SECOND PHASE, AUTOMATION ROTATION

For visual observation, a manual dome is a good solution. But if you want to do photography, photometry or astronomical spectroscopy, it is necessary to automate the dome since the opening of the dome has to be oriented automatically towards where the telescope is oriented. This is especially important if you take long photo sessions, which can last for hours, or even all night if you plan to photograph several objects. At least the rotation of the dome must be automated. The automatic opening of the roof is more complicated an was done in a next step.

 

The first step is therefore to motorize the turn of the dome. For this purpose I hired a metal ring with a diameter that fit exactly with the inside diameter of the base of the hexagon of the dome. The ring is fixed to the 6 sides and we already have something to transmit the force of the motor.

 

 

Of the many systems that I evaluated, the one that seemed easiest to implement was the one based on friction. To do this, the engine (power of 17Nm and 6 rpm) is fitted with a semi-rigid wheel that presses on the metal ring. At the point where the pressure is applied to the inner face of the ring, a wheel is placed on the outer face to withstand the pressure. At the beginning the friction was made rubber on metal, but over time I discovered that it worked much better to fix both the metal ring and the wheel of the engine an adhesive friction band of those that are placed on the steps to prevent them from escaping.

So that the assembly of the ring, fixed to the dome, stays centered, I installed some pieces of pressure with a neck as shown in the photo. Four pieces distributed every 90 degrees.

The electrical automation works with MaxDome II control board. The system requires the detection of the movement of the system by means of an optical detector. For this detection, Maxim DL suggests the placement of alternate black / white marks on the motor shaft, but it seemed safer to use the marks on a wheel that also rotates by friction. In this way, if at some point the engine turned but was not dragging the dome, the system detects it and jumps the corresponding error. The system also requires a detection of the "home" position of the dome, which was resolved by a reed-type relay and a magnet.

The aspect that turned out to be more complicated is to solve that, due to the power of the motor, the relays that integrates the MaxDome II board did not admit the sufficient current. This was solved using external relays and a circuit like the one shown below.

The operation of MaxDome II is very reliable and stable. Since I installed it, I have not had any problems.

This scheme allows the use of external relays with higher amperage to the internal ones available to MaxDome II.

 

In the case of the 17Nm motor, the MaxDome II internal relays were frequently blocked.

 

Only 4 diodes are needed and the connection can be made directly to the relay terminals without the need for a circuit board, as it is a fairly simple scheme.