SkyWatcher Esprit 150ED

The SkyWatcher Esprit 150ED at InFINNity Deck
Figure 1: The SkyWatcher Esprit 150ED at InFINNity Deck
The main imaging scope of InFINNity Deck is a SkyWatcher Esprit 150ED apochromat of 150mm aperture and 1050mm focal length (f/7). I acquired this scope from a previous owner even before the construction of the observatory had started. The previous owner used it on a portable set-up at his balcony, but was looking for something lighter, not wanting to carry around with this 15kg scope (without camera that is). For my fixed set-up the weight is no issue and I am still very pleased that I managed to get this scope.
In combination with the ZWO ASI1600 camera, the SkyWatcher Esprit 150ED delivers an image of 57' x 43', not exactly wide-field, but very nice to image smaller objects like the headphones nebula.
The Esprit came with its original 3" focuser. Although this is a nice and sturdy focuser, I was not too pleased with the original 1:10 planetary drive. For imaging I focus using Starizona's MicroTouch focuser-motor, which drives the focuser on the 1:10 input shaft. Failing to produce enough friction, the focuser could not be operated reliably.


The planetary drive of the SkyWatcher Esprit 150ED, at the centre the replacement geared drive..
Figure 2: The planetary drive of the SkyWatcher Esprit 150ED, at the centre the replacement geared drive..
I replaced the original friction type planetary drive with a geared one. For this I bought a robot motor with a 1:16 gearbox. The gearbox was fastened with two screws to the motor, so was quickly taken apart. Then I needed the shaft of the motor as that had the first gear that I needed to connect it to the MicroTouch motor.
On my lathe I created a new hub to give the gearbox the correct outer diameter to fits the focuser and the MicroTouch motor. Once assembled the whole contraption is only 7mm longer than the original drive. It now easily holds the weight of the camera and never slips a single focuser count.
After having the new drive assembled I made a special tool to dismantle the original friction type planetary gear. After cleaning and reassembling it functions much better than before, now easily holding the heavy TeleVue Ethos eyepieces. Still I prefer the geared drive-train for imaging. Exchanging the two only takes a few minutes.


The original SkyWatcher planetary gear fully disassembled.
Figure 3: The original SkyWatcher planetary gear fully disassembled.
In figure 3 the fully disassembled original SkyWatcher ball-type planetary gear is shown. The three balls at the left clamp the axis that is shown in the front. The brass part at the upper left holds the three balls together and is locked into place by the brass ring at the right (the one with the two notches).
The construction sits in the biggest metal part at the top, which is conical at the inside. The more the brass ring is tightened the more the three balls are pressed together and thus the more pressure is applied to the axis, resulting in more friction. Care should be taken not to overtighten the brass ring as that may cause the balls to create indentations in the axis, rendering it useless.
Also special attention should be given to the workspace when disassembling the gear as it contains a bearing with 18 tiny balls. When the gear is opened these ball roll freely! Assembling is made easy when the balls are glued in place with a thick layer of grease.


The steel tool for turning the brass locking plate next to the planetary gear.
Figure 4: The steel tool for turning the brass locking plate next to the planetary gear.
In order to disassemble the planetary gear I made a special tool (see figure 7). A piece of 25mm diameter steel rod was used for it. First I made a small depression in it to accept the protruding brass part of the planetary gear. Then I created the rim at the same inner and outer diameter of the brass ring. Finally I mounted the tool under a milling machine and machines the two notches by taking away the surrounding material. In order to be able to apply enough force a hole was drilled through the side to allow to use a screwdriver or a pick as a handle (See figure 7).
The final test is holding the gear and tool together, which should form a perfect match (see figure 8). Before tightening the brass ring, it should first be loosened as it is fixated using a retaining compound liquid.


If you have any questions and/or remarks please let me know.

The original ball type 1:10 drive (front) and the new geared planetary drive.
Figure 5: The original ball type 1:10 drive (front) and the new geared planetary drive.
 
The new geared planetary drive for the SkyWatcher Esprit 150ED assembled with the motor attached.
Figure 6: The new geared planetary drive for the SkyWatcher Esprit 150ED assembled with the motor attached.

The steel tool for turning the brass locking plate.
Figure 7: The steel tool for turning the brass locking plate.
 
The tool fits snugly into the brass ring.
Figure 8: The tool fits snugly into the brass ring.

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