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Monday: Grey again. 6" H-alpha OTA: Summary:
I have decided to use the steel tube rather than make a skeleton tube at this stage. At 160mm bore the 168mm rear of the cell cannot enter the tube for support. That means a fixed ring must be added to the outside of the tube. The ring must overhang beyond the lens end of the tube.
I have decided to use the steel tube rather than make a skeleton tube at this stage. At 160mm bore the 168mm rear of the cell cannot enter the tube for support. That means a fixed ring must be added to the outside of the tube. The ring must overhang beyond the lens end of the tube.There is a small, curved "flange" on each end of the main, steel tube. One flange will disappear when I cut the 2m tube shorter to match the 1.5m focal length. The best end of the tube will be chosen for the main telescope tube for cosmetic reasons.
Removing one flange offers a distinct advantage. Because the whole outside of the tube becomes accessible to slide tightly fitting, closed plywood rings along its entire length. Until stopped by the remaining flange behind the objective.
The collimation screws will push and pull on the plywood ring but will not affect the fixing of the plywood ring to the main tube. So radial fixing screws will be needed. Removing the lens cell will allow access to internal nuts.
The lens cell side of the plywood ring will be faced with aluminium to resist the collimation push screws. The central hole in this aluminium ring will be sized at ~168mm to help support the rear of the objective cell. This will require a reasonable overhang beyond the face of the tube flange.This can be achieved by adjusting the thickness of the plywood ring or by lamination of different sized rings.
The plywood ring can be produced in different ways: Using a router with a center pin to form a circle cutter. Or a rough, jigsawed ring can be cut out, trimmed and smoothed on the lathe. The latter depends on the lathe's maximum capacity for external cuts. Also on my ability to hold the center and outside with a 3 or 4 jaw chuck.
The plywood ring must be at least 195mm diameter to support a dewshield. Preferably without any contact with the outside of the objective cell. Freedom to reach the M5 collimation screws is essential. Or the dewshield made easily removable?
Long series hex drivers for collimation adjustment are highly desirable. I struggled with the 7" refractor on this point. The dewshield was fixed by the cell and too deep for my existing tools. So I would stack the M5 hex driver on the end of driver extensions. Which made them too floppy and very difficult to locate in the distant hex screw heads. Particularly in the confines of the deep, dark, matt black painted dewshield.
Matt black, thin aluminium baffles will be spaced on long threaded rods with opposing nuts for location, fixing and adjustment. This worked well on the 7". Edge notching of the baffles would allow air currents to move freely inside the main tube. Failing to notch forces thermal air currents to pass through the main baffle holes within the light beam. The notches must be rotated on each, consecutive baffle. This avoids direct, stray light from passing right down the length of the tube from the objective.
Long series hex drivers for collimation adjustment are highly desirable. I struggled with the 7" refractor on this point. The dewshield was fixed by the cell and too deep for my existing tools. So I would stack the M5 hex driver on the end of driver extensions. Which made them too floppy and very difficult to locate in the distant hex screw heads. Particularly in the confines of the deep, dark, matt black painted dewshield.
Matt black, thin aluminium baffles will be spaced on long threaded rods with opposing nuts for location, fixing and adjustment. This worked well on the 7". Edge notching of the baffles would allow air currents to move freely inside the main tube. Failing to notch forces thermal air currents to pass through the main baffle holes within the light beam. The notches must be rotated on each, consecutive baffle. This avoids direct, stray light from passing right down the length of the tube from the objective.
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