COMPREHENSIVE MOUNTS DESIGNS GUIDE

Posted by Daniel Amado on

The most important part of the astronomy gear setup deserves a whole article describing its purpose, designs differences, functionality, and features.

What is called “The mount” is the combination of both tripod legs or pier section and the mount head mechanism section that is comprised by 2 or more axes and has the purpose of allowing the movement of a telescope on a steady structure to frame or spot targets on the field of view of an eyepiece or camera.

The mount head mechanism is the foundation to locate and track objects in the night sky.

 

When we are choosing a mount for a telescope, we should look for stability, sturdiness, and smooth operation.

 

 

TELESCOPES MOUNTS DESIGNS

 

1. Alt-Azimuth Mounts

 

Manual Alt-Azimuth mounts (Aim and See)

 

 

The Alt-Azimuth mount is a simple design that consist of two moving components, a base with a rotatable axis for horizontal (panning) movements, and a single or dual arm with a rotatable axis for vertical (tilting) movements. The axes housings can be comprised by simple smooth friction padding material, bearings or can be more sophisticated with a worm wheel/worm screw drive gears mechanism with flex cables control. This whole structure assembles what is called the mount head intended to be attached to a tripod or pier. The name Alt-Azimuth is a contraction of Altitude and Azimuth terms. The Alt-Azimuth scheme is based on a horizon perspective coordinate system. Altitude are horizontal imaginary lines parallel to the horizon, and Azimuth are vertical imaginary lines that converge on the Central straight up imaginary point in the sky called Zenith. Both altitude and azimuth coordinates are visually measured in degrees, arc-minutes, and arc-seconds. The measurement range for altitude is 0 to 90 degrees, and the measurement range for azimuth is 0 to 360 degrees.

 

Dobsonian mounts

 

 

Like Alt-Azimuth mounts, they are composed by a flat ground base that allows horizontal movements, and rocker box that sits on the flat ground base and holds a Newtonian telescope and allows vertical movements. The horizontal and vertical movements are possible with smooth friction materials like Teflon. Dobsonian mounts are very steady compared with many “tripod-based” telescope mounts like single-arm Alt-Az or lightweight equatorial mounts. They also are unexpensive to manufacture. In combination with a Newtonian telescope, becomes the “Best Bang for the Buck” as the popular denomination reference for the best value regarding light gathering capability.

 

Star hopping

In this technique the observer spots bright stars as a reference to find faint objects in the night sky. With the help of a star chart, the observer first locates one or more bright stars in a finderscope, reflex sight, or, at a low magnification, with the telescope to be used for observation. The telescope is then moved by one or more increments towards the desired target that is near to the reference stars. The star hopping technique is used to locate space objects with manual Alt-Azimuth and Dobsonian mounts.

 

Smart “Push-to” Alt-Az and Dobsonian mounts

 

 

The “Push-to” Alt-Az and Dobsonian mounts only have encoders without motors. The computerized objects locator indicates the user where to move the telescope to spot the desired target.

 

 

Go-to Alt-Azimuth and Go-to Dobsonian mounts

The go-to Alt-Azimuth and Go-to dobsonian mounts include encoders and servo or stepper motors on both worm wheel/worm screw driven altitude and azimuth axes with computerized object locators hand controllers or WIFI connectivity with smartphones or tablets with planetarium apps to facilitate electronically the slewing to and tracking of space objects visible on the sky.

 

Dual-Fork Arm Design

 

 

Provides increased stability when holding the optical tube with two joints instead of one, but are considerably heavier.

 

Pros:

  • Easier and faster setup than equatorial mounts
  • Do not require polar alignment
  • No counter-weights required in most cases (except sometimes for the optical tube when imaging)
  • Objects in the night sky can be found using manual Alt-Az or dobsonian mounts with the star hopping technique without the need of batteries or any other power source
  • Dobsonian mounts are unexpensive and usually steadier than many “tripod-based” equatorial mounts
  • Usually lighter than equatorial mounts on single-arm Alt-Az design
  • Optional Equatorial Wedge for Polar Alignment available for some models
  • Alt-Az mounts with field rotators are a good solution for observatory roofs with short clearance

 

Cons:

 

  • The movement of both axes is required to track celestial objects
  • There are no motor drive tracking options for manual Alt-Az mounts due to its design nature
  • Field rotator is mandatory in deep sky astrophotography for exposures longer than 2-5 minutes
  • Limited movement range when imaging due to short clearance with Schmidt-Cassegrain telescopes and astrophotography gear interfering with the Alt-Az mount base when pointing straight up 
  • Alt-Az mounts usually have lower payload capacity than equatorial mounts
  • Dual arm fork design mounts are heavier than equatorial mounts and cannot be broken down into smaller sections easily like Mount head and optical tube for transportation

