Recommended Mounts for Beginning Astrophotography

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.

Primary Considerations

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.