Crosshair Repair

I had a little surprise the other night trying to find Saturn with my 5" refractor. I looked through the finderscope--and there were no crosshairs. There was a single line across the field of view--and after a bit more investigation, I found a squiggly line near the top of the field of view that looked like what was left of the vertical crosshair.

This surprised me. I've read of World War II crosshair manufacturing where they literally stretched a strand of spider web. But many years ago, my father and I modified a World War II era tank range finder telescope. It had a series of horizontal lines across the field of view. When we disassembled the optics, these lines were scratched onto a glass reticle. All we had to do was scratch a vertical line across the horizontal lines, and voila! Crosshairs! The next step was to drill a small hole in the tube next to the glass reticle; I could hold a small flashlight (the kind that used "grain of wheat" bulbs) against the hole, and get an illuminated reticle--very useful for finding objects against a very dark sky.

If even a World War II era scope had a glass reticle, it seemed implausible that even the cheapest Chinese-made finderscopes used fine threads for crosshairs, instead of reticles.

So I took the eyepiece apart, and indeed, there were two very fine pieces of thread or whatever (for all I knew, spider web strands) that formed the crosshairs. They were sufficiently fragile that sometime after disassembly, the remaining crosshair also snapped.

I wasn't thrilled with my used finderscope options, and I also didn't want to wait several days for a replacement to arrive, so I drilled four holes in the eyepiece assembly .495" from the end, where the crosshairs had been located. (The advantages of having a vertical mill and edge finder!) Then I ran the finest thread in my wife's sewing kit through the holes, and used a single drop of epoxy to prevent the threads from working loose.

I put it all back together again, and it works! Because I didn't have a good way to clamp this eyepiece assembly (it was too fragile to clamp hard, and being round, tried to rotate), two of the holes are across a chord, not the diameter of the tube. Still, all that matters is if the crosshairs align with the aim point of the telescope--not if they are perfectly centered.

The thread I used is obviously much coarser than the original crosshairs--one might even say grotesquely so. On the other hand, because this finderscope didn't have illuminated crosshairs, against a dark sky the fine crosshairs just disappeared. These coarser crosshairs won't have that problem.