Solar Filters

Although eyepiece projection is a convenient and safe method for Solar Viewing, particularly for groups of people, it has its own problems of screen contrast (for outdoor setups) and loss of definition. There is also the ever present risk of damage to the eyepiece, particularly if the image moves off centre. There are numerous Solar Filters available which allow direct viewing, and of course also facilitate photography. Some are quite expensive, but I have found Baader Solar Film to be perfectly adequate and reasonably priced. An A4 sheet (available from a variety of sellers) provided sufficient film for me to make filters for both my telescopes and finders, with some left over. This film received a very favourable review in the September 2000 issue of Sky and Telescope. Also good for binoculars with suitably constructed frames for the film - very useful for Solar eclipses!

24 Kb-19 Kb

The standard method of construction is to make two cardboard rings to fit the front of the telescope, one sliding on top of the other, and then trap the edge of the film between the rings. I also smeared adhesive on the inner and outer mating surfaces to ensure a safe assembly. This was fine for the 80 mm 'scope and the two finders, but the C8 aperture was too wide for the A4 sheet, so I had to reduce the diameter of the film mount. Possibly a slight loss of contrast and resolution due to the increased ratio of central obstruction to aperture, but not a problem in practice - atmospheric distortions on a sunny day are far more noticeable.

The image is a nice pearly white colour - very natural looking. Before using the filters I always check them for faults, but the film is surprisingly strong and easily withstands normal handling.

Red Dot Finders are frequently used on shorter focal length telescopes. And can also be used to centre the Sun - with Solar film in front of the screen, the Sun is safely visible, and the red dot reflects off the film so is also visible in daylight. Usually it is necessary to set it to the maximum brightness. Again a suitable frame needs to be constructed for the film, to suit the design of the red dot finder.

In all cases when using a solar filter it is imperative that the filter is safely secured to the instrument - if it falls off while viewing the magnified Sun, serious damage to the eye can result, even blindness!