Mount guide for SHGs

While the SHG can function by mounting it on a stationary tripod and allowing the Sun to drift across the slit due to sidereal motion, using an equatorial (EQ) mount is highly recommended for a smoother imaging experience.

Since the SHG reconstructs images through successive "slices," it is essential for these slices to align perfectly without rotation or interpolation. To achieve this, the Sun’s motion must be perpendicular to the slit so that the scan covers the solar disk uniformly, with each slice joining seamlessly. This level of precision is only possible with an EQ mount.

In contrast, an alt-azimuth (Alt-Az) mount introduces field rotation as it slews, which complicates image reconstruction. Although software tools can correct for these distortions, they rely on interpolation to produce a clean, round solar disk, which can slightly degrade image quality. Therefore, we recommend using a sturdy, high-quality EQ mount that has been properly polar aligned (PA) for SHG applications. Additionally, it is crucial to orient the slit perpendicular to the Sun’s movement across the field to ensure optimal scanning and the highest image quality (generally, a perpendicular error of 1 degree or less is recommended).

A higher-quality mount with smooth slewing will significantly reduce mosaicing artifacts, as it ensures better alignment of the successive slices. In contrast, a poorly performing mount with high periodic error (PE) and vibrations during slewing will amplify the "saw-edge" artifact often seen in SHG images.

This "saw-edge" effect occurs due to slight side-to-side misalignments between slices, which may result from atmospheric turbulence (seeing), wind, or mechanical vibrations from the mount. Using a sturdy, smooth-slewing mount helps eliminate these mechanical vibrations, leaving only environmental factors—like wind or seeing conditions—as the primary challenges. Thus, investing in a reliable mount improves image quality by minimizing these alignment issues, allowing for cleaner, more accurate reconstructions.

This artifact can be mitigated by stacking multiple images captured within a short time frame (a few minutes). Stacking works by averaging out the misalignments, often reducing or even eliminating the "saw-edge" artifact.

Below is a comparison between a single image and a stack of five. Notice how stacking not only reduces noise and smooths out artifacts but also provides a more visually pleasing result. The overall image becomes cleaner and more refined, enhancing both aesthetic quality and detail.

Every mount has an optimal slew speed that minimizes artifacts and provides the best image quality. It’s essential to experiment with your specific setup to find the ideal scan speed. However, slower slew speeds—such as 4x or 8x sidereal—tend to yield better results. Starting with these speeds and adjusting as needed can help you identify the best setting for your system.

Mounts with sturdy construction and smooth drive trains are generally more suitable for SHG imaging. I personally use a friction drive mount (the JTW Trident GTR), which performs exceptionally well. Smaller harmonic drive mounts, such as the ZWO AM5, have also been reported to work effectively. Your worm driven mount will also work well as long as it sturdy and smooth slewing.

Since mechanical vibrations can degrade the quality of the scan, the placement of the SHG on the mount is critical. It is preferable to mount the SHG as close to the declination (DEC) plate as possible, rather than far from the center of rotation. Placing it farther away increases the risk of resonance and periodic vibrations from the drive mechanism, which can negatively impact image quality.