23.4.15

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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2.4.15

10" f/8 April 1st: Collimation is no joke!

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After a stormy day it suddenly calmed and cleared by 6pm. I could see the gibbous moon in the East, with Jupiter much higher up further to the South. A brilliant Venus was so high it easily cleared the trees to the West!

I had dinner first, to allow the sky to darken and then set up the 10" on the MkIV mounting. It was obvious from the start that the primary cell was sagging on its rails. Collimation changed with each movement of the telescope. So I roughly re-collimated for each object in turn. The Moon was blindingly bright with Plato well placed but very low contrast due to the almost overhead lighting. I had a fairly constant view of the central crater and glimpses of two more. There was obvious high frequency thermal boiling on the limb and terminator. I pushed the power up to 200x but sharpness was best at 150x. I returned to observing the Moon at intervals for the next hour and half without much improvement in sharpness.

Jupiter's belts were higher contrast than usual but the three visible Galilean moons were cross shaped and fuzzy. Jupiter changed shape each side of focus. Re-collimation did not help to tidy things up.

Venus was so bright that it was difficult to focus properly. A well rounded half ball of intense brightness at all powers which I tried.

It clouded over at 9.30pm as a light frost settled on the grass. I had been crunching remaining patches of snow underfoot from yesterday's 3-4" fall.

I really have to take the primary mirror out again and fix the cell really firmly to its rails. I just needs another large coach bolt further down the cell to improve its attachment.

I now use an old, parallel sided, artillery shell casing to lift the mirror clear of its cell. I set up the shell casing vertically and just lower the cell over it. The mirror rises out of the cell until the cell base is resting on the workbench. Exactly the same technique as is used to safely lift an objective lens out of its cell. Though the lifting device needs to be well padded in that case to protect the lens surface and any coatings. The primary cell needs to be removable of in both cases. Otherwise it might involve carefully lowering a 2 metre long OTA over the cartridge shell!

The brass shell casing has the advantage of being bottom heavy and is wider at the base for superb stability. I would hate for the mirror to tip as it was being lifted. Luckily the cooling fan aperture in the cell base provided a nice big hole for the shell casing to enter. My original intention was to use a bit of PVC plumbing pipe as a lifting device until the shell casing presented itself as the perfect tool. I used to collect cheap shell casings from flea-markets in the vain hope of being able to use them for the construction of an all-brass refractor one day. Unfortunately I have only got as far as an all-brass 7 x 50 finder.



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