1. The Mount

1.1 The Night Sky

The night sky has captured the imagination of countless millions over the past thousands and thousands of years. With today's busy everyday life, some of us are not aware just how the night sky changes over the course of several hours and several months. For example, consider looking North from Europa Point in Gibraltar (Northern Hemisphere) on the night of the 26th July 2014 at 23:00. 

Highlighted above are the so-called North Star, Polaris and the constellation Cassiopeia. Important to note at this point in time is that Polaris is named as such because the star sits very, very close to our Northern celestial pole (the axis of rotation of the Earth as the day progresses through its 24 hour cycle). Due to this position, looking at Polaris over a number of hours shows extremely little change in position. For example, consider looking North on the same night but four hours later, at 03:00:

Indeed Polaris sits pretty much in the same place of the night sky but the constellation Cassiopeia has moved significantly. The bright star Capella has also appeared over the horizon at this point. This demonstrates how the night sky changes over the course of one night - objects appear through the East horizon and disappear through the West horizon, everything appearing to spin counter-clockwise around the axis pointed out by the star Polaris. During this motion, all objects pass the Meridian - an imaginary line that extends from North to South completely overhead (looking straight up). 

Above, Polaris is again highlighted and the view is this time toward the North-West, on the same night at 03:00. The green line going overhead is the Meridian and the grid shown all over the night sky shows the axis of rotation of the Earth. Red arrows drawn above demonstrate the motion of the objects visible in the night sky as the night progresses. 

Moreover, it is common knowledge that as the year progresses from day to day and month to month, the Earth moves around the Sun in its elliptical orbit. Together with the tilt of the Earth's axis of rotation, we have the seasons. Put all this together and given the different position of any particular observation site on Earth (e.g. Europa Point in Gibraltar), different constellations, stars and objects are visible in different times of the night. Take for example the night of 27th June 2014 at 01:00, looking East. The constellation of Orion is not visible because it is actually below the horizon and only visible on an alternate point of observation on the Earth. 

Effectively speaking, the Orion constellation is below the ground and cannot be observed until much, much later that night. Compound this with the fact that during the summer months, dawn arrives earlier and there is little chance of observing the Orion constellation during this time of the year. Fast forward the time to the 27th October 2014 at 1:00 (four months later), however and the Orion constellation is up and over the horizon. 

We conclude, therefore, that to photograph the night sky we need to take into account that our target will be moving constantly and may indeed not be visible until a certain time of the night or even a certain day later in the year. 

1.2 Types of Mount and the Axes

As discussed above, the night sky is constantly moving and objects appear to rotate counter-clockwise around the celestial axis, defined roughly by the position of the star Polaris. Since objects are generally very faint, they cannot be photographed in snapshots - exposures must be long and commonly of several minutes. This requires the imaging equipment to be accurately tracking the object of interest throughout exposures. It is both common and recommended that astrophotographers start their journey with a DSLR camera mounted on a simple tripod. 

The issue here is of course, that there is zero tracking of the night sky and pretty quickly, exposures taken start to show star trailing - the tracing of their path of motion across the night sky. Some amazing images of this have been captured, of course, mainly centered around the star Polaris, demonstrating how everything in the night sky appears to revolve around Polaris. 

It is noted however that amazing as some of these images may be, they only demonstrate one thing and they naturally destroy any detail that can be discerned from deep space objects such as galaxies and nebulae. At this point is when we invoke the idea of having a tripod that actually tracks the night sky. The first kind, which most people are aware of, is the Alt-Azimuth mount. There are dozens upon dozens of models by different manufacturers, so the following is just one of them (not necessarily the best or the worst available in the market). 

The mount is known as an Alt-Azimuth mount because it only moves the telescope in Altitude (up and down) and Azimuth (left and right). A common capability these days is of course, GOTO, whereby the handset controller contains a large database of thousands of objects and on a simple initial alignment procedure (commonly with Polaris so the mount starts pointing North), can be used to slew the telescope to any target you desire to view or photograph. Alignment to various stars is usually key to achieving a higher degree of GOTO accuracy. This involves the telescope being pointed at a star, failing and you centering it on an eyepiece for the handset to note down corrections. On pointing the telescope at a desired target, whether from the GOTO database on the handset or manually, the motors on the mount will track the object to ensure it remains in view throughout the night. Though this is not going to be 100% precise anyway, there is actually an inherent problem with Alt-Azimuth mounts that is much more important to address - field rotation. Below is a view of the night sky with two grids enabled. 

