Last week I offered the simple and easy to use altitude-azimuth mount. This week I’ll discuss the more complex, not-so-easy to use, but essential for astrophotography, equatorial mount.
To reiterate, the altitude-azimuth is simple and easy to use. It works best for visual astronomy, not so much for astrophotography. Why? Its design cannot accommodate the apparent arc that celestial objects take as they go from east to west in our sky. This is called field rotation and makes astrophotography challenging as the apparent position of objects seems to rotate. This is not an issue for visual astronomy so the alt-az mount is an ideal visual observer’s mount.
Funny thing about equatorial mounts, they still get marketed to budding astronomers as a higher-level option. That’s unfortunate because the last thing a new astronomy buff needs is added complexity.
Equatorial mounts operate in two axes the same as the alt-az. That’s where the similarity ends. The alt=az uses simple linear left-to-right motion on a level, rotating platform, and up-to-down motion. The equatorial mount platform is set at an angle, equal to the local Latitude. The motions of an equatorial mount are called Right Ascension and Declination. In a sense they are both left and right motions, but right ascension motion follows the arc of objects as they move in the sly. So right ascension also goes up and down because it is set on an angled platform. It’s a more complex motion than alt-az!
It’s on an angled platform, equal to the local latitude, and aimed at Polaris. This is because our Earth rotates, and Polaris is very close to the axis of rotation. If we could drive a rod through the Earth’s axis it would nearly hit Polaris. By having the platform angle set to local latitude and aiming the mount at Polaris the mount is called Polar Aligned. That’s the ticket for equatorial mounts. Once polar alignment is achieved an object will stay in the field of view by simply adjusting the right-ascension control. The object stays in the field of view and the mount follows it’s arc. No field rotation. That’s the ticked for astrophotography!
But that’s also a recipe for frustration. The equatorial mount is tricky to adjust manually because all motions are curved vs linear. For a beginner, even for a veteran, trying to locate an object manually requires numerous curvy moves as you get closer. That requires some patience.
If it’s so much more complex and trickier, why is the equatorial mount considered advanced vs the altitude-azimuth mount? As mentioned, it affords the user a suitable platform for astrophotography. However, the altitude-azimuth mount can be set up for astrophotography too, at additional cost.
Whichever mount you choose, they both benefit from automation/computerization.
A major key to enjoying your telescope is a well-made mount, no looseness, no wiggles.
What’s in the Sky?
THIS IS A TREAT!
December 7; start looking at about 8:45 pm. Very bright Mars slips behind the Full Moon and reappears a little later.