Alt-Azimuth Guidescope Mount

Introduction:

This specialised guidescope mounting is more convenient to use than the standard 3-screw ring mounts. Just two control knobs are used to move the guidescope in altitude and azimuth so it's easier to center a star within the field of view. This mounting is designed to hold the guidescope securely and rigidly within it's box framework. The guidescope is held by two close-fitting split-rings attached to it's tube (these need to be ordered specifically to suit the tube diameter of your own guidescope). The split-rings will normally be left in place should the guidescope is removed for any reason. The two split-rings provide the pivot points for the two-axis Alt-Az movement. This method of construction ensures there is plenty of metal available to provide for substantial bearing surfaces, and also that no stress is applied to the thin telescope tubing.

The bar carrying the two outer rings is hinged at the front (the front mounting block is clamped to the dovetail bar), the rear of the bar is raised and lowered by a jacking screw. A locking knob is provided to clamp the hinge. The front inner ring rotates to provide azimuth pointing, the two pivot points are at the top and bottom. The knurled screw seen at the top of the front ring is adjusted to remove any free play.

At the rear can be seen the large diameter hand-knob of the jack screw which is used to make adjustments in altitude. On the right-hand side of the rear outer ring (shown here) is the hand knob which controls azimuth adjustments. There is a spring-loaded plunger on the opposite side.

Another view of the rear of the A-Z mounting, showing the rear mounting block and the altitude adjustment screw. (Note that in all three pictures the guidescope mounting is shown attached to a dovetail bar, this is the standard accessory mounting bar shown elsewhere).

The above mounting shown attached to a customer's 10" LX200, and carrying an 80mm guidescope (pictures courtesy Richard Winter).

Assembly Instructions:

The Alt-Azimuth guidescope mount is supplied pre-assembled with the two split-rings temporarily held in place in their correct positions. To mount your guidescope the split-rings need to be fitted onto the guidescope's tube, they need to positioned the correct distance apart to fit inside the outer rings, and in the right orientation to suit the front pivot points and rear guide channels. To achieve this please use following instructions as a guide:

Release and remove both split-rings from the outer rings. For the front ring unscrew the knurled screw at the top until the pivot point is retracted and the ring can be removed. The lower pivot is also threaded and can be adjusted to centralise the split-ring within the outer ring using a screwdriver inserted from below (the slotted head is accessible through an access hole). The rear split-ring is released by unscrewing the adjuster knob on the right-hand side all the way and then gently pushing the ring against the spring plunger on the other side whilst pulling it out (alternatively, you can temporarily unscrew and completely remove the spring plunger housing which makes re-assembly a little more convenient). The spring plunger is retained in place and will not fly out! Note that the front split-ring has pivot points above and below and is split horizontally, whilst the rear split-ring has two slots - one on either side - and is split vertically.
Remove the clamp screws and separate the two halves of each split-ring. Important: the two halves of each are stamped to indicate their correct mating orientation for re-assembly (marked 1:1 and 2:2 on one side near the split).
Establish the approximate fore-aft position you wish to have your guidescope within the mount. It will be possible to change this later but to do so will require releasing both split-rings again (so it's as well to get it right first time). Most users should aim to have the center of gravity above the mount DEC pivots (fork mounts) to aid balancing the main telescope afterwards.
Secure the front split-ring in place on the guidescope tube. Important: Do NOT over-tighten the clamp bolts, there may be a small gap remaining where the ends of the ends of the two halves of each split-ring meet, if this is the case do not attempt to close that gap. Great pressure can be applied with the clamp bolts and in the circumstance described it's possible to crush the guidescope tube by over-tightening these bolts. If your guidescope has a rack and pinion focusser you will have to make sure this is in a suitable orientation - remember, the two pivot holes will be vertical and this will govern the rotational orientation of the guidescope tube.
Slip the front of the guidescope through the outer rings and position it in place between the two vertical pivots, use the front knurled screw (at the top of the outer ring) to remove most of the play but don't try to remove it all at this stage - at the moment you just want to get distance between the two split-rings correct. It may be necessary to unscrew (retract) the lower front pivot point to get your dewshield through (if it's a refractor), in which case you will need to re-adjust it later so the split-ring is central again.
Now place the two halves of the rear split-ring on the guidescope tube, have the split vertical judged as near as possible by eye, and adjust the tension of the two clamp screws such that the split ring will stay where it's put but can still slide in response to hand pressure. It needs to move a little to align it exactly. Slide the rear split-ring backwards until it is within the rear outer ring (both are 3/4" thick), engage the two plungers in the two slots (of course, you'll need to screw the plunger housing back into place if you removed it earlier).
When you are satisfied that the rear split-ring central and concentric with the rear outer ring, then this is the correct position in which it needs to be clamped. However, you can't get a wrench on the clamp screws so you will have to carefully release the guidescope from the front pivots, slide it backwards a little (without moving the rear split-ring), then you can get a wrench on the two clamp screws to secure it. Once these are tightened up slide the whole guidescope forwards again and this time you can remove all play from the front pivot using the top adjusting screw.

That's it. I recommend a little molybdenum-based grease be used on all the pivot points, and also the guide channels at the rear. Occasionally check the tensioning of the front AZ pivot although after a short while it will bed-in and further adjustment will not be necessary. The jack-screw is pre-lubed during assembly and will not need further attention for a year or two. Should backlash develop in the jacking screw this can be adjusted out by slightly tightening the setscrew located at the top/front edge of the rear block assembly (on the front face just under the large jackscrew hand knob). This setscrew bears on a ball bearing which in turn seats in a groove in the jackscrew shaft. You can lubricate this by removing the setscrew, placing grease in the threaded hole and replacing the screw, this will force grease past the ball bearing into the shaft bore (but don't bother for a few years as it's pre-lubed!)

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