Mirror cell issues
This covers:
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The use of a steel sling vice fabric to support the mirror on the 18 point cell. (Fabric stretches with temperature and humidity changes.) |
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the use of teflon tape on both the mirror edge and the steel sling to essentially eliminate astigmatism induced by lateral sheer pressure from a sling. ("Teflon on Teflon" appears to have a coefficient of friction that approximates zero, hence the mirror should be free to move against the sling.) This arrangement might still be prone to "potato chip" astigmatism at some angles, and I plan to convert the cell to use RTV/silicon to cement the primary into place and get away from edge supports at all. I want to be sure that I've got all the modifications and tweaks done to the scope first, since I'm not keen on iteratively and frequently removing the cell with the mirror glued in. |
| The use of velcro (I believe I first saw Lee Stock describing this on the Astromart ATM forum) in lieu of mirror clips. I had successfully experimented with RTV'ing nylon webbing to the back side of the mirror to secure it in proximity to the cell, but this is a much elegant solution. |
| Collimation bolts. I prefer to use larger diameter nuts on my collimation bolts. The standard sized furniture levelers typically used in K&B style Dobs are barely 1" in diameter. The larger you make the bolt head, the larger the turning radius and the finer the control you have when rotating it, which can translate in to being able to make much more accurate collimation adjustments. On Frankenscope I used 2 3/4" long flanged wingnuts; on Natasha, I wanted something larger. As a joke, I ended up using wooden stars nearly 5" across. They have a hex nut JBWelded to the backside. |
| Here's a hint applicable to virtually any collimation knob. If you find it tending to unthread itself from the shaft of the collimation rod under tension, put a hex nut and lock washer on the rod shaft before screwing the knob in. The "sandwiching" holds securely, and the hex nut gives you something other than the threaded shaft to hold onto if you need to remove your knobs for any reason. |
As noted above, I eventually plan to use RTV to attach the mirror to the cell and obviate the need for the sling and the velcro, but I won't do this until I'm certain the mirror won't have to come out for a long time... which I can't promise as Natasha goes through her "shake down" viewing season. If the current combination proves satisfactory, I may not switch at all.
I knew I would have to counterweight the scope considerably, in light of the large and overbuilt secondary cage, heavy focuser, and the fact that I favor the Nagler 31/Paracorr combination (nearly 4 pounds of weight) for my wide angled viewing.
I used mild steel bar stock for the initial counterweighting, cutting pieces to drill and attach to the tailgate. Lead weights would be the most efficient feasible source of dense metal (depleted uranium would be nice too, but has its own 'issues' <g>). Melting and casting lead weights wasn't appealing to me, since I didn't feel like losing any extra brain cells to lead poisoning. I eventually came up with the idea of using BB-filled aluminum square tubing as a reasonably dense source of weights that could be "stacked" as needed on various parts of the tailgate to ensure proper balance at all elevations.
My goal is to balance the scope well enough with my heaviest viewing setup (the Nagler 31/Paracorr) that using anything lighter can be accompanied by hanging a BB-filled bag of a predetermined weight from one of secondary cage rings to keep perfect balance. Alternatively, I may go with the belt-drive Dob Driver II arrangement, which in its quadruple torque version can deal with a weight imbalance of 20 pounds!
The steel sling, with the portion that will cover the lower 200 degrees of arc of the mirror covered with the teflon tape (see tape roll at left). The primary mirror's edge likewise received a teflon tape treatment.
The mechanics of the velcro alternative to mirror clips
The array of 4 velcro holders, spaced along the top edge of the mirror
A completed (but unpainted) BB-filled barstock weight, with wood plug in place, plus BB's and the filling funnel
Counterweights in place. Note the retaining eyebolt at the lower right edge holding the mirror box at an angle for transport or access to the tailgate. And yes, those wooden stars are my collimation knobs!
