October 6, 2008

The OMI Evo-30


The Evo-30 Collapsed and Nested


The 48” scope and Truss

For about a year now OMI has been developing and planning the introduction of a 30 inch Dobsonian telescope. We just made the first formal introduction of the scope at the Okie-Tex Star Party last week. At this point the scope exists only in virtual reality as CAD drawings. We will begin fabrication of the prototype this month and plan to demo the working prototype at the Texas Star Party in April of 2009.  I’ll post regular updates here as we work our way through the production of the first scope so stop by occasionally to see our progress.

This month at OMI we will successfully complete a production run of thirteen 30 inch F/4.5 primary mirrors, all made for Dave Kriege at Obsession Telescopes. When Dave and I first discussed making a run of 30 inch mirrors and scopes we were both a bit apprehensive about taking on the project. I expected that developing the process to fabricate the 2 inch thin mirrors would be a challenge, while handling the large scope parts would be a challenge for Dave’s shop.  In the end I found that scaling our process to produce the 30 inch mirrors was quite manageable and we were able to turn them out at a reasonable pace. It made sense for us to keep going. Dave, on the other hand, decided that this would be the last run of Obsession 30 inch scopes. After a bit of discussion we decided that OMI would take over making 30 inch Dobs, offering a scope that we would design and fabricate at OMI.

Obsession 18 inch Ultra Compact

We decided right away that the Evo-30 would be an all metal scope. All of the parts will be fabricated from 6061-T6 aluminum on our CNC machines and assembled with stainless steel fasteners. All aluminum parts will be anodized.

As you can see, the inspiration for the Evo-30 design is the Obsession 18 inch Ultra Compact telescope, which Dave introduced three years ago at the Texas Star Party. In a round about way the Evo-30 is also based on the design of the 48 inch Dobsonian that we built last year for Jimi Lowrey in Fort Davis, TX. The virtual mirror box with folding rocker bearings and nesting secondary cage are based on the Obsession UC while the dual stage truss assembly is based on the 48 inch Dob.

The OMI Evolution-30 is intended to be a well rounded package out of the box. To that end, it will include the following features as standard:

• Argo Navis digital setting circles with 10K encoders, wired and wireless hand pads
• ServoCat Goto drive system
• Powered ground plate to run drives, DSC and other 12V accessories
• Feathertouch focuser
• Light shroud
• Telrad finder
• Wheelbarrow handles

The Evo-30 Secondary Cage

The secondary mirror cage will accommodate an optional 8x50 finder scope and laser pointer. A removable upper light baffle is included 4-vane spider 4-point orthogonal secondary collimation 5.5 inch minor axis secondary mirror with built in off-set

The secondary cage attachment brackets feature two sets connection points; one set to accommodate most eyepiece types for visual observing and one set to provide additional out-travel for photography, video cameras and bino-viewers. The secondary mirror is sized to provide good illumination for photography but presents a central obstruction of less than 20%.

Slotted Truss Connections with Beveled Interface System

The telescope trusses break down into eight permanently assembled triangles. The corner of each triangle has a slotted bracket that slips over a threaded post as depicted in the drawing. The end of the slot features a concave conical bevel to which the convex conical bevel of the knurled knob interfaces. This system of interfacing bevels provides positive location of the truss members and a very rigid connection. In actual use, there and no loose parts and no tools required for assembly.

The Primary Cell Featuring Cable Sling, 36-point Support and Exhaust Fans

The images on the right show the internal structure of the primary mirror cell. The cell features a 36-point whiffle-tree for back support and a cable sling for edge support. The cable sling is adjustable from outside the mirror box to allow centering of the mirror in the cell. Incorporated into the inside walls of the cell are four Nylon blocks to retain the mirror on center, provide anti-tip-out protection and to maintain cable alignment. (The cable runs through slots in the Nylon blocks.) The back of the cell contains five low vibration Mag-lev fans that exhaust air out the bottom of the mirror cell. This is done, in part for mirror cooling, but primarily to pull convection currents away from the mirror, preventing currents from traveling up the telescope tube. As can be seen in the additional images below, the primary mirror is fully enclosed aside from the entrance aperture. Ambient air is pulled in through the entrance aperture around the edge of the mirror and exhausted out the bottom of the cell. From practical experience with large mirrors, the fans produce a noticeable improvement in image quality while running.

The Folding Altitude Bearings

The altitude bearings feature cross bracing in the folding half for lateral stability. They are also braced across their diameters when unfolded to provide stiffness when the scope is pointed toward the horizon. The bearings are hard-coat anodized for durability and ride on sealed roller bearings in the rocker box.

The Friction Drive Motors Mounted Inside the Rocker Box

The Altitude and Azimuth axes feature friction drives. The drives consist of pressure rollers that are pressed against an altitude bearing and the azimuth drive ring. They will be engaged by external pressure adjustment screws. The details of this system will be drawn in soon and they will be similar to the pressure wheel systems we use on our professional observatory scopes and Jimi Lowrey’s 48 inch Dob.

Nested Cage Front and Side View

The azimuth axis features a central tapered roller bearing that carries most of the load of the telescope. There are three out-rigger bearings that carry part of the load of the rocker box and provide additional stability to the telescope. These out-rigger bearings turn against a stainless steel track attached to the bottom of the rocker box.

Nested Cage Isometric View

The secondary cage nests securely on top of the primary mirror cell using the truss attachment points. The nested components weigh 350 lbs and can be easily moved with the included wheelbarrow handles. The dimensions of the nested components are: 38”x38”x31”

The Evo-30 is designed for quick assembly with no tools required. There are no loose parts aside from the truss triangles and secondary cage. The basic assembly process goes as follows:

1. Unfold and brace side bearings
2. Install four lower truss triangles
3. Install four upper truss triangles
4. Pull scope over to engage assembly latch
5. Install secondary cage
6. Install tube shroud
7. Disengage assembly latch

The assembly latch mentioned above automatically engages and holds the scope in the near horizontal position while the secondary cage is installed. After assembly it is depress and locked out of the way. It is released and reengaged for disassembly. The scope includes a mirror cover that should be locked in place during assembly, disassembly and transport.