Observing in Red and White

 

Canada Day 150 is less than one week away so it is good time to respect the colours of flag and do some solar observing in red and white light. At the Canada Day 150 celebrations in Maple Ridge, members of RASC will have telescopes, equipped with red and white filters, available for viewing of the Sun (weather permitting).

Warning: NEVER look directly at the Sun through binoculars, a telescope or with your unaided eye. Serious eye damage and even blindness can result!

High-quality solar filters that remove infrared and ultraviolet radiation and allow only a small fraction of the Sun’s visible light through are required to safely view the Sun. The most popular solar filters are a white light filter where the Sun appears white and a Hydrogen-alpha filter where the Sun appears red.

White Light Solar Filters

White light solar filters are very dark neutral density filters – they transmit only 0.001% of the light. These filters allow you to see sunspots on the surface of the sun and are ideal for viewing solar eclipses and transits of Mercury or Venus. Some granulation (texture) on the surface of the sun can also be seen. However, solar prominences can not be observed with a white light filter and require the use of a hydrogen-alpha filter. Filters made from Baader Astro Solar Film are popular, inexpensive, and high-quality.  Baader solar film is a thin material similar to Mylar that gives a pure white image.

Sun through a white light filter
Sunspots visible using a white light filter made with Baader solar film.  Taken with a cell phone through a Skywatcher ED100 refractor.  Image Credit:  Ken Jackson, May 2012.
Hydrogen-Alpha Solar Filters

Hydrogen-alpha (H-alpha) filters transmit only one specific wavelength of light: a deep red light that is emitted by the hydrogen atoms which make up the bulk of the sun.  Like white light filters, the overall light transmission is only 0.001% for safe observing, and potentially harmful infrared light is blocked completely.

H-alpha filters are much more expensive than white light filters. The least expensive ones; from companies like Lunt, Coronado, or DayStar; start at about $700.

Prominence visible using a Hydrogen-alpha filter.
Prominence visible in the deep red light from a Hydrogen-alpha filter in a Lunt 50mm solar scope.  Image credit:  Ken Jackson, June 25th, 2017

These companies also offer specialized solar scopes which are normally small refactors with an H-alpha filter already built-in. The sun is bright and does not require a large aperture telescope for viewing it in detail so most solar observers use 40-90mm diameter refractors.

White light filters provide an inexpensive way to start observing the Sun but H-alpha filters reveal its ferocious dynamics. Prominences, spicules, fibrils, and flares become visible. Prominences can be seen at the limb as brilliant blobs, as loops or as sprays. Structural changes can occur in as few as 10 minutes. They’re also capable of shooting higher up in the corona before the material recollects and falls back into the chromosphere.

Images taken through Hydrogen-alpha filters reveal details on the Sun’s surface and can be processed to create a yellow image that is expected by most people.

The Solar disk in Hydrogen-alpha
The Solar disk in Hydrogen-alpha using a Coronado PST. Image Credit: Lorenzo Mezzimi

 

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