Why are Equatorial mounts better for AP

Started Nov 22, 2012 | Questions thread
RustierOne
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Re: Why are Equatorial mounts better for AP
In reply to Hotphotons, Nov 22, 2012

Hotphotons wrote:

I have always heard equatorial mounts are better for astrophotography but have never seen an explanation as to why. A computerized and motorized alt azimuth type mount can still track a celestial object so why is it not good for AP?

PCwheeler's comments are quite correct. To help understand why, one needs to understand how the stars appear to move across the sky due to Earth's rotation.

An observer located on Earth's north pole would see the stars appear to rotate around a point directly overhead, where the Earth's rotational axis intersects the sky at the north celestial pole. An alt-azimuth mount at that location would function quite well since its vertical (azimuth) axis is aligned with the Earth's rotational axis. Tracking a star would take place by just turning the mount's vertical  axis. There would be no field rotation since the mount's rotating axis is aligned with Earth's rotational axis.

An observer located at a mid northern latitude (say 30°) would see the north celestial pole elevated 30° above his northern horizon. An alt-azimuth mount located at this latitude would have neither of it's axes directed at Earth's rotational axis. Tracking a star would require turning on both of the mount's axes. While the star would be tracked quite well, the field would rotate around the star due to misalignment of the axes.

In contrast, an equatorial mount located at 30° latitude is tipped 30° so that it's polar axis is aligned with Earth's rotational axis. By rotating just it's polar axle, the mount is able track a star with no field rotation.

I hope this explanation helps in understanding why an equatorial mount is best for long exposure astrophotography.

Best Regards,
Russ

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