A 4-1/4 Inch f/4 Newtonian
I built this telescope for photographic use before entering the digital age. Since then it has served primarily as a handheld RFT (Richest Field Telescope) for scanning the Milky Way.
The 5 inch aluminum tube has 1/16 inch wall thickness and is fitted with a turned front end ring for extra stiffness and partial light baffling. The large secondary mirror is mounted with a firm 4-vane spider, and the primary mirror cell has large holes for rapid cooling.
The Crayford-type focuser is built with a very low profile and is fitted with two draw tubes, a short one for photography and a longer one for visual use (pictured below). This eliminates the need to move the primary mirror up and down the tube when switching between uses. The knurled ring on the draw tube is a safety device to prevent it from being accidentally run too far into the telescope tube and hitting the secondary mirror.
First light for this telescope was done on this pipe-and-ball-bearing mount. This mount is fitted with slow motion controls and setting circles but no electric drive (yet). In the photo below you can see details of the adjustable slip clutch and the eyepiece of the through-the-polar-axis alignment scope, the objective lens of which is mounted on the "T" of the declination axis housing.
Visually, the scope's image stands up well to about 150x. The coma inherent in these short-focus Newtonians is rather severe, requiring much image cropping when used photographically, but with a low power eyepiece it is a real pleasure to use in RFT mode. The scope, drawtubes, and a couple of eyepieces and a Barlow fit nicely into a canvas "tanker's toolbag".
- Jim Sapp