 

2. Equatorial Mounts

 

The purpose of the equatorial mount is compensate or neutralize the apparent sky motion due to earth’s rotation with the movement of just one axis, which is called Right Ascension. Equatorial mounts have a moving structure with four axes in total. The Latitude and Azimuth Axes are made for the Polar Alignment of the mount with the Earth rotation Axis. The Latitude axis allows the vertical alignment of the mount with the North Celestial Pole, and the Azimuth axis allows the horizontal alignment with the North Celestial Pole. The measurement range for Latitude is 0 to 90 degrees, and the measurement range for azimuth is 0 to 360 degrees. The other two axes are Declination and Right Ascension. The 4 axes’ housings assemble what is called the mount head intended to be attached to a tripod or pier. The equatorial mounts operate with the Celestial Coordinates System principle.

 

 

Celestial Coordinates System

 

 

Like the earth surface with the imaginary lines to determine specific locations like latitude which are parallel lines to the equator, and longitude that are imaginary lines that traverse the Earth between the north and south poles, the Declination (Celestial "latitude”) are imaginary lines in the sky parallel to the Celestial equator, and the Right Ascension (Celestial “longitude”) are imaginary lines that traverse the sky between the North Celestial Pole and the South Celestial Pole. With the Equatorial mounts, space objects are located with the celestial coordinates system which is based on the sky's perspective surrounding the earth’s surface. Declination and Right Ascension axes are used to move or slew the telescope to spot the desired targets, and right ascension axis enables the tracking of objects in the sky to keep them in the field of view. Declination and Right Ascension coordinates system axes are visually measured in degrees, arc-minutes and arc-seconds. The measurement range for declination is 0 to 90 degrees, and the measurement range for Right Ascension is 0 seconds to 24 hours.

 

Manual equatorial mounts

 

 

Are driven by a worm wheel and a worm screw drive gears mechanism with flex cables control to allow the user to locate and track space objects manually in the sky using the Declination and right ascension coordinates setting circles. For most manual models there are available motor drives to keep objects in an eyepiece or a camera sensor within the field of view.

 

Entry level manual equatorial mounts classification:

EQ-1: Too Wobbly and flimsy. User must deal with constant vibrations while tracking and focusing.

EQ-2: More decent structure than EQ-1. Can handle with better stability 60mm and 70mm refractors and short tube 80mm refractors or 90mm maksutov-cassegrains.

EQ-3: Suitable to for 4.5-inch and 5-inch Newtonian reflectors and 90mm refractors.

EQ-4: Good for 4-inch and 5-inch refractors and 6-inch Newtonian reflectors.

 

Go-to Equatorial mounts

 

The Go-to equatorial mounts include encoders and servo or stepper motors on both worm wheel/worm screw gears driven declination and right ascension axes with computerized object locators hand controllers or WIFI connectivity with smartphones or tablets with planetarium apps to facilitate electronically the slewing to and tracking of space objects visible on the sky.

 

Dual Go-to Alt-AZ Equatorial Mounts

These mounts can be used in both Alt-Azimuth and Equatorial modes. The computerized object locator has the capability of operate with the Alt-Az and Equatorial coordinate sytems allowing polar alignment plus stars alignment for equatorial mode or just stars alignment for the Alt-Az mode. 

 

Astrotrackers

The Astrotracker is tipically a mount head with just the Latitude, Azimuth and Right Ascension axis with a motor drive. It is made for astrophotography using a DSLR or mirrorless camera with lenses to take pictures of the Milky Way, nightscape, time-lapse photography, the moon and the sun. There are available some Astrotrackers (Like Omegon Minitracker) that work with just a clockwork and no power is required. A ball head camera mount adapter lets position the camera at any angle.

 

Pros:

  • Only one axis (right ascension) is required to move for tracking celestial objects
  • Objects in the night sky can be found with the right ascension and Dec setting circles coordinates on manual equatorial mounts without the need of batteries or any other power source
  • Plenty of motor drive options available for manual equatorial mounts
  • Field rotators are not mandatory for long exposure astrophotography with Equatorial mounts
  • No clearance spacing limit on the back of the optical tube for imaging train compared to Alt-Azimuth mounts with SCT or other Cassegrain design telescopes
  • Optical tube and mount head can be easily detached for easier transportation compared to dual-arm fork Alt-Azimuth mounts
  • Usually higher payload capacity than Alt-Az mounts
  • Go-to equatorial mounts are typically the preferred choice for long exposure astrophotography

 

 

Cons:

  • More difficult to setup than Alt-Az mounts due to the equatorial design complexity
  • Requires polar alignment to locate and track space objects with the equatorial coordinate system
  • Longer setup time than Alt-Az mounts
  • Mount Counter-weights handling and balance is required
  • Higher overall weight (including mount and counter-weights) than single arm Alt-Az mounts
  • Higher height clearance required for observatory roofs compared to Alt-Az mounts.

