After using the stock 10" Meade LX-50 for several months, it became apparent that several areas needed improvement, particularly if I was going to do any prime focus photography even at f/6.3. These include, the Dec. axis, the Dec. motor speed control, the tangent arm screw (10-32 thread), Dec. clutch and the large R.A. periodic error in the R.A. drive system. The basic design is very sound and many of the componts are shared by the 8", 10", 12", Meade LX-50 LX-200 SCT's.

The first step was to look at what was available on the after-market. Unfortunately there is very little except for the Blessing DEC-FIX kit. After reviewing the current literature Meade user group etc. it was determined that there were some improvements that could be made without a great deal of expense.

Below is a list of changes made, In order of mechanical simplicity, and methods used.

#1 Install The Jordan Blessing Dec-Fix kit, (note Blessing also provides a very good quality 108 tooth gear in a version of his kit). This is a much better gear than the stock plastic gear Meade supplies.
This is the easiest mode to make and results in much better control of DEC. motions in particular with the ST-4 autoguider.

Up-grading to the Ver 6.1(PROM) from Meade will help in the use of an autoguider such as the ST-4, and from what I can learn the new chip corrects for the actions of the autoguider only.

After making these changes I could get good guiding with the ST-4 used at F/10 prime focus of up to 1 hr. doing piggyback photography with a 400mm lens with good results, even though the ST-4 issued a lot of corrections, largely in the DEC. axes. These corrections where of a oscillatory type, first a correction in one direction then a correction in the opposite direction, which I think was due to the low temperatures (around 18 degrees) and the DEC. stiction. This will be dealt with later.
 

#2 Adding the JMI NGF-S focuser coupled with the mirror lock down screw (Chris Vedeler's article) (http://www.isomedia.com/homes/cvedeler/scope/mirrorlock.htm) helps to stabilize the mirror for tracking and prime focus photography. Even without the JMI focuser, using the lock down screw provides an added measure of mirror stability.

#3 The next modification is somewhat controversial as to its value, (from threads on APML
and MEADE user groups), but after consultation with a knowledgeable machinist it made sense to LAP the R.A. gear, the fact that it would not hurt the gear, but could smooth out any rough spots from the manufacturing process and hopefully offer some improvement to the smoothness of the R.A. drive and reduce drive jumps due to burs etc.

  The bottom plate and the drive electronics are removed and grease on the gears is cleaned as much as possible. A mix of white Lithium Grease and 400 grit (I used grit from a mirror kit), about equal parts grease to grit is mixed together, and applied to the WORM GEAR with a small brush, taking care to stay away from the worm bearings. A 9-volt DC source is applied to the motor terminals, make sure the polarity is such that the motor turns in the same direction i.e. R.A. gear clockwise, as normal, view from the top. The 9-volt D.C. source will cause the WORM to turn the R.A. gear at the rate of one rotation per hour (Instead of one rev./day). 8-10 hours of lapping with frequent renewal of the grease/grit mix should do the trick. At this point the R.A. motor assembly is removed for cleaning, Mineral Spirits and a clean brush seems to work the best. The R.A. gear is carefully cleaned taking care to not let the grease get into other areas of the drive. It might be noted that there was a definite color change to the grease after lapping, indicating some material was removed. It started out pink in color and when finished a dirty gray. Regrease and reassemble the drive and electronics. That's it.

Improvements noted The original P.E. was in the order of 48-64 arc/sec. After lapping the P.E. was reduced to 38 +/-4 arc/sec. The overall R.A. drive is much smoother with no jumps or jogs. The P.E. error is somewhat hard to determine because of a small overall tracking error that prevents the star from returning to the original starting point in the 8-min. gear cycle. This error shows up mostly when using the 9mm eyepiece. Overall the improvements made by lapping the gears are very worthwhile, and would highly recommend the procedure to LX-50 owners. This would also be worth while for LX-200 owners also.
 
 

#4 The next change is more of convenience for balancing the scope for/aft. See (fig. 1) for illustration. Two Alum U-channel brackets about 2" long 1.25"x1" are slotted for a long hose clamp, that wraps around the front of the scope. The brackets are drilled and taped for a 1/4-20 screw about 1.5 inches long so that the screws are at right angles to the surface of the brackets. The brackets are secured to the scope, in line with the DEC. axes with a very long hose clamp with a strip of felt to protect the finish. A wing nut is used to secure the weights. Add weights as needed (these can be made from almost any material, but should be in pairs). I made mine from lead molded in an old cut down soup can, two, four and six oz. pairs. This allows for very fine balancing for/aft.

