7" f/12 iStar refractor 26: Bin there. Done that.

*

Repeated breakdown of affordable tarpaulins drove me to try something else to cover the MkIV mounting on its even taller pier. A 300 liter rectangular water butt! 55x75x95cm [22x30x38"] in green plastic. Fits like a glove, rather difficult to lift that high and looks completely daft, but protects the drive motors and their wiring. Whether it will prevent birds roosting is quite another matter. This thing is huge when it is on the ground but looks tiny when covering the mounting.

The polar axis has never been cut short since I replaced the rust-prone, original 1.25" [31.8mm] steel shafts with stainless steel. I had vague ideas for mounting larger wormwheels or even thrust bearings down there but nothing came to fruition. The projecting PA shaft pushes the water butt enough to stop the edge dropping neatly over the counterweights on the far corner. Turning the tub on the diagonal helped a little at the expense of looking rather casual, untidy and haphazard. Removing the tube rings and tatty tarpaulin would help. I have made the tube rings easily detachable in the past using wing-nuts for fasteners. Though I haven't quite reached that point with the new rings yet. The extra labour required before mounting the OTA and difficulty of reach makes it less desirable now. I will just have to try the water butt without tube rings on the mounting. Though it is likely to place even more demand on cutting off the over-long PA shaft.

Pictures tomorrow if it ever gets light again. Today was yet another, dark grey day but with added rain. Talking of water I just thought of black plastic, flexible pond liner material as a longer lasting cover. Too late! The CIA is bound to have satellite pictures of my inverted [water] butt by now. Any claims that I now have the worlds smallest [lift-off] observatory are unlikely to hold water. With a storm coming tonight I hope the tub won't catch the wind enough make the pier tip over! I have lowered the wheel jacks as much as possible. It is possible that the untidy tarpaulin would have acted as a sail but the weight of the pier seemed enough to keep it stable.

I decided to remove the rainwater tub during the storm and also removed the objective from the OTA to take it indoors. The tub proved to be a water collector even when inverted. I narrowly missed a soaking as the base and rim rapidly emptied as I tipped the tub over while lifting it off the mounting.

Another image with the tube rings removed allowing a much lower, protective cover. The tube ring bolts can be fitted with wingnuts for rapid, tools-free fixing to the saddle.

Another thought has occurred to me regarding the objective counter-cell. The present plywood rings are are prevented from being pulled off the main tube by the narrow, main tube, pressed flange. It follows that I could make another ring which fits in front of this flange. This will help to stiffen the counter-cell arrangement.

The short dewshield flexes slightly due to its dual purpose in offering resistance for the collimation push screws. There is a small gap between the flange and the plywood rings on the main tube due to the thickness of the pressed steel flange. This narrow gap offers the potential for air currents and insect ingress. The gap also denies the rather thin dewshield flange any support from the plywood rings which accept the collimation, pull screws.

Image of the MkIV mounting showing [in orange] how locking/drive control rods might use standard flexible, slow motion stalks to allow control extensions right back at the focuser. The red arrow shows how the PA locking stud and its Bakelite knob face the "wrong way" to allow a flexible stalk to connect to a control rod. It will require a new hole be drilled and tapped for a new control stud at 180 degrees to the original. In practice I shall drill the other side of the PA casting to avoid any conflict with the present stud. The tapped hole is strangely skewed which will require considerable care to achieve the correct angle. It will probably be easier to drill from the rim which supports the wormwheel. To ensure the active, plastic plug is sited against the inside surface of the annular wormwheel rim.

There is a fair seal from the main tube flange pressing against the back of the dewshield. A new plywood ring would provide solid resistance to the collimation screws pressing the plywood rings together. The ring's rear face would have to be relieved slightly to accept the main tube flange. It could then be glued to the other plywood rings over the main tube flange to close the gap. The inner diameter of this ring should match the rear, tubular section of the objective cell. With just enough freedom to allow the cell to be collimated. So the fit doesn't want to be too tight. The collimation screws push the cell away from the dewshield flange allowing airflow around the cell. This may not be a good thing unless the cylindrical rear section of the objective cell is better sealed into a "proper" counter-cell. It was not until I studied my rather odd counter-cell design [using a flange on the back of the dewshield] that I realised I had left the rear of the objective cell open.  Fortunately the dewshield flange is a close fit on the rear extension of the objective cell.

Click on any image for an enlargement.

*