Patent Description:
An optical telescope, it will be appreciated, is a combination of two optical systems, such systems cooperating to bring a distant object into view. The systems, namely an eyepiece and an objective, are relatively positionally adjustable via an optical focuser. Optical telescopes use focusers to adjust the position of an eyepiece or imaging sensor to achieve a fine focus of astronomical objects. Several types of focusers are commonly used for astronomy: rack-and-pinion, helical, and Crayford. Most commonly, the focuser is mounted to the telescope objective and the eyepiece, or imaging sensor, is carried by the focuser and moved relative to the objective to effect telescope focus. Focus is achieved when the primary principle focal point of the eyepiece, or imaging sensor, is brought into coincidence with the secondary principle focal point of the objective.

Document <CIT> B1discloses a lens support for a temperature compensating focuser, the lens support including a base and a draw tube that is guided in and out with respect to the base on a set of roller bearings and a shaft which is frictionally coupled to the draw tube, wherein the lens support is further configured to use many of the principles of a Crayford focuser. Document <CIT> discloses a Crayford focuser that is secured to the telescope's objective and includes an eyepiece-carrying drawtube which is selectively moved through the aperture to effect telescope focus. The drawtube substantially matingly conforms to the shape of both the aperture and the upstanding member. The drawtube, and thus the eyepiece, is held by frictional engagement with a radially adjustable control shaft and a pair of spaced roller bearings.

A Crayford focuser according to the invention is defined in the appended claims.

The disclosure will provide details in the following description of preferred embodiments with reference to the following figures wherein:.

Prior art Crayford focusers can be complex and costly to fabricate. Some embodiments of the present invention provide a novel Crayford focuser having unique structures that facilitate construction of the Crayford focuser, thereby reducing manufacturing costs.

A Crayford focuser <NUM>, according to an embodiment of the present invention, is shown in <FIG>. The Crayford focuser <NUM> includes a body member <NUM> having a bottom cover <NUM> and a top cover <NUM>. An aperture extends through the top cover <NUM>, the body member <NUM> and the bottom cover <NUM>. The aperture is dimensioned to receive a drawtube <NUM> that is configured to slide along a longitudinal direction within the aperture. The longitudinal movement of the drawtube <NUM> is accomplished by rotation of a focusing knob <NUM> that causes rotation of a focus adjustment shaft 120a extending laterally through the body member <NUM>. The focus adjustment shaft 120a is held in direct contact with a flat region <NUM> of the drawtube <NUM>. By way of a frictional force between the focus adjustment shaft and the flat region <NUM>, the drawtube <NUM> can be extended from or retracted into the body member <NUM> based on the rotational direction of the focusing knob <NUM>.

An end of the drawtube <NUM> extending outward from the top cover <NUM> is capped by a top ring <NUM>. The top ring <NUM> includes one or more thumb screw <NUM> configured to extend radially inward and impinge on an optical element, e.g., eyepiece, imaging device or optical adapter <NUM>, for example, thus holding the optical element securely within an eyepiece aperture <NUM> of the top ring <NUM>. In some embodiments, the thumb screw <NUM> contacts a compression ring that deforms when the thumb screw <NUM> is screwed into the top ring <NUM>. The compression ring, so deformed, clamps onto the optical element and secures the optical element within the eyepiece aperture <NUM>.

In some embodiments, the Crayford focuser <NUM> also includes a brake bolt <NUM> that can be extended or retracted into the body member <NUM> by way of appropriate rotation of the brake bolt <NUM>. By extending the brake bolt <NUM> into the body member <NUM>, an end of the brake bolt <NUM> applies a normal force onto the flat region <NUM> of the drawtube <NUM>. In an embodiment, the brake bolt <NUM> pushes the drawtube <NUM> away from focus adjustment shaft 120a eliminating any existing friction therebetween. Thus, the shaft is either engaged (generating friction) or loose. Alternatively, the brake bolt <NUM> applies a normal force to the flat region <NUM> sufficient to prevent movement of the drawtube <NUM> either by rotation focusing knob <NUM> and/or by the weight of one or more optical elements (e.g., eyepiece, digital imaging device, filter carousel, etc.) mounted to the drawtube <NUM>. The brake bolt <NUM> can be loosened to allow easy focus adjustment and tightened to prevent unwanted movement of the drawtube <NUM> that can cause a loss of focus when a heavy optical element is mounted to the drawtube <NUM>, for example.

