SHG Scope guide

So, you've just got your hands on the brand-new MLAstro SHG and are now wondering if your current telescope is the right fit. Here’s some friendly advice to help you optimize your setup!

Optical Design

First off, make sure your scope is a refractor. Reflectors are generally frowned upon for solar imaging unless you have an expensive front-mounted full aperture ERF. The focused light cone hitting the secondary mirror holder can potentially cause damage, especially if it’s misaligned and misses the secondary mirror. So, refractors are probably your safest bet for solar observations, including with the SHG.

For solar imaging, the optical quality requirements are less stringent compared to other types of astronomical imaging. A decent $400 secondhand 6" doublet can perform nearly as well as a high-end $10k ED APO when paired with the SHG. Since you’re working with extreme monochromatic light, chromatic aberration isn't a major concern, meaning you don't necessarily need triplet or ED optics. Furthermore, because you're only utilizing a small portion of the field of view, issues like astigmatism and coma are less pronounced. The main optical concern to keep an eye on is spherical aberration, which can be mitigated by masking down the aperture.

Remember, not all scopes are created equal. Be sure to perform a star test on your secondhand scope before buying. A well-collimated scope will provide the best sharpness and contrast.

F-ratio

The SHG works best with optics that have an f-ratio above F/7. Budget doublet scopes typically come with more spherical aberration, which can be lessened by stopping down the optics. While f/4 or f/5 scopes can also be used, they usually require higher-quality optics to achieve satisfactory results. If you’re using a budget doublet, there’s a good chance it will perform better when stopped down to F/8 or F/10. When the 12mm slit version of the MLAstro SHG became available, usage of this device on a telescope longer than 700mm in focal length will require stopping down the aperture to F/10 or more.

Focal Length

This topic can get a bit tricky. . While there isn’t a strict limit on focal lengths, SHG 700 has a recommended maximum focal length for achieving a full disk image in one scan. For the SHG-700, it’s 730mm. There's another version that's on the work with 10mm slit. This will allow you to use scopes up to 1000mm in focal length with your SHG. The SHG 700 users can choose to buy the 10mm slit and instantly upgrade their SHG to the SHG 1000 without any accessories.

The SHG-700 employs a compound lens design, which provides a larger corrected field, ensuring that the edges of the field of view are fully corrected. This means you can capture the entire Sun's disk in sharp focus with a 730mm focal length scope.

Additionally, since the SHG reconstructs images from consecutive slices, seeing conditions will affect the overall resolution. If you're observing in average seeing conditions (about 2-3 arcseconds), I’d recommend keeping the focal length around 1000mm at most; going longer than that may limit your resolution due to atmospheric distortion.

Focuser

Lastly, consider the focuser. The SHG is quite heavy, with the SHG-700 optics and camera weighing about 1.1-1.2kg. This creates significant torque at the focuser, so a sturdy, non-shifting focuser is crucial for maintaining image quality. Since the SHG scans across the Sun, any vibrations or shifts in the focuser can severely impact your images.

Feel free to reach out if you have any more questions or need further help optimizing your setup!