Dovetail Bar Fitting Instructions
Introduction:
Fitting the twin dovetail bars and counterbalance weight system is quite simple, so in all likelihood most owners will not require any instructions at all. However, there are a few areas where mistakes can be made and I hope these notes will help clarify the procedure. If they don't, or you have a problem not covered here, then please email me for further help.
Dovetail Bar Kit Contents:
Each kit comprises the following parts:
- The two dovetail bars: Note that one end of each bar has a machined cutaway on the underside - this end should always go towards the front of the telescope. Note also that one screw hole is elongated into a short slot, this allows for a small degree of adjustment to accommodate the exact length of your own OTA (yes, it varies a bit!)
- Pair of upper brackets: These are the low-profile brackets (the ones without the two auxiliary tubes). The bases are radiused to fit just one size of telescope tube so the brackets for 8", 10" and 12" scopes are all different. Note that when fitted correctly the ends of the dovetail bar sits nearly flush with the bracket ends - it IS possible to make a mistake and have these brackets fitted the wrong way around, such a condition will be obvious because you'll find the ends of the bars overhang the brackets substantially and the holes don't line up. Note also:
- On the 8" set the REAR top bracket is asymmetric - that is, shorter on the left-hand side to avoid fouling the finder bracket. The front bracket is full width. On the 8" set therefore the front and rear brackets are NOT interchangeable.
- Both the 8" and 10" brackets are machined from 1" thick alloy plate to provide a full 1" seat for the ends of the dovetail bar, this width is reduced either side of the seat to around 3/4" to better match the telescope.
- The 12" top brackets are the full 1" depth for their entire width, they are also much wider than the other models as on this OTA the 1/4"x20 accessory mounting screw holes are placed 3" apart instead of 2".
- Pair of lower brackets: These are more complex than the upper brackets, incorporating as they do the mounting holes for the two auxiliary tubes carrying the bias and trimming weights, and also the clamp knobs for the trimming weight extension. I normally supply these pre-assembled but they will likely still need minor adjustment for your particular OTA.
- Set of 12 stainless steel mounting screws: These are replacements for the factory fitted round-headed screws (which are of poor design, the hex socket being too small and consequently the corners tend to round off, and they rust if left exposed to the elements!). By all means keep the originals after removing them in case you wish to sell your scope at a later date and need to return it to original condition. Note that there are 4 long screws supplied (for the outer ends of the lower bar brackets) and also 8 shorter screws for the upper brackets and remaining 2 center holes in the lower brackets. Do NOT try fitting the longer screws anywhere other than the ends of the lower brackets.
- 2-D Weight Bracket and Weight: You will find this packaged separately, the dovetail slot will fit either the upper or lower bar.
- Camera Bracket: Again, this is packaged separately, the stainless steel 1/4" x 20tpi screw is temporarily fitted to the slotted plate for shipping and will need to be detached. After removal, thread the locking ring back on all the way, then place the camera body onto the slotted plate and thread the screw into it from the underside as far as it will go into the camera body, then use the lock ring to hold it in place. The slots allow you to position the camera body on the plate for best balance and support (the position of the mounting hole varies with different makes of camera). Note that the locking ring is perfectly flat on one side, whilst the other has it's center part raised - it's the latter side that bears against the underside of the slotted camera plate (this prevents the knurled edge scratching the plate).
- Trimming weight: This is in two parts, the extension rod and the weight itself. The rod can be fitted in one of 4 possible positions in the lower brackets (at either end of the 2 tubes). 2 clamp screws are supplied although only one is actually needed - but it's useful to keep one mounted at the front and one at the back for quick balance adjustments.
- Bias weight: This is normally packaged already assembled onto the right-hand auxiliary tube, if required it can be removed and fitted onto the left-hand tube.
Attaching the Kit to your Telescope:
Start with the upper dovetail bar. If the two brackets are not already attached the ends of the bar then attach them now, leave the two 5mm screws loose at this stage. Remove the top three accessory mounting screws from each end of the telescope (see separate paragraph below on doing this) and then move your scope so it's in a horizontal position. Place the bar and brackets in their approximate position, making sure the cutaway on the underside of the bar is at the front. Put two short mounting screws into the ends of each of the brackets, then tighten up the bracket at the end of the bar having a round hole for the 5mm screw (ie., the opposite end from that with the slotted hole). Now adjust the bracket at the slotted end until the holes in the bracket align with the threaded holes in the telescope rim, then tighten the other two mounting screws up. Now remove the two 5mm screws holding the bar to the brackets and lift the dovetail bar away. This reveals the center screw holes (one each bracket), put a short screw into each of these and tighten them up. Finally, replace the dovetail bar again. Note that it is neither necessary nor desirable to over-tighten any of the screws, the radiused bracket base and bar seating have ample surface area to hold the bar solidly without relying on overly tight screws (that's why they are made that way). It is not desirable to place anything between the radiused underside of the brackets and the telescope rim, The metal to metal contact will ensure the mounting is solid and cannot flex, if you don't over-tighten the screws then the original finish will not be harmed in any way. If you insist on putting something between bracket and tube rim then a thin sheet of paper will suffice but it's not really necessary. Do not use anything thick or flexible (felt etc.,) for this job, a thousandth of an inch flexure is enough to ruin high-magnification film shots through differential flexure of the guidescope.