In <FIG>, the Crayford focuser <NUM> includes a mounting adapter <NUM>. The mounting adapter <NUM> shown in <FIG> is dimensioned to join the Crayford focuser <NUM> to a curved surface of a Newtonian optical tube assembly (not shown). Other mounting adapters <NUM> can be coupled to the bottom cover <NUM> configured to join the Crayford focuser <NUM> to other types of telescopes. For example, a mounting adapter <NUM> forming a threaded female end can be attached to the bottom cover <NUM> to join the Crayford focuser <NUM> to a threaded rear cell of a Schmidt-Cassegrain (SCT) optical tube assembly (not shown). In yet another embodiment, the mounting adapter <NUM> can be formed as an extended tube configured for insertion into a visual back found on some SCT optical tube assemblies.

Turning to <FIG>, an exploded view of a Crayford focuser <NUM>, in accordance with an embodiment of the present invention, is shown. The body member <NUM> includes a pair of bearing rails <NUM>. Each bearing rail <NUM> holds one or more bearings <NUM>, such as roller bearings, for example. The bearings <NUM> are oriented to exert a normal force on a surface of the drawtube <NUM> and to allow motion in a longitudinal direction (indicated by line A) of the drawtube <NUM> through the body member <NUM>. The bearing <NUM> can be roller bearings, for example. In other embodiments, the bearing <NUM> can be linear bearings. The bearing rails <NUM> are held in place by rail holding structures <NUM> (shown in <FIG>) formed on an interior of the body member <NUM>, the bottom cover <NUM> secured to a bottom surface of the body member <NUM>, and the top cover <NUM> secured to a top surface of the body member <NUM>. The bottom cover <NUM> and the top cover <NUM> can be secured to the body member <NUM> by way of bolts, welds, industrial adhesives, or other appropriate securing means.

The bottom surface of the body member <NUM> herein is the side of the body member <NUM> that is to attach to a telescope optical tube assembly (not shown). The top surface of the body member <NUM> is the side of the body member <NUM> opposite the telescope optical tube assembly.

As shown in <FIG>, a through hole extends through a central opening of the bottom cover <NUM>, a central channel <NUM> (shown in <FIG>) of the body member <NUM> and a central opening of the top cover <NUM>. The top cover <NUM> and the bottom cover <NUM> can be joined to the body member <NUM> using interference fittings, also known as press or friction fittings. For example, pegs <NUM> can be formed on a surface of the top cover <NUM> facing the body member <NUM>. Each peg <NUM> corresponds to channels <NUM> (shown in <FIG>). Each peg <NUM> is aligned and pressed into the corresponding channel <NUM>. Because the peg <NUM> has a slightly larger diameter than the channel <NUM>, pressing the peg <NUM> into the corresponding channel <NUM> can create a secure interference fitting therebetween. The bottom cover <NUM> can be similarly joined to the body member <NUM>. However, once so joined the top cover <NUM> and the bottom cover <NUM> cannot be easily removed without damage. Thus, in some embodiments the bottom cover <NUM> can be secured using bolts <NUM>, for example, configured to removably screw into the corresponding channels <NUM>.

In other embodiments, industrial adhesives or epoxy can be used to bond the top cover <NUM> and/or bottom cover <NUM> to the body member <NUM>. In other embodiments, the top cover <NUM> and/or bottom cover <NUM> can be secured to the body member <NUM> by way of one or more screws or bolts.

