This telescope is a four-inch f/15 refractor. I built this in the 90’s. It offers many of the advantages of the really expensive fast APO refractors in this aperture but cost a small fraction of such exotic telescopes.
My home built Four Inch Folded Refractor
An easy way to visualize the telescope’s geometry
This diagram shows the symbolic layout of this folded refractor. It’s a simple diagram which allows you to figure out the required mirror or baffle dimensions. The sizes you measure on this diagram should be increased by about 10 percent to allow for placement errors and possible vignetting. Note: the scale on the horizontal axis need not be the same as the scale on the vertical axis. If this bothers you make both the same on your version. The dimensions depend on the backfocus you need for your focuser. The size of the box depends on the dimensions of your mirror holders. Build this from the inside out and put the box around all the components when you have them built. Do a complete diagram of the telescope with the folds included before you begin construction.
The only thing I would do differently on this scope would be to slightly increase the size of the folding mirrors. They have to be big enough to adjust a bit during collimation and that adjustment could shift them a tiny amount causing a trace of vignetting…not a serious problem, but to be avoided if possible.
The final design, closed and open views
In the pictures you can see that I’ve put some baffles in my telescope. These are fairly important in any refractor and they may be more important in this telescope.
Here’s a schematic diagram. Compare this to the above photos.
Lens and second folding mirror
Eyepiece and first folding mirror
The mirrors are mounted in simple adjustable cells much like the primary of a reflector except that they are tilted slightly.
This was a really fun ATM project. And the result is a telescope that costs about half as much as an APO and performs as well or better.