Recommended Mounts for Beginning Astrophotography
This page is a work in progress. It is not complete.
I'd like to state at the outset here that this is not an exhaustive discussion, and these are not the only options. However, the mounts discussed here have been widely used for AP and have proven capable of the task.
It is also critical to state that not all individual production units of the same mount will behave the same. Simply put, there are variations in production quality and while one mount might perform extremely well, another mount of the same model may not perform as well. This doesn't even take into account the fact that the way the mount is cared for and stored, the payload it carries, the camera used with the payload, and the climate and weather conditions in which it is regularly used all play a part in the overall result a given mount provides.
With this all in mind, there are a handful of mounts that do stand out as better than others.
The purpose of the mount is to hold the telescope and camera stable, aim it at a target, and keep that target fixed in the field of view. To do so, a mount must cancel out the apparent motion of the stars caused by the rotation of the earth by matching that motion.
For the most part, this requires an equatorial mount. Though there are some ways of doing this with an alt-az mount, to do so requires additional technology and a higher degree of precision. Normally this is only found in high-end research-grade telescopes.
But not all equatorial mounts are made the same, and this has a significant effect on accuracy. For visual use, a mount need only be accurate enough to keep the object of interest in the field of view while the object is being observed. But for the kind of long-exposure imaging required for capturing deep sky objects, the object must remain absolutely still in the field of view during the duration of the exposure. This is strongly affected by the focal length of the telescope and the size of the image sensor.
Image Sensor and Focal Length
Consider a fixed camera for a moment. As the Earth turns, the stars above appear to move. The rate of motion is approximately 360 degrees per day (it's a little shorter than this by about 4 minutes, but we'll round up for the sake of argument). Divide this by 24 and we get about 15 degrees per hour. Divide that by 60 minutes and we get about 0.25 degrees, or 15 arcminutes, per minute. Divide by 60 again and we get about 0.25 arcminutes, or 15 arcseconds, per second. So, the star you are pointing your camera at moves about 15" in a single second.
Let's say your camera is a Canon T7i. It has an APS-C sensor that measures about 22.3 mm by 14.9 mm with a resolution of 6,000 by 4,000 pixels. To that, let's attach a 50 mm lens. When you calculate out the field of view, you get about 25.7° by 17.2°. This gives you about 15.7" per pixel. Let's say in a particular part of the image there is a small star that's only 1 pixel wide/tall. If you take a 1 second exposure,that star will start to bleed into a second pixel, creating a small streak. Of course, with this level of resolution, this is not going to be very noticeable unless you zoom way in. But the effect is already there.
The commonly-used principle known as the "500 Rule" basically states that the maximum exposure time in seconds you can get with a DSLR before the motion of the stars is noticeable in the image is approximately 500 divided by the focal length of the lens used. So for a 50 mm lens, you can get about 10 seconds of exposure time before the motion is noticeable. For a 100 mm lens, it's about half that, or 5 seconds. For a 300 mm telescope or long telephoto lens, it's 1.67 seconds.
This demonstrates that the longer your field of view, the less room for error you have. The longer the field of view, the more important your mount's ability to properly track on the target. This is because the field of view decreases as you increase focal length. But the focal length is only half the problem. The size of the image sensor
The relationship between sensor size and field of view is similar to the relationship between the telescope focal length and eyepiece focal length. The larger the sensor size, the wider the field of view will be, but as you decrease sensor size, the field of view contracts. As the field of view contracts, smaller errors in the mount's motion have a proportionately larger effect. If your camera and telescope give you approximately 1 arcsecond per pixel resolution at 500 mm focal length, at 1,000 mm focal length this will be 0.5 arcseconds per pixel. If, then, your periodic error is approximately 1 arcsecond, then the longer focal length has double the effect. This should be kept in mind when selecting a mount for the type of AP you want to do.
The amount of payload weight a particular mount can handle is limited and determined by such factors as the gear ratio, motor torque, and materials used in manufacture. Manufacturers usually publish the mount's capacity in their specifications documentation, and exceeding the mount's published limits may cause damage to the mount that is not covered by the warranty. However, there is no official and universal procedure used by manufacturers to determine this amount. The method used by Orion, for example, may be different from that used by Celestron, which may be different from that used by Astro-Physics. Even though many of the mounts made by common brands are produced in the same factory by the same workers as those from other brands (for example, Celestron and SkyWatcher are both produced by Synta), they may label their capacities differently.
In general, the capacity refers to the payload capacity and does not include counterweights. For example, Celestron lists the payload capacity of the CGEM as 40 lbs. The mount comes with a 17 lb counterweight, and many owners purchase a second such weight to counter-balance the payload. When sold with some of their heavier scopes, such as the C11, Celestron includes a second weight. In the case of the C11, the OTA is approximately 28 lbs and the combined weight of the two counterweights is 34 lbs. The combined weight, then, of 62 lbs would well exceed the 40 lb limit if it were to apply to the total weight. Nearly all manufacturers specify payload, but it is important to read their specifications carefully, as a few manufacturers may specify total weight on the axis.