LX200 - SuperWedge Modifications

Replacement azimuth dog and T-bolts
Altitude adjustment thrust bearings
Note the ledge on the tilt-plate for the scope base to rest against.
Modified wedge mounted on my new permanent pier.
SuperWedge mods
Note: Details of an "UPGRADE KIT" which I can supply are given here.
The Meade SuperWedge (SW) is of excellent basic design, constructed from 4 heavy plate castings (the base, 2 sides and tilt-plate) and solidly bolted together. With all clamp screws tightened it has ample strength and stability to support the 12" LX SCT even when heavily loaded down with accessories. The castings actually aid in reducing vibration more than construction from plate would do. The SW is the way to go for long-exposure film photography or CCD imaging, it's superior to the de-rotator for a variety of reasons (the piggy-back guidescope also tracks equatorially, the PEC only works in Polar mode, and the wedge-mounted scope has access to all areas of the sky), but the penalty of course is the extra weight to move around. A fine adjustable wedge is only really necessary for portable use, where it needs to be re-aligned each session. In contrast, a permanent mounting only needs to be setup once, though it must be at least as solid and stable of course. However, the adjustments to polar align it are only made once and therefore need not be as sophisticated as a wedge which needs constant adjustment, and simple fine-threaded jacking bolts will probably do the job for the one setup.

Whilst the design of the SW is excellent the execution of that design leaves something to be desired, and the production item suffers from several cost-saving compromises in it's construction which (in my opinion) impact severely on it's performance. I'm sure the designer never specified the clearances that exist between the various moving parts. Just to clarify though - the stock SW will do the job when eventually setup, but there are several modifications to the mechanics that will make fine adjustments much easier when polar aligning. Also, the 10" and 12" LX200s are very heavy items to lift into position on top of the wedge, the simple addition of a ledge to the bottom edge of the tilt-plate makes it much easier to mount the scope, align the bolt holes, and to take the weight whilst the bolts are screwed in place. My 10" LX200, with it's balance weights and other bits which I tend to leave permanently attached, weighs at least as much as a stock 12" LX200, yet I have no trouble mounting my scope on it's wedge on my own. I leave the front bolt in place, lift the scope and engage that bolt with the slot on the tilt-plate (probably the trickyest part) then let the scope base slide down until it's bottom edge rests on the ledge. The ledge takes the weight and also straightens up the scope base so that the remaining 2 bolt holes are exactly aligned, and these can then be screwed in by hand quite easily. Final job is to torque the bolts up, and the job is done.

General Finish

No real problems with this, the powder coat finish is very durable. The underside of the SW base is left as-cast, and whilst this may not be a problem with your particular example, mine hade a couple of high-spots which prevented it bedding down nicely onto the tripod top. You could use a flat 10" second-cut file to knock these down, but I simply up-ended it onto my milling machine table and used a surface cutter on it. You can check yours by looking through the eyepiece whilst tightening down the 3 mounting screws which bolt the SW to the tripod. If the SW base and tripod top match there will be very little movement as the pair are bolted together, if the SW rocks at all as alternate bolts are slightly loosend and re-tightened, this will be evident by substantial movements of an object in the FOV. A marked improvement can be made by interposing a thin sheet of teflon (about 1/16" thick) between SW base and tripod top after any large bumps have been removed, or even thin oiled card (the hard variety) will allow the two to bed togther better but card will not last as long. This Teflon sheet or card will, of course, need to have a central hole and 3 curved slots cut in it to pass the 3 locking bolts and the central stud.

Making Alt-Az adjustments for Polar Alignment

This is what the adjustable equatorial wedge is all about (the key word here being 'adjustable'), and it is also here that the Meade SW falls down the most. To be useful, it is essential that the wedge be able to point the polar axis very precisely, and to achieve this the adjustments made via the two jacking screws also need to be fine and predictable. For good polar alignment we are talking of making very fine adjustments of only a few arc-seconds, frequently in one direction and then the other, and this requirement demands very little backlash in the adjusting screws, and even more important, tight tollerances in the various lever arms and pivots that move the wedge. The stock SW has horrendous backlash in both screws - perhaps not too much of a problem on it's own, but combined with this the pivots exhibit large amounts of slack, whilst the azimuth dog threads are loose on the jackscrew and it's drive pin does not fit the slot on the tripod. The combined effect of these mechanical defects is that movements of an alignment star (as observed through the eyepiece) are not as predictable as they should be. Turning an adjuster knob clockwise (for example) can cause jerky movements in one direction, whilst turning it counter-clockwise results in no movement at all for part of a turn, and then the object suddenly shoots off in another entirely random direction (but never in the direction where you actually want it to go!) You need the patience of a saint not to curse the thing just trying to centre Polaris. Making drift alignments is even worse because tweaking the wedge in ALT can affect it's alignment in AZ (and vice-vera), and you are only measuring one of these at a time.

