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A purely solar telescope.

Note of Caution

Direct observation of the Sun with any optical instrument can be very dangerous. The optical design presented here minimizes the risks, but it is essential that all the instructions in this article be followed carefully. Remember that the primary mirror must have any reflective coating removed. The secondary and tertiary mirrors must also be uncoated, and the observer has to set them to the correct orientation for safe dimming of the Sun's potentially blinding light.

BROWSING through old issues of this magazine, I read with much interest Robert Pike's description of his solar telescope. I had just completed my own telescope, which is similar in concept to Pike's but differs in many details. Both our instruments dim the Sun's glare to a safe and comfortable viewing level without filters -- relying instead on reflection by three uncoated mirrors and the principle of cross-polarization.

My instrument is basically a Newtonian reflector. But instead of a single diagonal mirror set at 45|degrees~ within the light path, it uses two flats inclined by a particular amount, called the Brewster angle, that causes the reflected light to be highly polarized. By adjusting the mutual orientations of these two flats I can make the polarizing effect of one nearly cancel that of the other. As a result, only a very small and safe fraction of the Sun's light passes through the optical system to my eye.


Sunlight has a warming effect on every optical element it strikes, and temperature differences within the tube make a solar telescope much more prone to bad seeing than any night telescope. To reduce the effect of heat absorbed by my 95-millimeter (3 3/4-inch) primary mirror, I ground and polished both its front and back surfaces. Neither surface has any kind of coating.

I figured the front surface to a sphere whose radius of curvature is 1,900 mm. So this telescope's focal length is 950 mm (37.4 inches), which makes it an f/10 instrument as far as the beam's convergence is concerned. (The effective photographic speed is much slower, of course, because the light is severely attenuated.) A sphere is the easiest mirror curve to achieve, and I carefully tested this surface by the Ronchi and Foucault methods covered in mirror-making books.

As for the primary's back surface, first I ground it with the tool off-center until the glass became slightly wedged, like a 3|degrees~ prism. Then I continued grinding and polishing to give it a convex curve with a radius of about 1,500 mm. This back surface has a good polish and probably a very bad figure.

Because the disk has the form of a meniscus wedge, it produces two separate images of the Sun. The useful one is made by the front surface, while a ghost image comes to focus in the air just below the secondary mirror. Nothing of this ghost can be seen at the eyepiece, so it has no effect on the view.

And what about heat buildup within the primary mirror? About 4 percent of the incoming energy is reflected toward the secondary mirror. Roughly 2 percent is absorbed within the glass of the primary itself, and a further 4 percent is reflected by the back surface to be lost out the front of the telescope. All the remaining solar energy passes through the primary and goes out the back, having no heating effect on the optics.


The secondary mirror is an uncoated flat, inclined at the Brewster angle of 57|degrees~ so that the reflected beam is linearly polarized. Rather than make this mirror in the form of a Herschel wedge, as is traditional in a solar instrument, I simply fineground the back to get rid of its ghost image and painted this surface black. Owing to the special tilt of this flat, the exit tube appears strangely cocked on the main body of the telescope.

The tertiary mirror, also uncoated, is fixed inside the elbow of the eyepiece holder, where it too is set for Brewster incidence. This holder rotates around the exit tube, allowing me to produce varying amounts of cross-polarization. I can't make the Sun disappear because the light in a converging beam never becomes fully polarized. But I can make the Sun dim enough for safe viewing. The contrast of the image is not affected, and the Sun has its true color at all times.

Most modern solar telescopes use a full-aperture filter in which light is reduced with a metallic coating. Such a filter, made either of Mylar or more costly optical glass, adds two surfaces to the system that can potentially reduce contrast through added scattering of light. By comparison, my instrument has only one more surface than a conventional Newtonian. The tertiary flat does very little harm to the image because it lies so close to the eyepiece.

Collimation is a little tricky, however, because the tertiary adds one more "jog" to the optical axis. I followed a common-sense approach. With the telescope pointed away from the Sun, I removed the eyepiece holder and peered into the exit tube. While the secondary is not set at 45|degrees~ in this instrument, the reflection of the primary should nevertheless appear centered, and so I adjusted the secondary accordingly. Then I examined the tiny silhouette of the secondary, as reflected in the primary, and adjusted the primary's mounting screws to center this reflection as well. Because both mirrors are uncoated these reflections are weak, but they are still easy to see.

Finally, after reinstalling the eyepiece holder (without an eyepiece), I made sure that the reflection of the secondary appeared centered in the tertiary. In any event, with an f/10 reflecting telescope the collimation requirements are not very stringent.

This instrument provides a good, crisp image of the Sun, showing far more detail than I can see with a conventional telescope by projecting the solar image on a screen. It is perfect for studying sunspots, faculae, and plage regions on the solar surface -- and, of course, it would offer fine views of a rare white-light flare, should I be so lucky!

I am now thinking of a second Sun telescope, using a 6-inch f/8 uncoated mirror that sleeps somewhere in my den. The new instrument will be able to support a camera body.
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Title Annotation:Telescope Making; construction techniques
Author:Herard, Marcel
Publication:Sky & Telescope
Date:Jun 1, 1993
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