#5 These next changes are designed to improve the action of the DEC. axes by lowering the axis stiction, increase load capacity, make dynamic balancing easier and improve the tangent arm movement by reducing motor load.

For the first change, I'm going to defer to Doc G's excellent article on adding roller bearings to the DEC. axes. (http://www.mailbag.com/users/ragreiner/index.html)

Even though this article refers to the LX-200 it applies equally to the LX-50, same fork arms and plasticbearings. (Fig. 2) shows the modified Dec. axes with the new bearings installed and for comparison the original plastic bearing.
 
 

Also note the assembly and alignment procedures that Doc G. describes on his web page, this should be followed closely. I was very lucky in the final assembly, when checked; the alignment was spot on requiring only minimal adjustment.

#4a The next change involves the tangent arm screw (Originally a 10-32 threaded rod), this rod is way to light for the work its intended to do, is easily distorted which changes the drive gear contact. The bearing supports (inexpensive oilite type) are way under sized and too sloppy. This is also common to Celestron units that employ this design. This change is the most expensive of the modifications detailed, but I think is very worthwhile in the long run.
 
 

(Fig. 3) Shows the old DEC. SCREW and it's associated parts alongside the new 3/8" 50 threads per inch rod, ball bearings and retainer plus the new clutch, for comparison.

The Tangent Arm design is able to provide a finer degree of motion than the typical DEC. worm gear setup found on the typical small mount designs. It's also used in a lot of HIGH-END mounts. Properly designed a tangent arm can provide a much smaller degree of shift in arc/sec. than the best of worm/gear combinations per motor revolutions.

In my case I replaced the screw with a new, (found in my odds an end junk pile), 3/8 dia.
50 threads per in. shaft 5" long with a brass split nut, resized the shaft ends and add ball bearing support on both ends. This is not an over the counter replacement part but is custom made. The easiest stock replacement, would be to have a 40-threads per in. thread cut on 1/4" dia. SS steal rod, by a machine shop with a custom split nut taped. (40-threads per in. taps are available in various sizes from industrial supply houses for around $12-$15 each) Also there are fine pitch die's available that could be used to create the threads instead of having them cut. This is not the cheapest of options but worth doing at the same time as the FORK arms are being bored for the new bearings. This new screw nut combo could also be lapped as per above to insure a smooth fit. For this part, having a good working relationship with a machinist helps. He should be able to work out the details of the modifications needed, such as new /enlarged holes for the support bearings, screw lock methods, thrust loading etc. The final change was to replace the original clutch with an adjustable spring-loaded pressure type clutch. This can be obtained from Allied Devices Inc. 2365 Milburn Ave. Baldwin N.Y. Part #DR33 For a 1/4" shaft, other sizes are available to suit.
 

As I say this part is not for the faint of heart, outside of PEC (there is PEC correction in the works from at least two sources that I know of) these modifications should do wonders for the MOUNT. When one looks at the problems with the LX-200, I still think that the LX-50 is not a bad telescope for its price. The optics is first rate and the basic mechanical design is very sound. The biggest problem, and this applies to many scopes in the market place, not just MEADE products, is coast cutting in the wrong places and exaggerated marketing claims. As an example the DEC. BEARINGS cost just $11.00 for the pair and sense the bearing hole has to be board anyway, this would not add that much to the final cost, verses the tremendous benefits gained.

FINAL RESULTS

The first change noticed when the scope was fully assembled in the workshop, was the freedom of movement of the DEC. axes. With the clutch OFF, the scope could be lifted to one side, released and it would swing like pendulum back and fourth, the chief resistance was from the tangent arm. This will make dynamic balance much easier to manage.
The new DEC. SCREW arrangement is silky smooth and the motor action shows no tendency to slow under load. Tests conducted with the ST-4 @ 1800mm fl showed average errors of 5-7 units, this is very good for this system.
 

Ted Agos N1SBM
lester@ziplink.net
Acorn Hollow Obs.
42' 01" 71' 48"