Solar Observation - a Sri Lankan perspective

Safe observation of the Sun

The sun can be one of the most interesting astronomical objects to observe with its ever-changing spots, faculae etc. Nevertheless looking at the sun without proper eye protection or methods can lead to eye damage and even blindness. With one of the rarest of astronomical events – the transit of Venus - due to occur on 6^th June 2012 there will be many people looking at the Sun. Here we will look at some common methods of observation.

Naked Eye + Filter

Smoked glass, CDs, DVDs can be dangerous.

Darkest grade welding filter can be OK for brief observation. (Do not use with optical instruments).

Projection Method

This is the safest method if carried out carefully. Aim the telescope at the sun, then rack the focusing tube out a little more than normal (experiment here) and project the Sun’s image on a sheet of white cardboard etc. Note: Do not look through the telescope to aim it at the sun. Adjust the instrument until its shadow on the cardboard is at minimum size – the telescope will then be aimed at the sun. Do NOT look through the finder or any other sighting device - for safety the finder should be removed from the telescope. Some cemented eyepieces may be damaged by the Sun’s heat so if possible use un-cemented oculars eg. Ramsden, Huyghenien etc. As most binoculars have cemented oculars they are generally unsuited to the projection method. Most reflector and catidioptric telescopes can be damaged when using the projection method as the cement securing the secondaries to their holders can be damaged by the Sun’s heat. In larger instruments; reflectors, refractors and catadioptrics it will be found useful to reduce the aperture with a mask which can be made of cardboard etc. Reflectors and catadioptrics will require 'off axis' masks. Altogether I do not favour the projection method as I find it cumbersome and images are of low-contrast, but note this is my personal view only.

White Light Solar Filters

In my opinion using metallized polyester film solar filters give the best images. I have tried various filter materials and have settled on the Astro-Baader solar film. This film can be obtained in A4 size sheets which the user must cut to appropriate size and mount in a suitable (home-made) holder. Note: the filter comes in two grades viz. for visual use and for photographic use. Make sure that you buy the film designed for visual use. Mounted filters are also commercially available for a range of telescopes. If a filter is not available for your instrument buy a slightly larger one and use cardboard or neoprene shims to fit it to your telescope. Make sure that the filter is a secure fit so that it will not fall off in use and expose the eye to dangerous solar radiation. I have found that the Baader filter in conjuntion with a Green G[X1] filter increases visibility of the solar faculae. Metallized glass filters are also available. They perform much as the polyester film does, but are much more durable. As might be expected they also cost more. Nevertheless, unless these filters are made of optically flat and plane parallel glass (some are not) image quality may be inferior to that of the film. Before using either type, the filter should be examined for damage, pinholes etc. These filters can also be used on binoculars although two are required (one for each barrel); alternately a single filter can be used on one barrel provided that the other barrel is securely capped for safety.

H Alpha Solar Filters

Hydrogen alpha solar filters give spectacular views of the sun in the spectral region of 656.28 nanometres (deep red). Using H. Alpha filters is the only way one can see the prominences, dark filaments and surface granulation. A downside is their high cost.

Eyepiece Filters

Sun Filters (i.e. solar filters caps which fitted over the oculars of refractor telescopes) were supplied in the past. These are dangerous as they are prone to cracking from the Sun's heat. NEVER use these filters.

When in Doubt

When in doubt do not risk your eyesight, do not look at the Sun. You can always view images from the internet.

Finally these are my personal views. Others may have different opinions and methods.