This first mod I came up with myself one day when I took the cover off

the declination drive assembly and watched as I moved the 'scope back

and forth in declination using the hand pad. While looking at your dec

drive assembly you'll note there is (for some reason that seems hard to

imagine) a rubber *O* ring that serves as a buffer between the assembly

holding the worm gear and the assembly holding the motor and the

gearbox.

To determine if you want to do this modification, simply watch the

junction where the *O* ring is while you move the 'scope back and forth

in declination. My jaw dropped the first time I did this as there was a

considerable amount of motion with the worm gear and gearbox compressing

the *O* ring and clearly adding some motion to the assembly that doesn't

do anything but make any slop in the dec drive worse than it needs to

be. I started poking around in here when I noticed that most of my

autoguided images had streaking along the declination axis. Using the

backlash control setting I was unable to significantly reduce this slop.

Anyway, while I couldn't be sure that the culprit was all the motion

happening in this joint, it was worth a try to reduce/eliminate it and

see if things improve. I took the dec drive out and using pipe cleaners

I cleaned the area around the *O* ring as best I could (some of the worm

gear grease had migrated into that area). I then mixed up some epoxy

and let it set for about 20 minutes so it was no longer so fluid. Using

a toothpick, I packed the outside of the *O* ring with epoxy. After

letting it dry for a day, I packed some more in, my objective being to

make that joint as inflexible as I could. The idea of the epoxy is not

so much that it sticks to the metal but that it's fairly incompressible,

and thus will stop those sides of the joint that want to move together.

After letting it harden, I reinstalled the dec drive and ran the OTA

back and forth. I was happy to see that I couldn't visually detect any

motion in the joint. I was also very happy to see that my backlash

problem had all but gone away...it was now quite controllable with the

telescope backlash control. Autoguided images improved dramatically.

The telescope has been in more or less regular use for well over one

year with this modification and still gives quite satisfactory results.

No harmful effects have been noted.

The second mod involves simply removing the *pressure* screw (the one

with the spring around it) that presses the worm gear assembly up

against the declination gear...taking the spring off, and replacing the

screw. I replaced the metal screw with a nylon screw that has a slight

amount of give. The good news about this mod is that without the spring

the worm gear no longer has the spring induced pressure variation that

caused variations in the dec drive performance. The bad news is that you

must be careful about how much you tighten the screw. Not tightening

enough results in slack between the gears, tightening too much stresses

the motor. I use the very unscientific method of running the dec drive

back and forth while noticing the change of the sound of the assembly.

It seems to be rather easy to tell by listening when the screw is too

tight. That probably sounds rather empirical, but if you give it a try

you'll see what I mean.

Where I live we'll have temperature variations from -5 to +90 degrees

over the course of a year. Clearly this is enough temperature change to

require that the pressure on the screw be adjusted as the seasons

change. I just take a small pair of needle nose pliers, take the cover

off the dec drive, and use the pliers to gently turn the screw while I

listen to the pitch of the drive assembly as an assistant rocks the OTA

back and forth using the hand pad. I probably do this once every few

months. I facilitate this adjustment by leaving the screws off the dec

drive cover. I use darkroom tape to hold it on, so it goes on and off

pretty quick.

These two mods have dramatically improved the performance of my dec

drive. If you have backlash problems, they may be worth a try.

John Ruthroff