10" f/8 As you were!

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Last night was incredible with "too many stars to recognise the constellations." I went outside at 11pm to check for the Borealis but there was none visible. The crescent moon was keeping company with a dazzling Venus. No doubt Mercury was nearby but I was too tired to set up a telescope.

I have given up trying to connect the beams to the lower cell sufficiently rigidly to cure my collimation issues. The cantilevered cell would sag, mostly due to the weight of the mirror applying torque. The misalignment would depend on which side of the pier the OTA was set. Having to re-collimate almost literally every time I moved the telescope was becoming a real bore.

I have resolved to rebuild the 10" F:8 OTA as a Dobsonian with a single beam on either side of the cells. Not having to hang the telescope from an equatorial will resolve many of the issues with which I am struggling. If I want a driven telescope for imaging I shall just have to build a platform. My earlier denouncement of such systems was based on a fear of heights when observing at higher altitudes.

In fact the arrangement of a Dobsonian on a driven platform might be the answer to my desire for a well raised platform or observatory to obtain a better view of the sky. An equatorial mounting needs a very stiff pier to completely avoid flexure. The taller the pier, the greater the chance of flexure and the greater likelihood of building a compound pendulum. That is a device with a very top-heavy arrangement above a lower mass. One which has a low natural frequency as the weight of the entire telescope and heavy mounting literally swing back and forth. The concrete block in the ground cannot be made massive enough to resist rotation about some point just above the foundation. The metronome is the most common type of compound pendulum. It's very low frequency can be adjusted by raising or lowering the upper weight. The natural frequency of a telescope and mounting would change relatively little due to adding or removing small weights like heavy eyepieces or cameras.

While the Dobsonian spreads the load more evenly through a ground board. It cannot swing because it has a very low natural centre of gravity relative to the ground. A raised platform can be made inherently stiff and then surfaced with foam to kill vibration from footfall excitement. There can be no extended vibration once the observer settles at the eyepiece. The Dobsonian mounting does not introduce beam flexure modes since there are no cantilevered masses suspended from long beams or rods. The platform itself must resist swaying on its multiple legs by careful triangulation by bracing each corner.

Meanwhile I shall return the 6" F:8 Celestron refractor to the Fullerscopes MkIV equatorial mounting. Just to have something ready to view the sky at short notice. Though still quite heavy the 6" refractor is relatively easy to drop into the open mounting rings and begin observing. 

Lifting the MkIV so high to place it on the 6'+ welded steel refractor pier has always been a struggle due to its weight. I just don't want to do this manually any more. So I set up my two builders folding stepladders as a support for a 3-way block and tackle. This allowed me to lift the MkIV straight off the low, angle-iron pier and hoist it straight up onto the much higher refractor pier. Once I had one bolt dropped through the MkIV's base into the pier flange it went very smoothly. I had to remove some counterweights due to the refractor having a lighter OTA. Or, more accurately, rather less overhang.

The stepladders have crossbars with rubber feet at each end making them incredibly stable. I am easily able to climb the fully extended stepladders with hardly a wobble once they are lashed together at the top. The untidy image shows how dense the trees are to the north of my usual observing site.

Click on any image for an enlargement.

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