Building a New Type Low Cost Refractor Focuser
When planning a focuser for a fast refractor, a long focuser tube allows different eyepieces (perhaps with diagonal and/or Barlow) to come to focus and extends to focus nearby objects for bird or wildlife viewing, while a relatively short focuser tube is needed to avoid cutting into the wide light cone and causing vignetting. The common solution to these obviously opposite requirements is to use a very large diameter focuser tube. Unfortunately, unless you can find something like the discontinued Jaegers 2.7" "Superfocuser" on the used market, the cost of such a focuser plus a similarly large external diagonal is high. These problems are good motivation to try a different ATM approach to the traditional refractor focuser setup.
An optimal focuser might feature (1) long focus travel to accommodate differing eyepieces and image distances, (2) a large mirror or prism with arrangement to minimize of "choke point" vignetting/partial edge-of-field illumination, (3) the ability to fine focus, (4) comfortable right angle viewing, (5) correct (or at least erect) images, (6) ease of construction without expensive machine shop equipment, and (7) low cost parts and materials. My attempt to meet those criteria has resulted in a scope that looks a bit strange compared to a standard refractor, but may have elements useful to other ATMs - hence this post. Like most "new" developments in telescope building, this type of refractor focuser is based on previous ones, but put together in a different way.
The first focus idea is from a 1950's vintage refractor spotting scope that focused by just sliding the chromed focuser tube in a ring of firm felt inside the scope tailpiece. (Idea: Sliding metal on felt) See photo A. I used the sliding idea to coarse focus my first ATM Newtonian back in 1963 by mounting the diagonal and eyepiece on a section cut from extra scope tubing that functioned as a "sled" focuser, sliding on felt pads and constrained by rails cut from narrow tube sections that are screwed to the tube. (Idea: Sliding focuser on main tube.) See photo B. The third idea is from a thread on CN in December of 2010 with posts by "Arnold3" and "Mirzam". They both use a large Newtonian-like diagonal in the bottom of the refractor tube and focus the eyepiece using a reflector type focuser mounted on the side of the tube, near the bottom. (Idea: Diagonal mirror within refractor main tube.) The thread containing their posts of this great idea is here: https://www.cloudyni...to#entry3882048
An article in the March 2020 S&T Magazine by Jerry Oltion features Art Gamble's nice ATM refractor using that same configuration.
Combining and modifying the above ideas results in a new refractor focuser design that uses a big diagonal mirror or prism within the tube, like the Arnold3/Mirzam arrangement, but has that diagonal, along with its attached eyepiece/focuser, able to move longitudinally in the tube. This is similar to my old external-sliding Newtonian sled focuser, but in this case, the sliding occurs internally, with a rigid diagonal-holding "cage" sliding inside the tube and carrying along the eyepiece and fine focuser that protrude out through a wide lengthwise tube slot. The diagonal assembly slides smoothly on attached firm felt strips that bear against the inside of the aluminum scope tube to permit coarse focusing. This design could be called a Longitudinal Slide Diagonal Focuser, or LSDF for short. See Photo C and D for LSDF refractor.
The "different" part of the LSDF is a hollow wooden cylinder that slides within the main scope tube and carries the large 45˚ diagonal mirror or prism and the fine focuser with eyepiece on top. The sliding assembly is built-up from wooden rings, screwed together to form the hollow cylinder of requisite length. The back ring and end disk are spaced apart by 3 large dowels, forming a diagonal holding "cage". The back ring and end disk are cut flat on top to support a focuser board atop the "cage". This board protrudes outside the tube through a wide slot cut all the way to the bottom end of the scope tube. The focuser board holds a short 2" helical focuser that carries the eyepiece. The focuser board also serves, along with the forward rings, to seal the slot from dust and light entry. Adhesive backed firm felt strips applied circumferentially to the cylinder allow the whole assembly with included diagonal mirror (or prism) and eyepiece to slide somewhat snugly fore and aft within the scope tube.
The eyepiece holder can be of any type, but needs only minimal travel, so a short-throw, low profile helical focuser is ideal. Besides having a large mirror, keeping the eyepiece field lens close to the diagonal mirror or prism also makes it easier to attain good edge-of-field illumination with no vignetting, since there's not much draw tube length to cut into the light cone. The bottom of the fine focus eyepiece holder baseboard is screwed to the rings inside the tube, barely overhangs the slot width, and can be gripped to move the whole assembly along the tube for coarse focus.
A flanged wooden end cap disk, fit BEFORE the tube is cut and screwed to the bottom end of the tube, holds the bottom tube walls concentric after the slot cut is made. This is important since the tube walls at the end tend to "splay out" slightly when the slot segment is removed and will be pulled back into alignment by the screws reattaching the end cap. The fact that most of the focuser is made of wood enables "low-tech" amateur construction as well as reducing weight. Since the sliding diagonal assembly moves within the rigid main scope tube, the problem of a "drooping" focuser tube when the eyepiece is at maximum distance from the objective is eliminated.
Captioned photos in the following posts show how the parts are made and go together, hopefully clarifying just how the refractor LSDF works. Although I've now made several refractors of the LSDF design, the one illustrated strives especially for low cost and building in a home-workshop setting. See following photo posts.
- Bill Blunk AKA BillB9430