by Jay Reynolds Freeman
I bought the NexStar 8 partly to use the mount for other telescopes, and I wanted to repaint the tube. So after three nights' use I threw caution and the warranty to the winds, seized a 15-inch crescent wrench in one hand and a fist full of Allen drivers in the other, and advanced relentlessly on the cringing telescope.
It wasn't obvious how to get it apart. I took the plastic cover off the inside of the fork arm -- perhaps I could get at the OTA attachment screws then. No luck, and I damaged the head on a screw that that would have been nearly impossible to drill out. Yet brawn triumphed where brains had failed -- I finally freed the tormented fastener.
The way in was through the recess where the controller parks. With a 1/16-inch Allen driver, I removed three tiny screws that secure the plastic part containing the main controller hook. Underneath lay a thin hex nut -- 3/16 inch thick, but 1-1/4 inches across the flats. You thought I was kidding about that crescent wrench. The nut was aluminum, and had deformed during installation, so it tried to strip its threads as I unscrewed it. I chased the threads with a tap successfully before rethreading it, but if it had been beyond repair, 3/16-inch aluminum is a hardware store commodity. I could have made a new nut with a hack saw, file, drills, and a 1/2-20 tap.
That nut and its washer held the OTA to the fork arm. With them out of the way, and after a little wiggling to separate the gears that drive the altitude shaft, the tube came clear of the mounting.
Still attached to the OTA was a black metal bracket with a wide bearing surface for altitude motion. The bracket included the long, thin, black bar, visible in NexStar 8 ads, extending along the side of the tube up to the corrector. Between the bracket and the fork arm were a thin fiber washer, a big aluminum gear, and three thick plastic washers recessed into the gear surface. The plastic washers and the surfaces they bore against were heavily greased, while the fiber washer and its side of the gear were clean. I kept the clean stuff sparkling as I washed the greasy parts, and bagged gear and fiber washer separately. I cleaned and regreased everything when I got around to reassembling the mount.
The black metal bracket attached to the OTA with three screws. The one at the top took a 1/8-inch Allen driver; the other two needed a 3/32-inch one. The top hole bottomed out in metal, the others went through the OTA wall; if you take them out, use caution you don't get something into the tube, or damage the primary.
With those screws removed, the OTA sat ready to mask and paint; I will describe that process in a subsequent report. I bagged parts and put things away for a while, then considered what to do next.
What I wanted was an easy way to swap the original OTA with others, without disassembling the fork arm. I toyed with designs for a fancy additional bracket, permanently mounted on the black metal one, that could accept the NexStar OTA and would be drilled to attach other telescopes. Fortunately, I was too lazy to start building one, so I hadn't wasted any effort when I saw the light.
I mean that literally. With the plastic cover and plastic hook gizmo off, you can see through the fork arm. It is a rather open casting, with brackets, gears, and motors inside, but the internal parts don't block all the space. With the plastic pieces removed, it is easy to poke a long ball driver through the arm from outside, and turn the screws that hold the bottom OTA end to the black metal bracket. Those screws are all but impossible to reach with the plastic parts in place. (The screw at the front end of the OTA is easy to reach as is.)
I left the hook gizmo intact -- the hole through it that I would have had to drill is a in curved surface, which would be awkward to plug. I filed a semicircular cutout, half an inch or a bit less in diameter, in the upper edge of the plastic cover for the inside of the fork arm -- I can fit a piece of tape there. So I need to take the hook gizmo off to get at the lower screws.
The modification to the black bracket, to install other OTAs, was equally simple. That bracket resembles a clenched fist, with an extraordinarily long middle finger rudely extended. The holes to mount the NexStar 8 OTA are a 1/4-inch one through the middle finger nail, and smaller ones through the thumb and little finger sides of the heel of the hand. I drilled through and tapped 1/4-20 at the mid-point of the heel of the hand, where there is plenty of metal, hence minimum chance of weakening the bracket. That hole position clears the fixed portions of the mount, so if a screw protrudes a bit through it in the direction of the fork arm, it doesn't rub up against anything.
I made a jig for locating that hole. I drilled 1/4-inch through a piece of thin bar stock, then measured the position corresponding to the requisite hole in the black bracket, and made a pilot hole through the bar stock there. To drill the bracket, I secured the jig to it at the "finger nail" hole, with a 1/4-inch bolt, then put my drill through the pilot hole in the jig to make a matching pilot hole in the bracket.
Next I used the jig to lay out and drill 1/4-inch holes in several lengths of 1/4 by 1-inch aluminum extrusion, at the same distance apart as the holes in the bracket. Thus I can use 1/4-inch fasteners to attach these pieces of extrusion to the black bracket; they fit exactly. The lengths of extrusion run from the "finger tip" to the heel of the hand, and about six inches beyond -- I made them as long as possible in that direction, because I anticipated needing counterweights there.
These pieces of extrusion serve as adapters to tube rings, camera attach fittings, and so on. I drilled 1/4-inch holes through them in several places, countersunk for flat-head machine screws from the side of the extrusion that faces the bracket, to attach things to the adapters. I made another hole near the end of the adapter opposite the "finger tip", for attaching counterweights.
I got a five-pound lead ingot from a plumbing supply store, and cast two counterweights, one of about 3.7 pounds and the other of the rest of the ingot, in 5-1/2 ounce cat food cans. I melted the lead in the cans and left it there; the less contact with lead the better. The big weight nearly filled its can, so I cut a plywood disc to fit where the can top had been, sealed the lead surface with silicone adhesive, screwed the disc to the lead, caulked around the edges, drilled through for a 1/4-inch bolt, and painted the assembly. The small weight filled only about a centimeter of the can, so I snipped off most of the extra metal and hammered the jagged rim down on top of the lead, before proceeding with the wooden disc.
Suppose I want to mount my Vixen 70 mm f/8 fluorite on the modified mounting. I attach its tube rings to an adapter, then temporarily install the OTA in them, and play with counterweight size and OTA position till I get the assembly to balance at the altitude axis. Then I remove the OTA, to turn the screws that fasten the adapter to the black bracket. After the adapter is in place, I put the OTA back, at the position for correct balance. I have several other OTAs that will mount similarly. I could probably come up with a way to attach stuff that uses dovetail bars, but I don't happen to have any such equipment.
When I want to use the NexStar 8 OTA itself, I reverse the process to remove the Vixen 70, then remove the plastic cover from inside the controller parking well, to get at the lower attach points. The entire swap takes about five minutes.
The NexStar 8 OTA weighs ten or eleven pounds, so many interesting telescopes, like my Vixen fluorite refractors, or the small f/5 achromats that have become popular, should not overload the mount, even with counterweights. The OTA of my Vixen 70 mm fluorite weighs four pounds, and that of my Vixen 90 mm fluorite weighs six pounds. Some telescopes that are too long, or that weigh too much to counterweight, might balance with the eyepiece far enough aft to prevent observing near the zenith, but as Dobson users know, that is not really a big deal.
My first impressions of this modification are that it is very clean and will be very useful. I suspect I am about to retire several old, cranky, hand-operated altazimuth mounts.
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