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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.

December 21, 2008

We’ve made significant progress on the Evo-30 project. Dave Pasley has completed the design work and we have begun production of the prototype. As you can imagine, there is a lot of detail work in the electrical systems including the drive motors and encoders. Dave has completed all mechanical details down to the last screw. There will still be some detail work after we assemble the prototype, like creating a clean wiring harness, and making the shroud but these things are better left until we have the real thing to work with.

With the aid of some rendered drawings I’ll try to describe how these systems are designed to work.

Electrical Pass-through

12V Power Lugs on the Under Side of the Ground Plate

This image shows the 12V power input lugs on the bottom of the ground plate. A power cable will run from the lugs to an automotive style 12V plug one the outside edge of the ground plate. This is where you’ll plug in the power from a battery or a 12V power supply.

Power Transfer Rings

Power is transferred to two copper power rings on the top side of the ground plate. Also shown in this image are the center stub shaft and bearing around which the telescopes Azimuth axis turns. The Azimuth encoder is also shown in this image.

Top Side Power Ring Contacts and Lugs

Power is picked up and delivered into the bottom of the telescopes rocker box via two posts that are spring loaded to press against the power rings. At the top of each post there is a power lug that will deliver 12V to a power bar that will distribute power to the ServoCat, Argo Navis and other accessories like dew heaters, etc...

Top Side Power Lugs shown with Rocker Box Plate, Center Bearing and Encoder

The Azimuth bearing consists of a shaft stub fixed to the ground board and a tapered roller bearing mounted in the bottom of the rocker box. The tapered roller provides some preload in the vertical direction as well as stability in the lateral direction. A large nut at the top of the stub shaft provides a means of setting the bearing preload.

The Azimuth encoder is coupled to a 0.25 inch diameter post at the top of the Azimuth shaft. The encoder housing is fixed to and rotates with the rocker box. All of these components are visible in these images.

The Ground Plate Showing Feet, Out-rigger Bearings and Drive Ring

In addition to the central Azimuth shaft and bearing there are three out-rigger bearings, each mounted directly above a ground foot. These radial bearings contact the bottom of the rocker box to provide stability.

Also visible in these images is the Azimuth drive ring and the drive roller protruding through the bottom of the rocker box. You can also see the Altitude drive roller.

Altitude and Azimuth Drive Rollers and Engagement latches

The image on the right is a slightly different view that shows the Altitude and Azimuth drive rollers and the engagement latches.

The next few images are more detailed views of the Altitude and Azimuth drives without and with the rocker box bottom plate in place. Each drive motor/roller assembly is mounted to a fixed pivot point on the rocker box. A lever attached to the drive assembly protrudes through the wall of the rocker box as show in the images below. The engagement latch pulls the lever to pivot the drive assembly and press the roller into contact with the drive ring.

The Altitude and Azimuth Drives Viewed Through the Bottom of the Rocker Box

The Azimuth Drive and Engagement Latch

The Altitude Drive and Engagement Latch

Typical Motor, Drive and Latch assembly

This is the Altitude drive assembly consisting of the ServoCat motor and gear box attached to the pivot/lever assembly. It is important to note that the drive roller is captured in a bracket with a bearing on either side of the roller. The roller is then coupled to the ServoCat gearbox output shaft. Supporting the drive roller on its own bearings avoids side loading of the gearbox output shaft when the drive roller is engaged.

Altitude and Azimuth Drive Assemblies Inside the Rocker Box

This is a view of the motors as they will be mounted on the inside of the rocker box.

January 27, 2009

Production has begun!

Last week Toney Mulherin started writing CNC programs and fabrication parts for the prototype Evo-30 on the bridge mill. There are three phases to production of the telescope parts. Phase one involves fabricating all of the parts that start out as flat plate. These parts are made on the CNC bridge mill. Included in this list are things like the ground plate, the rocker box bottom and walls, the primary mirror cell, spider vanes and secondary cage rings. Toney will complete phase one by the end of this week.

In phase two, Toney will edge drill any of the parts made in phase one that require blind or threaded holes in their edges and he will make most of the smaller parts such as truss connections and drive motor mounts. Phase three is all lathe work where Toney will make things like shafts and drive rollers. Following all of this we will do a full up test assembly then send the parts out for anodizing. We expect to complete the test assembly the first part of March. In the mean time, here are some of the parts that Toney has made so far.

I'll add photos as parts are made during this first phase of production...

The ground plate

Toney Mulherin holding the rocker box bottom plate

The rocker box bottom plate and the front and back side walls

The primary mirror cell top and bottom skin plate with fan mounting holes

The spider vanes

One of the secondary cage rings on the CNC bridge mill table

Toney running the Azimuth drive ring on the mill

Cutting a rocker box side wall

The rocker box bottom and walls

Optical technician JR Smith polishing a 30 inch primary mirror

Cutting a rocker bearing

Another view of the rocker bearing on the mill

Edge drilling jig with rocker box side wall

Spider vanes, drilled and tapped

Preparing to edge drill the rocker box bottom plate

A rocker box side wall and folding rocker bearing parts

Assembling the rocker box

Dave Pasley and Toney demo the rocker box and a folding rocker bearing

The rocker bearing in the folded position

This completes Phase 1 of the fabrication process. Toney will move on to
vise work on the CNC milling machine next week. Stay tuned...

February 11, 2009

Phase two fabrication is under way.

Here are a few photographic updates.

