CORRECTING DS SERIES(OTHERS?) ALTITUDE SCALE SLOP & ERROR Last updated: 4 July 2008

Sent:	Monday, June 30, 2008 22:16:37
From:	stange (stange34@sbcglobal.net)
      
The DS-series DEC knob scale assy can be accuratized when the mechanical
problems are identified. The ETX series (may be fixeable the same way or
similar.)

The drawback is....since the scale needs to show zero when the scope is
level and to show accurately the scopes true Altitude angles upward it
cannot be done without having the base mount level to begin with, for
starting accuracy. And the Alt. scale must never move independantly when
the clutch knob is rotated in either direction, and must never
independantly move during normal up-down gear driven motion.

The problem is the Altitude scale has a couple of degrees(or a lot more)
slop because it is not firmly fixed in rotation & locked to the hub
assy. If it were, the scope mount would have to be leveled to make the
Alt. scale accurate afterward.

Meade's promotion was the mount need not be leveled because it uses Star
alignment with only the tube needing leveling in the beginning. So the
Alt. scale stayed sloppy no matter how the tube was aimed after
alignment.

There are two problems that must be corrected (if both exist), to
eliminate scale slop.

1) The outer Altitude knob SKIRT must NOT TOUCH the Alt. scale at any
time. If it does, it will cause a dragging on the Alt. (SCALE) with
increasing scale error in either direction depending on the scale slop.
Even by removing the slop, it can later cause the slop to return.

2) The Alt. scale sets somewhat loosely on a special hub assy without
any means to lock it to the gear driven hub accurately and permanently.
This is the source of the scale slop.

Correcting problem #1:

After removal of OTA clam-shell, (4 screws underneath the clam shell
padding at the top, "carefully" loosen the locknut you see and remove
it. Hold the hub so that you are not damaging internal gearing etc.

Gripping the Altitude scale clutch knob firmly, unscrew it completely
and withdraw it. Notice you are unscrewing a shaft which goes THRU a 3
pointed clutch pressure plate which detents in the housing at 3 points
on the other side inside the clamshell hub. It must be returned that way
back too.

Add a washer to the Alt. knob shaft and re-insert it back into the hub
assy to check that you now have a clearance between the knob skirt and
the Alt. scale surface. Remove the knob and shaft from the hub after
checking. Do not reassemble anything yet.

Correcting problem #2

Level the telescope mount, and attach a permanent bubble level.

Now this gets "tricky" so bear with me. You need to temporarily
re-attach the clamshell with the 4 screws and put the OTA back in the
clamshell. It will not hold any verticle position because the clutch
assy has not been reassembled. What we intend to do is make the Altitude
scale read WHATEVER angle the OTA is at whether level(0) or any other
angle even a negative angle.

(If) you have a digital level you can just let the scope sag down until
it rests against the mount. Place a digital level on the OTA and note
the exact reading. Leave the scope at that angle and move the scale with
damp thumb or whatever to match that reading in minus degrees. Sometimes
it is necessary to move the OTA up and down a little bit to get the
scale to move easier. BUT...the OTA needs to be remeasured to insure
your sag angle is still unchanged. When the OTA(scope) angle and the
Altitude degree scale read the same....you can GLUE the top & bottom
CENTER edges of the scale to the inner hub surface it is rubbing
against. Use a really good glue. Nothing that is soft when cured. Maybe
Radio Shack Super-Glue or a small amount from a tube of commercial
contact adhesive like Scotch-Grip #1357 from a hardware store. In one of
the two Autostars I have, I used BOTH at different places on the hub.

Now if you only have a bubble level you need to do the same thing. Only
you must support the OTA level, and wiggle it up and down until the
scale you are trying to move with your thumb says "0" when the supported
OTA(scope) shows level with the bubble device. Let the glue(s) cure
before reassembling the telescope back.

TO REASSEMBLE--

Remove telescope from clamshell, unscrew the 4 screws holding the
clamshell and remove it. Re-insert the Alt. Clutch knob shaft (and
washer if it was needed), back into the Alt. scale hub. Thread the 3
point clutch pressure plate on the shaft as you rotate the knob. Be sure
the pressure plate seats in the hub at the 3 recessed points. You do not
need to screw the knob down tight, just snug. Now add the locknut back
on the threaded shaft and snug it up too. The hub can be rotated with a
little hand power and tightened down with the knob just like normal
again. Having done all this.... reassemble the clamshell back into the
hub with the 4 screws, tighten the clutch knob a bit and put the scope
back in. Level the scope with your bubble level and look at your scale.
It should be DEAD ON and it will stay that way regardless the angle,
reading accurately. BTW -the glue is not permanent. You can chip it off
with a razor if needed at a later time for any reason. You have not done
anything that altered your AutoStar unless you forced that knob off
against the internal stops inside and something broke. You were supposed
to be hand holding the hub during nut removal and reassembly to prevent
any internal damage to those stops. Go slow, do it carefully, and enjoy
the results!

Thankyou Mike for hosting this.

L.Stange
YubaCity Sentinel -Meteor research station

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