Designing a Fibre Coupler 

When designing such a contraption, there are a few key functions it must fulfill. Firstly, the alignment must be extremely precise, as the core diameter of a standard optical fiber is only a few microns wide (for initial tests, I opted for a custom-made 1 mm core multimode fiber). Additionally, the placement of the optical fiber must not obstruct the observer’s view through the eyepiece or camera. This is essential for efficiently measuring specific objects that either move quickly or are difficult to align properly.

To achieve simultaneous light input through both the eyepiece and the fiber, a beam splitter is required. Among the various types of beam splitters, the most cost-effective solution is to use two right-angle prisms cemented together to form a cube. In an ideal case, where the refractive indices of both prisms are identical, the output intensity on each side equals exactly 50% of the input. When light rays strike the diagonal interface of the prisms, half of the light is reflected at a right angle to the input direction, while the other half passes straight through. It’s important to note that this configuration does not affect the divergence of the input beam, as the outer surfaces of the cube are flat.

To focus half of the light and form an image that can be observed, a simple eyepiece is sufficient. The other beam is focused onto the tip of the fiber using a biconvex lens (L1). Based on my tests, chromatic aberration was negligibly small (otherwise, the lens would have to be replaced with an achromatic doublet). Since this is the first prototype, I measured the parameters of my Maksutov–Cassegrain telescope to determine whether adjusting the focus, and thus changing the position of the primary mirror, affects the focal point at the fiber tip. This means that the current design is not yet guaranteed to work with every telescope. For my scenario, the focal point showed almost no visible change in size or position.

To choose the correct size of beam-splitter, I first measured the diameter of the telescopes output beam relative to the distance from the eyepiece slot. By then using some simple trigonometry we can calculate the divergence of the beam. I calculated a full divergence of 2.2° and a starting beam width of 22.5 mm which led to me choosing a 25.4 mm cube beam-splitter for my design.