By Chris Fellows, Serenity Mobile Observatory
Getting you pointed in the right direction.
Last week we looked at Optical Tube Assemblies (OTA) and talked about the different designs and uses of each. This week we are going to concentrate on what I think is a more important part of any astronomy equipment setup, the telescope mount. The mount is basically the part of the rig that attaches the OTA to the tripod or pier. This bit of equipment holds and points the OTA at the targets you are going to be looking at. Mounts come in two basic designs, Altazimuth and Equatorial, and we need to understand the differences between these before we can make an informed decision.
Altazimuth, or Alt/Az, stands for Altitude and Azimuth, which basically means up/down and left/right. Altazimuth mounts are simple in design and consist of two rotatable and perpendicular axes, up/down (vertical) degrees above the horizon, and left/right (horizontal) degrees east/west. The mounting head on your satellite dish, if you have one, is an Altazimuth mount. The main advantage of an altazimuth mount is its simplicity. It has a simple mechanical design, is simple to setup, and simple to use.
A second advantage, at least in a standard two-arm “fork” or “C” arm altazimuth mount, is its rigidity. Most altazimuth mounts hold the OTA between two “arms” – one on either side of the OTA – which is a very ridged design and tends to hold the OTA very steady. Several newer low-end altazimuth mounts remove one of the arms to reduce material costs and in the process lose this advantage.
An additional advantage of an altazimuth mount is that due to its configuration it tends to keep the eyepiece at a convenient position at all observing locations. This means you typically won’t need a stepladder or to be a contortionist to get to the eyepiece no matter what you are pointing at.
Altazimuth mounts have a few disadvantages as well. For me, as an astrophotographer, the biggest disadvantage is field rotation. Due to the design of the altazimuth mount, the arms must be moved or rotated in both axes simultaneously and at differing rates in order to track an object across the sky. Many modern altazimuth mounts are motor driven and computer controlled so this task is handled by the mount itself. This doesn’t, however, solve the field rotation problem where the object you are observing seems to slowly rotate in the eyepiece. For visual observations this isn’t a big deal – you will probably not even notice the rotation as you observe. But for photography this is disastrous, and your photographs will be a smeared blob in the final photograph.
A second disadvantage is called gimbal lock. This only happens when the scope is pointed at the zenith or straight up, but any target within about a degree of the zenith is all but unobservable because the azimuth axis will have to rotate faster and faster the closer you are to the zenith. At the zenith this speed is effectively infinite. Obviously no mechanical device can move at infinite speeds, so objects very near the zenith are out. Again, for visual observations this isn’t a big issue. If the object you are hoping to observe is close to the zenith, just wait a few minutes until it moves – due to the rotation of the earth – away from straight up, and you can observe it. Manufacturers have developed and sell an add-on device called an equatorial wedge that effectively transform altazimuth mounts into equatorial mounts to avoid these issues.
Equatorial (EQ) mounts add two additional pointing positions to the mix, called right ascension and declination. They have altitude and azimuth settings like altazimuth mounts, but in the case of equatorial mounts those are semi-permanent adjustments made during the initial mount setup and left there until the telescope is taken down or moved to a new location. All pointing at targets is accomplished by moving the mount in right ascension and declination and all object tracking is done by moving only in the right ascension axis. The main advantage of this design is it eliminates the problems of field rotation and gimbal lock and also requires only one motor to track celestial objects.
The downside is they are generally much more complex to set up and get aligned. The right ascension axis must be pointed directly at the celestial pole (near Polaris, the North Star). This is accomplished by moving the azimuth and altitude adjustments on the mount, basically turning a few screws to move the mount head into the right position. This adjustment is not intuitive, must be very precise, must be done in the dark, and can take a long time to get right. Software developers have created tools to assist with this step, so if you are running your telescope with a laptop, help is a download away but, of course, we just added a laptop to the mix.
Now, I don’t want to give the impression that equatorial mount alignment is impossible. I set up and tear down my scope several times a week as I move from park to park, and I can get “good enough” polar alignment in just a few minutes. Like anything else, practice makes perfect. Once the mount is polar aligned, it will be moved in right ascension and declination to point at your target. Once a target is acquired, only the right ascension axis needs to be moved to counteract the rotation of the earth in order to keep a target centered in the eyepiece with no field rotation.
Whether you select an altazimuth or a equatorial mount you will need to check its payload capacity and make sure it can carry your scope and other equipment you may attach to your scope. In my case, I had selected an equatorial mount made by Celestron called a CGX, but I had to move up to the CGX-L (almost double the price) because the CGX capacity was 50 lbs. My scope by itself is 40 lbs., add on guide scope, a couple of cameras, autofocuser, filter wheels, eyepieces, dew shields, finder scopes, blah, blah blah, and I would be overweight. So, I stepped up to the CGX-L, which has 100 lbs. of capacity. More than I need, but it is always good to have some spare capabilities. Most mounts are sold with an accompanying tripod or pier to sit on, but make sure you ask. Some high-end mounts don’t include a tripod but offer you a variety of choices at additional cost, of course.
Mount selection is possibly the most important decision you will make when buying a telescope. It is important to understand the features and limitations of your mount and how you will use it. Take your time, do some additional research, and talk to others who have made this journey ahead of you. There are several good discussion groups on the Internet that will be more than happy to give you their opinions.
Chris Fellows, Serenity Mobile Observatory
Find Chris on Facebook (or, if you’re lucky, at your campground). (Editor: Check out his amazing photos on his Facebook page!)