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Making a Large Image Safe Solar Viewer (LISSV)

The original web page for the LISSV had details for making the viewer with lenses that were difficult to obtain or simply no longer available. This web page offers a simple approach but the original information is archived here for those who wish to access it.

Making a LISSV is a little more challenging than one projecting a smaller image because the supports must be more robust and getting the alignment of the lenses correct and keeping them that way requires more effort. Still, such a viewer can be made in a woodworking shop with modest equipment or using another simple method.

If you lack access to a woodworking shop, one of the Uline boxes with additional reinforcement will be sufficient. That reinforcement could be as simple as extra layers of cardboard glued to the sides and bottom of the box or already sized lumber purchased from one of the big-box stores and glued to the bottom and sides of the cardboard box. Uline offers a 48 x 10 x 10 box that seems ideal, especially if it is reinforced with wood or hardboard sheets.

Some members of our community with access to 3-D printing facilities have printed their entire viewer except for the lenses and obtained excellent results. With that approach the alignment of the lenses is almost automatic.

People have had trouble finding a suitable objective for the LISSV until now. The objective lens needs to be of sufficient size and focal length to do the job. Fortunately Surplus Shed had listed a new SSV package that is perfect for a LISSV. That lens is a 71.9 mm x 700 mm achromat that is offered with a suitable −19 mm Barlow for only $23 plus shipping. (Product # 14955)

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 with the lenses we recommend. Even if you find an achromatic Barlow lens, the combination will not produce an entirely color-free result. I treat the blue fringe as both a teaching opportunity and means of adjusting the lens alignment.

If the objective and Barlow are both parallel to the screen, the blue fringe will be uniform. If it is not, then slightly adjusting the tilt of either or both of the lenses is necessary. With an SSV made of cardboard, we simply flex the cardboard hold the objective lens to adjust its tilt. With the Barlow adjusting the tilt of the Barlow holder up and down or left and right does the trick.

When anyone inquires about the blue fringe during a solar viewing session, I present a short optics lesson about dispersion, prisms and lenses. If you are concerned about chromatic aberration, then the finding a lens with a higher f/ratio than f/10 will theoretically show less of the effect but also make your LISSV longer.

In constructing 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 larger image but the projection distance will be longer with increasing solar image size.

Our tests with the new Surplus Shed 71.9 mm objective gave an image size of about 8.5 inches in a LISSV measuring 48 inches from the objective to the screen. A longer device would produce a bigger solar image but that image would be fainter. It is a matter of personal taste as to how bright the image must be.

One caution should be observed with any LISSV. Since the objective lens is necessarily larger with these viewers, the greater light gathering power of these objectives will make the concentrated sunlight striking the Barlow hot enough to ignite paper and char wood if the beam drifts off the lens and onto the holder. For that reason we suggest covering the face of the Barlow holder with aluminum foil that is attached by some means. We use double stick tape.

As with the 2-lens SSV, the best image will result when the screen, the Barlow holder and the objective holder are kept parallel and the Barlow is centered on the optical axis defined by the objective. For other details in making a LISSV, please refer to the pages on making the 2-lens SSV.

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 one brand of AV projection screen material with adhesive backing. Applying it required skill that we did not possess. 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, but you may have better ideas and, if you do, let us know so we can advise others.