Friday/Saturday build diary:

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Friday: With the OTA horizontal on the mounting, I hung a 5kg weight on a long cord from the nose of the 7" OTA. This was to simulate the weight of objective without risking the lens. Then I added the focuser and moved the sliding weights around to even them out.

I was one weight short of a set so made another yellow brass one in the lathe. 65mm Ø x 50mm L x 16mm bore. I was lucky enough to get a load of shortish lengths of round bar, yellow brass in a scrap yard some years ago. Dumped as leftovers from a college metalwork class I was told. There isn't much brass left now.

Being so close to the focuser, with a long rail and so far from the pivot, this weight will have a major effect on the main tube balance. Hopefully allowing preset positions for adding binoviewers, lightweight cameras and 2" star diagonals. Not to mention enjoying its major task of helping to balance the heavy objective.

The weights on the cradle are so near the axes that they act as fine adjusters rather than serious counterbalances.

Then I brought the long OTA back down from the mounting and fitted the objective lens. With the OTA propped off the floor of the observatory for safety in handling. It is an awkward load inside the stumpy dewshield feeding the long fixing screws through the plywood ring on the OTA. That's all I had time for today.

I'll have to start looking for a 24cm diameter saucepan or baking tin. To cover this thick, birch plywood, cell retaining rings. They work fine but aren't very pretty. I will need to make an 8" hole in the bottom of the pan and shorten the sides to make it look like a solid metal ring. It will be purely cosmetic so ought to be light enough not to affect the OTA's balance.

The ugly plywood ring behind the lens has been bugging me since I built the thing. The ring relied on the small flange on the end of the thin, steel, main tube for extra retention. The main tube came from old stock from a furniture factory, dust extraction system. Available in all sizes for mere beer money. Or rather, the weekly cake money fund for the factory staff.

Saturday: In between opening and closing the shutters, between heavy showers, I managed to lift the 7" OTA back onto the mounting. I used the block and tackle though I am still rather wary of the telescope falling. Because the pulleys are so "dinky" and free running. Their rating is at least ten times the OTAs weight. While I am holding tightly and supporting the OTA in case something breaks.

After that I looked at ways to mount the 6" without using the primary instrument's own tube rings. I seem to have enough room beside the cradle. With direct fixing of the 6" rings to the side of the cradle.

It will mean removing the sliding weight and bar from that side of the cradle. The weight can be added to the rail on the other side of the cradle to help to balance the offset 6".  Though it will be very close to the axes. With minimum need for more counterbalancing on the Dec shaft. A couple of holes were drilled in the newly bare side of the cradle and the 6" rings bolted on and the black OTA fitted.
Job done. It should have been more difficult. Like everything else.

I also fitted a 9x50 finder by bolting directly down through the little dovetail base onto the main tube. It meant filing a rectangle for the base stop projection in the steel tube. I also drilled through the tubular section to allow a long hex key/wrench to reach the screw hidden in the base with the finder removed. Better pictures tomorrow in daylight if the weather cooperates. We seem to be having a bit of a monsoon season for the next few days.

Talking of which: I checked the humidity of the inside of the dome cladding, 4mm birch plywood. Despite a run of wet and cold weather it has dropped right back from the maximum possible reading of 35% to around 20%. It may not be enough to kill the black mold but it is certainly a positive and quite unexpected improvement.

I have also been watching the digital thermometer and it shows a differential between inside and out of only 1-2 degrees F. It is presently hovering at just over 40F. It is very draughty in the dome as the wind blows readily up through the gaps and under the rubber skirt. The weather hasn't cooperated in allowing me to cut arcs of plywood to try and reduce the airflow around the edge of the dome.

My wife thinks I should make something more solid. Though I think the rubber actually helps by closing the gaps during a gust more than allowing them to open. Only a truly round skirt and top ring to the octagon would really help. I'll keep thinking about this in case a better arrangement occurs to me. The air movement should help in sunny weather. I just don't have any control over the degree of air movement at the moment.

Click on any image for an enlargement. 

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