The drawtube <NUM> is inserted into the through hole, such that the outside surface of the drawtube <NUM> is in contact with the bearings <NUM>. Additionally, a PTFE (polytetrafluoroethylene) block <NUM>, is positioned between the inside wall of the body member <NUM> and a focus adjustment shaft <NUM> that is inserted through a bore hole extending laterally (in the direction indicated by line B) through the body member <NUM>. The PTFE block <NUM> can be adjusted to push the focus adjustment shaft <NUM> against a flat region <NUM> of the drawtube <NUM>.

The adjustment of the PTFE block <NUM> can be accomplished by operation of one or more adjustment bolts <NUM>. Increasing contact pressure between the focus adjustment shaft <NUM> and the flat region <NUM> of the drawtube <NUM> can facilitate movement of the drawtube <NUM> when a heavy optical element, such as a digital imaging sensor, for example, is mounted thereon by increasing friction at a point of contact of the focus adjustment shaft <NUM> and the flat region <NUM>. A focus knobs <NUM> can be mated to an end of the focus adjustment shaft <NUM> to allow manual focus adjustment. In other embodiments, an electric motor (not shown), e.g., stepper motor or servo motor, for example, can be coupled to the focus adjustment shaft <NUM> in place of, or in addition to, the focus knob <NUM>. In some embodiments, the PTFE block <NUM> can be replaced with another low friction material.

The drawtube <NUM> can be capped with a top ring <NUM> configured to hold an eyepiece, imaging sensor or eyepiece adapter <NUM> to the drawtube <NUM>. The top ring can include one or more thumb screws for securely holding an eyepiece, or other optical device to the drawtube <NUM>. The top ring <NUM> can be joined to the drawtube <NUM> using interference fittings, also known as press or friction fitting. In other embodiments, industrial adhesives or epoxy can be used to bond the top ring <NUM> to the drawtube <NUM>. In other embodiments, the top ring <NUM> is secured to the drawtube <NUM> by way of one or more screws or bolts. In still other embodiments, the top ring <NUM> can be secured to the drawtube <NUM> using matching threads formed on respective surfaces of the drawtube <NUM> and the top ring <NUM>.

Turning to <FIG>, an isometric view of the body member <NUM> shows internal structures thereof. The unique structures shown in <FIG> facilitates construction of the Crayford focuser <NUM> and can reduce the time and cost for manufacturing Crayford focusers in accordance with the present invention. As shown, the body member <NUM> has a central channel <NUM> extending through the body member <NUM> in a direction corresponding with line A. The perimeter of the central channel <NUM> defining an inner wall of the body member <NUM>. A first bearing holding structure <NUM> extends outward from the inner wall, and a second bearing holding structure <NUM> extends outward from the inner wall. The first bearing holding structure <NUM> and the second bearing holding structure <NUM> are open to the central channel <NUM> such that bay-like regions are formed. The first bearing holding structure <NUM> and the second bearing holding structure <NUM>, in some embodiments, are positioned at a predefined separation along the perimeter of the central channel <NUM>. In embodiments, the separation between first bearing holding structure <NUM> and the second bearing holding structure <NUM> form an angle between <NUM>° and <NUM>°.

The first bearing holding structure <NUM> and the second bearing holding structure <NUM> each have a generally inverted trapezoidal cross-sectional, as can be seen in <FIG>. The shortest side of the trapezoidal cross-section is open to the central channel <NUM>. Each bearing rail <NUM> slides into the trapezoidal cross-section of the respective first bearing holding structure <NUM> and second bearing holding structure <NUM>. In other embodiments, first bearing holding structure <NUM> and the second bearing holding structure <NUM> can have cross-sectional shape other than trapezoidal.

