How to make your own Plössl!
OEP for “Open Eyepiece Project”
For some time now, I’ve been working on an idea: designing an open source 3D printed eyepiece. The initial goal is to make a Plössl eyepiece, a fairly common model appreciated for its simplicity and optical quality. Later, I would like to explore other optical designs like the double Plössl in order to offer a small library of free and reproducible eyepieces.
A design made for DIY
The structure is entirely designed to be 3D printed, allowing everyone to easily make their own eyepiece. On the optics side, I start with lenses available on AliExpress, to prototype quickly at low cost.
Red Henry
In my approach, I was greatly inspired by the work of Red Henry, a passionate amateur based in the United States, who makes his own eyepieces and shares his experiences and advice. There is even a Facebook group dedicated to his work and that of the community: ATM Eyepieces (Red Henry)
The optical diagram
It consists of two symmetrical doublets, each formed of two cemented lenses. These two doublets are placed facing each other, separated by a small space called a spacer.
A tool?
A Plössl eyepiece consists of two symmetrical achromatic doublets. Its effective focal length (F) can be approximated by the general formula for compound optical systems:
- f1 and f2 = focal lengths of the two doublets,
- d = distance separating the two doublets (measured between their principal planes).
Note: this formula is only an approximation. To precisely model a Plössl, you need to use ray tracing methods or optical matrix methods (ABCD method), and not just limit yourself to the two-lens formula.
Following his recommendations and calculations, I developed my own tool to calculate an eyepiece’s focal length, accessible online: My focal length calculator
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Note: this formula is only an approximation. To precisely model a Plössl, you need to use ray tracing methods or optical matrix methods (ABCD method), and not just limit yourself to the two-lens formula.
The first prototypes
The eyepiece assembly diagram:
The different parts:
The files are not yet available on my Printables page.
Good evening!
Raphaël - La 3ème Dimension