7" f/12 iStar folded refractor 14: Square tube objective bayonet fitting.

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"Round" is far too narrow a definition of the meaning of a lens bayonet.

For a square tube there is absolutely no need for cell rotation. Simple, vertical 'keyholes' are cut in the permanently mounted, OTA, objective support plate instead. See image alongside for a rough idea of what I mean. Choose your own dimensions and layout to suit your own circumstances, materials, skills and OTA construction.

A second, 'bayonet' plate is attached to the objective cell to deal with both collimation and attachment to the square OTA. This plate [or ring] acts as an intermediary device to hold the projecting 'bayonet' screws as well as provide collimation. [Just as it did with the round tube, bayonet form.]

Most larger refractors will attach the objective cell via a counter-cell and collimation screws. This arrangement may not readily lend itself to a bayonet fitting due to the small flange size involved. The socket head, 'bayonet' screws must project equally from the back of the bayonet plate completely independently of the collimation screws.

Exactly as with the 'round' rotating bayonet system, the bayonet screw heads pass straight through the larger, round part of the vertical keyholes. These keyholes are cut into the square OTA's objective support plate. Then the entire objective assembly is gently lowered until stopped by the bayonet screw shanks reaching the bottom of their respective slots. Now the screw heads are safely retained by the narrower dimensions of the OTA slots. Only by lifting the entire objective assembly will the screws pull freely out of the round tops of the keyhole slots.

If you are prepared to use larger headed screws ensure that they have flat undersides to their heads. Definitely NOT conical countersunk screws which is just asking for trouble! You must also ensure that you have enough 'meat' in the OTA support plate to accept much larger keyholes without weakening the plate. I chose socket head screws primarily because they offer neat, round heads, often with small chamfers to easily pass through the round 'tops' of the vertical keyholes. They have easily enough shoulder width, under the heads, not to be easily pulled out though the narrow slots. Your mileage may vary, depending on your own skills in making reasonably accurate slots. Milling is unnecessary and filing or fret sawing will probably do for aluminium if care is taken in marking out the slots first.

The bayonet screws could be reversed so that they project from the OTA's front plate but I strongly advise against it. The jutting screws could very easily scratch the back of the objective glass while fumbling with a heavy objective assembly in the dark. Only if one could guarantee that the bayonet screws can never reach the glass would I ever consider this arrangement. A suitably thick bayonet plate might ensure safety in this matter. But! Do exercise great care if you do choose to have the screw heads projecting from the OTA.

It is also vitally important that the objective cell does not project behind the bayonet plate. Otherwise the large hole in the OTA objective support plate would need to be made oval. The projecting rear of the cell would stop the objective cell from dropping down if it was sitting inside a large round hole. Which would deny the bayonet screws the chance to safely enter their narrower, locating slots.

In the case of the iStar 7" objective the rear of the lens cell extends by about one centimeter beyond the rear face of the collimation screw flange. The length of the collimation screws and thickness of the square bayonet plate must be arranged to allow the rear of the cell to safely clear the OTA lens mounting plate.

A ring could be attached to the bayonet plate for the cell's collimation push-pull screws to act against. With that ring then fixed securely to the square bayonet plate behind it. A ring is lighter than a full thickness square backplate since both should ideally be made of metal to accept screw threads. Those with the skill may use a round ring for the bayonet plate despite it being fitted to vertical keyholes. The smaller dimensions are of a ring may allow slight rocking compared with a larger, square plate.

The 'pull' collimation screws, which hold the objective cell to the bayonet plate, should not project behind the bayonet plate either. Or they will bind against the OTA's lens support plate. Instead of allowing the square bayonet plate to lie flat before sliding safely downwards into the keyholes and thence the locating slots.

Add a locking thumbscrew between the bayonet plate and the OTA plate and the lens will never shift in use. This is a far simpler system, better suited to square tubes and altazimuth mountings. The rotational bayonet, discussed earlier, is much better suited to round tubes and equatorial mountings. A suitable arrangement should be provided for supporting a proper dewshield in both forms of bayonet lens mounting.

The cell and its bayonet ring [or square bayonet plate] can be kept safely indoors in a sealed and padded, plastic food container with a snap-on lid for safety. When the telescope is going to be used the food container and its precious cargo are brought out to the telescope. Only then is the lens removed from the container and fitted to the OTA.

Carrying a 'naked' and heavy objective from indoors is not really sensible in the dark.  Particularly if there are steps, changes in level and doors to be opened and closed [often with elbows] on the journey out to the telescope. [And back again later when it clouds over.]

The heavy glass lens may need to cool to the outdoor temperature before it performs well. So early fitting to the telescope is often advised to allow it to cool naturally from the higher indoor temperature. Experience will suggest how long your own lens needs to cool under different conditions.

When the observing session ends the food container is, once again, used to safely carry the precious lens back indoors. Dealing with dew might require the container be left open, once indoors, to allow the moisture to evaporate. It may be that the food container seals well enough not to allow dew to form when the cold lens is brought back indoors. Only direct experience will dictate how you should deal with the dew problem. Silica gel is sometimes used to absorb moisture in sealed spaces but do check regularly that the lens isn't deteriorating inside its own 'lunch box.' Moisture can attack the lens coatings sometimes and can even lead to acid etching fungus! The lens must be dry before long term storage.

Anything cold will automatically attract condensation when subjected to the typically warm and moist conditions indoors. Storage in a secure shed or garage might then be tried if you enjoy 'tropical' central heating but live in a cold climate. The problem now is that the lens might actually be colder by lagging behind the outdoor temperatures. Causing the lens to instantly dew up on on all four surfaces when suddenly exposed to slightly warmer conditions. Safely warming and drying the objective enough to get rid of the dew can present quite a problem. A hairdryer would need to be used very gently indeed not to shock some ED and other exotic glasses into actual breakage!

My first ATM f3.8, 8.75" mirror lived permanently and snugly in a round food container with snap-on lid at the bottom of the skeleton OTA. Food for thought, methinks? ;)

Click on any image for an enlargement.

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