7" f/12 Istar folded refractor 21: Objective 'bayonet' plates completed.

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I spent a rainy day working on the bayonet plates. These are for mounting the objective on the front of the Porsa framework. The previous domed screws were first replaced with countersunk screws. This was to ensure no screw heads would interfere with the sliding action.

The first image shows the location 'keyholes' in the OTA plate for the projecting, 'bayonet' socket head screws. The screw heads go easily though the large holes at the top of the keyhole slots. When the bayonet plate is lowered, the screw heads cannot pull back out through the much narrower slots. The square shoulders under the screw heads provide a firm grip and ideally fixed location without any 'slop.' [Any unwanted movement might affect collimation.] The screw heads are 8mm in diameter so I made the large holes 9mm to allow easy fitting and removal. Trying to be mean with the top hole sizes of the keyholes will only make extraction of the 'bayonet' plate screws foolishly difficult. Moreover, they have no effect [at all] on locking the 'bayonet screws safely in the lower end of the grooves.

Then I decided to pack out the bayonet plate to produce a fixed distance from the OTA plate. Self-adhesive, hard felt, furniture protectors proved to be the ideal solution. These have no "give" in them so provide repeatedly accurate location of the objective via the bayonet plate.

These packing pads also offer a wonderfully smooth sliding action. Thereby avoiding the likely grating which would occur between two, soft, aluminium surfaces. Adding further pads will ensure that the 'bayonet' plate cannot sag nor compress the existing pads. As a side effect the pads also allow room for lock nuts on the ends of the collimation 'pull' screws. Thus offering a secondary level of security in the unlikelihood of a thread stripping in the bayonet plate.

Here the bayonet plate is laid on the OTA plate with the screw heads in the keyholes. The plate is not yet slid downwards to lock the screw heads in the lower ends of the slots.

The ordinary hex nuts on the 'bayonet' screws will be replaced with stainless steel. Zinc plated fixings soon rust.

Note the eccentricity of the two 8" holes until the 'bayonet' plate is slid down relative to the OTA plate.

I had to snatch pictures between showers or I would have orientated them more logically.

In this image the bayonet plate screws are fully home in the bottoms of the narrow slots. Where the square shoulders of the undersides of the bayonet screws ensure secure location.

The objective will be collimated on the 'bayonet' plate when they are both seated on the mounted OTA. After that there should [ideally] be no need for re-collimation between removal and refitting of the objective. The cell will be attached to the 'bayonet' plate via its collimation 'push-pull' screws as if attached permanently to the OTA

In fact the bayonet plate acts just like a counter-cell in normal, "straight tube" refractor practice. Except that it slides vertically downwards to fix and locate the objective on the front of the OTA framework.

I chose the plate size to provide a safety margin beyond the 198mm / 8" holes necessary for the objective cell clearance. The OTA plates rests on the recessed, Porsa tube flanges for neatness. I thought this far better than mounting the plates on the surface of their plain, square tubing. 

The objective was then assembled to the 'bayonet' plate with the collimation 'push-pull' screws. Only to find the projecting rear of the cell obstructed the OTA plate! This required a combination of doubled sticky felt pads and deeper adjustment of the 'push' screws. The cell then clears the OTA plate nicely and slides very smoothly on the hard felt pads.

The 'bayonet' plate makes for much safer carriage of the lens indoors and out but feels rather sharp on the bare hands. Carrying the bare cell always felt really insecure due to its considerable weight and shape. I thought I might add some U-shaped, metal drawer handles to the top and bottom of the 'bayonet' plate. This will give a comfortable and secure grip when handling the objective and its plate. I will need to ensure the handles do not interfere with the 10" diameter dewshield and its fixing. If I re-use the saucepan base, to hold the dewshield, I may be able to attach handles to that.

The necessary gaps to ensure rear cell clearance are obvious in this picture.

The pull screws are only temporary until I can obtain some more stainless steel, socket head fasteners of the correct length.

It will be important that the lens in its cell is treated gently between attachments to the OTA to avoid spoiling the collimation. The lens, in its cell, will remain permanently attached to the bayonet plate. Being brought out only when the bare OTA is already placed on the mounting. The quick release 'bayonet' system will save me having to carry the 10lb burden of the objective while still attached to the OTA. That extra weight is not insignificant when the entire OTA must be carried in and out. Not to mention struggling to lift it onto the mounting.

Click on any image for an enlargement. 

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