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[12]
The affordable aluminium spar seemed almost too good to be true. It certainly offered stiffness with light weight. The profile is exactly twice the width of the common builders level. A central web increases stiffness across the narrower 18mm section. Ensuring the 100mm wide faces do not fold under load. The highest strength and stiffness is obviously in depth when the profile is on edge. The torsional stiffness is still an unknown until I have a length of straight edge to play with.
I have discovered an alternative and lower priced source of these straight edges. I can buy a 3m length for the price of the 2m elsewhere. I just have to confirm that there is a central web before purchase. My thoughts are to fix the spare 1m length to the MkIV's 60cm saddle if it proves necessary. Thereby extending the supported length dramatically with only a small increase in weight. (About 1kg or 2lbs) This saddle extension would best be biased towards the primary mirror end to help reduce torsion effects from the primary as the optical tube assembly rotates in use. It might be worth bonding the two profiles with epoxy rather than bolting them together. In practice it may not even be necessary to double them up.
The first and most obvious design problem to overcome was mounting the focuser without drilling the beam. I want the focuser to slide up and down the spar. Carrying the curved spider and secondary mirror along with it. Offsetting it to the side of the beam would require the primary mirror was also offset similarly. This would greatly increase torsion loads on the beam. The minimum offset would be over 3" or 75mm.
I could use two spars. Either in parallel or at right angles to each other. Double the weight and you double the stiffness. The weight would still be manageable but the clean, ultra-minimalist lines of the single spar would be gone.
I quite like the idea of two spars at right angles to each other with a decent gap between them. The focuser could look between the spars and still slide up and down. Having the beams at right angles greatly increased the tube stiffness in the weakest direction of the original beam profile. It would allow the primary mirror to be better supported without introducing torsional problems due to offset. Or, the focuser clamping/sliding block could be arranged at an angle to point at the optical axis.
The single spar had one other functional drawback. It placed the focuser on the underside of the beam when the telescope was pointing north. I obviously wouldn't want to use a star diagonal on my optimised skeletal Newtonian. This would limit the telescope to looking in an arc across the southern sky. Which, rather fortunately, is where the planets and moon normally reside. Though not exclusively.
Here is a simple drawing of the twin spar OTA, with one beam clamped to the saddle as before. This turns the focuser at 45 degrees to the saddle. The EP is now at 45 degree pointing upwards when looking directly north. Looking southwards the focuser is at a more comfortable 45 degrees to the OTA. The actual angle will vary depending on the direction and elevation of the object under study.
The problem now is that the spars need to be a long way apart to point the focuser at the optical axis of the primary mirror. (see the fine lines)
This greatly increases the difficulty of solidly connecting the two metal beams. The complexity may also increase the weight significantly. Again, the focuser clamping block could be arranged at an odd angle to point at the optical axis.
I think I prefer the single beam for the prototype. The focuser can be arranged at any angle to the beam using a dog-leg clamping arrangement to point at the optical axis. It could even run around a metal arc to allow a change in the angle of the eyepiece simply by loosening a clamp holding the focuser to the arc. Similar to the rotating head but simplified and more limited in its angle of coverage. The spider and secondary would be carried on the same clamp to ensure stability and collimation.
The distance between the single beam and the focuser arc is rather exaggerated here. The greater the distance of the beam from the optical axis, the more counterweighting would be required.
The focuser arc's radius should be close enough to the optical path to ensure full field illumination with an undersized secondary mirror. It can be treated as a small section of a complete 12" tube. Hopefully more lightweight and stable than a piece of the cardboard tube. Stiff aluminium is the most obvious material to use here. It must be capable of remaining round and stable while attached only to the sliding block on the beam. It must also be capable of carrying the spider and secondary mirror. Without distorting as the tube is moved all over the sky on an equatorial mounting.
A Dobsonian would be super easy (and stiff) using a spar on each side of the tube. The altitude bearings could be very simply attached to the spars using wooden clamps. This remains a serious fall-back option if the single spar fails miserably on an equatorial mounting. Which I still doubt thanks to the impressive size of these aluminium profiles.
I am quite excited at the possibility of having such a light tube but progress has been severely hampered by the snowy weather, gales and subsequent drifting. Hopefully I can get out in the car tomorrow. To purchase one of these straight edges from the local builders merchant.
Then I can clamp one end and check the lateral and torsional deflection under various loads. Fixing laser pointers to the free end of the beam during these trials will be very useful to amplify any resulting flexure. Though I do have both digital and analogue dial gauges to provide real numbers. I believe the lasers will best imitate the function of the spar while working as a telescope. If the spar flexes enough to throw the laser spot off a target then something must obviously be done.
