ISO 9001 - AS 9100 - ITAR Standard & Custom Optical Filters and Coatings


Solar cycles are measured by the ebb and flow of naturally occurring sun spots. These spots appear as dark cooler spots. As Solar Cycle 24 progresses towards its minimum solar activity with over 100 days of displaying zero sunspots, astronomers and solar researchers alike, look forward to the sun “waking up” and ushering in Solar Cycle 25. Solar cycles have an average 11-year life span. During this new 11-year cycle we can expect to start seeing a heightened number of sunspots and solar events in the upcoming years. Activity should peak around the year 2025. This paper is intended to illustrate the variety of optical filters available for viewing solar activity and how they can improve the precision and value of the data collected. 

Astronomers can choose to study the sun across a broad spectrum (e.g.; visible “white” light, or infrared) or choose among several narrow band wavelengths, H-Alpha and CaK, being the most prominent.

Space Weather Prediction Center.

White light solar filters are the most affordable for the amateur astronomer. They are typically made from a durable glass or polymer film that attenuates the transmission of light from the sun to a fraction of a percent. Utilizing white light filtering, one sees the sun’s photosphere as a soft yellow solar disk and is able to see the solar granulation of the surface, as well as occurring sun spots. White light is good for viewing and documenting day to day or week to week changes.

If you are looking for real time excitement, one needs to go no further than the Hydrogen-Alpha line filter at 656.28nm. According to the Observer’s Guide to the H-Alpha, written by Bob King, “Viewed in a narrow slice of the spectrum centered in the ruby red H-Alpha, the sun broils with activity. Prominences, spicules, fibrils, Ellerman bombs, flares and more seethe and boil before your eyes. Unlike white light, the H-Alpha pulls back the curtain to expose the beast for what it is, a ferocious, unpredictable ball of incandescent hydrogen gas”. The H-Alpha bandpass filter is utilized in many solar telescopes in conjunction with a red glass filter, dichroic filter and Fabry-Perot etalon to cancel out the undesired wavelengths and giving an artificial solar eclipse any sunny day of the year. Being able to see the edge of the sun will reveal a close up view of solar flares, mass ejections and other solar activity not visible through white light filtering.

The CaK emission line, short for Calcium-Potassium, is found at the edge of the visible spectrum at 393.4nm. Most people cannot see detail at this wavelength. However, digital cameras capture it very well resulting in excellent images differing greatly from the H-Alpha line. These images will appear a deep blue and show excellent surface detail - at the cost of loss of definition in sun spots.

Additional wavelengths less commonly used are the Sodium D-Line doublets at 589.0nm and 589.6nm along with the Helium D3-Line at 587.6nm. The Helium D3-Line will show better sun spot detail, supergranulation and other solar activity.
Whether you are an amateur astronomer, sun gazer or scientist, Andover Corporation’s knowledgeable sales team and engineers can help provide the right type of optical filters for your solar observation in this upcoming and exciting solar cycle.

1.     Bob King (2015). Observer’s Guide to the H-Alpha Sun. Sky and Telescope:     Cloud Break Optics. Best Telescopes & Gear For: Solar Viewing.     David H. Hathaway (2015). The Solar Cycle. Springer Link, Living Reviews in Solar Physics valume 12. Article number: 4 (2015).
4.     Damian Allis (2014). CNYO Observing Log: Baltimore Woods Solar Session, 22 February 2014.