12.10.19 Mounting re-alignment continued.

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Saturday: Mounting alignment adjustments aren't a daily occurrence in a fixed observatory. So there is really no need to apply micro-adjusters to the mounting itself. All it really needs is some means of monitoring azimuth rotation of the base plate during adjustment. An arm extending from the plate, provided it is stiff enough, can be carried over to the observatory wall for measuring rotation relative to a simple indexing mark.

The means of rotating the entire mounting becomes almost irrelevant provided it is smooth and finely adjustable. An indicator moving across a simple scale will suffice to check movement. The sheer weight of the loaded mounting makes small radius adjusters likely to be rather hard work. The accuracy of any measurement directly on the mounting is likely to be equally as poor. While a 1.5m [5'] radius arm can show really tiny changes in the mounting's pointing direction. The arm would be removed once the adjustment is made. There being no need for it until it is considered worth changing the alignment again.

The sheer effort required to remove the three telescopes, counterweights and heavy shafts is a very serious hurdle to making any changes. I would like to change the central pivot point on the base plate but it would mean hours of heavy lifting just to be able to do so. It would only mean a slightly better balance point relative to the base plate.

A counterbalance would have to be really massive to balance the present offset. But it occurs to me that I could run a couple of screws [or studs] down from the baseplate. The thick plywood top of the pier can then resist any tipping forces on the base plate itself. This would only be be done once the mounting is accurately re-aligned. To avoid the screws or studs resisting base plate rotation.

With a little more foresight I could have had a tipping top plate for the pier. Though I do have a fine altitude adjuster on the PA in the form of a turnbuckle. The 16mm PA altitude pivot, axle clamping nuts just need to be slackened off prior to making any changes.

We are getting some weak sunshine now. So I may be able to make a start on re-alignment using SharpCap and the sun. It's a shame about the gusty, southerly wind blowing straight into the slit. At least the rain has stopped for a while. I completely filled a standard bucket this morning with the contents of the sandwich boxes resting on the base ring.

This followed 40mm [1.5"] of overnight rain. At least I feel in control of the dome leakage now. What is collected in the shallow plastic tubs can't do any harm to the structure of the building. Fortunately the telescopes haven't been threatened with drips yet. The spiders seem to like the fact that the centrally mounted instruments stay dry. Because I am usually sitting and facing the light the fine strands of their webs are beautifully highlighted.

As soon as I was set up after lunch, just after 14.00, the sky darkened and it started raining again! Hours later I was still struggling to find an objective with a short enough focal length to match the solar disk to SharpCap's ring & crosshairs reticule.

I took a quick snap of the SharpCap screen with the ZWO camera fitted to the 90/11 1000mm Vixen. The mismatch in solar disk diameter is clearly visible. Imagine how large the solar disk appears with longer focal lengths and added Barlows! The 7" has a focal length of 2.16 meters. The sun on the monitor when the ZWO has a Barlow on its nose is huge!

Fortunately I remembered I had an old, possibly Skywatcher, 9x50 finder in my optical junk collection. This would do but had an odd tube diameter. Just undersize for a 2" push fit. Bigger than 2" OD. With the screw-in eyepiece removed the camera just focuses on infinity when pressed up against the end of the finder's main tube.

Now I just need to turn down a 2-1.25" eyepiece adapter to fit inside the finder tube. It would make the finder much more useful if it had a standard 1.25" eyepiece fitting with locking thumbscrew. So I might as well aim for that. The finder base can easily slot into the Vixen 90's focuser dovetail.

The sun finally broke through the cloud at 16.45! For just long enough to confirm the solar image diameter matched the reticule. I'll need a scrap of Baader solar foil to safely filter the finder for the camera. Then all I need is more sunshine.

Sunday: All day cloud with rain later. So I took the opportunity to turn a 2/1.25" adaptor down to fit inside the finder tube. It was very close to not reaching infinity focus. I even had to turn a recess for the camera nose plate in the outer face of the adapter before I could get a sharp view of distant woods. That also required I remove the objective focusing, locking ring. After that it worked perfectly with just a margin of excessive travel and a nice, sharp image of the neighbourhood.

Now all I need is some sunshine to put the mounting alignment procedure into practice. It is not looking at all promising at the moment. With wall to wall, solid grey overcast and spots of rain in the air.

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