ERF for SHGs

Despite Christian Buil's recommendation to use ND filters as ERFs, I have observed that any ND filter can negatively impact image quality, including the Hoya ProND he suggests. This can be attributed to the fact that these filters are constructed with very thin substrates relative to their diameter. In contrast, solar Energy Rejection Filters (ERFs) typically have a thickness-to-diameter ratio of around 1:15. For example, a 100mm ERF should be at least 7mm thick, while a 130mm ERF would be approximately 9mm thick. These dielectric filters reject unwanted energy primarily through reflection, absorbing only a minimal amount of heat.

ND filters, on the other hand, are absorption filters that work by absorbing heat rather than reflecting it. To withstand the thermal stress caused by substrate expansion while maintaining wavefront integrity, absorption filters must be particularly thick. For instance, Daystar’s absorption ERFs are very thick, with a thickness-to-diameter ratio of about 1/8 to 1/10 (meaning a 130mm ERF would be around 13mm thick). In comparison, typical ND filters are only about 2mm thick for any diameter, such as the 82mm filter I purchased.

Furthermore, ERFs on solar scopes are usually mounted in a loose cell, allowing for movement and expansion as they heat up. This design is crucial; if the edges of the filter are held too tightly, thermal expansion can cause warping, which degrades the wavefront. ND filters are not designed for solar observation. While the Hoya ProND exhibits good surface quality (lambda/4 or better), prolonged use under the Sun ultimately leads to a decline in image quality.

For these reasons, I advise against using ND filters, particularly for larger apertures. Instead, I recommend using a solar wedge (Herschel prism) as an ERF. A high-quality solar wedge effectively rejects most of the heat while maintaining excellent image quality, and they are generally affordable—options priced around $120 from China can work well.

In my experience, the front optical cell absorbs very little heat (less than 0.5%) since the glass is essentially transparent in the visible and infrared spectrums. It becomes opaque only in deep UV (below 200nm), but those wavelengths have low intensity in the atmosphere and contribute minimal heat. After 4-5 hours of solar observation, the front cell of my 6" scope usually remains cool to the touch.

The objective lens faces minimal risk, as it is transparent and positioned in an unfocused beam. The only exception occurs with oil-spaced objectives, where prolonged exposure to direct sunlight can cause the spacer oil to become cloudy. While the objective glass may heat slightly and expand within its housing, potentially reducing figure accuracy, this primarily affects image quality without threatening the safety of the optics.

The real risk comes from components downstream of the objective. In larger scopes with additional glass elements (such as correctors or reducers) at the end of the focused light cone, there is a risk of focused sunlight damaging coatings of unknown quality. Although the likelihood of this is low, uneven heating could lead to cracks if the coating absorbs too much heat.

If using a subaperture narrowband filter as an ERF, there is a risk that the reflected beam could focus in front of the ERF. If the Sun is at an angle to the tube, this focused beam might strike the side lining of the tube and potentially damage the flocking material, although such occurrences are rare. The focused image typically remains only a few centimeters above the ERF surface, making it difficult to hit the tube wall directly. The beam expands rapidly, losing its damaging potential as it spreads.

If no ERF is used and only the slit is exposed, the reflected beam from the slit will likely strike the side of the 1.25" nosepiece of the SHG (since the slit is mounted at an angle), which poses no danger to the scope, although the nosepiece may become warm to the touch.

The safest method for ERF implementation is to use a front-mounted ERF, which rejects heat before it reaches the tube, though these can be more expensive and they limit the wavelength that you can observe with your SHG. The second safest option is a Herschel prism, which directs heat out the back of the scope while producing minimal reflected light. This also improves exposure time management, which is essential for SHGs. The Herschel wedge allows you to observe in full spectrum.

For our MLAstro slit, we utilize a specialized material with a Quartz substrate and a unique coating process. Some lime-soda slit, however, is not designed for solar use; it was originally intended for nighttime applications. Its substrate is made of lime-soda glass, which expands significantly under heat and can crack easily. Additionally, its thin coating has a low blocking rating (OD5), making it susceptible to melting with excessive heat.

In contrast, our Quartz slit features a low thermal expansion coefficient, making it highly resilient to solar heat loads. Its thicker coating provides a much higher blocking rating (OD7, which is 100 times better than that of the lime soda slit) and is more durable overall.

I have successfully used the Quartz slit without ERF filters on a 4" scope (102mm) for extended periods without issue. So usage of an ERF on scope smaller than 4" is not a must, but if you choose to use the ERF on your setup, it'll definitely reduce the heat flux that hit the slit and extend it's life span. A UV-IR cut filter in front of the slit is enough to significantly reduce the chance of slit damage due to over heating.