Visual solar observing with an eyepiece graticule

Geoff Johnstone

Solar observing is normally performed by one of two methods: projection without an objective filter, or direct visual with a filter. Using the projection method it is possible to determine the latitude, longitude and size of active areas. The following describes a direct method by which it is possible to carry out the same measurements. In order to measure the size and position of active areas, the normal procedure is to project an image onto a ruled template (Figure 1). The sunspots are then copied onto a thin piece of paper clipped over a card on which is drawn a similar template. The drawing can then be analysed as required. When I bought my ETX 90 RA I was immediately disappointed in not being able to record the sunspots in the same way as I had done previously. As I do not believe that there are any suitable graticules or eyepieces available on the open market, I decided to make a graticule that would fit inside an eyepiece so that more accurate drawings could be made. After a period of blood, sweat and tears, and a number of graticules later, I believe I have devised the easiest, quickest and cheapest method of construction. Materials A Nylon bead thread obtainable from art and craft shops. This is made up of a large number of individual strands, each of which is thinner than a human hair, and almost transparent. The strands are surprisingly strong, and very difficult to see. I used magnifying glasses as used for intricate craft work, but even then some pieces of thread disappeared without trace. B A washer that will sit on the eyepiece diaphragm to reduce the field of view – the object being to make the field size such that the image of the Sun virtually fills the field of view. It might be possible to make a washer, but this is beyond my capability, and so I merely found something suitable from the washer tin in my garage. C A spacer between the body of the telescope and the eyepiece to render the image, in mid-winter, the correct size to fill the field of view created by the washer. This requirement is dependent on the telescope–eyepiece combination. The eyepiece will need to be raised and lowered to compensate for the variation in solar diameter throughout the year (Figure 2). Method I tried several ways of setting out and fixing the threads to the washer, and the following proved comparatively easy. 1 Cut off about 150 mm of thread and remove a few individual strands. My graticule required ten of them. 2 Measure the diameter of the hole in the washer, and divide the result by 5 to determine where the threads will subsequently need to be positioned. Set this out on a piece of A4 card (Figure 3).

3 Position the washer at the intersection of the lines, and hold it in place with tiny pieces of Blutac or similar material. Fix a thread to one end of line number 1, then stretch the thread across to the other end of the line and fix it into position. Rotate the paper through 90 degrees, and repeat the process with thread number 2 and line number 2. Continue this procedure until all the threads are in position. Check the position of the threads over the washer, and adjust any that are misaligned with the point of a pin. Glue into place with superglue. 4 When the glue is dry, use nail scissors to trim the threads close to the washer, and with matt paint or a marker pen blacken the rim of the washer to prevent internal reflections when it is in the eyepiece (Figure 4).

In order to position the threads I found that four hands were needed. As I only had two, I had to borrow two from my wife (although she insisted on keeping them on the ends of her arms). The individual strands of thread slip through the hands very easily, and I held each one over the lines while my wife stuck them down with small pieces of sticky tape. I tried doing this on my own with double-sided tape and with Bluetac, but neither method worked. Using the four-hand system, the entire job was finished in a few minutes. With the graticule in the eyepiece it is rotated until a sunspot trails along one of the threads. The spots are then recorded as with the projection method, using the template (Figure 4).