There are several articles which detail simple modifications one can make to improve matters (see references below). However, from an engineering perspective I really *hate* such lack of mechanical precision, and to my mind there is really only one cure, and that is to tighten up the tollerances on *all* the moving components. Adding washers or bearings will not give you the needed improved pointing accuracy on their own, the pivot points need to be re-machined or bushed out to remove excessive clearance. The modifications I list here do not go all the way, but they do address the immediate problems. Below is a list of the principle modifications I have made to my own SW:

As an example, consider the last item listed above. That 25 thou clearance in the ALT pivot bar recesses will allow either end of both bars to move up and down, or forwards and backwards for that matter. Loose tilt-plate pivots merely add to the extent of potential movement. With the jackscrew tensioned to move the tilt-plate upwards both bars will be firmly at their upper travel within these clearances. Reversing the screw direction may well result in the clearance of one side being taken up before the other (it may even 'drop' suddenly as the screw is reversed and all of the clearance is taken up at once). The result? The tilt-plate drops to one side, you get movement of the star in the FOV either sideways or obliquely when you intended it to move vertically. How many times have you seen that sort of behaviour?

I think the above modifications address the major mechanical concerns, and certainly from my experience a SW with these modifications is much easier to adjust. The volume of cursing is much lower. It's not perfect, but much better. It also helps to lube all moving/sliding surfaces (tilt-plate pivots, jackscrews, wedge base/tripod top interface) with a high-pressure Lithium or Mollybdenum-based grease to reduce friction to a minimum.

Other modifications which I would place in the 'Nice to Have' catagory are replacing all the socket bolts with T-bolts. This enables all adjustments to be made by hand without the use of tools - a definite advantage on cold, dark nights. I also added Nylon washers under all the T-bolts and keep the 3 mounting bolts on the SW base permanently tightened up (there is still enough play such that the AZ adjuster can move the base side-to-side, it just doesn't need to be further tightened after an adjustment is made).

Using The SuperWedge

Just a few comments on how I use my SW in the field. All that weight, tripod, wedge and scope (+ any accessories) is difficult to deal with on soft ground (i.e., anything other than concrete or similar). There is absolutely no point in aiming for precise alignment for photographic purposes if the whole lot is going to sink into a soggy field! Remember the few-arcsecond aiming precision? it doesn't take much movement to throw that right out. A common observing site I use is gravel based, and for this I use 12" disks of 3/4" timber with a central indent to accept the tripod feet (without rubber ends). This works rather well for me. The ALT scale on the SW is a waste of space, it's accuracy is nowhere near anything useful. The compass is also of little value unless you are perhaps setting up in daylight. I don't bother aligning until I can see Polaris, and then I just place my little torch on the tilt-plate and sight along it's body to point at polaris. This gets me surprisingly close. With everything loaded on the tripod I add my weight to really bed it down (no, I don't actually climb on top of it, I just press down on the wedge side plates). Then I level the tripod and repeat if necessary. Then I check the tension of the SW clamp T-bolts and adjust if required. I also used to check alignment of the finder with the main scope at this point, but this is no longer necessary since replacing the finder mount with a dovetail mount. With the scope pointing at 90 degree DEC and 0 RA I power it up and check date and time settings, and also the site details. Then select Polar align and center Polaris in an 18mm reticle eyepiece (a non-adjustable one - little point in using the Meade adjustable one as it won't indicate center of FOV. You can set it of course by turning it in the holder with reference to a star until the cross looks centered). The modified SW is almost pleasant to adjust now, I can turn the jack screws forwards and backwards and... the star moves forwards and backwards too :) Then allow the scope to slew to it's second alignment star, center it and press enter. I will usually iterate Polaris-to-second alignment star 5 times, finally checking against a third alignment star. This gets me close enough for 1-2 min exposures without guiding. More accuracy demands drift aligning which takes a little longer. I do not tighten the wedge clamp bolts up at all during or after this procedure, it is not necessary.

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