Dome build: Observation slit mock-up.

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The observation slit provides the view of the sky from within the enclosed confines of the dome. Its minimum width is important for a number of reasons. Not least the difficulty of keeping an object in view as the sky passes across the slit over time. Though in reality it is the rotation of the Earth which gives this illusion of sky movement from East to West.

If the slit is too narrow then the dome must be moved so often it becomes a bore. Many amateurs prefer to drive the dome to avoid having to monitor and manually rotate the dome during observations. Those who image from a remote location will demand a dome rotator which closely follows the telescope's field of view. This will change depending on the altitude of the object being imaged or studied.

For simplicity I have chosen a slit width of 60cm or 2' which is the same as the normal gores. The mock-up was to see how such a slit, or rather its framework, would affect the rest of the ribs.

The gores on either side of the slit have no need of an adjoining rib so these nearest slit can simply be left out. Though the opposite side of the adjoining gores will still have a normal rib which stops at the slit framework. The two gore's horizontal struts can be carried right over the intervening inch to stop at the slit framework. To be fixed there as if they were normal ribs. Though the miters on the slit end will be at different angles to normal ribs. 

As mentioned previously, the slit framework must be strong enough to compensate for the missing struts which would otherwise cross the slit. More than that, the framework carries the tops of all the ribs where they are cut away to allow viewing at the zenith. [i.e. The slit must be open overhead.]  So none of the ribs actually reach the dome's pole. In the image above I have pulled the base ring forwards by a 30cm [one foot] to simulate the open zenith. The arched sides of the slit's framework have been extended to compensate. Perhaps a foot [30cm] extra clearance, for zenith viewing,  is too much?

It would be safer to match the slit framework to the complete dome skeleton to ensure geometric accuracy. The top of the slit can be left square. Or a rounded collar could be formed from laminated plywood glued in enough layers for adequate strength. Weatherproofing at the zenith is obviously important as any leaks will fall directly onto the mounting or telescope, or both.

Click on any image for an enlargement.

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