A Refractor in the Round.
FOLDED REFRACTORS HAVE LONG been popular with telescope makers seeking to combine the fine performance of a long-focus refractor with a smaller package that is easier to use, mount, and transport. Indeed, there are many ways to accomplish the goal of shortening such an instrument, as Ernie Pfannenschmidt's article on the subject demonstrates (S&T: March 2001, page 120). My solution results in a compact instrument that has a convenient eyepiece location and is simple to mount. For reasons the accompanying photographs make obvious, I refer to my 80-millimeter folded "refractor in the round" as the Drumscope.
My Drumscope employs three reflecting elements between the objective and the eyepiece, producing an image that is right-side up but reversed left to right--just like a normal refractor equipped with a star diagonal. Some refractor purists may shudder at the prospect of using three reflective surfaces, assuming that an unacceptable amount of scattered light would result. Bear in mind that most refractors already have one reflective surface (the star diagonal) and that the popular Schmidt-Cassegrain and Maksutov telescopes have three reflective surfaces when used with star diagonals. One doesn't often hear complaints of scatter in these instruments. The modest light losses resulting from the use of these mirrors seem to me to be a small price to pay for the comfort and compactness of a telescope whose only drawback is the expense of obtaining good-quality optical flats of the required size.
It's All Done with Mirrors
Beginning with a circle as the basic pattern for the optical system, I decided that the objective and eyepiece should be at right angles to each other, on the periphery of a cylinder (the drum), as seen in the diagram below. Two flats, located just inside the wall of the cylinder, are positioned along the circumference of a second, smaller circle along with a third flat at the midpoint between objective and eyepiece. The angles of the flats are adjusted so that the converging light cone from the objective is reflected to the midpoint flat, and from there, via the third mirror, to the eyepiece. A scale drawing makes it easy to determine the angles and sizes of the flat mirrors relative to the objective lens and focuser.
To design your own Drumscope, start with the focal length of the objective and choose the size of the drum. A good starting point is a drum whose inside diameter is roughly 1/3 to 1/4 the objective lens's focal length. Next, select the radius of an inner, concentric circle so that your flats will have plenty of room for mounting inside the drum. Once these dimensions are known, it is a simple matter to figure out the final position of the focal plane.
Begin by calculating how much of the objective's focal length will be taken up by bouncing off the three flat mirrors. This amount is 7.6 times the radius you selected for the scope's inner circle. Using my telescope as an example, this equals 33.4 inches (7.6 x 4.4). To this value we add the distance between the center plane of the objective and the inner circle radius (2.7 inches), and the distance between the inside diameter of the drum and the inner circle beneath the focuser (1.7 inches), for a total of 37.8 inches (33.4 + 2.7 + 1.7). Since the objective lens of my telescope has a focal length of 39.4 inches, I know that the focal plane will lie 1.6 inches (39.4 - 37.8) outside the drum.
If I had used a larger drum, I would have had to make the radius of my inner circle smaller; a drum of smaller diameter would have placed the focal plane farther outside the scope. When determining these dimensions, you will probably compute a series of successive approximations until you find an optimal inner-circle radius that works well with your objective's focal length, the size of your drum, and the height of your focuser.
Construction Hints and Details
A drum of the necessary size can be made from a number of materials ranging from a short length of cardboard tubing (Sonotube) to something custom made from sheet metal. A large aluminum pot, a metal wastebasket, or a flower pot might also be suitable--use your imagination. For my Drumscope I had a sheet-metal cylinder made and attached a pair of slightly oversize plywood disks to close off the open sides.
The objective lens for my Drumscope is a commercially available 80-mm (3.1-inch) objective mounted in an aluminum cell, which I affixed to a 28-by-28-inch plate contoured to match the curve of the drum. This ensures that the optical axis of the lens is aimed at the center of the drum. A similar plate holds the focuser 90[degrees] away from the objective along the drum's circumference. Suitable achromatic objectives are available from numerous sources, or one could always cannibalize the lens and focuser of an existing instrument.
One side of the drum is permanently attached and acts as a kind of optical bench on which the three mirror supports are anchored. Each support has adjustments for collimation.
You can align the entire optical system quickly and precisely by inserting a laser collimator into the eyepiece holder and making the beam hit successively the centers of the three mirrors and then a target at the center of a translucent cap placed over the objective. Alternatively, using a standard collimation peep sight in the focuser, adjust mirror 3 so that the reflection of mirror 2 is centered, then adjust mirror 2 so that the reflection of mirror 1 is centered, and finally adjust mirror 1 until the objective is centered. If the mirrors are initially grossly out of alignment, it might take an iteration or two to get all three perfectly aligned.
Standard Newtonian diagonals can be used for the flat mirrors, though less-expensive flats can be bought at swap meets and from surplus-optics outlets. These mirrors should be of the highest quality you can afford. Precision optical flats in diameters up to 8 inches are available from Edmund Industrial Optics (www.edmundoptics.com).
I covered each mirror with a round mask that acts as a glare stop, with size diminishing from a diameter of 75 mm for the mirror nearest to the objective lens to 60 mm for the mirror closest to the focuser. Again, a scale drawing of the optical path will come in handy when you are determining how big these glare stops should be in your telescope.
Mounting the Drumscope
The drum design readily lends itself to an ordinary Dobsonian-type mounting. Although the usual Dobsonian bearing combination of Ebony Star laminate and Teflon pads will work admirably, I took a slightly different approach. My scope is mounted in an aluminum cradle and rides atop two stainless-steel axles, each carrying a pair of 50-mm plastic wheels with 4-mm-thick rubber O-ring tires glued to grooves cut in them. Each shaft-and-wheel assembly rotates freely on ball-bearing supports attached to the walls of the cradle. The rear shaft protrudes through the cradle wall and is equipped with a knob that provides an altitude slow-motion control. An adjustable friction pad bearing against the side of the drum near the altitude knob provides just enough resistance to prevent the drum from moving on its own.
The cradle assembly is attached to an aluminum disk 13 1/2 inches across and 3 mm thick that is equipped with three rubber feet to permit the unit to be placed on a table. A central boss drilled and tapped 1/4-20 also allows the instrument to be supported by a sturdy tripod when used in the field.
A layer of nylon or Teflon is placed between the base of the cradle and the aluminum disk to reduce drag. Fine motion in azimuth is achieved by means of a spring-loaded friction drive that bears against the edge of the disk, controlled by a large knob.
I added a few extra conveniences to the basic design. For example, there is enough room between the cradle and the drum for a small foam-lined drawer for storing a set of eyepieces. In addition, the base and the drum are equipped with metal handles so that they can be carried comfortably. The base also has a rectangular plate with a bubble level and a magnetic compass, and the removable side of the drum supports an altitude scale. These refinements allow me to aim the scope at a given altitude and azimuth position.
The pleasure of having a compact and sturdy refractor with a relatively long focal length has more than justified the construction effort. This Drumscope really does pack the performance of a big telescope into a small package.
EMMANUEL CARREIRA holds doctorates in physics, theology, and philosophy. He is a member of the staff of the Vatican Observatory and the faculty of John Carroll University in Cleveland and Comillas Pontifical University in Madrid. He is also the inventor of the Sky Window binocular support reviewed in the January issue, page 55.
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|Title Annotation:||the Drumscope is compact and easy to mount, but performs like a large telescope|
|Author:||Carreira, Emmanuel M.|
|Publication:||Sky & Telescope|
|Date:||Oct 1, 2002|
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