The only variable I hadn't tried was the choice of wood. I was using pine as I didn't have any handy hardwood.
Today I did a quick and rough job using an Australian Hardwood ?? possibly blackbut, from my builder next door neighbour (handy) and it works just fine. The close grain timber must exhibit more flexibility than pine. It required very little force and i can even use a nylon wing nut (short term test).
Thanks Matt you guessed it!
Today I also started tidying up my spreadsheet for the basic balance test. Its the Krieg and Berry weight and distance calculation from the balance point. As I am using much heavier timber on the prototype its not going to give me the desired result however it provides a framework for future use.
The pivot kits from AstroSystems arrived. They include:
Asimuth movement:
Pivot Bolt - pre drilled with 0.25 hole for encoder if required.
T-Nut
Pivot Bolt Bushing
Locknut
PTFE Bushing
Hex Key Washer
Washer
Asimuth and Altitude Pads pre countersunk holes
Screws
Both with very clear instructions on instillation.
Looking at all the alternatives in creating a workable, reliable lower truss block. Had to test simply splitting the block in two with a hinge on one end and the tightening bolt on the other. Conclusion:
Provides a more satisfactory clamp around the pole.
Introduces new issues, challenges, skills such as, countersinking and installing a hinge (8 of them).
Cost of 8 hinges and screws.
The gap between the two halves has to be managed carefully to provide enough room to clamp around the pole and yet not to much force to loosen the hinge screws.
Thanks to Matt again for the idea. Your going to tell me "I told you so".
Its looking more like the best solution for many reasons is using Delrin manufactured blocks.
Having a look at the alignment of struts between the lower strut block mounted on the outside of the mirror box with the lower secondary ring. Should help to determine the style of connection to use.
The software program Newt helped determine an optimum ID of the lower ring of the secondary cage to provide the least obstruction of the light path.
Looking like an L bracket mounted on the lower ring so the strut can connect to the outside of the ring. Will have to have a good think about this one.
Neil, you can always angle the poles in a little if you want them to line up with a particular place on the UTA. You can either use a very thin shim between the block and the mirror box or once you have worked out how much you need to shim, you can sand the back of the block to suit. Sand the top to move the pole inwards, bottom to move it out.
Only very small amount of shim/sanding is needed to move the top of the pole quite a bit. A 1mm shim on a 50mm block will move the top of a 1.5m pole in 30mm.
Thanks Matt
That's good information. The text talk about using a shim on the blocks but I hadn't realized the effect of a small adjustment by the time it travels 41". It would be better if the middle of the pole lines up with the middle of the lower ring rather than the outside. In my case a two inch wide ring. It certainly gives you more options of connection and is better aligned for stability. Highe goes to incredible lengths explaining the engineering attributes of the truss and connections.
While i'm at it, what techniques do you use to measure that the focus board is mounted and pointing exactly to the center of the UOA? I am presuming I will use two small blocks mounted on the upper and lower rings to support the focus board, probably 1/4 " ply.
I have noted that AstroSystems use a piece of paper wraped around the tube and subdivide it into 90 degree elements when mounting the spider. We would have to use one or both of the rings in absence of a tube.
Small degrees or mm in the shape or position of the LOA strut block can mean LARGE misalignment by the time the Strut reaches the UOA.
Requires:
Square blocks from the start.
Drill press to drill the required angel of hole. In my case no angel, just a straight hole from the top to the bottom inside the block.
Testing of the exact degrees of rotation of the block when positioned on the mirror box so as to point each strut to the centre of each side of the UOA.
Use of a shim (narrow chock) to point the strut inward or outward to align with the LOA bottom ring.
This will have to be repeated for the required number of struts (8).
Also note my final version of the split block using hinges.
Neil, I plan to mount all blocks and install poles, shim to correct position. Then measure the shim thicknesses, remove blocks and sand the back of block the required amount before final fitting. The shims are thin strips of plastic sheet which is 0.2mm thick.