Now turn the OTA over so that the underside is on top and horizontal. Fitting the lower bar is a similar job to fitting the top bar - but it is a little more complicated due to the presence of the two auxiliary tubes. You will need a small hex wrench for the two set screws which retain the tubes (I use 4mm hex setscrews), the two setscrews at the slotted end of the bar need to be loosened so that the bracket can slide to accommodate the adjustment for length. It's likely that the bracket will not slide easily so you may need to tap it (with a piece of wood) to move it into position. Note also that the trimming weight thumbscrew is threaded into the brackets and pass through an oversize hole in the tube wall, this thumbscrew locks the extension rod which carries the weight in place. It is possible (if both the thumbscrews are removed) to accidentally rotate the tubes such that the thumbscrew no longer passes through the hole in the tube, in which case the extension rod cannot be locked. Take care this does not happen by leaving the two thumbscrews in place whilst adjustments are made, this will prevent the tube from rotating. Remember that the four longer mounting screws fit into the outer edges of the lower brackets whilst the remaining two shorter screws fit into the center holes. Follow the fitting routine for installing the mounting screws as described for the upper bar, again make sure that the cutaway on the underside is at the front of the telescope. When the brackets are in place don't forget to tighten up the two small setscrews.
Miscellaneous Problems:
It's possible, based on the small sample of LX200's I've managed to actually get my hands on, that fitting the brackets may cause problems. On one example of an 8" LX200 I found that the accessory mounting screw spacings were not accurately positioned. The error was such that the 3 holes in the top bracket would not line up correctly with the threaded holes in the telescope rim. The error was some 0.050" (the nominal distance between these screws is 2 inches). It is not possible to account for this discrepancy by simply making the holes larger during manufacture, it would be necessary to resort to using slots in the brackets and that's something I don't find attractive. I think this problem is fairly rare, apart from the one instance I'd had a few others where the brackets were reported as being a 'tight fit' - which they should not be given the amount of clearance I allow. What I've done is to make the holes in the bracket as large as practicable consistent with being mechanically sound. If you should encounter this problem with your own telescope you will need to get a special replacement (exchange) bracket from me with the holes positioned to suit.
Similarly, it's possible for the length of the OTA vary (or rather, the distance between the front and rear accessory screw holes varies). I account for this using a short slot (the slot being unavoidable in this circumstance!) in one end of the bars. Should your particular OTA fall outside the range provided by this slot then you will have to contact me for a replacement bracket where the 5mm threaded hole has been repositioned. I've never had this happen yet but knowing Meade I'm sure there's one out there somewhere!
Removing the Factory Accessory Mounting Screws:
If the factory screws have never been removed before they might be snugged up very tight. In any case, before you start make *absolutely sure* that the wrench you use fits the hex sockets exactly, if the wrench is a loose fit then there's an excellent chance of rounding the corners off which will make the screw very difficult to get out. If you're lucky, it's the corners on the wrench that get rounded (the wrench is made slightly softer than the screw head for this purpose), but if you're unlucky it's the screw head that gets damaged. It's worth buying a T-handle Allen Wrench just for this job. To remove the screws, press the end of the wrench firmly into the socket (this is easier done with the T-Handle wrench) and apply increasing torque until the seal breaks and the screw can be undone. Should the worst happen and the socket ends up round instead of hexagonal (!) it is necessary to use a hacksaw to cut a slot across the head of the screw and use a normal flat-blade screwdriver. Before you start to panic I should point out that, provided you use the correct sized wrench to begin with, it is unlikely you'll need to resort to the hacksaw. Nevertheless, it's good to know there is a Plan B just in case! The replacement screws have a much larger socket compared to the factory ones and are hence much easier to get out - these will never get stuck in service (they won't rust either).
Caring for the finish:
The anodised finish requires little attention, an initial polish with a wax-based or silicone-based compound will help keep water at bay. Whilst all other metal parts are stainless steel you should remember that stainless steel is better described as 'rust-resistant' steel - i.e., under some circumstances it can still corrode. There are steels that are more resistant to corrosion (example: those for marine use where there is contact with seawater, or for surgical instruments) but this stuff is expensive, painful to machine and is never used where it's not essential to use it. The frequent damp conditions that your astronomical telescope has to put up with can sometimes be a problem, but it's unlikely that any serious corrosion will result even with continuous exposure. Again, a coating of wax or silicone compounds applied to the weights will prevent corrosion completely, if a faint patina of rust does occur it can easily be removed with metal polish or WD40 on a cloth.
The clamping face of the 2-D bracket and camera bracket hand-knobs are made from Nylon so they are unlikely to damage the dovetail bars. It is not necessary to tighten these knobs more than finger-tight to hold the brackets rigidly.
