A plant-pot solar projector

Cliff Shuttlewood

At the BAA Exhibition Meeting in June 1999 I saw a ‘plant-pot’ projection screen exhibited by one of the participating societies. I had heard of this method of solar projection, but had not at that time seriously considered the construction of such a device. I realised then that such an instrument might be very useful for demonstrations to school children and other groups of young people. This proved to be the case, and people of all ages (including members of Leicester Astronomical Society) have been very interested to see the solar image projected in this way. I searched through my remaining optical bits and pieces, and found a 60-mm aperture 320-mm focus OG, a 90-degree prism from a Porro pair, a 12.5-mm Ramsden eyepiece and an old RAS-threaded helical focusing mount. I then had the basics, and could proceed with design and construction – which was quite easy. For the tube I used a piece of plastic tube from a redundant vacuum cleaner, with thin rubber packing to provide a snug (but not tight) fit for the OG, which was retained by a plastic split ring. I have a wood-turning lathe, and so was able to turn part of the prism box to fit the end of the OG tube. The block was cut to 45°, and was slotted to house the 90-degree prism, which was held in place by packing blocks, with a retaining screw fixed to the 45-degree face of the prism box. The helical focusing mount was modified to take the Ramsden eyepiece by fitting a plastic liner (a coupler for electrical conduit plastic tube) to the prism box – and the optical set-up was complete. The design of the carrier for the plant-pot and the screen was, of course, dictated by the required size of the solar image and the eyepiece-image distance. In the Solar Astronomy Handbook there is an empirical formula for calculating the eyepiece-image distance for a given set of optics, and this formula was used in the design of the projector; but it applies only when the focal length of the eyepiece is very much less than the distance between the eyepiece and the screen. I decided on an 80-mm diameter image, and the calculations produced an eyepiece-image distance of 312.5 mm.

The screen was made of a piece of drafting film (as used instead of paper in many drawing offices). I had a small sheet available from my design office days (so long ago), and mounted it in two cardboard rings to hold it flat and rigid, the assembly being of such a diameter as to fit neatly about 20 mm inside the mouth of the plant-pot. The plant-pot has a depth of 180 mm, the diameter of the mouth being 152 mm. Wooden discs with 32-mm-diameter central holes were fitted each side of the base, and the plastic base blocking the central hole was removed to provide clear passage for the projected image, the block outside the pot having flats to which the extended arms from the prism box were attached, these arms being of a length such that the eyepiece-image distance is automatically set. The screen assembly was glued into place on three equally spaced pieces around the inside of the pot. The shade was added because the best view is obtained from slightly to one side. (A direct view produces only a bright patch with no detail.) This completed the assembly, and all that remained was to determine whether it worked. The opportunity came at a public exhibition in which Leicester Astronomical Society had various instruments on display. The Sun appeared briefly during the afternoon, and I was pleased (and relieved) to see the calculated-size image, complete with sunspots, appear on the screen. And on the day of the eclipse – 11 August 1999 – the instrument was used very successfully to show the progress of the Moon over the disc of the Sun to the 93% partial phase observed from the Leicester area.

Applying the equation A = (B[1] x f[EP])/B[0] where A = eyepiece-screen distance B[0] = 1/100 focal length of OG B[1] = diameter of solar image f[EP] = focal length of eyepiece to this instrument where B[1] = 80 mm f[EP] = 12.5 mm B[0] = 320/100 then A = (80 x 12.5)/(320/100) = 312.5 mm (12.3 inches)