The low cost and relatively light weight of these profiles do give the option of doubling up on the entire beam if it proves absolutely necessary. The resulting weight will still be a fraction of that of the 2m cardboard tube. I just have to perfect the distribution and triangulation of the loads applied to the spar in practice. Particularly at the mirror end. A 10lb weight (primary mirror substitute + cell) at 6" nominal radius from the spar top surface is not something to be ignored. We shall see...
I have discovered an alternative and lower priced source of these straight edges. I can buy a 3m length for the price of the 2m elsewhere. I just have to confirm that there is a central web before purchase. My thoughts are to fix the spare 1m length to the MkIV's 60cm saddle if it proves necessary. Thereby extending the supported length dramatically with only a small increase in weight. (About 1kg or 2lbs) This saddle extension would best be biased towards the primary mirror end to help reduce torsion effects from the primary as the optical tube assembly rotates in use. It might be worth bonding the two profiles with epoxy rather than bolting them together. In practice it may not even be necessary to double them up.
The first and most obvious design problem to overcome was mounting the focuser without drilling the beam. I want the focuser to slide up and down the spar. Carrying the curved spider and secondary mirror along with it. Offsetting it to the side of the beam would require the primary mirror was also offset similarly. This would greatly increase torsion loads on the beam. The minimum offset would be over 3" or 75mm.
I quite like the idea of two spars at right angles to each other with a decent gap between them. The focuser could look between the spars and still slide up and down. Having the beams at right angles greatly increased the tube stiffness in the weakest direction of the original beam profile. It would allow the primary mirror to be better supported without introducing torsional problems due to offset. Or, the focuser clamping/sliding block could be arranged at an angle to point at the optical axis.
The single spar had one other functional drawback. It placed the focuser on the underside of the beam when the telescope was pointing north. I obviously wouldn't want to use a star diagonal on my optimised skeletal Newtonian. This would limit the telescope to looking in an arc across the southern sky. Which, rather fortunately, is where the planets and moon normally reside. Though not exclusively.
Here is a simple drawing of the twin spar OTA, with one beam clamped to the saddle as before. This turns the focuser at 45 degrees to the saddle. The EP is now at 45 degree pointing upwards when looking directly north. Looking southwards the focuser is at a more comfortable 45 degrees to the OTA. The actual angle will vary depending on the direction and elevation of the object under study.The problem now is that the spars need to be a long way apart to point the focuser at the optical axis of the primary mirror. (see the fine lines)
This greatly increases the difficulty of solidly connecting the two metal beams. The complexity may also increase the weight significantly. Again, the focuser clamping block could be arranged at an odd angle to point at the optical axis.
The distance between the single beam and the focuser arc is rather exaggerated here. The greater the distance of the beam from the optical axis, the more counterweighting would be required.
The focuser arc's radius should be close enough to the optical path to ensure full field illumination with an undersized secondary mirror. It can be treated as a small section of a complete 12" tube. Hopefully more lightweight and stable than a piece of the cardboard tube. Stiff aluminium is the most obvious material to use here. It must be capable of remaining round and stable while attached only to the sliding block on the beam. It must also be capable of carrying the spider and secondary mirror. Without distorting as the tube is moved all over the sky on an equatorial mounting.
A Dobsonian would be super easy (and stiff) using a spar on each side of the tube. The altitude bearings could be very simply attached to the spars using wooden clamps. This remains a serious fall-back option if the single spar fails miserably on an equatorial mounting. Which I still doubt thanks to the impressive size of these aluminium profiles.
I am quite excited at the possibility of having such a light tube but progress has been severely hampered by the snowy weather, gales and subsequent drifting. Hopefully I can get out in the car tomorrow. To purchase one of these straight edges from the local builders merchant.
Then I can clamp one end and check the lateral and torsional deflection under various loads. Fixing laser pointers to the free end of the beam during these trials will be very useful to amplify any resulting flexure. Though I do have both digital and analogue dial gauges to provide real numbers. I believe the lasers will best imitate the function of the spar while working as a telescope. If the spar flexes enough to throw the laser spot off a target then something must obviously be done.
The low cost and relatively light weight of these profiles do give the option of doubling up on the entire beam if it proves absolutely necessary. The resulting weight will still be a fraction of that of the 2m cardboard tube. I just have to perfect the distribution and triangulation of the loads applied to the spar in practice. Particularly at the mirror end. A 10lb weight (primary mirror substitute + cell) at 6" nominal radius from the spar top surface is not something to be ignored. We shall see...
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