For this particular application I suggest an automotive battery and a single high wattage resistor sized to limit the voltage to between six and eight volts, with two automotive starter contactors, the type that would be used to directly power a starter motor. The first contactor to apply power through the high wattage resistor and the second contactor to bypass the resistor after a very few seconds. Selecting the direction would be done by a separate reversing switch that never would switch under load. The reversing contactor should engage to allow opening, so that manual operation for closing can be done even if there is a power failure. And certainly there should be a suitable circuit breaker as the first device in line with the battery. The battery can have a low current float charger connected constantly.
No little processor module needed.
Consider that at the first moment of startup that entire 900 watts is converted to heat in the controller, although if the supply voltage drops the power will be a bit less.
So the scheme with the two heavy relays and the resistor will make sense. There will need to be a means of assuring that the resistor is bypassed as required every time after the motor is started..So really the move should start and then run to the stopping point, rather then constantly cycling on and off..
also, that observatory opens much faster than I have seen in the videos of observatory opening and closing.

 

I will remove the DC power supply and will try this with a lead-acid car battery. probably this week. What is the recommended ramp up / slow down time? 2 seconds, 3 seconds, higher? Also is a linear ramp-up./slow-down ok, or are we talking about some non-linear curves?

 

You do not mention the gear reduction ratio?
When you ramp the PWM up, do you reach the maximum operating RPM of the motor, if not what % Max RPM?
Ideally your controller should have current sense, limit, and current fold back if necessary.

 

It is a 90:1 gear reduction and it turns the sprocket at 38 rpm. I do rev it up to 98% under load.