1.  Equatorial mount

The equatorial mount for an astronomical telescope that I had built in a previous project was operational for astronomical observation with a small telescope. However, since the payload was about 5 kg, it was impossible to mount a telescope with an aperture of 20 cm class, which is effective for planetary observation, and furthermore, it was completely impossible to mount a telescope with an aperture of 30 cm class. To solve this problem, I decided to build the YTM-4 with a mechanical part that is 1.5 times larger than the previous YTM-3. Figure 1 shows the YTM-4 with a 30cm Newtonian telescope mounted.

Fig. 1   YTM-4 single arm equatorial mount with a Orion Optics UK's 30cm F4 Newtonian optical tube.

    The equatorial mount is a single arm type, which was adopted this time as well, since it was understood in the final state of YTM-3 that the single arm type, in which the optical tube is mounted on the inside of the arm, can increase the payload capacity with a smaller load on the motor. The advantages of this type are that it does not require a counterweight, which is almost the same weight as the optical tube in the German type, and that the optical tube does not need to be moved from the west side to the east side of the equatorial mount when the observed object passes south-central. On the other hand, the disadvantage is that the length of the arm needs to be considerably increased in order to increase the operating range in the Dec. axis.

    In YTM-3, the structure was built by machining a 10mm thick, 100mm wide aluminum plate, but this has been increased to 150 mm wide. The length of the arm structure has been changed from 200 mm per side to 300 mm. The thickness of the plate is the same as before, 10 mm, for ease of fabrication, but the arm is made of two layers and add L-angles to increase its strength. I used a small band saw and a drilling machine to process the aluminum plates.

    Figure 2 shows the appearance of YTM-4. The polar axis body is fixed on to the tripod. The R.A. axis rotary stage is bolted to the polar axis body surface of perpendicular to the polar axis. The arm structure is bolted to the top of the R.A. axis rotary stage. And the arm structure is made up of the plate 1/2 and the plate 2/2 joined at a right angle. The Dec. axis rotary stage is bolted to the plate 2/2, and a connection plate for fixing the optical tube is bolted to the top of the Dec. rotary stage.

Fig. 2  YTM-4 Single arm equatorial mount.

2. Gear section

The gear section of R.A. and Dec. axis of YTM-3 used a motorized rotary stage Y200RA60 with a diameter of 60 mm, but since the strength of this section needed to be increased, an HT03RA100 with a diameter of 100 mm was used for this section as shown in Fig. 3. The stepper motor is the same 42-type as the 60-mm-diameter one, and the connector pin connection for the coil terminals is also the same. Although a larger gear diameter would have been preferable, the next largest type of rotary stage available is 200 mm in diameter, and its weight, price, and the entire equatorial mount would have been too large for this concept, so I did not use this type. The reduction ratio of the worm gear was increased from 90:1 to 180:1, so the control pulse had to be adjusted to the new ratio.

Fig. 3  100mm diameter motorized rotary stage and the polar axis body.

4. Controller section

The control unit and counting unit are from the MCT-6 project. To cope with the difference in worm gear reduction ratios, I first considered changing the clock frequency divider circuit. However, it turned out that this would require changing the frequency divider circuit of the counting unit as well, resulting in a loss of compatibility with the existing system as a whole. I decided to test the system with the micro-step driver DM542 with the division...