How to Use an Equatorial Mount
by zane landers
Equatorial mounts are commonly sold with telescopes meant for astrophotography or for small inexpensive beginner instruments. In spite of their complicated appearance, equatorial mounts really aren’t all that complicated nor are they much more difficult to use than an alt-azimuth mount.
An equatorial mount works by allowing you to move the telescope in only one direction – the right ascension axis – for tracking, which means the telescope can be driven by a simple synchronous motor or the turn of one knob. This is extremely helpful for astrophotography.
Equatorial Mount Parts
An equatorial mount consists of three main parts. The right ascension axis or polar axis is the main axis that the mount swings on as it moves across the night sky from east to west, tracking with motors or by hand. The right ascension axis lines up perpendicularly with the celestial pole around which stars seem to rotate and Earth’s axis points to. The right ascension axis usually has a T-shaped exterior, with the right ascension shaft going down into the base of the mount with adjustments for latitude.
Right Ascension Axis(Polar Axis)
The declination axis usually goes right through the right ascension axis (or sits atop) and is opposite the counterweight shaft used to balance the right ascension axis. It adjusts pointing north-south. Most telescopes sit on top of the declination axis and attach to it with bolted-on tube rings or a dovetail plate.
Declination Axis
Altitude Motion
Azimuth Motion
The final part of an equatorial mount is the tripod or pier which the mount rests on to keep it steady and the telescope at a comfortable height.
Some equatorial mounts are all-manual, but most can at least have a simple drive attached to move the right ascension axis along automatically, and many have computerized drive systems with motors on both the right ascension and declination axis for automatic pointing and to allow for guiding adjustments during long-exposure astrophotography.
All equatorial mounts have clutches. For manual mounts, you’ll frequently find yourself locking and unlocking these clutches when aiming the telescope around the sky. A computerized mount must have its clutches locked at all times during operation for safety and accurate pointing/tracking, and should only have the clutches unlocked during initial balancing or assembly/disassembly.
Counterweight
Assembly & Balance
Any equatorial mount needs to of course be assembled and the tripod leveled before use. Once you’ve done so, you attach the right ascension axis counterweights to the shaft provided in the quantity needed to balance the telescope. When you unlock the clutches, the telescope is going to swing either in the direction of the counterweights or the optical tube; slide the counterweight(s) until the telescope no longer moves on the right ascension under its own weight. You can mark the counterweight shaft with tape or a permanent marker to keep track of the balance point every time you set up.
Once the right ascension axis is balanced, you’ll need to balance the telescope on the declination axis. You do this by unlocking the declination axis clutch and sliding the telescope tube – either in its tube rings or with the attached dovetail bar – until it is no longer front-heavy or back-heavy. You can mark the tube, rings, or dovetail to keep track in the future.
The process of balancing is the same with any equatorial mount, computerized or manual, and it’s extremely important. An unbalanced manual mount is frustrating to aim and an unbalanced computerized mount will have inaccurate slewing and put strain on the gears, causing damage over time.
Polar Alignment
Once your telescope/mount combination is assembled, leveled, and balanced, the next step is to polar align it. In the case of smaller and cheaper equatorial mounts, you simply set the latitude adjustment to as close as you can and aim the right ascension axis in the direction of due north or south (in the case of Northern Hemisphere observers, as close to Polaris as you can get). A small equatorial mount is probably all-manual or driven with a simple right-ascension drive so exact accuracy is not critical.
Polaris will seem fixed compared to other stars.
Larger equatorial mounts, especially computerized ones, almost always have a hole in the polar axis to sight through at a minimum. This is enough to get you close enough for visual use with shorter focal length instruments, but for astrophotography and viewing with larger or longer focal length telescopes where more precision is required you’ll need either a polar scope or to use computerized polar alignment tools. Some GoTo equatorial mounts have software which will allow you to polar align without even having a direct line of sight to the north celestial pole or stars near it such as Polaris. Alternatively, a polar alignment camera like the QHY PoleMaster can attach to your mount and assist in polar alignment.
Using Your Equatorial Mount
Once you’re polar aligned, leveled, and balanced, your mount is done with the setup process – though a computerized equatorial mount usually needs a basic 2- or 3-star alignment for GoTo functionality. With a computerized mount, you’ll control movements with buttons either on the key pad, a smartphone/tablet or a connected computer. A manual mount generally requires you to unlock the clutches to move the telescope coarsely across the sky. Fine adjustments are then made with either a key pad or by turning cables to adjust fine-motion gears on the mount’s right ascension and declination axis to line up your target as well as provide tracking functions in the absence of a drive.
You might find that your telescope’s eyepiece ends up in uncomfortable positions in some parts of the sky with an equatorial mount. The solution to this is to either rotate the telescope’s attached star diagonal or the entire telescope in its rings if possible. However, this may require you to re-center your target afterwards and can ruin the polar and/or GoTo alignment of a computerized mount entirely.
You might also find that with an equatorial mount your balance (and thus accuracy in the case of a computerized mount) is affected by off-axis loads like a heavy finder or guide scope, or stuff in your focuser in the case of a Newtonian reflector. For photography, the key is to line your focuser and any finder/guide scope up with the right ascension and declination axis as closely as possible – this may mean having your camera be “below” the telescope at times with a reflector. However, for visual use you’ll probably have to put up with it if you want to have any comfortable viewing.