7" f/12 iStar refractor 25: Fixing problems:

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Another update: I found 3 shorter, but much stronger springs, for the focuser backplate collimation. Until now the focuser collimation has been rather ineffective due to a lack of resistance from the soft springs. Unfortunately the socket head bolts I ordered online have not arrived yet. I will wait until they arrive before refitting and re-collimating the backplate/focuser. Having visited or searched the websites of many potential outlets I find it ridiculous how difficult it is to obtain springs.

I am still struggling with backplate/focuser collimation. Trying to align the cross-hairs while looking through the Cheshire EP shows how imprecise the adjustments really are despite the solid drive from socket head bolts on the very stiff springs. I shall have to Measure the space available to see how much room the springs really need. Or even if they are having any effect at all. It certainly doesn't feel like it as I adjust the new screws. I have already had to reduce the diameter of the ring which allows the focuser to rotate. The springs and any packing tended to lock the rotation due to the proximity of the plywood ring.

I had another thought on the objective [dewshield] cap. Anything which projects makes the telescope unstable when standing upright. The previous saucepan lid made it impossible to stand the OTA on the dewshield. There is also the problem of bringing the OTA upright [for storage] with only the edge of the dewshield resting on the floor. While simultaneously lifting the heavy and very awkward OTA upright between the ceiling joists. I really must add a central handle of some form to aid lifting and carrying.

If I made a thick plywood cap which fits inside the dewshield and had one disk thickness projecting then the cap will reinforce the dewshield when at its most vulnerable. The lift will start while resting on the edge of the plywood cap rather than the unsupported dewshield.

A circle, bored out of the center of the cap, will allow a sunken handle which does not strike the floor. While still allowing a firm grip when removing the cap from the dewshield once the OTA is safely mounted. Thick polystyrene could be sandwiched [and glued] between two thinner disks of plywood to reduce weight while retaining stiffness in the lens cap.

But then again, the OTA would be lighter and shorter if the outer dewshield was easily removable. This would return the OTA to the stumpy but much firmer, inner dewshield for storage. The OTA could then be lifted onto its nose without effort or having to worry about the ceiling joists intervening. Not to mention the ease with which objective collimation could be achieved without stretching to reach into the bottom of the huge dewshield. It's all a matter of being willing to rethink and even to rebuild where actual hands-on experience suggests it is necessary.

Working out in the garden in thick mist I removed the outer dewshield and replaced the collimation screws. It was much easier working inside the shorter dewshield.

The backplate was removed and the clearances with the main tube plywood rings measured. I then used some plated Nyloc nuts as spacers under the springs to take up the slack. The backplate [and focuser] now respond nicely to screw adjustments. Obtaining a central image of the cross-hairs on the objective was now very simple and precise. 

Putting the shorter and slightly lighter OTA back into storage was very much easier without having to worry about the ceiling timbers getting in the way. The missing foot of dewshield seems to make a large difference in handling in confined spaces.

I am still thinking about fitting control rods extended back to the eyepiece for the wormwheel/axis locking systems. The mounting is now too high for a comfortable reach and these locking/drive controls want to be much more accessible. Preferably without having to use a stepladder. The Fullerscopes MkIV has radio knobs on short threaded rods.[studs] These rods force a nylon plug into contact with the inside of the ring-like [annular] wormwheels. Tightening the plugs into the wormwheels locks them to the castings. The worms then drive the RA and Dec castings around the stainless steel shafts in their bronze [shell] bearings.

Since the OTA is removed from the mounting between observing sessions the remote control rods cannot be fitted permanently to the OTA. So some sort of open/split rings or hooks must be arranged to support the focuser end of the control rods. My original idea to use universal joints has given way to using flexible stalks as the intermediary angle changers between the studs and the long rods. Once the telescope is mounted I shall clip the control rods onto the rings provided on the OTA. Then un-clip them again ready for placing the OTA back into storage after use.

I removed the [very] bent and [very] rusty studs which lock the slow motion wormwheels to drive the axes. The studding thread is coarse so I tried a 1/4" BSW [Whitworth] die and it ran down smoothly until it reached a bend. I have now ordered a length of 1/4" BSW studding, in stainless steel, online.

Making a remote control rod for the Declination axis stud seems simple enough. It just needs a short flexible stalk to allow the slight change in direction. At the moment the stud is so bent that it gives a false sense of locking as a bend strikes the casting. Trying to straighten the rod is fraught with danger of it snapping from repeated work hardening.

The Polar axis locking stud is quite another matter altogether. It faces the "wrong way" to be able to add a simple rod and stalk back to the focuser. Ideally it probably would need bevel wheels to connect the long rod to the control stud. The rod would rotate freely while being driven by the bevel wheels when the telescope was moved. Only when the telescope was accurately pointed would the rod need to be tightened to engage the RA drive. At least that is how I see it at the moment. The alternative is a very long, flexible stalk. This would hang down beside the pier an allow locking and unlocking of the drive without the use of a ladder. Or struggling to reach up find the stud and its Bakelite control knob in the dark.

I have ordered a new, 1/4"BSW tap intending to make a new thread in the polar casting facing towards the South. The active thread length for the stud is about 5cm or 2". Hopefully the new tap will be long enough to reach right through the newly drilled hole in the polar casting without making it too bell-mouthed or over-sized. Perhaps I should just order an 8" [or even larger] PA wormwheel and matching worm from Beacon Hill in the UK? My Fullerscopes RA worm is plain steel and rusts readily despite frequent lubrication. The early type, MkIV open, worm base casting is difficult to adjust and prone to looseness between adjustments. Since the worm acts as the axis lock this is not an ideal state of affairs! Hopefully a Beacon Hill worm housing would be much better thought out! I suppose I could drill new holes in the castings and have MkIII type knobs to lock the axes directly. Though they would be even more difficult to reach from the ground. Is it even worth continuing with the MkIV mounting when a new Beacon Hill mounting would do a much better job? I am already looking at large, pillow block bearings on the small ads websites.

Click on any image for an enlargement.

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