*
Sometimes the gods of the stars do smile upon us. Perhaps in compensation for missing the Lunar eclipse: I was searching for a suitable rail on which to have a sliding counterweight. And there it was, hiding under the bench in the shed with two neat brackets all in shining stainless steel. It was once a towel rail, I think, but I have absolutely no recollection of buying it on my endless rounds of the flea markets and charity shops looking for ATM materials. I just needed to curve the "feet" and find a suitable weight which would clear the main tube. A delve in the brass stock bin provided a nice sized stump to be bored and turned to make it tidy. I have a wheel and screw borrowed from another counterweight to allow it to grip the rail securely. A plastic plug will avoid marring the rod and make sliding more pleasant..The counterweight rail is good length and the weight sufficient to make a difference in the OTA's balance point. I just have to decide where to put it. If I fix it on top of the tube there is a danger I might use it as a lifting handle. Sturdy as it might be, this would probably shorten its useful life. Underneath makes sense if I remove the handle. This will allow the weight to come a lot closer to the focuser. It would then become yet another handle for pointing the telescope. Or, I could fix it between two of the handles.
If I put it in the wrong place it will be more distant from the mounting's counterweights on the declination axis. Which will require some compensation to bring the OTA back into balance around the polar axis. If the weight is fixed too high on the tube it will tend to rotate the OTA when it is being carried. In the end I fixed it below the finder without removing any handles. The problem then was inserting my large hand through the smallest baffle to fit the lock washers and nuts on the inside of the rail post screws. Not quite "blood everywhere" but it required some dexterity to retrieve my hand. There were far too many projecting screws inside the tube holding the handles on. All of which would need removing, with one hand, before I could slide the baffles out to work on the rail screws in the bare tube. The split, lock washers will ensure the counterweight rail doesn't work loose over time. The OTA is already gaining weight rapidly. I haven't tried to weigh it yet because I wanted to have everything fitted first. By "top and bottom" I mean relative to the finder being on the left side of the main tube for comfortable, right eye use.
With the new tube counterweight midway on its rail a check on the new balance point had it at 80:100cm in favour of the objective. Not too bad really but I'd much prefer a midpoint for a nicely "balanced" look when fixed to the mounting. A short "top end" above the mounting rings always looks wrong to my eye. It's as if the OTA has slipped down through the rings while pointing high in the sky. Which probably means I will have to add more weight at the focuser end. It is much easier to balance the tube on the mounting, with the axes unlocked, rather than resting it on a fulcrum bar on the workbench. The OTA tends to want to rotate on its axis to find its natural balance point. Which means the finder and counterweight try to rest on the bench and won't allow the tube to see-saw.Before I packed in for the night I bridged the OTA on the benches with a stepladder and brought out my old Salter dial scales. The OTA now seems to weigh 17kg or roughly 37lbs complete. Given its size and construction that isn't too bad. [If I were a young muscle builder on steroids!] I'm using rubber faced workman's gloves to ensure a good grip on the smooth tube while I am carrying it about.
In anticipation of using 235mm tube rings I have been routing out plywood packing rings. My cheap plywood circle cutter began to break up at the center pin clamp as I was making practice cuts. So I had to modify the alloy angle and rails device I had made previously. I need to be able to cut circles down to 200mm internally. Which required a concave radius on the piece of alloy angle to bring the center pin close enough to the cutter. A millimeter too much in radius needs a larger external radius on the plywood packing ring. [and vice versa] Otherwise the tube ring would not close sufficiently to clamp the tube firmly. By the time I was finally ready to cut my rings to the desired size it was getting too dark to continue. The weather has been most favourable for working out of doors with daytime temperatures reaching 60F in bright sunshine.
Click on any image for an enlargement.
*
No comments:
Post a Comment