Choosing the Right Mount for Astrophotography: Equatorial vs. Alt-Azimuth

When it comes to astrophotography, the choice of telescope mount is crucial. In this article, we will discuss the differences between alt-azimuth and equatorial mounts, and why an equatorial mount is almost mandatory for professional and amateur astrophotography.

The Importance of Mounts in Astrophotography

Almost all astrophotography requires long exposure imaging, where shutter speeds can be measured in seconds or minutes. For these long exposures, even small movements of the subject can have a significant impact on the image. Stars, for example, can appear elongated or as streaks if the mount does not compensate for the Earth's rotation. The Earth rotates at approximately 15 degrees of arc per hour, and this rotational movement must be accounted for to avoid blurring and streaking of the images.

Understanding the Earth's Rotation and Mount Compensation

To counteract the Earth's rotation, an alt-azimuth mount or an equatorial mount is required. However, the nature of celestial objects moving across the sky in arcs rather than straight lines complicates the situation for non-equatorial mounts.

Alt-Azimuth Mounts - The Fundamentals

An alt-azimuth mount (alt-az) allows the telescope to move in two axes - altitude (up and down) and azimuth (left and right). To track a star or celestial object across the sky, an alt-az mount must slew in both axes at varying rates. This requires computerized control to achieve accurate tracking, as manual control would not be precise enough.

Typically, an alt-az mount is computerized to track celestial objects. This involves the following steps:

The mount is first positioned to know its exact location on Earth (latitude and longitude) and the current local time, often obtained using GPS. A model of where specific stars should be in the sky at any given time is generated. At least two and often up to three stars are used to calibrate the model. A known star is centered, and adjustments are made to ensure it is properly aligned. Once the object to be imaged is selected, the computer calculates the required position and sends commands to the motors to move the mount to the correct position. Encoders on the mount help to verify movement and ensure accuracy.

Even with these measures, the accuracy of an alt-az mount for astrophotography is limited by various factors such as the precision of the star alignment, GPS accuracy, and the inherent errors in the calculations. These inaccuracies can accumulate over long exposures, leading to degraded image quality.

The Limitations of Alt-Az Mounts in Astrophotography

The primary limitation of an alt-az mount is the need for continuous alignment. Unlike an equatorial mount, an alt-az mount does not fully "see" the target. It relies on initial and continual adjustments to keep the target centered. Any error in these adjustments will affect the final image.

A further complication is frame rotation, where the field of view rotates as the object moves across the sky. This is particularly noticeable with wide-angle fields like constellations or clusters of stars. Frame rotation can be significant and can severely degrade the quality of the photograph, leading to distorted or misaligned images.

Compensating for frame rotation is possible with the use of autoguiding devices, but these are typically expensive and not always entirely effective. As a result, alt-az mounts are generally not recommended for deep-sky astrophotography.

Equatorial Mounts - The Solution

Equatorial mounts, on the other hand, are designed specifically to counteract the Earth's rotation by aligning one axis parallel to the celestial poles. This allows the mount to track celestial objects with precise pointing, leading to higher-quality images without the need for continuous alignment or correction.

Equatorial mounts use a primary axis that is aligned with the Earth's rotational axis, allowing for continuous tracking without the need for constant adjustments. This makes them highly suitable for astrophotography, especially for deep-sky objects.

Conclusion

While alt-azimuth mounts can be used for astrophotography, they are generally not as effective as equatorial mounts. Unless the user can afford observatory-class mounts with advanced features like t-point models and autoguiding, an equatorial mount is the recommended choice for producing high-quality astrophotography images.

Equatorial mounts offer better precision, consistent tracking, and longer exposure times, which are all critical for successful astrophotography. While they may be more expensive, the quality of the images they produce makes them a worthwhile investment for serious astrophotographers.