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Building a Solar Projection Box

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Building a Solar Projection Box Appendix A Building a Solar Projection Box As discussed in Chap. 2 , a projection box offers some distinct advantages for the solar observer. It allows the Sun’s image to be viewed safely without the inconvenience of having to hold a screen behind the telescope. It leaves your hands free to make notes, drawings and adjustments. It is much easier to make accurate records of sunspot posi- tions with a projected image than it is by looking directly at the Sun through a fi lter. If a projection box is attached to the telescope, the telescope’s fi eld of view does not move relative to the projection screen, and sunspots and other solar features can be observed and recorded very accurately. A projection box also allows the Sun to be observed by groups of people and, if thoughtfully constructed, can prevent curious children from putting their heads to the eyepiece (or at least make it very dif fi cult!). The projection box described here was built entirely from 6.4 mm (1/4 in.) thick balsa wood bought from an art and crafts store. The interior of the box was lined with thin black card (also obtainable from art stores) in order to reduce internal re fl ections, which can spoil the contrast of the image. Two sides of the box are parallel, so that the pieces could be joined together easily using balsa wood framing of square cross-section, 12.5 mm (1/2 in.) on a side. However, the sides of the box were made so that they taper down to 75 mm (3 in.) at the eyepiece end, as other- wise the front of the box would collide with parts of the telescope tube and the whole thing would look bulky and ungainly. Note that because balsa wood is intended for making such things as model airplanes and other small objects and not large pieces of apparatus like projection boxes, it often comes in strips no more than 75 mm (3 in.) or 100 mm (4 in.) wide. If this is the case with your balsa wood, you will need to cut two or more pieces of wood and then glue them together. Glue all the joints using ordinary wood adhesive from a local hardware store. The most important dimension to establish when building a projection box is its length. Using an eyepiece of a given magni fi cation, moving the screen further L. Macdonald, How to Observe the Sun Safely, Patrick Moore’s Practical Astronomy Series, 197 DOI 10.1007/978-1-4614-3825-0, © Springer Science+Business Media New York 2012 198 Appendix A behind the eyepiece increases the size of the projected image. To do serious solar work you need an eyepiece that will show the whole solar disc comfortably within the fi eld of view. Avoid using an eyepiece that barely accommodates the whole Sun and so shows the limb of the Sun’s disc very close to the edge of the fi eld, as the view at the edge of the fi eld is distorted in most eyepieces. On the other hand, using too low a power gives a very long projection distance for a good-sized disc and results in an inconveniently long projection box. An eyepiece giving a magni fi cation of between 50× and 65× gives the best results. The standard diameter used by ama- teur astronomers for a projected solar image is 152 mm (6 in.), although if you use a very small telescope you may fi nd a 100 mm (4 in.) disc easier to work with. To fi nd the correct length for a projection box on your own telescope, draw a circle of your chosen disc size on a sheet of plain paper. Here we will use a 152 mm (6 in.) disc size as an example. Insert your choice of projection eyepiece in the tele- scope so that it is not quite fully placed in the drawtube but the eyepiece barrel sticks about 3 mm out. Determining the correct projection distance for a 152 mm disc should in theory be a simple matter: just measure the distance behind the eyepiece you have to hold the paper in order for the Sun’s image to exactly fi ll the circle. You should note, however, that Earth’s orbit around the Sun is slightly elliptical, which means that Earth’s distance from the Sun varies during the course of the year. Earth is furthest from the Sun in July and closest in early January. 1 For the solar observer, the upshot of this is that the Sun’s apparent diameter is slightly larger in January than it is in July. More precisely, its January diameter is 32.5 arc min, whereas in July it is just 31.5 arc min. The average between these two extremes is 32 arc min, so the variation of 0.5 arc min either way means that the diameter of the projected image varies by about 1.56 % either way, which for a 152 mm disc translates into a variation of about 2.4 mm on either side. This is enough to make the real solar image too small for a 6-in. circle in summer and too large in winter, and will compromise the accuracy of solar drawings. Therefore, you need to allow the projection distance to vary by a corresponding percentage. The easiest way to determine the maximum and minimum projection distances needed throughout the year is to simulate the variation in the Sun’s apparent diam- eter by drawing three concentric circles: one of your chosen disc diameter (in our case 152 mm), one of the maximum disc diameter (152 mm + 3 mm = 155 mm) and a third of the minimum diameter (152 mm − 3 mm = 149 mm). The increment has been rounded up from 2.4 to 3 mm in order to give slightly more tolerance than you actually need. You can then simply measure, and note down, the projection distance required to fi ll each circle. It is essential to measure the projection distance accu- rately: it is de fi ned as the distance between the rear surface of the eyepiece and the surface of the projection screen . In the case of the given setup, with an 80 mm 1 This may seem paradoxical to inhabitants of the northern hemisphere, who experience winter when Earth is closest to the Sun! But the cooling of the northern hemisphere in winter caused by the tilt of Earth’s axis away from the Sun totally overwhelms the very slight heating effect caused by Earth’s closer proximity to the Sun at this time of year. Appendix A 199 refractor of focal length 910 mm projecting using a 15 mm Plössl eyepiece (magni fi cation 61×), the distances were: Average 263 mm Maximum 270 mm Minimum 258 mm You need to build your box so that it allows for the maximum projection distance required. If the box is a little too long you can shorten the distance slightly, and thus get the image down to the correct size by pulling the eyepiece out a little in its drawtube, but if it is too short you cannot extend the box! The fi nal length for your projection box is this distance (marked d in Fig. A.1 ) plus the distance by which the Pipe Bracket Star Diagonal (seen from behind) Eyepiece 6.4 mm– thick Balsa Wood d Projection Screen CornerJoint Fig. A.1 Diagram showing the construction of the author’s projection box. The projection distance d determines the diameter of the projected solar image 200 Appendix A eyepiece protrudes into the box. When you use the box, you can make small changes to the projection distance to allow for the variation in the Sun’s apparent size by adjusting the position of the eyepiece in its drawtube. Inserting the eyepiece fully will give the maximum projection distance, whereas pulling it out by a few millimeters and then clamping it again will give the minimum. The screen end of the box can be square. When determining its size, allow for 5 mm of clearance around the solar disc (i.e., 5 mm on each side if measuring its cross section), plus the thickness of the square corner joints holding the box together (another 26 mm or 1 in. in total) and the thickness of the sides of the box (total 13 mm or ½ in.). For the box in the example, then, the total width of the base comes to: 152 mm+++ 5 mm 5 mm 26 mm + 13 mm = 201 mm Thus the screen end of the box should be 201 mm square. The top piece, through the center of which the eyepiece mount should be inserted, has the same width as the tops of the side pieces (75 mm, 3 in.) and the same length as the internal width of the box (188 mm). The hole for the eyepiece or drawtube (see below) should be just wide enough for the tube to slide through with as little play as possible. If you have access to a lathe or other precision cutting device, boring a hole of the correct diameter should not be a problem. If you only have access to hand tools, you can make a very functional eyepiece hole quite easily by drawing a circle of the correct diameter in the exact center of the top piece and then drilling a series of small holes around its circumference. If the holes are spaced reasonably closely together, you can then break down the walls between them with a chisel until the central piece of wood falls out.
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