The optical system of the microscope consist of three main components: (i) an objective (A) which is responsable of magnification, (ii) a biconvex focal lens for focusing the sample at a desired distance, and (iii) the optical tube holding all components of the optical system together. For the implementation of (iii), there are two different approaches; we can use 3D printed components or we can buy Edmund Optics (or other supplier; we are looking in TaoBao) tubes as recommended at the OpenLabTools website. The tube has itself five components. Below we expand on each of these systems with more detail.
To magnify the sample, we used cheap microscope objectives widely available in the Chinese online market and depicted here in gray (A) in the sketch of the optical system. 4x, 10x, 20x and even 40x objective work with the system.
Bi-convex focal lens
To focus the image (coming from the sample and captured by the objective) onto the Raspberry Pi camera sensor, we use a bi-convex focal lens with a of focal length of 50mm and with a diameter of 25mm. We used the focal lens made by Comar Optics 50VB25. This lens has AR coating blocking light except in the 450-900nm range. Blocking deep UV light is useful to protect the camera sensor when using blue light for illumination and excitation of fluorescent proteins. It is interesting however to also test the lens which are not coated. From Comar Optics catalog we can see they have one lens, product number 50VQ25 with this specs and which is a bit cheaper than 50VB25. We also have suppliers in the Chinese market.
An optical tube is needed as backbone of the optical system. We have two options, an expensive and a cheap one.
Metal tube from Edmund Optics (EO) is expensive. It consists of five parts listed above: a DIN/C-mount Adapter (B), EO part #03-627; a C-mount 40 mm Tube (C), EO part #54-631; Lens mount (D), EO part 56-353; C-mount 10 mm Tube (F), part #54-629; Ajustable length Tube (G), part #58-756.
3D printed tube. This is a 3D printed version of the parts in the optical tube, which .stl files can be downloaded from here. This is the cheapest option and saves considerable amount of money as Edmund is not cheap.