So I attempted to to build a flexible focus extender for the ETX-70AT telescope. The concept is solid, and the price is right but mistakes were made.

I spent a grand total of $2.42 at Ace for an aluminum spacer, a foot of cable, and some set screws. As I drilled and tapped the holes in the spacer, mistakes #1 and #2 became evident.

I had selected #8-32 set screws, just because I already had a tap for #8-32. It was quickly apparent that this was way too big for this particular spacer. I could also see that I probably should have gone with a steel spacer rather than aluminum, so the threads would be stronger, as there turned out to be very few of them. The set screws are also waaay too long, as you can see here:

The long set screw worked beautifully in the knob, however. I had printed the knob with 20% infill, so the tap was only making threads on the inner and outer walls. I probably should have modified the STL file to include a hole to tap, or just printed it with 100% infill, but the set screw was actually just long enough to grab the few threads that were there, and get a nice solid bite on the cable.

Despite the mistakes that’d I’d encountered, it was shaping up nicely, until I tried to attach it to the focus rod on the telescope. The few threads I was able to catch before the set screw hit the rod just peeled right out of the spacer because the aluminum was too soft.

Debating at this point about either redoing the spacer in steel with smaller set screws, or using a file to create a flat spot on the focus rod. Or maybe both…