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Observing life would be so much easier if I had no serious lifting or carrying to do.
Or, the heaviest and most fragile bits could be easily removed for safe, indoor storage.
So, what about a bayonet fitting for the objective cell on the OTA?No need to carry that extra 10lbs along with the already heavy OTA. Just bring the lens out separately from safe storage and fix it straight onto the OTA without further ado, or fiddling about. An instant 10lb reduction in the carrying weight of the OTA. Read on:
Push in, twist and then
safely lock in place with a separate thumbscrew. Socket screw heads would project on a fixed radius from a dual purpose ring sitting behind behind the objective cell. This added ring would provide collimation AND attach itself to the OTA via the projecting screw heads and the keyhole shaped slots. The rough drawing gives some idea of what is needed on the end of the OTA. The keyhole slots would need to be true arcs of the correct width and radius to match the shanks of of the bayonet screws. Jamming between the screw shanks and the radiused slots must be avoided. There must also be unquestioned, fail-safe security of objective lens on the OTA. The undersides of the screw heads need only project by the thickness of the OTA ring plus just enough clearance for easy rotation. Flatness of the OTA ring obviously helps here. Warping between the rings must be avoided.
The keyhole slots could be cut in a fixed OTA ring or a modified counter-cell. The screw heads go straight through the larger round holes at the start of the arcing 'keyholes' on the OTA. Then the entire objective cell and its attached bayonet ring are rotated. This rotation brings the screw heads under the much narrower sections of the arced slots.
To aid rotation of the cell, into the locking position, the collimation pull screws could be seated in deep cups [used as handles] to give the user better grip on the cell. And to aid later removal, of course.
It is strongly advised that the round holes of the keyholes are made suitably over-sized to allow easy entry of the protruding 'bayonet' screw heads. There is absolutely no need for a tight fit on the screw heads. All that will do is make initial location of a heavy and fragile object exceedingly, and quite unnecessarily, difficult! Particularly in the dark!
It is the larger heads of the screws passing behind the narrower slots on the OTA which do all the real work of locating the objective assembly safely in place. With at least one locking screw for absolute security! An equatorial might eventually put rotational forces on the lens cell which the altazimuth mounted OTA would never experience. Consider a bayonet locking device absolutely essential in both types of mounting to avoid catastrophic disasters! Just carrying the OTA around with the bayonet mounted cell in place, particularly when you are tired, might dislodge an unlocked objective cell.
Once the locating screws reach the limits of their clockwise travel [in the slots] a simple thumbscrew in the bayonet ring is screwed in. This action must safely lock the bayonet ring to the permanently fitted OTA plate or ring. Most refractor counter-cells will not have remotely enough enough room for arced, keyhole shaped slots. The normal counter-cell normally provides threaded holes for the collimation 'pull' screws. So rotation is impossible without the addition of the added, cell backplate ring.
Several screws could be used to hold the bayonet ring firmly against the OTA plate to ensure exact collimation but this adds further complication. It is assumed that the socket head screws are chemically locked into their screw threads in the sturdy bayonet ring. Stainless steel screws seem appropriate to avoid wear problems and metal dust from the usual zinc coated fixings. Not to mention rapid rusting of the cheaper option on repeated exposure to moisture.
With reasonable care in handling, the objective collimation should remain undisturbed between the objective cell and its 'bayonet' ring. If the OTA's objective support ring is stiff enough [sturdy aluminium plate or brass plate] there is no reason for the collimation to change between removal and refitting of the objective.
Plywood would probably be too soft and changeable to act as a bayonet ring. It might also require a metal rear surface to prevent the bayonet screw heads from literally pulling out through the much softer slots in the plywood. It could work with harder birch plywood if much larger screw heads were chosen. Though this would require matching, over-sized holes for the large bolt heads to pass through.
Great care must be taken to avoid the lens, in its cell, form literally falling off the OTA. Which might occur if the telescope was pointed horizontally, or even downwards. The latter behaviour is quite commonplace with larger refractors when checking for objective dewing on the front surface of the glass. Which is usually caused by far too short a dewshield.
A bayonet fitting would probably require that the lens was replaced in the same orientation each time. [Unless the slots were perfectly matched radially.] The bayonet ring must fit flat against the OTA keyhole plate. No need for "This way up" markings with the iStar name and lens details neatly printed on the front ring of the 10lb objective cell. Always assuming you have enough light to see the lettering!
Click on any image for an enlargement.
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