Adapter to use eyepiece with a 1¼" barrel with any optics with Pentax K-mount

The basis of this adapter is a cheap 2X tele converter lens. These are second-hand often cheaper than extension tubes.

The tele converter lens was dismantled. Everything except the housing and the bajonet mount on the lens end was taken away. The outer diameter of a short aluminium tube was milled to fit exactly in the empty housing. Three screws that held the converter lens hold now the aluminium tube in place. The inner diameter of a second aluminium tube was milled to take eyepieces with 1¼" barrels and the outer diameter of that second tube was milled to fit exactly into the first tube. Only the end of the outer diameter of the second tube was not milled and got the thread for a set screw to hold the eyepieces. Three set srews hold the second tube in the first tube.

This adapter was made to serve several purposes. First, it should be usable for focussing instead of using the viewfinder of a SLR camera. Second, the adapter should take all other eyepieces, for example those with longer focal lengths that enable finding and framing of dimmer objects. Of course all eyepieces had to be homofocal and, most important, the focus of the eyepieces and the SLR camera had to be exactly the same.

The latter was achieved by using the SLR camera and focussing the telescope on a distant target with the help of the camera's focussing aids. Then the camera was exchanged against the adapter. The inner (second) tube holding a 6.3 mm eyepiece was moved in and out until the object was in focus. The focussing knob of the telescope was not touched this time. Now the three brass set screws were locked. The right picture shows the adapter with a 6.3 mm Plössl eyepiece.

This adapter fulfills all my requirements. It is not only usable as a focussing aid but enables me to use any eyepiece with many lenses sporting a Pentax K-mount. My 500 mm tele lens can be used as a rich field telescope now. Luckily the dimensions of the inner aluminium tube are such that even eyepieces with a hybrid 2" and 1¼" barrel can be used.

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