MENU
NexStar Home
Slippery Slope
NexStar Links
Observing Log
NexStar Tech
NexStar FAQ
Astrophotos

This is the arrangement with the camera and extension tube that was causing slippage:

1

The camera weighs 17 oz and is about 6" behind the rear cell.

Here is a view of the dec nut with the fork arm cover removed:

2

It's a really big nut (a little under 2" if I recall), you need a huge wrench to adjust it properly - I would not recommend pliers.   I used a 14" adjustable wrench.

Removing the nut you find behind it a washer:

3

That washer is cone shaped:

4

so that it acts like a spring as you tighten the nut and it flattens out.

Of course, when you remove the nut, the OTA immediately comes off the other side revealing the dec gear:

5

What you see here is a big gear with 3 thick washers (probably nylon) set in small recesses on it.  For the curious, this assembly weighs between 6 and 7 lbs.  A closeup:

6

That's some kind of black grease that is all over everything.  This gear is not fixed to the central shaft you see in the upper right corner.  If you look at the back side of the gear there is a thick fiber pad between it and a plate fixed to the OTA:

7

While I didn't remove it, I have been told the gear just slides off the shaft and if you remove the fiber pad, behind it is a plate that is held to the OTA by 3 screws.  Some owners have received scopes in which these screws are loose.

With the OTA removed, the fork arm looks like this:

8

What you are looking at is the metal casting of the fork arm.  This has a circular depression in it.  At the "bottom" of the depression are two washers, which are still in place here.  Next is a roller bearing and then two more washers.  Those have been removed in the picture above and this is what they look like (they are sitting on the conical washer):

9

This is the other side of the fork arm:

10

The wide groove is where the white washers slide as the scope turns - this is basically another bearing though not as nice as a roller bearing.   This is not the clutch, that being the fiber disk between the dec gear and the plate bolted to the OTA.  The bolt on the OTA sticks through the hole in the middle.   If you move the OTA manually, the fiber washer slips against the back side of the dec gear while the dec gear and encoder stand still.

So the OTA rides on a pair of bearings: one is shown in picture 9 above and is clamped under the dec nut; the other is composed of the white washers sliding in the groove above.

Here is a close up of the spur gear that drives the dec gear:

11

This gear is mounted on a shaft that goes through the arm to a right angle gear-set the bottom of which is the motor gear:

12

And here are the electrical connections for the handset jack:

13

The only thing I found wrong in all of this was the presence of some brown "crud" on the conical washer, the dec nut, and the OTA shaft.   You can see the crud in the threads here:

14

This stuff reminded me of plumbers putty but all dried out.   I cleaned it off of the conical washer and dec nut and put it all back together.   The is no "right position" that the OTA needs to be in relative to the drive gears, you just stick in back on making sure the gear teeth line up. 

When I tightened up the dec nut, hand tight was all it needed to hold position with the camera on the back.  In fact, I set a 1 lb. weight on the camera and it still didn't slip.  Even if I wound the camera it didn't slip.   Haven't tried taking pictures through it yet though.  I did notice that with the camera on the back, when slewing at rate 5, the motion was not even, as if the white washers weren't sliding smoothly in their groove.  The motion was much more even when just the eyepiece was in the back though still not perfect.  However, I never checked for even motion before so I don't know if that is any different.  Obviously, one sign of an over tightened dec nut would be too much friction between the white washers and the fork arm, leading possibly to jerky motion.

February 16, 2002