Originally, smoked glass was employed as a filtering media by astronomers to observe features of the Sun (sunspots, Faculae, etc.) through the early telescopes. However, while the black carbon attenuated the brightness to a comfortable level, these filters permitted the then unknown harmful portions of the spectrum to pass and cause injury to the retina of the eye. Since there are no nerves sensitive to pain in this photosensitive area of the eye, one could be going blind gradually and feel no pain. As a result, many of the famous early astronomers (including Galileo) died blind; this blindness was accepted by some people as proof that "God" was angry with people who would attempt to peer into the heavens. It would be some centuries later when the components and hazards of sunlight were fully understood, and suitable solar media developed to permit safe observing of the Sun. However, even today it is possible to find some filters marketed for solar observing that are simply not safe.
Left: Examples of the better metal coated glass solar filters in cells. The lower two filters are "Off-Axis" arrangements, above these is a Full Aperture filter; each is in a machined aluminum cell. (63,163 bytes).
Along came some cheaply made commercial glass solar filters made from common window plate glass with an aluminum or other metal allow coating. While acceptable for persons who looked for a durable and economical way to safely observe the major solar features, we have tested a number of these filters and found them to generally have such a poor optical figure that we could not recommend them for use with high resolution telescopes to any of our customers.
Appearing in as long ago as 1978 or 1979, and readily available from major telescope makers (Tuthill, and others) in the USA by the early 1980's, flexible aluminized Mylar was considered good for high-resolution visual and photographic applications, certainly showing finer detail than cheaply made glass filters. The aluminized Mylar is thinner than a sheet of paper, and sold in economical sheets. However, this material had drawbacks: when metallized only on one side it was prone to showing multiple pinholes and so it therefore required two sheets of this Mylar (laid back to back) in order to make an acceptable filter, but this arrangement allowed rubbing of the two surfaces in windy conditions resulting in degradation of the metallization. There was a form of Mylar which was made available aluminized on both surfaces, and this is described in detail in an article published in 1979 by L. M. Dougherty (then Director of the BAA Solar Section) in the Journal of the British Astronomical Association. But the mylar film has a grain structure and produced a strong cross pattern of a pinhole test source (it was not diffraction-limited). This produces a hazy bluish, low contrast final image, although sunspot detail was quite good. In the following years, the original thin aluminized Mylar was discontinued and replaced by a thicker double-coated film that did not have the sharpness of the original and the contrast was degraded.
For those wishing the ultimate resolution and definition there are few well made glass solar filters which employ polished, smooth surfaced, plane parallel flats made of optical grade glass in production. Such filters including those which Company Seven offers by Questar Corporation which do afford excellent views and supreme durability, reliability, and safety. However, these are not inexpensive ranging in price from $350 for a 3.5 inch diameter filter, up to well more than $1,000 for a 7 inch diameter filter housed in a precisely machined cell. We sold a number of limited-production glass filters in the mid 1990's that were made in Germany. These were diffraction-limited, produced a pleasing neutral color of the sun and showed extremely high contrast.
The alternative was to use a Hershel Wedge prism (available from Baader Planetarium in Germany) but the problem with this approach was one of safety; the full sunlight enters the telescope aperture and 95% of the energy is then sent out the side of the prism, leaving 5% to be filtered further before entering the eye. If this filter was chosen incorrectly, it could pass a tremendous amount of infrared and visible light to the retina. The potential for eye damage was too great. In addition, the intense heat that was directed out the side of the prism would burn clothing or skin. For these reasons Astro-Physics and Company Seven chose not to not sell these prisms.
Left: Baader AstroSolar film with instructions, with optional scissor (158,210 bytes).
Just in time for the last total solar eclipse to cross the Northern Hemisphere for some years to come (August 11, 1999) enter the new BAADER AstroSolar safety film. This new high-strength polymer is unlike anything else sold today. Thomas Baader tested hundreds of films on a double-pass auto collimator before discovering this special material that produces a high contrast, scatter-free diffraction-limited image. He told us that all the other films, including Mylar, produce a significant cross pattern of a pinhole source, and show a high degree of scatter due to internal strains and grain structures. To produce this special film, the manufacturer uses a patented high-temperature process similar to annealing to eliminate internal strains. The material is then ion implanted and metallized with a tough, color neutral layer on both sides of the film. This ion implantation/metallization process (also patented) produces a pinhole-free, high-contrast film that stands up to considerable abuse, unlike the metallized mylar materials used in virtually every other commercial solar filter today. The coating cannot easily be rubbed off, and in that respect this is actually safer than coated glass filters. Unlike commercial film and glass filters, Baader's AstroSolar will not significantly degrade the imaging capabilities of your telescope. We have evaluated samples of this material on a double-pass interferometer and measured a 94% Strehl Ratio when placed flat over a 6 inch diameter test optic. Compare this to Strehl ratios of 45% for aluminized Mylar and less than 24% for inexpensive glass solar filters (yes, cheap glass filters are far from being diffraction-limited!).
