Solar Filter Material from Baader Planetarium
Astro-Physics has been chosen sole US and Canadian importer for BAADER AstroSolar, an exciting solar filter material that has revolutionized high-resolution solar observing. The following comments are from Roland Christen of Astro-Physics.
The Quest for High-Quality, White-Light, Solar Filters
For years our customers have asked us to recommend a solar filter that would not degrade the image of our fine refractors. Years ago, aluminized Mylar material was sold in very thin sheets, and was quite good for high-resolution visual and photographic work. However, the material had several drawbacks - it was metallized on only one side and so was prone to multiple pinholes. It required 2 sheets back to back which allowed rubbing of the surfaces in windy conditions causing degradation of the metallization. The film had a grain structure and produced a strong cross pattern of a pinhole test source (it was not diffraction-limited). This produced a hazy bluish, low contrast final image, although sunspot detail was quite good. In the following years, the original thin aluminized Mylar was discontinued to be replaced by a thicker double-coated film that did not have the sharpness of the original. On top of that, the contrast was even more degraded.
Along came inexpensive commercial glass solar filters made from plate glass. We tested a number of these and found them to have extremely poor optical figure and thus could not recommend them to any of our customers. This situation has not changed to this day.
For those wishing the ultimate resolution and definition, we sold a number of limited-production glass filters several years ago from Baader Planetarium that were made in Germany. The filters were diffraction-limited. They produced a pleasing neutral color of the sun and showed extremely high contrast. These filters are no longer available.
The alternative is to use a Hershel Wedge prism (available from Baader Planetarium in Germany). The problem with this approach is 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 is used incorrectly, it can pass a tremendous amount of infrared and visible light to the retina. The potential for eye damage is too great. In addition, the intense heat that is directed out the side of the prism will burn clothing or skin. For these reasons Astro-Physics does not sell these prisms.
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 autocollimator before discovering this special material that produces a high contrast, scatter-free diffraction-limited image. 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 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 is actually safer than coated glass filters. As mentioned above, the material is coated on both sides. Even if you find a pinhole on one side, it is likely that the other side is still coated, providing the protection of that single layer. Since it is nearly impossible for any manufacturer to produce filter coating that is absolutely pinhole-free, a few pinholes may be found.
Unlike commercial film and glass filters, Baader's AstroSolar will not significantly degrade the imaging capabilities of your telescope. Roland Christen of Astro-Physics has checked out samples of this material on a double-pass interferometer and measured a 94% Strehl Ratio when placed flat over a 6" test optic. Compare this to Strehl ratios of 45% for aluminized Mylar and less than 24% for inexpensive glass solar filters.
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 Apo refractors in their evaluations. Gordon was indeed impressed by the resolution and contrast, and actively spread the word to all his solar colleagues. When Wally and I tested the filter at Astro-Physics, he noted the pleasing, almost neutral color on the surface, and the black background sky due to the lack of scatter and few, if any, pinholes. We could see 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, 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.
The September 2000 issue of Sky & Telescope declared that AstroSolar is the "new standard in solar filters." The review compared AstroSolar with Solar-Skreen, the winner of the July 1999 review, in which five of the leading filters available at that time were evaluated. Alan MacRobert of Sky & Telescope determined that the AstroSolar material leaves the sun a more natural white, provides a brighter image, reveals sharper and more contrasty detail, and is a better value than the competing filter.
BAADER AstroSolar film is available in two densities: ND 5 ( 0.00001 transmission) for direct visual use, and ND 3.8 (0.00016 transmission) for photography only. Each order will come with these instructions: "Making an Inexpensive Filter Cell for BAADER AstroSolar Material" and "Cleaning AstroSolar Film."
BAADER AstroSolar Density 5 - Visual, A4 size, 200 x 290mm (7.9" x 11.4")
BAADER AstroSolar Density 5 - Visual, 1/2 Meter size, 500 x 1000mm (19.7" x 39.4")
BAADER AstroSolar Density 3.8 - Photographic only, 1/2 Meter size, 500 x 1000mm (19.7" x 39.4")
MacRobert, Alan. "A New Standard in Solar Filters", Sky & Telescope, September 2000, pp. 63-64.
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If you would rather purchase a completed and ready-to-use filter made with BAADER AstroSolar film, we now offer the filters made by Kendrick Astro-Instruments.
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This page was last modified: March 9, 2012
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