 

 

Specifications to consider when choosing a Go-to mount for observations and astrophotography with telescopes are:

 

Payload Capacity: The maximum instrument weight capacity that the mount is capable of operate smoothly.

Autoguiding capability: The Auto-guider port or Auto-guiding feature allows the mount to receive the guiding signal from the Auto-guider camera through a ST-4 cable or USB cable (pulse guiding) to correct the mount’s periodic error and polar alignment errors.

Tripod or Pier: Fixed Piers are the best choice for permanently installed telescopes. Portable piers or “Tri-Piers” tend to have superior stability with shorter legs than tripods and are easier to level than a massive tripod.

Worm wheel diameter/teeth count: The larger the diameter of the worm wheel with higher teeth count, the smoother and more precise the slewing and tracking will be.

Encoders Resolution: Encoders with higher resolution will increase tracking accuracy and minimize residual tracking errors.

Periodic error/Periodic error correction: Is the inverval error induced by mechanical imperfections on the worm wheel and the worm screw drive gear because they do not have an exact circular shape. This error repeats in every revolution as a periodic oscillation even if the mount is perfectly polar aligned, hence the term "periodic". The periodic error can be seen graphically represented in any autoguiding software like PHD2 as a sine wave pattern and is measured in arc-seconds. For mass produced mounts, the periodic error is not posted by the manufacturer, but its range is typically between 15 to 30 arc-seconds. When the manufacturer advertises the periodic error, the mount is guaranteed to meet or exceed that periodic error smoothness performance. Many Go-to equatorial mounts made by Celestron, Sky-watcher and Meade have a programmable periodic error correction built-in the mount’s electronic mainboard or hand controller.

Backlash: is the play between the worm wheel teeth and the worm screw thread. Too much backlash makes the stars “jump” in the field of view and increases the mount’s periodic error with a “bouncing” behavior. Increased backlash is caused when the worm wheel and worm screw gears are far from each other making the engagement of the gears mesh loose. On some mounts, it can be compensated physically by the user adjusting the distance between them. Besides the manual adjustment, many Go-to mounts include an electronic backlash compensation option on the hand controller. If the mount head has been dropped, knocked or is handled roughly during transportation or shipping, it could affect the factory calibration of gears meshing. Due to the nature of the worm wheel/worm screw gears design there must be an appropriate distance for minimal play. If the gears are completely tight, they will bind and eventually cause a motor stall. To reduce periodic error with proper engagement of the gears mesh with an equatorial mount, the counter-weights and the optical tube center of gravity have to be slightly out of balance towards the east on the right ascension axis and towards the back of the optical tube on the declination axis. Additionally, when the go-to mount is aligned with the hand controller or a computer, is better to finish the star alignments with the up and right directional buttons to improve tracking performance.

 

 

Reduced backlash and Zero Backlash mechanism mount designs

Higher-end performance mounts with more advanced mechanical system, reduced periodic error and minimal or no backlash are available from some manufacturers.

 

Belt-driven:

 

 

Instead of direct contact between the motor drive gear and the worm screw gear, belt-driven mounts have a neoprene belt that engages both gears while being apart over 1 inch. The belt system relieves the motor drive torque needed to move the mount’s axes reducing backlash and smoothing the native periodic error. Examples of Belt-driven mounts are the Sky-Watcher EQ6-R, Celestron CGX and iOptron CEM26.

 

Harmonic drive (Strain Wave Gear):

The Harmonic drives, also known as Strain Wave Gear, are composed by 3 elements, the wave generator, the flex spline, and the circular spline. Compared to the worm wheel/worm screw design, the harmonic drive has a significantly improved performance with virtually no backlash, higher torque, more compact size, excellent repeatability, and position accuracy. Manufacturers of mounts with harmonic drives have developed a weightless design able to work without counter-weights and with a high payload capacity. Because the compact size of the harmonic drive design housing, mounts with this gear design are extremely portable.  Rainbowastro and Crux mounts have the Strain Wave Gear design.

 

Direct drive:

The direct drives have a direct torque generation on the axis via a magnetic coil. This direct torque generation system with no transmission is implemented with the same principle on zero emission electric cars like Tesla. Direct drive design is absolutely gear free and provides zero backlash and zero periodic error when combined with absolute encoders while slewing and tracking. Mounts with direct drives have a maximum slewing speed up to 50 degrees per second and can counter against wind gusts. This mount design has the highest performance among all mount designs, superior to worm gear and strain wave gear designs. Mounts like Planewave L-500, 10micron AZ5000 and ASA DDM200 have direct drive technology.


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