03 July 2004

Spider Construction - Phase One

Following an idea about re-designing the spider mount, I created a prototype adjustment jig (the actual rigs will be made of aluminum - this is made of steel). The concept springs from the idea that adjustments are easier made around the outside of the telescope than with tiny adjustment screws above the secondary mirror. The adjustment rig must also incorporate the mount points for the truss tubes in order to minimize hardware and weight at the upper end of the tube assembly. I won't cover this much detail about this subject when the actual rigs are made and applied. So, here are some concept drawings and explanations. For scale, the graph paper shown is 1/4 inch grid (though the actual construction is done with 1 1/4 inch tubing.

This details the adjustment mechanism from the side. In practice, the adjustment is limited by the size of the square tubing. I could only find 1 1/4 inch tubing, so the range of adjustment is 1/4-inch in and out and 3/8-inch up and down. Because of leverage, adjustments on the outside of the ring create very fine movements in the center. Springs (not drawn) create opposing tension against the adjustment screws so the adjustments can be made and then locked-in with a lock nut. These small adjustments should be adequate to accurately position the secondary mirror as long as great care is taken to accurately position the mirror in relation to the eyepiece holder in the first place. Here are some pictures to demonstrate the motion of the adjustments:

spider vane is UP	spider vane is DOWN	spider vane is IN	spider vane is OUT

This view shows the rig from above in relation to the ring and the mirror. The truss attachment screws are seen pointing down on the left and right. In the pictures below, they are shown with wingnuts attached. The spider adjustment screws (center) are sawed in half and joined in a pivot. The spider vane (a hacksaw blade) is joined by a second pivot and extends through a slot in the rig that keeps the blade perpendicular. .

Viewed from the inside of the ring, you can see the slot where the spider vane is held perpendicular. The vertical adjustment screw/nut is seen in the center. The two holes are used to tighten the truss screws on the outside (not seen in this picture). Since the screw holes on the other side are tapped (just like a nut), the screws are threaded from the inside to the outside to allow the truss rod holes to be held in place by wingnuts.

Viewed from the outside of the ring, the truss screws can be seen with the wingnuts attached. The vertical adjustment and horizontal adjustment screws can be seen in the middle. In application, the truss rods would be held in place by the wingnuts, but the adjustment screw/nut is still accessible from the outside of the upper tube assembly ring.

Here is a look at the parts for the assembly. I have photographed them on a piece of 1/4-inch graph paper and a ruler for scale. You can click on the image to see a larger view. The large screws and nuts are 1/4-20. The small ones are very small threaded (I can't recall the exact size). You could probably use a cotter pin instead of the small screws. You can see the two adjustment screws that were sawed in half. The process of sawing the screws was a little tedious, but not that hard. I used a vice to hold the screws and then a metal file to smooth out the rough spots. Assembly was a little tricky because it had to be done INSIDE the metal tube. Again, it wasn't difficult, just a little tedious. Needlenose pliers are a must! The two nuts next to the springs are standard nuts, but in practice, we may use nylon-threaded lock nuts. That way we will not have to 'lock' in the adjustments when made. The springs are 3/4-inch tall. I recommend these pyramid springs because they collapse all the way down to the just the thickness of the spring wire.

I'll be showing updates of the rest of the assembly, including the secondary mirror mount, as construction progresses. Again, I won't be covering this much detail about these rigs in future descriptions.

Clear Skies!

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