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Making a Large Image Safe Solar Viewer (LISSV)

At the time of the August 2017 total solar eclipse I was contacted by a number of individuals wishing to make an SSV projecting a solar image up to 16 inches in diameter. At the time I was swamped with queries and conducting workshops for children with the 1-lens and 2-lens SSVs. I managed to get enough information out to those individuals to help them make a Large Image Safe Solar Viewer (LISSV) but did not post the results on the web. Since then I have evaluated the lenses available to make such viewers and have tested some of them in in solar observing. First, a LISSV will have to be longer than a normal SSV to get the large image size. Second, a larger objective will be necessary so that the image brightness is sufficient for proper viewing. With the rule of thumb that an image should be no more than about 4 times the objective diameter, a LISSV producing a 16 inch image would require an object with a diameter of 4 inches (100 mm) and a solar image size of a foot requires an objective of 3 inches (75 mm).

Making a LISSV is a little more challenging than the smaller one because the support must be more robust and getting the alignment of the lenses correct and keeping them that way requires special tools and some skill using them. Still, such a viewer can be made in a woodworking shop with modest equipment.

If you lack access to a woodworking shop, all is not lost. For some of the smaller objective lenses listed in the table below, one of the Uline boxes with additional reinforcement will be sufficient as long as the objective is one of the smaller ones. That reinforcement could be as simple as extra layers of cardboard glued to the side and bottom of the box or already sized lumber from one of the big suppliers glued in the same manner to the bottom and sides. Uline offers a 48 x 10 x 10 that would be sufficient for all but the largest and heaviest lenses in the table, especially if it is reinforced with wood or hardboard sheets.

The objective lens needs to be of sufficient size and focal length to do the job. Usually, since the larger an objective the higher the price, to control costs it might be best to select the smallest diameter lens that will produce your desired image size. The sizes noted here are suggestions and not the result of any rules of optics. For comparison, the image sizes listed are based on a fixed objective to screen distance of 48 or 61 inches. A shorter distance will give a smaller solar image while a greater distance yields a larger one. If the distance is too short, image formation cannot be achieved.

Below is a table listing the Surplus Shed lenses that can serve as an objective for a LISSV. Even though these lenses are achromatic, there will be chromatic aberration (a blue fringe around the solar image) from dispersion just as there is with the smaller SSVs that are made with single element spherical lenses; and, unless you find an achromatic Barlow lens, the negative lens will not produce a color free result anyway. I see the blue fringe as a teaching opportunity when anyone inquires about it. If you are concerned about chromatic aberration, then the lenses with a higher f/ratio will inherently show less of the effect.

For a choice of Barlow lens, a focal length between -18 mm and -26 mm has worked best for our large image viewers. As to the diameter of the lens, that parameter depends on the focal length of the objective. The light from the Sun will, on entering the Barlow, be reduced to a bundle of rays with a diameter of about a hundredth the focal length of the objective. So for the 80 mm by 325 mm where that bundle is less than 4 mm, a Barlow 10 mm in diameter is a fine choice. Now for the 153 mm by 1200 mm objective a 15 mm diameter might not be enough but 20 mm should work well.

Rather than make specific recommendations of lenses we have not personally tested, we give this advice. Go to the Surplus Shed home page (www.surplusshed.com) and select the lens finder from the menu bar. Just know the search function is a bit quirky on this page. Do two searches. Search first for plano concave (PCV) lenses from -21 to -19 (don't reverse these numbers or you will get an empty result) and specify a minimum diameter based on your objective and enter 30 mm for the maximum. Do the same for double concave (DCV) lenses. If there is a choice between coated and uncoated, select the coated lens, though it is doubtful you will see a difference.

In construction of your LISSV, know that the distance from the objective to the Barlow is almost the same as the focal length of the objective. The location of the Barlow changes very little in a LISSV with a small image compared to one with a large image but the projection distance increases with solar image size.

One caution should be observed with the LISSVs. Since the objective is larger with the LISSV than with the original design that produced a smaller image, the greater light gathering power of these objectives will make the concentrated sunlight striking the Barlow holder hot enough to ignite paper and char wood. For that reason we are covering the face of the Barlow hold with aluminum foil that is attached with double stick tape.

With a larger solar image it is important for the projection surface to be perpendicular to the optical axis and as flat as possible. Copier paper will ripple. With the regular SSV we have used heavy card stock and others in the community have been successful with poster board. We have tried a sheet of AV projection screen material with adhesive backing. It required skill that we did not have. We simply could not get the sheet to adhere to smooth plywood without bubbles or ripples. We think white card stock or poster board works well.