The rocker box assembly with the side bearing rollers installed. The side roller bearings consist of four pairs of sealed bearings. The Altitude bearing will ride on the roller bearings. Also visible in this photo are the center tapered roller bearing and the Azimuth encoder mounting bracket. Note also the square hole in the rocker box bottom through which you can see the edge of the Azimuth drive ring.

A close up of an Altitude roller bearing pair.

A close up of the Azimuth tapered roller bearing and the encoder mounting bracket. The holes to the left of the bearing are for the electrical leads that pass through to contact the power rings on the ground plate.

This is a shot of the ground plate and the Azimuth drive ring. Again, note the hole in the bottom of the rocker box. The Azimuth drive roller will protrude through this hole to engage the drive ring.

A close up of a ground contact foot and pad and an Azimuth outrigger bearing. Most of the load of the telescope is carried on the center tapered roller in the photos above. The three outrigger bearings, each directly above a foot, provide stability and very smooth motion in Azimuth.

The primary mirror support triangles

The folding altitude bearing hinge plates

The primary mirror cell internal frame structure made from 1"x1" solid aluminum square bar stock.

Test fitting the primary cell bottom skin plate. The five square holes will each hold a Sunon Mag-lev fan.

The primary cell assembly showing the internal frame structure.

Adding the top skin plate to the primary mirror cell.

The finished primary cell structure: light and strong.

Primary mirror whiffle-tree pivot bars. Note Heim bearings in center and at each end of the bars. The center of the bar attaches to the top of a collimation bolt. Each end of the bar holds a mirror support triangle.

A collimation screw. Note the internal thread. This is where the whiffle tree pivot bars attach.

A view of the primary collimation screws from the bottom of the cell. The screws will eventually have knurled knobs and locking nuts.

The assembled primary support whiffle-tree. Pivot bar rotation is prevented by a post that comes up from the main cell assembly. A similar anti-rotation post prevents rotation of the support triangles.

The secondary mirror cage assembly

The secondary mirror cage assembly

Here we've added the X-brace to the front half of the rocker bearing

May 14, 2009

2009 Texas Star Party Report

James Mulherin and the Evo 30 set up on the north end of the upper field at TSP 2009

As promised we made it to the Texas Star Party with the Evo-30 prototype in tow. But, as is our usual custom, we took it right down to the wire in terms of schedule. We only had first light at Jimi Lowrey's place, about 3 miles from the TSP site, on the Saturday evening before the start of TSP. Once again, I owe Jim Chandler a debt of gratitude as he was instrumental in setting up the Argo Navis and ServoCat. I had forgotten to bring a critical component required to initialize the drive system: a USB to serial converter to connect to and load parameters into the ServoCat. Fortunately Jim keeps one in his tool kit and we were on the sky in no time. Saturdays first light test was a quick one; just to make sure the scope would point and track. We looked as some eye-candy with Jim's wife Anna and Barbara Wilson. Satisfied that all was well we broke the scope down and packed it back into the trailer for transport to the star party the next day.

On Sunday afternoon we set up on the north end of the upper field next to Jim and Anna Chandler (30" and 18" Obsession), Buster and Barbara Wilson and Larry Mitchell (36" Obsession). This being the first night, most of the attendees were busy with setting up their own equipment but we did have quite a few visitors stop by to have a look at the Evo. That night, after working out a couple of bugs I spent a few hours observing by myself and with a few others who stopped by for a look. The scope was performing very well both optically and mechanically.

I hadn't brought any observing guides with me to the star party so I asked for some suggestions as to what I could pick up at the vendor building. For general purpose observing, Anna Chandler suggested The Night Sky Observers Guide by George Kepple and Glen Sanner. There are two of these; one for summer and one for the winter constellations. Both authors were at the vendor building so I picked up an autographed copy of each. For some more challenging objects Jimi Lowrey suggested a set of Alvin Huey's guides: The Abel Planetary Observers Guide, Hickson Group Observers Guide and Observing the Arp Peculiar Galaxies. With guides in hand I had a few very productive nights at the scope which I have to say performed almost flawlessly. (More on this below...).

After the thorough trial run at TSP we've decided to make a few refinements. Some of them were already planned but we ran out of time to implement them before the star party. Here's the list and the rational:

1. Lighten the secondary cage. As you can see in the photos we've added some bar-bell weights to the bottom side of the primary mirror cell.  Removing weight from the secondary cage will eliminate most of the bar-bell weights. There are a couple of changes that we'll make to the primary cell, replacing some aluminum parts with steel, that will fully eliminate the need for the bar-bells.
2. We have yet to add the assembly latch that will hold the scope near the horizontal position while the secondary cage is installed during assembly. This will be done ASAP as the scope currently requires two people to set it up. Our goal is to make it a one person operation.
3. A minor issue is the current location of the wireless transmitter for the ServoCat hand pad. There is a switch on the transmitter that you flip to select between the wired and wireless hand pads. The transmitter is mounted low on the rocker box. We'll move it to the Argo Navis stalk where it will be more convenient to access the switch.
4. We will make the Kydex light baffle at the top of the secondary cage taller. In our haste we accidentally cut it too short which will allow stray light to make it into the eyepiece.

Buster Wilson standing next to the Evo 30 on Sunday afternoon

We will make all of these changes in time for the Cherry Springs Star Party in June. So far so good...

The Evo 30 set up and ready to observe.

Some close up views