Theory is fine, but what will this new material show you? We asked two prominent solar observers, Wally Piorkowski and Gordon Garcia to test the material and give us their opinion. Both observers used high quality Apochromatic refractor telescopes in their evaluations. Gordon was indeed impressed by the resolution and contrast, and is actively spreading the word to all his solar colleagues. He believes the images are every bit as good as those seen through a high-quality Hershel Wedge. When Wally and Roland Christen tested the filter at the Astro-Physics facility they noted the pleasing, almost neutral color on the surface and the black sky background due to the lack of scatter and pinholes. The filter revealed extensive solar granulation all over the surface as in any high-resolution glass filter. Sunspots showed detail deep within the Umbra and the Penumbral Brushes were so finely resolved that they looked like eyelashes. But what surprised Wally most was that he could see for the first time white filamentary Faculae all over the sun's surface, not just at the edge.
This article was written at the time of the introduction of these products. And so, for the most current availability and prices information please refer to our on line Astro-Physics Pricing information page.
BAADER AstroSolarTM Film Availability and Ordering Information
Available in two densities:
AstroSolarTM is available in the following sizes:
- ND 5 with 0.00001 transmission suitable for direct visual use, and
- ND 3.8 of 0.00016 transmission suitable for photography only where the fastest possible shutter speeds may be desrired in order to shoot in brief moments of good "seeing".
1. Density 5 - Visual
Each order of these AstroSolar films will include instructions written in English describing to make a simple and effective solar filter for your telescope. Astro-Physics will not offer solar filter cells, however Company Seven may offer some machined ventilated cells in the near future.
#AS50A4 A4 size 200 x 290mm (7.9" x 11.4"). Introduced at $20.00 per sheet
#AS50HM 1/2 meter size 500 x 1000mm (19.7" x 39.4") rolled in 20 inch long poster tube.
2. Density 3.8 - Photographic
#AS38HM 1/2 meter size 500 x 1000mm (19.7" x 39.4"). Introduced at $55.00 rolled in 20 inch long poster tube.
BAADER AstroSolarTM Filter In Cell
Company Seven is now making the BAADER AstroSolarTM film available not only as sheets for you to adapt, but we also make it available cut to size and installed into a very lightweight circular metal Cell. The cell is fitted to slip on to most of the popular telescopes which are sold at Company Seven. The Cell is a bit oversize and lined with felt so that it does not mar the telescope, each Cell is furnished with three nylon set screws to hold the filter into place more securely even though gravity and the slip fit usually alone may hold the filter in place. The filter Cell is painted to match closely the paint finish of the Astro-Physics telescopes as shown in the following illustration. Each Cell is provided with a simple plastic case with snap on lid for protection of the filter. These filters in the Cell are available in the customers choice of either visual or photographic density, and each filter in the cell will have a small indicating label ("Visual" or "Photo") to prevent confusion.
Right: Astro-Physics 92mmf7 "Stowaway" Apochromat Telescope equipped with optional Baader Astro-Solar Filter in fitted slip on Cell, and the Baader Binocular Viewer attached (184,912 bytes).
Company Seven routinely stocks the most popular sizes of the BAADER AstroSolarTM filters at our Laurel, Maryland showroom. For further information about pricing and availability of these items please contact Company Seven.
BAADER TurboFilmTM Film Availability and Ordering Information
This diffraction-limited polymer film is also available in clear transparent form under the trade name TurboFilm for those wishing to protect their telescope optics from heavy dewing or windblown sand, salt spray or dust. TurboFilm is made from the same material as the BAADER AstroSolar solar filters, however it is not coated for solar filter use. Many people use a clear filter in front of their camera lenses to protect the coatings from dust and fingerprints. Use TurboFilm to make a protective filter for your telescope optics that will serve the same purpose. Since this clear material is diffraction-limited, it will not degrade the quality of your image. DO NOT use the TurboFilm material as a solar filter to observe the sun or irreversible eye damage will occur.
3. TurboFilm (transparent, not solar filter)
CLEANING & MAINTENANCE INSTRUCTIONS:
#TF2050 51 x 127cm (20" x 50") rolled in 20 inch long poster tube.
- It is best to protect this accessory from sharp edges or any item that could cause a tear in the film. While the film is insensitive to shock, or vibration the filter cell may be vulnerable (depending on the source and construction).
When not in use, store the filter rolled into a tube. Or if the filter is mounted into a cell then store it in a rigid container (such as a "TupperwareTM" or other plastic air tight container), or a fitted carrying case. Ideally the container should hold the filter in place without contacting the filter media, a soft velour like pouch or lining may accomplish this goal. Do not store the filter into an airtight containers when it is wet (from dew, etc.) for long periods of time; whenever possible let the filter "dry out" in order to reduce the potential for problems associated with Fungus, etc.
- Draw any large, loose bits of foreign matter from the surface of the prisms with an air bulb or small vacuum. Stubborn particles can be removed with a soft camel's hair brush.
- Place a few drops of an approved lens cleaning solution (by Kodak, etc.) or mild soap solution of clear dish washing liquid diluted with distilled water onto a cotton swab. Then gently wipe in a circular motion with little or no pressure. The filter can be damaged if there are tiny abrasive particles on the surface which are pressed in by rubbing hard.