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Thursday: A dry, but windy day with sunny periods. I decided to take the dome stepladders down. This allowed me to swing the telescope [tube] freely. First I set the tube vertically on the bare tailpiece resting on the floor. Then clamped the top of the tube in the lower tube ring. This allowed me to fit the objective lens in its stumpy dewshield on top of the tube while working comfortably from a short stepladder.
Next I added two 5kg counterweights to the end of the Dec shaft. During this step, the tail end of the tube was tied firmly to one leg of the pier to avoid the tube taking flight. The next stage was pushing the tube bodily up through the rings while pointing at the Pole. I could now safely pinch the tube rings to stop the tube sliding up or down.
This was followed by my fitting the focuser in its collimating and rotating tailpiece to the tube. It needed a small longitudinal adjustment of the OTA in the rings to lift the focuser to a better height. Before balancing with the tube weights on the cradle. Finally, I could clamp the tube firmly in the rings and the mounting was done.
I still need to collimate the optics of course. Then decide on the best position to fit the 6" H-alpha, solar telescope. I don't think it would be sensible to mount the 6" directly outboard of the 7". This would considerably increase the moment [mass x distance from the pivot] and require many more balance weights.
A more sensible, side-by-side arrangement needs some stiff mounting bars to fit on the cradle. The moment rises but would be far more modest in proportion. Probably requiring only one extra 5kg counterweight over the two already required for the 7".
Or, I could use two 8" rings to mount the 6" beside the 7". This would save me having to make crossbars for the two instruments to sit on the cradle. The 6" can be adjusted to fit snugly beside the 7" simply by rotating the rings as appropriate.
In the end I fitted the 6" onto the side of the saddle in place of one tube balance rail and weights. I checked carefully and the 6" doesn't contact the mounting even in the 'weights up' horizontal setting.
The images show the telescopes pointing at the zenith. On both the east and the west of the mounting. When it is to the west I could stand upright on the stepladder for a more comfortable viewing posture.
The 7" could still be lifted by another 6" and even more if the pier were not so far to the south in the dome. I placed the pier in the center of the dome. Which means the dewshield is much nearer the northern upper dome but clears the southern side by a good margin. This is the result of the northern offset of a GEM. [German Equatorial Mounting.]
I could overcome this problem by adding a new, very thick, plywood top plate with a supported overhang to the south. Only time will tell whether the offset becomes a noticeable nuisance.
The present situation certainly shifts a refractor eyepiece nearer the northern observatory wall. Which matters far more than a safe dewshield clearance up high. Lifting the tube through the rings reduces clearance at the top but progressively increases eyepiece clearance at lower pointing angles.
I'll take some daylight images of the big GEM to confirm the northern offset.
Next day: Such images merely confirmed that the offset lies in the area of 3-4". Worse, a check of the pier, relative to the octagon, proved an error of 7cm in a northerly direction. So moving the whole mounting towards the south would require a 6" movement to cancel both offsets.
Unfortunately the entire [huge] pier cannot be moved. Nor can it be tipped so far towards the south without serious woodwork changes. Which would throw the top plate well out of level anyway.
The northerly offset is also the CofG offset from the center of the pier. So, in theory, the mounting and its heavy burden of instruments and counterweights is also offset to the north. Which, in a paranoid moment of pure pedantry, might suggest the pier is being pulled slightly northwards.
How to support the massive load while I replant the mounting base plate and 16mm central stud? I have no idea, but I'm certainly not risking lifting it by a couple of eye-bolts at the top of the dome!
This was followed by my fitting the focuser in its collimating and rotating tailpiece to the tube. It needed a small longitudinal adjustment of the OTA in the rings to lift the focuser to a better height. Before balancing with the tube weights on the cradle. Finally, I could clamp the tube firmly in the rings and the mounting was done.
I still need to collimate the optics of course. Then decide on the best position to fit the 6" H-alpha, solar telescope. I don't think it would be sensible to mount the 6" directly outboard of the 7". This would considerably increase the moment [mass x distance from the pivot] and require many more balance weights.
A more sensible, side-by-side arrangement needs some stiff mounting bars to fit on the cradle. The moment rises but would be far more modest in proportion. Probably requiring only one extra 5kg counterweight over the two already required for the 7".Or, I could use two 8" rings to mount the 6" beside the 7". This would save me having to make crossbars for the two instruments to sit on the cradle. The 6" can be adjusted to fit snugly beside the 7" simply by rotating the rings as appropriate.
In the end I fitted the 6" onto the side of the saddle in place of one tube balance rail and weights. I checked carefully and the 6" doesn't contact the mounting even in the 'weights up' horizontal setting.
The images show the telescopes pointing at the zenith. On both the east and the west of the mounting. When it is to the west I could stand upright on the stepladder for a more comfortable viewing posture.The 7" could still be lifted by another 6" and even more if the pier were not so far to the south in the dome. I placed the pier in the center of the dome. Which means the dewshield is much nearer the northern upper dome but clears the southern side by a good margin. This is the result of the northern offset of a GEM. [German Equatorial Mounting.]
I could overcome this problem by adding a new, very thick, plywood top plate with a supported overhang to the south. Only time will tell whether the offset becomes a noticeable nuisance.The present situation certainly shifts a refractor eyepiece nearer the northern observatory wall. Which matters far more than a safe dewshield clearance up high. Lifting the tube through the rings reduces clearance at the top but progressively increases eyepiece clearance at lower pointing angles.
I'll take some daylight images of the big GEM to confirm the northern offset.Next day: Such images merely confirmed that the offset lies in the area of 3-4". Worse, a check of the pier, relative to the octagon, proved an error of 7cm in a northerly direction. So moving the whole mounting towards the south would require a 6" movement to cancel both offsets.
Unfortunately the entire [huge] pier cannot be moved. Nor can it be tipped so far towards the south without serious woodwork changes. Which would throw the top plate well out of level anyway.
The northerly offset is also the CofG offset from the center of the pier. So, in theory, the mounting and its heavy burden of instruments and counterweights is also offset to the north. Which, in a paranoid moment of pure pedantry, might suggest the pier is being pulled slightly northwards.
How to support the massive load while I replant the mounting base plate and 16mm central stud? I have no idea, but I'm certainly not risking lifting it by a couple of eye-bolts at the top of the dome!
Click on any image for an enlargement.
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