My poles will be paired up using offset plates on top of the poles with a quick release skewer to clamp onto the receiving block.
(going skiing tomorrow but will try to do some photos on the weekend)
Learning how to adjust pole angles.
Only anchor the strut block with one tightening screw at the start. This gives you a free rotating block you can use to establish the correct angle for each pair of struts to join.
Shim the top or bottom of the block to turn the strut in towards the mirror or outward.
Managed to remove the small portion of the Alt Bearing to provide room for 2 of the eight blocks.
Testing the wooden pole seat and wedge. Pole seats had to be made out of a hole drilled at 8 degrees. Interesting to find out if this works!. Have to find a suitable bolt with a cam-lever or similar to clamp the wedge into the secondary ring.
After gluing the anchors and installing a bolt through the wedge (currently held with a wing nut) the assembly is holding the poles quite firmly.
Alignment is another matter that will require the full installation of the 4 trusses before a satisfactory result can be tested.
There was a question of whether the two anchor blocks should be positioned parallel to each other or slightly twisted to align with the the edge of the ring. Pictures I have observed use the former.
Very pleased with the decision to build a prototype, many lessons learned.
Using threaded inserts to pair the poles to a bracket on the UOA will be a better system> easier to get the angles! and to attach with one horizontal bolt to the bottom ring.
I have not trimmed the poles to the same size so I used the hole through the bottom blocks to make adjustments.
Going away for a number of weeks so will have time for thought! and to save.
Very pleased with the decision to build a prototype, many lessons learned. Using threaded inserts to pair the poles to a bracket on the UOA will be a better system> easier to get the angles! and to attach with one horizontal bolt to the bottom ring.
I have not trimmed the poles to the same size so I used the hole through the bottom blocks to make adjustments.
Going away for a number of weeks so will have time for thought! and to save.
Neil, if your going with the top mounted brackets on the poles remember to offset them so that when they are closed up for storage you have a 20mm gap between the poles to allow for the foam insulation the poles.
While I'm saving for a focuser, spider and bits, I have started on the truss connections. I have tested a variety of solutions using wood and found the solutions not convincing. So here is my Alloy solution using C Channel and L lengths. Using a bike Cam lever for the quick release solution.
Have started marking out the mirror box using Marine Ply AA 18mm for top, bottom and mirror cell and 12mm for the rest of the mirror front, back and sides.
Enjoying taking my time when I have the money and when my order for spider, holder, focuser and secondary arrive. Have ordered a Moonlight CR2 focuser and spider and holder from AstroSystems.
Testing accuracy of build on mirror box. Decided to use 12mm ply on sides and top with 18 mm on the bottom. 17 inches square all around. Now to plan the side clearance for the rocker. Probably 1/8 either side or 1/4 in total bigger than the mirror box. Scale drawing on graph paper to sort depth and clearance of box inside rocker.
Having to live with the design decisions that you make.
I have decided from the start to make a light weight 10" high mirror box.
Using the spreadsheet provided by the Albert Highe "Portable Newtonian Telescopes" weighing most of the components x distance from the bottom of the mirror box points to a 15" radius for the Alt Bearings. This is the distance from the Balance point or M point to the bottom of the mirror box. Other methods of balancing the scope outlined by Krieg and Berry indicate that the secondary cage is going to be much heavier than the mirror box when distance weight is calculated.
So I am going to go with a 17" Balance point. Actually a 17 1/4 to allow for the teflon pads.
The second decision to make is balancing the clearance of the mirror box with the bottom of the rocker and how deep to set the box inside the rocker. The first is easily achieved by a scale drawing on graph paper and a compass. The second decision is dictated be requiring at least a 60 percent arc from the M point, (30 degrees either side of the middle of the box) to space the teflon pads.
Now time to pause and think about it before making saw dust!!
Here is my marking out of the two rocker sides. Used a thin MDF stick compass to create the arc.
Marking out mirror box
