*
I decided that being close to completion the folded refractor should have some direct competition. I wanted another [useful] telescope to confirm the seeing conditions and to make comparisons between the two instrument types.
The problem with the 10" f/8 reflector was
flexure between the duplex spar/beam and the lower pot which contained
the primary mirror in its collimation cell. I had used heavy sections of
alloy channel to space the twin beams but only thin angle to support and align the cell 'pots.'
The spar design on a slightly taller pier is shown here. While it was simple to fix to the equatorial mounting it was unsuitable for Dobsonian mounting. For scale, the spar/beam is 2m or 6'6" long.
Today I took down the OTA from the shed roof space where it had been stored away from potential damage. I quickly removed the channel sections by removing the screws and socket head furniture nuts. Then I fitted another 2meter [6'6"] length of builders straight edge section between the two original beam sections. Trapping the new section with short lengths of studding and furniture socket head nuts produced an even stiffer beam than before. I have the equivalent of a simple 'plank' OTA but immensely stiff without having added a lot of weight.
The problem still exists that I had used rather flimsy channel sections to support the primary and secondary pots. These could be replaced with heavier angle sections. Which will not flex so easily when the pots are firmly clamped to the beam.
The alternative is to cut some curved strips to match and support the pots directly. These strips could be made of 18mm [3/4"] plywood on the bandsaw or using a circle cutting jig on the router.. Several strips will help to support the alloy pots without distortion. The relatively flimsy, angle sections would then become more cosmetic that structural. It is important that the lower and heavier pot [with primary mirror] does not flex relative to the beam.
Previously, this flexure tended to throw the optical alignment out as the OTA was moved to the east or west of the pier on the MkIV equatorial mounting. Having to re-collimate the OTA every time it was moved to a new celestial object was an exercise in futility and frustration! Which is why I became interested in building a refractor instead while I pondered the reflector design.
I think it would be best to have the lower pot easily removable to save my having to carry its 13+lbs weight [with mirror] out to the mounting. The pot provides excellent protection for the primary mirror and has a safety lid/plug lined with genuine chamois. The mirror could then be housed safely in unheated accommodation to avoid long waits for acclimation to the outside temperature. The plywood curved strips should provide a repeatable, location bed for the pot requiring very little collimation after fitting.
It is a long walk and/or climb from the Cheshire eyepiece to the mirror collimation screws on a 10" f/8! Hence my sudden desire to build something more user-friendly. Only to find a 7" f/12 refractor is a huge and heavy beast used in a straight line. Folding only added considerable expense and complexity. While the finished weight actually increased! Though the easily removable objective is a huge help in reducing the weight needed to be carried it is an added chore. If I had my time over again I would simply have made a circular bayonet system for the straight tube! Hindsight is always so perfect.
Click on any image for an enlargement.
*
5 comments:
Hooray! Life was beginning to become dull without the regular constructional updates from Fyn. I had mentally given you a month between pretty much finishing the folded refractor and announcing that you were starting another telescope project: you managed it within 3 weeks.
Reflectors are another kettle of fish, but a 10" ought to give the 7" refractor a good run for its money at least, particularly with a long focal length and relatively small flat. Is the all-up weight of the reinforced reflector going to be low enough for the MkIII mount to carry it so that you can do some real side-by-side comparisons? I think those are really enlightening and we see far too few of them in the hobby magazines, which often report scope trials at the level of "good views obtained of moon and Jupiter" which are hardly going to add to the body of knowledge.
Just one word of caution: don't get confused on a dark night and end up fitting the detachable mirror cell to the refractor and the lens cell to the reflector!
Best,
Andrew
Second comment following your overnight doubts and subsequent re-working: if I have understood correctly, we're looking at a full-length cardboard tube with two or three plywood surrounds that support an exoskeleton of flat aluminium cladding strips? Sounds plausible in terms of not bending - the only thought which crosses my mind is whether there is any possibility of the exoskeleton undergoing a twisting motion and managing to deform that way? The alternative to this I suppose is a truss design with two sets of trusses extending either side of a central balancing box positioned where the mounting meets the scope. There is always more food for thought...
Best wishes, Andrew
Hi Andrew,
No, the cardboard tubes will provide short cells but nothing more. The side beams are double box sections and remarkably stiff. Forcing these through close-fitting slots in four 12mm plywood rings provided excellent stiffness. When the rings are glued to the cardboard tube there would be considerable resistance to twisting. Dobsonian mounted I don't see the need for two more beams at top and bottom until proven wrong in [high magnification] practice.
Regards
Chris
Third time lucky - the design is changing more rapidly than my comments can keep up, and the photo of the nice long cardboard tube has now gone again. Is it being reserved for the project after thus one we wonder!
My comment on the two side bars and the partial tubes top and bottom is that I'd be very surprised you can achieve and sustain the very high rigidity needed between the side-pieces and the plywood rings, without at the least bolting a rigid right-angle bar in place at each joint to stop inline and twist distortions. But let's await developments...
Best, Andrew
Hi Andrew
The slots in the plywood are deliberately tight fitting. The rings are not even fixed/glued to the tubes yet but relying only on friction.
I placed the resulting 'OTA' across the raised sides of my trailer as a test bed and it really is difficult to twist or distort the present structure.
Once the real rings are cut very precisely and glued to new cardboard tubes the structure should be stiff enough.
A center section to support altitude bearings might be an idea and add further
stiffness.
I have already mentioned using opposing wedges to ensure no longitudinal slippage of the alloy beams in the ring slots.
Regards
Chris
Post a Comment