* -- Gravity Assisted Stabilization
Here's a thought and prototype I'd like to share on a modification to focuser designs that I think may have merit. The concept could be used on both helical Crayford designs (which have the axes of the rollers only slightly tipped to the focuser axis) or even as a variant of the more traditional Crayford (which have all roller axes perpendicular to the focuser axis). It can be slid to close focus, then rotated for fine focus. The thought for this struck me immediately after reading this thread in which Pierre Lemay shared a link to Kineoptic Helical focusers and showed his similar handmade ones on his large ball scope.
The main design concept is to have a pair of widely spaced rollers (and a provision to apply gentle pressure against them) in two different sections as with traditional focusers, but to have the rollers in one plane reversed 180o to allow gravity to always work (in varying degrees to each roller) to increase the pressure of the drawtube against ALL the focuser rollers for ALL telescope altitude angles. The design intent is to use this on any alt-azimuth arrangement where heavy loads of big eyepieces, Paracorr, cameras, etcetera create large moments that make collimation maintenance difficult.
The prototype I'm about to show was built to fit on my folding hexagonal 10" dob which was originally designed with a 3" tall fully racked-in 1 1/4" focuser. As a result, it is taller than most 2" focusers and has a full 2" range of adjustment. Despite its heavy-looking design, it weighs in at just under 10 ounces, with the 1/8" wall thickness aluminum drawtube being more than half of the total. Shorter versions would easily be in the 6-8 ounce range with these materials and considerably less if one wanted to start optimizing the design with better materials. It needs just a few finishing touches including paint, a drawtube thumbscrew (possibly spring-loaded), and an eyepiece thumbscrew, but preliminary shop tests with a laser collimator without those are VERY encouraging. I'd prefer it if the tube were anodized for surface hardness, but for my use and these low forces I think it should suffice as is.
First a few photos of the prototype, then a description of its construction and how it works. It is shown in the orientation it would be for a scope altitude of 45 degrees:
That tan material the body is made of is a urethane board left over from a water jet fixture project at work. It is one of the lightest density versions of the product. At 38 pounds per cubic foot it is about the same density as dried birch or ash wood. I used this material because I don't have a machine shop, just woodworking tools, and wanted to make this in my own garage. I used my Shopsmith as a table saw, wood lathe and drill press to build this. This material turns, drills and sands almost like wood, and taps well for 1/4-20 threads or coarser. The focuser body is 1/2" thick, so it is rigid enough despite being made from such a plastic.
Directly opposite each of the four rollers is a Vlier screw with a Delrin tip.
These are stainless steel body spring-loaded plungers of the lightest spring force available in the 1/4"-20 size.
The smooth spherical tip requires a force of only one pound to start depressing it, and 3.5 pounds to fully bottom them out. I've adjusted them near the middle of the range. I'll explain more on the construction below, I just mentioned these details early to explain how the focuser functions.
The design is optimized for a dob aimed at an altitude of 45 degrees. I'd probably make future ones with slightly smaller angles about the vertical plane (perhaps 50o rather than 60o) to further reduce stresses on the roller screws now that I see how well it works, but all rollers and their opposing pushers are 60 degrees from the vertical plane that passes through the focuser axis at a scope 45 degree altitude for this prototype.
On this prototype at this scope angle the rollers furthest from the eyepiece are about 1 1/2" below the top surface at 10 o'clock and 2 o'clock. The closer rollers are 1 1/4" further from the base of the focuser, just below the top, and are at 8 o'clock and 4 o'clock. This CAD wireframe view may make the roller arrangement clearer, although it shows the cylinders that correspond to the pushers 120 degrees from each roller (another option that should work):
Here's the same view shown opaque:
All rollers are angled 3 degrees to the focuser axis, so the focuser advances at about half the speed (0.370" per turn) of a Kineoptics focuser. I made mine left-handed to turn in the opposite direction of my Paracorr--not necessary, but I wanted to slightly INCREASE focuser turning friction when pulling and pushing while turning the Paracorr. The bottom rollers are elevated more than an inch from the base of the focuser to increase my focuser range. For low-profile focusers these lower rollers could be mounted just above the plane of the base, which would be stiffer on lightweight versions.
Note that regardless of what angle the focuser is mounted on the side of the tube, as long as the focuser is mounted 45 degrees to the primary axis of the scope, gravity always works in the focuser's favor. When the scope is pointed at zenith the rollers nearest the eyepiece move from 8 o'clock and 4 o'clock to 6:30 and 2:30 for right-side focusers, or 9:30 and 5:30 for left side focusers. The rollers never cross a vertical plane through the focuser axis. Similarly, the lower rollers move from 10 o'clock and 2 o'clock to either 8:30 and 12:30, or 11:30 and 3:30, again depending on whether the focuser is on the left or right side.
