Solar Filters

If done safely, solar observing can be a joy. Old Sol, especially at the height of the 11-year sunspot cycle, is endlessly fascinating. How does the CAT owner observe the Sun safely, so that neither eye nor telescope is harmed? This is possible using a full-aperture solar filter from a reputable manufacturer. A full-aperture filter fits securely over the corrector assembly and reduces the intensity of the Sun to a level that is safe for visual observing. The finder scope on the CAT should be left capped or removed so no one is tempted to use it. Find Sol by observing the shadow of the scope. When it is smallest, the Sun should be in the field.

Safety of the telescope also needs to be considered. Keep the CAT safe by ignoring the advice found in older astronomy books. They usually suggest "projection" is the safest way of observing the Sun. Projection is easy: place an eyepiece in the telescope, hold a white card a suitable distance behind it, point the scope at the Sun, and view the projected image of Sol. The problem is that a closed-tube SCT or MCT heats up very quickly, when the unfiltered Sun is allowed into the OTA. In almost no time, temperatures can climb high enough to cause severe damage. The secondary mirror holder can warp or melt, the baffle tube can be burned and distorted, and lubricants can vaporize and condense on the mirror and corrector. So do not use a CAT for solar projection.

What is visible with a safe solar filter? One thing that will not be seen is a solar prominence. The great fountains of fire spewing out from the solar limb require a very expensive hydrogen alpha filter for viewing. What a normal white light filter will mostly show are sunspots and the granulated "faculae" that form the Sun's photosphere. Truly, this is enough. Sunspots can be amazing, forming huge complexes that slowly move across the Sun's disk as it rotates. When things are hopping on the Sun, different, often bizarre-looking sunspot groups are on display almost every day. Occasionally, a solar flare may be intense enough to be visible in white light, but that is a fairly rare occurrence.

The first question a prospective solar filter buyers usually asks is, "Mylar or glass?" A solar filter's substrate can be either optical glass or thin sheets of Mylar plastic. It seems natural to expect glass filters to be better optically, but surprisingly, that is not the case. Mylar filters are capable of producing sharper images than glass ones. Mylar solar filters are made of (usually) two sheets of the plastic material stretched loosely across a filter cell that is fitted over the telescope's corrector assembly. Despite their wrinkled appearance (tightly stretching the Mylar in its holder to eliminate wrinkles actually harms filter performance), they deliver fine images.

The major problem with Mylar filters is that they do not produce a realistically colored image of the Sun. Glass filters often deliver a yellow or orange Sol, but most Mylar filters deliver a bluish or greenish image. In practice, that is not really a problem. As long as appropriate detail can be seen, who cares if the Sun is blue? If that is annoying, though, an appropriately colored eyepiece filter (used in conjunction with the solar filter, naturally) can give the Sun a more normal hue. Since they are made of thin plastic film, Mylar filters tend to be less durable than glass ones, but with reasonable care one filter should last for years.

Which filter, specifically, is best? If Mylar is the choice, one filter (or material) stands out: Baader Planetarium's AstroSolar film. It is available from various dealers mounted in filter cells for various aperture telescopes, but it is also commonly sold as unmounted film with instructions for building a simple cell that will fit snugly and safely over the corrector. The views of the Sun produced by AstroSolar film are probably superior to that of the best and most expensive glass filters. The color produced by AstroSolar is not natural, but it is not a disturbing blue or green, either, being a faintly bluish gray. Baader AstroSolar film in a commercially made cell for a C8 will cost about $100. Kendrick Astro Instruments sells AstroSolar filters in a huge variety of sizes to fit almost any aperture and type of scope. Prices vary depending on aperture.

Glass filters are still my choice, though, mainly because of their durability, and there are excellent ones from Orion, Thousand Oaks, and other manufacturers. The glass solar filter at the top of the list for quality, however, is the J. M. B. Identiview ($147 for the C8 model). This filter (Plate 44) seems almost as good optically as the AstroSolar film and presents a pleasing orange-colored Sun.

Identiview solar filter ready for some fun with old Sol. Credit: Author

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