Building the Octagon Pt.64 Kung Fu wobbling.

*

I have delayed publication of two posts on building the roof until I can obtain some Cellotape. Paper models of bi-pitched octagonal roofs, using only copying paper, proved hard to work with in a Cellotape vacuum. You need tabs for gluing, as any child brought up on cardboard cereal packet models, knows full well. Moving swiftly on:

It was late afternoon when I spotted a pale half moon only 20 degrees up and almost due south. The earlier rain had gone with well spaced plates of thin cloud crossing from the SW. 

Contrast was bound to be low but it must be worth a try. So I dragged a large cardboard box of drive boxes, cables and eyepieces 'upstairs' and ran an extension lead.

The milky Moon didn't want to stay in the field of view for ten minutes so I needed to improve the haphazard mounting alignment. My laser rangefinder has an accurate clinometer so I started by getting the PA up to 55° instead of whatever it had drooped to since first being perched on its pyramidal stilts. I arranged the telescope over the mounting and pointing [roughly] at the pole to use that as a huge lever. Just one of the advantages of oversized ATM refractors. Don't do this at home without adult supervision! 

The tracking was slightly better after that but still wandering gently away. I had set up the azimuth to some arbitrary pointing at the solid canopy of trees. So out came my compass. Once I had distanced myself from the massive steel OTA [tube} I could safely assume the needle was pointing north. Then I had to turn the whole mounting so that it almost matched my arbitrarily aligned pyramid. Magnetic deviation? No idea until I look it up. I'd have to stay up until midnight for it to get dark enough to see the Pole Star and it was getting cloudier by the minute. There was also the matter of showering before dinner and could I possibly fit both in before July became August?

There followed further trials with the AWR paddle and several unexpected bleeps from crossing multiple meridians. The Moon now stayed "almost put" in the field of view of a 32mm Plossl at 69x. As I returned to the eyepiece after going back down to fetch a hex key I noticed the image was wobbling briefly. Only for about 2 seconds with rapid damping but it shouldn't be happening. I checked carefully around the floor's clearance gaps from the pier's pyramid posts. Weird, but everything looked fine.

I stamped my foot. Image wobble. I stamped again. More image wobbles. Then I notice my Bailey Bridge of 2x8s were almost invisibly shorting the ladder to the pier over the stairwell. I swapped one 2x8 for a 2x6 and moved them both away from the pier legs. Stamping my feet and even jumping up and down now showed no image movement. It's lucky I have almost total privacy in my back garden or the men in white coats would have been only a phone call away by now. "He's having another temper tantrum up on his tree platform!!" Little do they know..

Time to check the pier itself for stability. Being so far from the pier at the eyepiece on the end of a 7" f/12 refractor called for special measures. My head would almost have been touching the observatory wall had one existed. So I gently tapped the pier with the bottom of my shoe at full leg's length. The Moon wobbled by 1/8th of the field of view but rapidly settled. Hmm. I gave it a harder kick. Much the same.

No more infantile sissy kicking! Time for a serious bit of Kung Fu testing! I gave the pier a couple of really hefty Hollywood style, full-on, martial arts kicks. The same moon wobble occurred, but died back to completely still within two seconds. Can you kick your pier viciously with such positive results? How tall is your pier? I think we can safely say that mine gets the job done.

Early wind was slowly reducing as I played endlessly up in my open tree house. Earlier, 15mph gusts had caused a short wobble of the moon in the eyepiece. Again it was rapidly damped. If I touched the focuser I could instantly stop the wind wobbles. There was no problem holding the focuser continuously as far as the image stability went. I wouldn't be doing any imaging with such [slight] wobbling but it was still perfectly usable for visual observation. 

There is definitely some backlash at the worms because I never finished the screw pressure adjustment. The unwanted freedom at the wormwheel teeth may have allowed enough slack for the crosswind to affect the 7' long 8" Ø tube and added 10" dewshield.

I pushed the power up to 110x and the moon remained sharp but still pale and subject to rapid thermal 'boiling.' The Moon was by now sinking ever lower to the West, the nearby trees beckoned and the cloud was becoming a case of more often than not. I packed everything back into its cardboard box, put a bin bag on the vertical OTA and went in for a late shower and dinner. This is not really my idea of instant gratification. I want to be able to leave the cardboard boxes upstairs safely protected from the weather.

Click on any image for an enlargement. 

 

*