The Itsy Bitsy Spider

Well, folks, I admit that I have been very intimidated by the secondary mirror holder and spider assembly. So, I decided to experiment a bit on the finder scope. Oh, did I mention I'm building a 4.25" f/10 finder scope for this Beast?

Anyway, I got my first inkling of a decent design from Barislav Bracun and his website: http://www.scopemaking.net/dobson/ota.htm

Granted, it was a pretty rough example made from PVC and such, but I decided to upgrade the design a bit for a 3-vane 1.0" diagonal spider mount, and the result was pretty satisfactory. I've included some pictures and directions below.

If you have all the tools (taps, dies, drill bits, etc.) then this spider only costs about $15 in parts from ACE Hardware! That is a far cry from the $155 quote I quote from another company for this part, so I'd say I've saved money once more.

• First, find a suitable hexagonal barb coupling to drill. Mine was a 1/2" Barb x 3/4" FPT Female Adapter in Brass (Watts Water PEX Model # P-516) - $5.99

• Next, mark the middle of the hexagon faces on three side, each 120 degrees apart (every other face) and drill an appropriate hole for tapping. In this case, I used 10-24 threaded rod, so I drill a #25 drill hole.

• Then, tap the hole straight through and test the threaded rod fit. In this case, I used a 10-24NF tap in the #25 holes I already drilled.

 

• Next, mark, drill, and tap three holes on the top of the adapter, perpendicular to the faces you did not already drill for the spider vane rods. Make sure you use a small screw size that will clear the inside threads of the main adapter but be close enough to ensure clearance for the guts of the spider mount. (In this case, I used a #30 drill and tapped a 4mm metric bolt hole). Then, test fit the bolts for clearance.

• At this point, you have a brass adapter that has been drilled and tapped to accommodate three 10-24 threaded rod spider vanes and three collimating bolts. Next, you need to populate the assembly. In this case, I used a 6mm metric center bolt with two washers offset with a #140 compression spring and 1" flat washer and two locks nuts. Don't lose me here; it's not that complicated and anything that fits will do. Then test assemble everything. At some point I will put together some better instructions for all you ATM's out there.
• The assembly order should be: 6mm bolt, washer, adapter, washer, spring, flat washer, nut, lock nut.

• Then, cut the threaded rods to length for your optical tube assembly. Use lock washers and a lock nut against the brass adapter to ensure it won't unscrew in the future. You can use Locktite in the nut in the final assembly, but remember that you'll have to leave 1 vane off in order to fit it into the tube at installation.

• One point of notice is that the upper washer above the spring might not fit around your bolts in a smaller spider like mine, so I had to file 3 round indentations in the washer at 120 degrees apart. This shouldn't be necessary if you have a large assembly.

• Next, you will cut an aluminum tube of appropriate size at a 45 degree angle for later mounting of the secondary mirror. Once the cut is made, trim below the cut at a length sufficient to clear the bolt and nuts inside. It doesn't have to be long, so long as it clears the bolt and mirror. Then test fit for clearances. At this point, you have constructed your spider and vanes. The next step will be to complete the secondary mirror holder.