As the scope is pointed to the horizon things rotate the other direction, but again, the rollers never cross a vertical plane through the focuser axis. On my hexagonal scope the focuser would be angled 30 degrees from vertical when the scope is aimed at the horizon. On a big dob with the focuser parallel or nearly parallel to the scope altitude axis the focuser would be nearly horizontal when the scope is aimed at the horizon. In all cases none of the rollers cross a vertical plane through the focuser axis for all ranges of altitude angles. The scopes with the focuser mounted more on the side simply see a greater amount of gravity loading at lower altitude angles.
Now, as long as the center of gravity of the combination of drawtube, adapter, eyepiece, Paracorr, camera, etc. are further from the base of the focuser than the point on the focuser axis midway between the planes of the rollers, gravity will always increase the pressure on each roller and never oppose any of the gentle pushers opposite the rollers for all altitude angles. A thumbscrew could be added in the plane of the top rollers and pushers to lock things down, but that thumbscrew would not be required except to keep the focuser from sliding or turning inadvertently. In the photos you may see the threaded hole I have already made that is at the top when the focuser is aimed at 45 degrees. When I add a thumbscrew there I may make it a spring loaded Vlier screw of a stiffer design than those used so far. I've already found a moderate pressure setting on my four screws that keeps my drawtube from sliding with two pounds on it when pointed vertically.
I think I'd like to loosen those four existing light-duty spring-loaded Vlier screws slightly, and use one stiffer spring-loaded one on a thumbscrew as a more traditional focus lock. That would also make it easier for me to slide the focuser to close focus, and then rotate it for fine focus. I can do that already, but I could do so more easily if I loosen those screws some. The coefficient of friction of the Delrin to the aluminum drawtube is only about 0.2, so as soon as I rock the tube slightly off the rollers I can slide it easily. Because those stainless steel rollers (1/8" ID, 3/8" OD, 5/32" long bearings) are almost square to the axis of the focuser the slightest pressure of the screws and gravity keep the tube from slipping.
Anyway, I want to encourage others to experiment along these lines. I've recently learned from the greats on these forums how critical collimation is as we progress to faster mirrors. With the heaviest accessories being added to the focusers of the biggest of these dobs, collimation becomes ever more critical. My laser tests in my shop have me increasingly confident that a design that inherently WANTS to stay against the steel rollers when all other pressures are lessened is preferable. This doesn't have to be much more complicated a change than a rearrangement of rollers. It is critical, however, that with a focuser with a drawtube that can be tipped a little manually, all pressure points must be in the same planes as the rollers.
As far as things I'd do differently in the future (besides building in a REAL machine shop with all metal), I'd like to move the rollers a little closer to that vertical plane--maybe five or ten degrees. This would reduce slightly the forces on the small screws that support the bearings and make the design slightly less sensitive to dimensional variation in the drawtube. As long as no roller crosses that vertical plane in use, loading should never be against those gentle pushers. I'd also likely increase the bearing size slightly to 1/2" with 3/16" holes (and 3/16" axles) and have them mounted on the outside of an aluminum body and extend through notches in it. That would allow the focuser body to be another aluminum tube with an ID only slightly larger than the OD of the drawtube. The top rollers and pushers could be mounted beneath a stiffening ring at the top of that body tube, making the structure resemble the "top hat" UTA shape known for a good stiffness to weight ratio.
This design could also be used even better on an alt-az scope with a rear-mounted focuser. Since there would be no rotation about the focuser axis those bearings could be separated by a more conventional +/- 45 degrees rather than 60 degrees. AT a stretch, if a locking swivel were built into the base that allowed rotation of the body about the focuser axis, this could be used on an equatorial mount. Opportunities to add error to collimation would occur with such an arrangement, but it could be done carefully. These concepts could also be used on a more conventional construction with longitudinal rollers and a typical Crayford drive shaft on the upper plane. I want others to think about setting things up not to require fighting gravity and subsequently applying point forces to drawtubes that exceed the weights of the accessories by several times just to lock them down.
Here are a few more pictures of construction of my prototype. The aluminum tubing arrived with some serious scoring on the outside and had to be sanded and polished on the lathe. I also gave it enough of a chamfer on the bottom edge to facilitate assemble. Parallelism of the outside of the drawtube is critical with this design; a taper would generate varying amounts of tipping of the focuser axis throughout its travel. Machinists would call this a tight cylindricity tolerance--not just roundness, but constant diameter along its length to a close tolerance. I used round headed brass wood screws to hold the bearings. I spun them against a fine flat file to taper the shaft from about 0.130" just below the head to about 0.123" just above the threads. I then tapped the screws through the 0.125" ID bearings until they jammed on the taper, raising a slight burr as they did so 0.040"-0.060" below the head. That eliminated longitudinal play in the bearings in the assembly. 1/8" shoulder bolts would be preferable if metal were used, but this body was made of plastic. All screw holes were drilled with the drill press table tipped 3 degrees.
Now we're cooking with GAS!
Edited by jtsenghas, 24 March 2015 - 12:42 PM.