Embodiments of the present invention, using the bearing holding structures <NUM>, <NUM> described herein can eliminate the need for screws to hold the bearing rails <NUM> in place, instead relying on interference fits. The top cover <NUM> and the bottom cover <NUM> (shown in <FIG>), once attached to the body member <NUM>, trap the bearing rails <NUM> within their respective bearing holding structure <NUM>, <NUM>. Additionally, industrial grade adhesives can be used to further secure the bearing rails <NUM> in the bearing holding structures <NUM>, <NUM>. In some embodiments, set or grub screws can be present to adjust a tilt of the bearing rails <NUM>, either from inside or outside.

A through-bore <NUM> extends laterally (in the direction of line B) through a portion of the body member <NUM> opposite the first bearing holding structure <NUM> and the second bearing holding structure <NUM>. A focus adjustment shaft ,<NUM> is inserted into the through bore <NUM> as shown in <FIG> and capped with focus knobs <NUM>. Additionally, a lower recess <NUM> is present below the through-bore <NUM> and configured to receive the PTFE block <NUM> as shown in <FIG>. <FIG> shows the body member <NUM> of <FIG> viewed head on along the direction of line A.

By fabricating a die having the cross-sectional profile of the base member <NUM> shown in <FIG>, the base member <NUM> can be produced using an extrusion process. The base member <NUM> can be constructed from, for example, aluminum, brass, or steel. Other materials, such as metals and plastics, can be used as appropriate.

Alternatively, cast aluminum can be used to fabricate the body member <NUM>. However, casting can be significantly more expensive since the cast tooling cost can exceed, by multiples of four to five, the cost of aluminum profile tooling. A <NUM>-foot-long aluminum profile can be cut to the require size by a metal bandsaw. Any necessary holes can then be added to the cut piece. Once the necessary holes are added the body member <NUM> is ready for final assembly. The application of interference fits, in the form of the bearing holding structures <NUM> and <NUM>, configured to hold the bearing rails <NUM> in place can provide another advantage, as the interference fits allows the bearing rails <NUM> to be sandwiched between the top cover <NUM> and the bottom cover <NUM>. This allows the bearing rails <NUM> to be fixed without the need for another set of fixing screws. Thus, embodiments of the present invention can decrease manufacturing costs of a Crayford focuser <NUM> by an estimate of <NUM>%. Also, the drawtube <NUM> presses against the bearing <NUM> and the focus adjustment shaft <NUM> adding more stability to the system, which relieves any pressure from the rails from moving from the existing position. Tooling costs can also be reduced, thus allowing for the tooling costs to be amortized faster than by traditional cast processes.

Turning to <FIG>, a fabrication process for an embodiment of a Crayford focuser <NUM> in accordance with the present invention will now be described with reference to <FIG>. Initially, the process begins, at block <NUM>, by generating an extrusion die corresponding to a profile of a body member <NUM>, as shown, for example, in <FIG>. It should be noted that the profile die need only be manufactured once, as the die can be used for multiple extrusion runs. With the extrusion die generated, a block of aluminum, or other appropriate material, can be fed through the die by an extruder at block <NUM>. The aluminum profile output can be cut at block <NUM>, to appropriate lengths by a bandsaw, for example. A through bore <NUM> (shown in <FIG>) can be drilled out, block <NUM>, using a drill press, for example. Additional bolt holes can also be drilled out as needed at block <NUM>. Once all holes are drilled out, cosmetic processes can be performed at block <NUM>, such as anodizing, painting, or polishing the body member <NUM>, for example.

Claim 1:
A Crayford focuser (<NUM>), comprising:
a body member (<NUM>) having an inner wall defining a central channel (<NUM>);
a first bearing holding structure (<NUM>) extending outward from the inner wall and being open to the central channel (<NUM>) at one end;
a second bearing holding structure (<NUM>) extending outward from the inner wall and being open to the central channel (<NUM>) at one end; and
first and second bearing rails (<NUM>) configured to be removably held by the first bearing holding structure (<NUM>) and the second bearing holding structure (<NUM>), respectively;
wherein each of the first and second bearing rails (<NUM>) include a bearing (<NUM>) oriented to facilitate movement parallel to a central axis of the body member (<NUM>).