Without a doubt the most important piece of gear in an astrophotographer's imaging chain is the mount. With the ability to track stars continuously for several minutes as the Earth rotates on its axis, a precision mount is the only way to capture some of astronomy's finest details. Faint stars, nebula, and galaxies require long exposures. Sans a mount, long camera exposures will result in images similar to the one below.
While the image above is very cool, it is the result of stacking many long exposures taken without a mount. One important aspect can be gleamed from this image, the star in the center of the swirling pool is Polaris, the North Star. Sailors of old would often use Polaris for navigating their ships at night as it did not change position with the passage of time. It is this star, that astrophotographers use for polar aligning their telescope mounts. By polar aligning a mount with Polaris, the mount will rotate at the same speed as the Earth rotates on its axis. The result will be pinpoint, sharp stars with little noise and brilliant details typically invisible to the naked eye. An example of an image only capable of being captured with the aid of an astronomic mount is shown below.
Life would be too easy were our discussion to end there. However, there are two general categories of mounts to choose from. Which one should you choose? The short answer is a German Equatorial Go-To Mount, and the best one you can afford at that. There are a number of considerations to take into account before laying out that large sum of cash though. For my recommendation on which mount to buy, stay tuned for a later blog post where I cover the equipment the beginner astronomer should purchase and why. But for the purposes of this post, let's just start out with an explanation on the types of mounts, some of their benefits, and any possible weaknesses.
Remember how I said there were two general categories of mounts. They are Equatorial and Non-Equatorial. The most popular Non-Equatorial mount is known as the Altitude-Azimuth mount, or Altazimuth mount. Another less popular type is known as the Ball and Socket mount, which I will not cover in detail here.
As you can see to the right, the Altazimuth mount will rotate on two axis. The azimuth axis rotates the mount in the horizontal plane. As the name implies, the second axis will adjust the mount and telescopes altitude. This type of mount is typically cheaper than an Equatorial mount and is usually included with beginner and Dobsonian telescopes.
While the mechanics of an Altazimuth mount are simpler than an Equatorial mount, the movement along both axis in order to track the stars is more complex. This isn't an issue for computerized / Go-To Altazimuth mounts; however, it becomes significantly more difficult when attempting to track the stars by hand. This challenge is not impossible to overcome, yet there is a larger concern when it comes to astrophotography. Due to the geometry of how an Altazimuth mount operates, the image being captured will slowly rotate about the center of the field of view. Long exposures will be impossible with this type of mount. While perfectly fine for visual observation, a German Equatorial mount does not suffer from this Altazimuth weakness.
A German Equatorial Mount, or GEM, is the optimal mount for astrophotography. As seen in the image below, there are also two axis, Right Ascension and Declination. The Right Ascension motor rotates the mount about the polar axis. While the Declination motor rotates the mount about the axis perpendicular to the polar axis.
Once the star or astronomical object to be photographed is centered in the telescope, the right ascension motor is locked down and will no longer need a variable speed to track the star. This is because the right ascension motor will rotate the mount at the same speed as the Earth rotates about the same polar axis. This is known as side real or clock drive, and is one revolution per 23 hours and 56 minutes. Therefore, the mount only needs to speed up or slow down the declination motor depending on the chosen astronomical target. GEMs also have the added benefit of not rotating the object within the field of view. This is crucial to astrophotography.
In addition to the type of mount, it is also important to ensure the mount can carry the maximum load. This will include your telescope, camera or eye piece, diagonal, field flattener, guide scope, autoguider, and any other gear you may want to put on the mount. Be careful to check the retailer's mount specifications, as some companies count the counterweight against the maximum capacity, while others do not. If the maximum capacity is 40 pounds, check to see if that includes 20 pounds of counterweights (effectively 20 pounds of gear only).
Other than a sturdy tripod, there are a couple features to strongly consider when making your choice about mounts. One is a built in port for autoguiding. An autoguider will make the difference between tack sharp exposures at 1 minute versus 7 minutes. Those longer exposures really help get the detail of faint deep space objects, and an autoguider will help get you there. Also a computerized Go-To feature is very helpful. Chances are, if the mount if Go-To then it will be compatible with the Astronomy Common Object Model (ASCOM). ASCOM is the defacto computer software for guiding your mount. It also opens a whole world of helpful astronomical software enabling you to get the best images possible.
As mentioned earlier, stay tuned to future blog installments, where we will discuss everything from Optical Tube Assemblies (OTAs), to acquisition software, processing techniques, and helpful tutorials to get you out the door and under the stars.