needed for collimation simulation).
You should also save the whole web page since it is the only help available.


This program is written in java. That means you can use it an any platform. The program works the same way with linux, windows, MacOS or anything else. You just need to have a java virtual machine installed on your computer. If you don't have one (or if you don't know) then you can go to and download one. It's free. Once it is installed on your system you can run the binewt designer. If you are lucky you just need to double click on the binewt.jar file you have downloaded. If this does not work, then you can right click on binewt.jar and choose "open with.." then select the java executable that has been installed on your system. If nothing works then run it from the command line (or a dos window for windows users) by typing "java -jar binewt.jar". This may sound complicated, but in most cases, The first try should be a success :-) . In case you have big problems with installetion then send me a mail at eroyer @ (without the blanks of course).

Program documentation

The program should be easy to use as soon as you are familiar with the kind of binocular telescope it is aimed at. The drawings and computations correspond to a binoscope with a rotating secondary cage. You can find an example on the image below (my 360mm binoscope). Other examples of these binoscopes can be found on these web pages : , .

The dimensions that appear in the program are displayed on the figure below :

IPD stands for Inter Pupillary Distance (the distance between eypeiece centers). IPDmed is the average value of the IPD. IPDmin and IPDmax are the smallest and the largest value that should be reachable by rotating the secondary cages. The angle alpha0 corresponds to IPDmed. Once you have chosen both alpha0 and the range of IPD you want to be able to use, the program computes by how much you will have to turn the secondary cages to reach IPDmin or IPDmax. The corresponding alpha angles are displayed in the top view and side view.

All the distances between mirrors start from the optical center of a mirror to the optical center of the next mirror. In the optics tab, you can view how much light reaches the focal plane depending on the off axis distance. Note that it is not possible to reach 100% illumination even at the center of the field because of secondary obstruction. The maximum illumination (which takes secondary obstruction into account) is displayed in cyan.

In the top and side view you can zoom in or out by using the mouse wheel.

A new parameter called "fold-angle" has been added in version 1.2. This parameters allows to study folded configurations where the secondary miror is not at 45 with reference to the optical axis of the primary. This configuration can be useful for large binocular telescopes in order to reduce eyepiece height. I don't know of any folded binocular newtonian telescope being built anywhere on earth. But with binoculars becoming larger and larger (20", 22", and even 24"), there may be some interest in this function. By default "fold-angle" is set at 0: this is the standard configuration. If the telescope is aimed at the zenith and fold-angle=0 then the optical axis is horizontal afer the secondary. Fold-angle refers to the optical axis. If you increase fold-angle by 1 then the optical axis after the secondary miror has an angle of 1 below the horizontal plane (and the secondary miror has been rotated by only 0.5).


Please note that the computation of the curve showing the illumination versus off axis distance is not exact. It should be a good approximation but I recomend that you double check the results before ordering or grinding the mirrors. A better computation will be available in a future version. If you want to know how it is computed, here is a small explanation. The program divides the primary mirror in small squares (10000 squares are currently used). For each square it computes if this part of the mirror is visible from the focal plane. To simplify things I assume the secondary mirror and the tertiary mirror are circular openings in planes perpendicular to the optical axis as shown on the figure below (the light ray shown in red does not pass through the secondary mirror). Thus decentering of the plane mirrors are not taken into account as they should be.

The program is free. I ask no money for it but if you use it, I would be happy to receive an e-mail from you.
Here are 2 snapshots, so that you can see what the program looks like before installing it.

Source code

The source code is now available on source-forge :

Release notes

v1.3 beta9 (27 October 2016)

- bug corrections - more information on the tertiary and eyepiece view

v1.3 beta

collimation simulation

magnified view of the tertiary and eyepiece


- source code released as open-source under the GPL license

- folded configurations

- improved user interface


Never released to the public. New features :

- secondary cage diameter to prevent vignetting for 1"1/4 or 2" eyepieces


first public release