Each year for several years now, my daughter and I have helped out at Starfest; Canada's largest star party. During the solar viewing on Saturday afternoon, I usually set up two refractors side-by-side on the same mount - one equipped with a Herschel wedge and the other with an h-alpha filter. This setup provides a nice opportunity for the attendees to compare the view in "white light" with the view in h-alpha and many have commented on how they enjoy making that comparison. During these sessions, I have also had several experienced amateurs remark positively on the outstanding view in the "wedge". So my preference for the Herschel wedge has been bolstered by these experiences and I have used it primarily for my observations in "white light".

     However, a recent thread on S.A.A. (sci astro amateur newsgroup) about the different types of solar filters got me thinking. Several felt the Baader Astrosolar film gave a better image than glass filters. I've been solar viewing for several years now, but I have never really taken the time to check out the different types of solar filters in a side-by-side comparison. Given the fact that I own all three types - a glass solar filter, solar film and a Herschel wedge - I decide that it would be interesting to make this direct comparison and see if my impressions were accurate or if indeed the Baader film could match the views of the wedge. This afternoon gave me the opportunity under some very good conditions and so I put the three types to the test and the results were not exactly as I expected.

     The scope used was a 4" refractor. The filters I compared were: 1) a Zeiss glass solar filter, 2) Baader Astrosolar film and 3) a Herschel wedge made by Intes. The Zeiss glass solar filter is a SFO63. It is a high quality filter that produces a yellow-orange sun. The Astrosolar film from Baader, which is very popular among solar observers, displays a white sun; the same is true of the Herschel wedge.

     There are some general advantages that accompany each of the three types of filters. The glass solar filter has the advantage over the solar film in that there are no worries about pin holes from wear or distortion from mounting the film material stretched too tightly (it should be mounted loosely with the "ripples"). The biggest advantage of the solar film is that it is much cheaper, especially if you make your own mounting cell (the Baader Astrosolar film comes with instructions to make a cardboard cell). The Herschel wedge has the advantage of "one-size-fits-all" - all refractors that is. Because it is a diagonal, it can be used on any refractor and therefore, if you have more than one refractor or you change refractors, you can still use the same wedge.

     Each filter in turn, has disadvantages. The biggest disadvantage of the glass solar filter is .... well, it's glass and it can break if dropped. It's also more expensive than solar film. I have already alluded to the potential handicaps of solar film - pin holes and distortion. The real disadvantage of the Herschel wedge is that it can only be used with refractors! Because the sunlight is not filtered on its path to the wedge, it adds heat to anything along the way, rising in temperature as the light cone narrows. This includes the small secondary mirrors of reflectors and compound scopes and damage can occur. This is not a problem if you use a refractor, but ...... you have to use a refractor. The Intes model also has another danger; the light escaping from the bottom of the diagonal is very intense and hot. Care must be taken that flammable material or curious eyes don't get in its path. The Herschel wedge is also the most expensive of the three types around - $300 for the Intes and over $600 for the Baader. Though this does include filters that help to dim the sunlight further as it emerges from the prism.

    To make the comparison as fair as possible, I used the same filter lens cap for all three views, as shown in the picture (produce through the wonders of digital imaging). It's the same cap - first with the Zeiss filter; then fitted with Astrosolar film and finally left "open" for use with the Herschel wedge.

    The conditions were ideal, a temperature of 64° F, a very light breeze and of course - clear skies. I set up on the lawn in an area that gave me a view across grass and trees to the south. My experience is that daytime seeing is not often the best; suffering from the effect of heat rising into the air as the sun warms the earth. However, I have found that this effect can be minimized by carefully choosing your viewing site. Grass, trees and water are much better than concrete, roofs and asphalt, so if possible, view the sun across a large lawn or field.

I used two eyepieces - a 20mm plossl for 50X, full-disk view and a 10mm plossl for a 100X close-up view. The subject was a reasonably active sun at about 48° above the horizon. On the surface was a "naked-eye" sunspot in active region AR0756, followed by a grouping of tiny spots making up AR0758. Each group was near the solar limb; AR0756 in the west and AR0758 toward the east.

    I began with the 50 power, full-disk view. Right away I noticed that seeing was pretty good yielding a steady image. Region 756 reminded me of M51 (the whirlpool galaxy), with two sunspots, one large and one small, whose penumbra connected together like the spiral arms of those galaxies. I was also able to pick out three pores (tiny spots without penumbra) - two to the east and one to the northwest. Active region 0758 had only one small sunspot, but made up for it with 17 pores surrounding it like bees around a hive. It was while viewing AR0758, when I began to notice some slight difference between the filter types.

     While I could count the same number of pores in each filter, I had to wait for the very best seeing moments before I could pick out the tiniest ones with the Astrosolar film. All pores were readily seen in both the Zeiss filter and Intes wedge. Faculae (bright areas mostly seen near the limb), were also apparent in all three filters, but once again the solar film seem to benefit the most from improvements in seeing. I attributed this impression to a small increase in contrast with the "glass" filters; the differences while noticable, were not dramatic.

     I then examined AR0756 at 100X. The detail was excellent, revealing that the "bridge" between two spots was actually a small loop of disturbance in the penumbra; another "peninsula" jutted out to the northwest from the penumbra of the larger spot. Again, while the views in all three filters were very similar, the view in the Zeiss filter and Intes wedge had slightly better contrast and sharpness, revealing details more readily than the film.

     After a session of about two hours, I drew my conclusions - my previous impressions were partially right. The Herschel wedge gave the best view, but it shared that distinction with the Zeiss glass filter; I could not distinguish virtually any difference between the two. The Baader Astrosolar film came in a very close second, but had slightly less contrast and sharpness. Though given it's relatively inexpensive price tag, the Astrosolar film would certainly be ranked first in value.