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Friday 12th 48F, very heavy overcast and raining steadily. The early mist has cleared to a dark and depressing day.
I went out to work on the new dome but even with the LED floodlight it was miserable in there.
The warped slit rib has now been put under heavy stress using large G-cramps and a 2x4. The bend is being overcompensated in the opposite direction by the pressure and a spacer at the bottom. If the rib doesn't straighten out after this I shall just have to make a new section. The difference is 96mm at the warped bend compared to a 93cm average slit width. It doesn't sound like much but the bottom of the rib is visibly sloping steeply inwards.
I was getting bored and didn't want to open the shutters in the rain. So I mocked up a cross-axis polar axis with a couple of planks. The 55° PA angle was set with an adjustable angle level and checked with a protractor and builders level. With the top of the PA just touching the dome I could judge where the PA's foot would fall on the observatory floor.
It must be remembered that the floor is 5' lower than the present ground level. So I measured the spacing on the ground and discovered the necessary extension as one meter towards the dome's skirt. It must fall well within the skirt. Because I need to support the foot of the PA with heavy posts rising from the ground under the observatory.
Some clearance is desirable to be able to walk easily around the foot of the PA. I also want to judge how much headroom there is under the PA for moving about freely. The telescopes sweep out a circle at various heights depending on the season and solar altitude. So it is not always easy to move about under the sloping PA beam. Even though it will be about 5' higher in reality.
If I conclude that the space available is too cramped for comfort I may just remake the big German mounting with a much longer PA. In the classical style. Though I'd much prefer a cross axis. Simply because it is the only design which allows long telescopes to balance each other on opposite sides of the PA.
Other mountings demand counterweights. Opposed instruments would collide with the mounting and pier. The great length of the cross-axis PA increases stiffness if made sufficiently strong. Easily achieved with a heavy plywood, tapered, box beam construction. Weight and bulk are not an issue because a cross axis isn't meant to be portable.
Later: Just before it became dark I set up the 360° laser. With a small mirror resting against the lower drawer slides' supporting 2x4. The vertical beam was reflected back to the middle of the rear of the dome. A useful exercise just to confirm the drawer slides really were square to the observation slit. I had repeatedly measured it with the laser rangefinder and the double cord plumb line and it seemed fine.
The shutters were almost beginning to move as desired but still had a 50mm [2"] gap in the middle. From experience gained, before the drawer slides were fitted, the outer shutter ribs need to be raised or bodily rotated.
Having had no luck with height adjustment I shall aim for rotation tomorrow. That will require a wedge between the slides and the shaped form of the bottom boards. If I can find a suitable arrangement then I can cut out new bottom boards from solid timber.
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