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Hi Mario
By "tilt" I assume you are referring to the angle of the polar axis on your mount? If so, then as SaturnC writes, the tilt should be the same as your Latitude and the mount should be pointed at the South Celestial Pole (SCP).
Why? Well, first of all, let’s imagine that the Earth is at the centre of a huge celestial sphere and for convenience, let’s also imagine that all the stars appear as points of light inside this celestial sphere and we are inside, at the centre, looking out.
Just as the Earth has an axis of rotation with a N and S pole, our imaginary celestial sphere also has an imaginary North Celestial Pole and a South Celestial Pole (SCP). If you now imagine sticking a knitting needle or skewer through the N-S axis of rotation of the Earth, then in the South, the skewer would touch our imaginary celestial sphere at the SCP. This is where we want to point our mount.
We know that the Earth rotates once in 24 hours (approx) and if you stand outside (at night) and look south for long enough, you would see that the stars appear to revolve around this imaginary point of the SCP. That is, they would rise above your horizon E of S, be highest above your horizon when due S and finally set below your horizon when W of South. (Except for those stars that are within 32 degrees of the SCP - these will never set in Perth, they are called circumpolar).
So, the SCP is an imaginary point on our celestial sphere around which the stars appear to rotate once every 24 hours due to the Earth’s daily rotation. If we make a drawing and use simple geometry, we can show that the Latitude of our observing location is the same angle as the angle of the SCP above our true horizon. In Brisbane the SCP is 27.5 degrees above my true horizon. If the Latitude of Perth is 32 degrees south, then the SCP would be due south of you and be 32 degrees above your true horizon.
Setting our mount so that the right ascension axis points to the SCP allows the mount to use its motors to track the stars as they appear to rotate due to the Earth’s daily rotation.
Hope that rather long winded explanation helps!
Cheers
Dennis
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