The grid shown above in orange is that corresponding to an Alt-Azimuth mount's motion (up and down, left and right). However, as discussed in section 1.1, the night sky does not move along this simplistic grid. The night sky actually moves along the blue grid shown above, known as the Equatorial grid. As a result, when one tracks an object in space with an Alt-Azimuth mount, it appears in a certain orientation at one point in time and a few hours later, will appear rotated. See the following example of Andromeda Galaxy captured at different times with an Alt-Azimuth mount. 

Imagine now that your exposure has to be of 5 minutes in length and that you are fairly zoomed in on the target with the use of a telescope. The effects of field rotation will be vastly pronounced and are shown as circular trails and generally blurred out detail. This places a restriction on how long exposures can be with an Alt-Azimuth mount before the detail suffers significantly from field rotation. The more zoomed in your optical system is (e.g. higher focal length telescope), the more restricted you will be in exposure length as the field rotation becomes increasingly pronounced. Unfortunately this does mean that fainter objects will not really be photographable to a viable extent. 

Enter the Equatorial mount. This type of mount is aligned precisely to the star Polaris and the motors then move the telescope along the axes shown in the blue grid - the Equatorial grid. 

Labelled above are Declination and Right Ascension. These are the names given to the axes in the Equatorial grid (compared to earlier when we called up and down Altitude and left and right Azimuth). An Equatorial mount has an iconic, standard design. The telescope and all other accessories are clamped on to the top and a bar with counterweights comes out the opposite side. These counterweights are there to balance the system well and ensure the gears in the Equatorial mount are able to move consistently and thus track the night sky accurately. Again, there are dozens upon dozens of models of Equatorial mounts and by different manufacturers, so the following is just one of them (not necessarily the best or the worst available in the market). 

Again, nowadays these mounts come with GOTO handsets for slewing to targets from a database as well as assisting in aligning the mount with Polaris and various stars. They work in much the same way as Alt-Azimuth mounts, except their motors track the night sky along the Equatorial grid (Declination and Right Ascension), meaning that in effect, the astrophotographer is able to capture much, much longer exposures and avoid field rotation. 

1.3 Choosing the Right Mount

Choosing the right mount for the job is key to building an astrophotography setup. The mount forms the most important component of the astrophotography setup as you can buy the most expensive and high-quality optics paired with an incredible CCD camera, but if your mount is not Equatorial, cannot sustain the weight put on it or keeps shaking, there is no point in even trying. There are therefore general guidelines to choosing the right mount:

• • 1. Type of Mount: The mount must be Equatorial. This is an absolute must if you are to capture exposures of any decent length, which is necessary to capture faint objects in deep space. Where this is relaxed is when you are only interested in astrophotography of very bright targets - the planets, the Moon and the Sun. An Alt-Azimuth mount suffices here. 

  • 2. Payload: The mount must be able to carry the payload you will be putting on it. If a mount is advertised as being able to carry 25 kg of payload, for example, then for astrophotography do not exceed 3/4 of this (about 18 kg in this case). This will allow the mount to keep up with its job - tracking the night sky - without shaking. 

  • 3. Polarscope: For an Equatorial mount, getting one with a polarscope is a great idea. This is a small scope generally at the rear of the mount that you look through to align the mount precisely to Polaris. Precise polar alignment is key to good tracking as the mount is able to slew knowing precisely the location of the axis of rotation of the Earth. 

  • 4. Computer Control: Mounts these days generally have a GOTO handset for slewing to targets from a database but a lot of these Equatorial mounts can be controlled straight through a Windows laptop, by use of an inexpensive USB interface cable that goes into the port where the GOTO handset normally goes, and connects to the laptop via USB. These are called EQDirect cables and allow the user to take full control of the mount and go well beyond the GOTO handsets' capabilities. 

  • 5. Autoguiding Port: Though Equatorial mounts can track the night sky pretty well (some better than others), the tracking is never 100% precise, especially for long exposures. A second camera is generally used to lock on to a star near your target and track its movement, sending corrections to the mount via the autoguiding port if it moves (to keep the mount tracking constantly on-check). Technically, if point 4 above is met, this autoguiding port is not required but it is handy. 

With the above information, one should be able to make a more informed decision of the mount to buy. The following are recommendations to date (22 / 12 / 2013) of Equatorial mounts. These vary in price and capabilities but meet all five points above.