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by Gordon Garcia
In 1981, with a newly purchased full aperture solar filter, I turned my C8 telescope to the Sun for the first time. I was absolutely amazed that one could actually view the fiery surface of our daytime star. I was also amazed at how quickly it changed. It only took a year to add a hydrogen-alpha solar filter to my observing arsenal. I was hooked forever on solar observing.
Years have come and gone and new solar observing devices have been introduced into the amateur market. One leader in my opinion at making quality solar observing equipment available to the amateur astronomer has been Baader Planetarium near Munich, Germany. Over the past ten years they have produced superb full aperture glass solar filters, energy rejection filters for H-alpha filters and an outstanding two-inch Herschel wedge prism. All are highly coveted by experienced solar observers. They have also produced a H-alpha Coronograph for viewing solar prominences (tested by Don Trombino, S&T, June 1994). I was surprised during a recent visit to Astro-Physics in Rockford, Illinois when owner Roland Christen informed me that Baader Planetarium had developed a very low cost foil type material for solar viewing. My initial response was less than enthusiastic. Many have tried to equal mylar solar filters sold by Roger Tuthill and up until that point none had succeeded. Roland insisted though that I accompany him to his observatory to view through a 5-inch refractor that he had prepared a solar filter for with the new Baader AstroSolar Safety Film. Well, seeing is believing the new Baader solar filter produced a very sharp image of the Sun with excellent contrast and little scatter. The sky adjacent to the solar limb was jet black. Roland gave me a sheet of the new filter material. It was of a photograph density (ND 3.8). The next day I contacted Astro-Physics to see about purchasing some AstroSolar film with a visual neutral density. I was pleasantly surprised to hear that Thomas Baader had a agreed to send me some of the material in a visual neutral density.
The first thing I must point out is that the Baader AstroSolar Safety Film is provided in unmounted sheet form. Instructions are provided for making a homemade cell to attach the filter to your telescope. The Sun is very dangerous to observe. Please do not under estimate this fact. When attaching your solar filter for the first time make sure it is securely attached and cannot fall off or be blown off the telescope by a gust of wind.
The manufacturer provided the following information on AstroSolar. The film is patent pending in all 19 European
countries. It also has been tested by the German National Bureau of Standards for eye-safety. The substrate is
not mylar. There are several pre-treatment processes used before the film is coated. The film is changed to a dark
grey color by an ionizing process. This coloring minimizes internal reflections.. After the coloring, the film
is first annealed similar to optical glass.
The substrate material is heated almost to the melting point. During the heating process the film shrinks and internal stress is released greatly improving its optical performance. Lastly, the film is then coated on both sides with a metallic coating. The double coating prevents pinholes from appearing. It would be interesting to know more about the composition of the substrate material and coatings, however, Mr. Baader is not talking. He did indicate that he tested several hundred types of films on a Zeiss double pass autocollimator before finding the right material that would withstand the treatment process.
My first test of the material I received from Baader came on an evening shortly before sunset. I mounted a piece of the AstroSolar in a cardboard cell and went outside. I first checked the material by holding it up to the Sun. The Sun could be seen through the film. I judged it to be about neutral density 5. The image was white in color with little scatter and no pinholes apparent. I then mounted it securely to my 5.1" f/8 Astro-Physics refractor and pointed the scope towards the Sun. I next viewed the Sun through the telescope with a star diagonal, but no eyepiece. I again examined the filter for scatter and pinholes. Again, there appeared to be little scatter and what appeared to be one small pinhole in one side of the coating. This small imperfection was faint and not needing to be darkened with a felt tip pen because of the two-sided coating process. I then placed a 19 mm Panoptic eyepiece in the star diagonal and brought the Sun into focus. At 53x the Sun's image was an off white color with a slight blue-grey tint simulating what you would see through a Herschel Wedge Prism with neutral density filters. The image was sharp with black sky adjacent to the solar limb. Although seeing wasn't particularly good this late in the day, granulation was easily seeing as were sharp boundaries between the sunspot's umbra and penumbra. Faculae were easily seen well into the disk. The image was bright, however, at low magnification the image should be fairly bright so that when higher magnifications are used the image is not too dim.
I again had the opportunity to test this material several days later on a day where I estimated the seeing to be about two arc-seconds with 10 % cloud cover and a slight haze. I first observed the Sun with a 10 mm Orthoscopic eyepiece with my 5.1" f/8 refractor at just over 100x. The image was very sharp with little scatter along the Sun's limb. Some scatter was seen from the slightly hazy condition that day. The sky adjacent to the limb was very dark. Faculae were again easily seen well in from the Sun's limb. I next switched to a 4 mm Orthoscopic eyepiece. At just over 250 x the image was still sharp. Granulation was easily seen as well as detail in both the umbra and penumbra. I next examined Active Region 8611. I counted a total of 53 sunspots in the group. Later examining the NOAA Space Environment Center Internet site I saw that the NOAA count was 39 for AR 8611. The image at 250 x was slightly dim. A head cover to cut ambient light would have further enhanced contrast.
In summary, I found the new Baader AstroSolar Safety Film to be of excellent quality for solar observing. Judging from the solar image at 250x, it appears that AstroSolar film is diffraction limited. Test results supplied by the manufacturer showed a Strehl ratio of 0.941 at 632.8 nanometers (uncoated film tested on a Zygo Interferometer). The test results can also be found on the Internet by visiting Adirondack Video Astronomy's home page at: www.astrovid.com. Follow the links to Baader AstroSolar.
The solar filter material will be offered in both a visual and photographic neutral density (ND 5 and ND 3.8). Astro-Physics, Inc. will be the sole U.S. distributor for individuals and dealers. Baader Planetarium is also offering this material in a clear uncoated format called TurboFilm to close open tubes and protect lens surfaces. The price of an unmounted sheet of AstroSolar film large enough to cover an eight-inch aperture will be $20 (U.S.). At this price and solar maximum being within a year away, it should introduce many more observers to solar observing. Again, be very careful when observing the Sun. Make sure your solar filter is securely mounted to your telescope and inspect it before every use.
I also had the opportunity to test the substrate material for the AstroSolar filter. Baader Planetarium is marketing the clear substrate material as "TurboFilm." I first looked through the clear film in broad daylight without a telescope. The TurboFilm showed no distortion and only a very slight but noticeable dimming.
As daylight faded to twilight I turned my refractor towards a three day old crescent moon. At 58x the Moon appeared very sharp. There was no noticeable difference in image quality with or without the TurboFilm. There was no scatter noticeable from the TurboFilm. Although, again, I noticed a very slight dimming with the TurboFilm. I then switched to a 10 mm Orthoscopic eyepiece. Again, I could not notice a difference in the image other than a very slight dimming. The image sharpness with and without the filter was confirmed by other members of my astronomy club. I noticed no color shifting or added scatter caused by the film.
My next test was to conduct a star test with and without the TurboFilm. My telescope has a very slight over
correction. The star test showed the same fresnel ring pattern both in a out of focus. I did notice a slight diffraction
spike with the filter material in place. Closer examination of the material showed a slight crease or fold mark.
I confirmed that the crease was causing the diffraction spike by rotation the TurboFilm and then reexaming the
image. My TurboFilm sample may have been folded somehow in shipment. This material is cheap enough to by a roll
and then select the best pieces for use.