Observing the sun requires a special type of protective filter called a solar filter. Such filters work by passing sunlight through specialized layer of metal, placed in between layers of glass or plastic film. Solar filters block more than 99.99% of sunlight, and must be placed on top of the telescope’s objective (on the main aperture of the telescope) to reduce the sunlight’s intensity before it gets focused by the telescope. Without a solar filter, sunlight’s intensity will be high enough to cause loss of vision and damage to equipment.
Unlike in more expensive glass filters which are already equipped with mounting frames, solar films are less expensive, but it must first be mounted onto a sturdy frame before it can be used. One popular brand known to deliver excellent results is the Baader AstroSolar Safety Film, used in both in visual observation and solar imaging.
Solar films are usually sold in various sizes. In the case of my telescope which has an objective of 4 inches (diameter), an 8 in x 11 in film should be more than adequate. The aperture of your telescope determines the size that you will need.
In this article, I will describe how to construct a mounting frame (also called a filter cell) for a solar filter, which will help protect the delicate film and allow safe viewing (and imaging) of the sun.
Constructing the Filter Cell
First, cut a styrofoam in the shape of a square with a large circular hole at the center. The hole must be just the right size to allow the styrofoam to securely hold on to the outside barrel of your telescope. You may cut a hole which is a few millimeters smaller than the diameter of the telescope’s barrel.
Cut two cardboard sheets of similar dimensions as the styroafoam, but this time, with a hole that is a few millimeters larger than the telescope’s barrel.
Using scissors, cut a section of the Baader Solar Film into a square with the same dimension as the cardboard and the styrofoam. It is important not to touch the filter with bare hands. Natural oil in our skin acts as weak acids that could eventually damage the film. Baader recommends placing the filter in between two sheets of clean paper to facilitate cutting.
You should now have four different square cut-outs that will make up the filter cell: the styrofoam, the two cardboard sheets, and the solar filter. Arrange them in the following order from top to bottom: cardboard-filter- cardboard-styrofoam.
Place the solar filter between the two cardboard sheets and then attach it onto the styrofoam. Secure everything in place using tape. Make sure that the filter is not stretched in any way since doing so could possibly damage the filter, degrade its optical properties, and allow more light than what is intended to pass through.
At this point, the solar filter is now ready for solar observation and imaging. You may mount it onto your telescope to test if it stays securely in place. The last thing you’ll want is a filter cell that can be easily blown away by wind.
While the filter is quite delicate, with proper care and storage, such filter should provide many years of continued use. Avoid exposing the filter to moisture and avoid touching it with your hands. For protection from dust and scratches, store the filter in a plastic case when not in use.
Sun/Sunspots, Venus Transit, Solar Eclipse, Solar
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© Anthony Urbano (Manila, Philippines)