In solar work, there is a less-frequently used alternative to the conventional large telescope on an equatorial mounting in a dome, as exemplified by Dave Tyler's setup. Es Reid's setup, based on a heliostat and fixed spectrohelioscope, is a good example of this. This arrangement follows the example of professional astronomers, who often use such complicated and extensive equipment to analyse the light of celestial objects that it cannot be carried on a moving telescope. The solution, then, is to arrange for the instrumentation to be fixed, and the light to be directed onto it. The arrangement often used in large observatories is known as the coudé layout,5 where mirrors in the telescope and mounting direct the light along the declination axis and down the polar axis, to the fixed point known as the coudé focus, where, most often, spectroscopes are used. There tends to be a lot of light lost in this process, but this is tolerable if the telescope is more than large enough for the object being studied. Due to the light loss, coudé arrangements in amateur observatories are very rare, but one variant of the same general idea that it is possible for amateurs to employ is the heliostat, a moving mirror that produces a static beam of sunlight that can be fed to other equipment, and used for any desired purpose.
Es Reid is an optician by profession, and has worked on many of the most sophisticated optical systems employed by amateur and professional astronomers, as well as by industrial companies, the military, and other security forces. He designs and builds telescopes and makes all sorts of optics, work that is done by a combination of computer modelling, handicraft, and simple mechanical aids, and, which is, despite the predictive powers of modern optical design software, still something of an art. His problem with astronomy is that, like so many of us, he doesn't like getting cold. This is a major problem on the bleak East Anglian plain of England, where the wind usually cuts like a knife.
5Do not ask who Coudé was - the word is just the French for "bent".
His solution is to concentrate on observing the Sun from inside a completely enclosed shed. The heliostat directs sunlight into the shed through a hole cut in the wall, about 15 cm (6 in.) across. The hole is blanked off, when not in use, by a shutter. The heliostat (Fig. 9.51) consists of an optically-flat mirror 20 cm (8 in.) in diameter, supported on an altazimuth mount that used to belong to a Meade LX200 telescope. This was modified into its present form by Brian Brooks of Astroparts, Milton Keynes. Es did use an equatorial mount in the past, but the current arrangement is more stable. The azimuth gear of this mount has been replaced by a better one (made by Beacon Hill Telescopes), and both axes have been fitted with powerful new stepper motors supplied by AWR Technology. The drive electronics box for them can be seen on the plinth below the mount. The whole is supported on a 15 cm (6 in.) aluminium column embedded in gravel and concrete. The column is abut 1.3 m (4 ft) high. To install it, a 60 cm (2 ft) deep hole was dug, and a wooden fence post was concreted in. The aluminium column was placed over the fence post, and more concrete and gravel was added round it. The space remaining inside the column was filled with sand to damp vibrations. When not in use, the heliostat is protected only with a plastic bag tied over the top of the column.
The tracking of the heliostat is controlled by a handset and a bespoke software solution supplied by AWR. The motor controller is connected to the handset, in the shed, by a long Category 4e cable. The power supply for the motors is also in the shed. The shed is a perfectly ordinary garden shed 2.4 x 3 m (8 x 10 ft) with the windows covered to make it dark inside. All the equipment in the shed, including the telescope, spectrohelioscope, camera and computer, is arranged on a laboratory-bench type table. This has steel legs, which are placed on concrete blocks (Fig. 9.52). The blocks go through holes cut in the floor of the shed, and are concreted to the concrete pad on which the shed stands, which is about 15 cm (6 in.) thick. The shed is thus mechanically disconnected from the table, and there is no detectable movement of the optics.
The equipment on the optical bench in the shed much more closely resembles an optical laboratory setup than conventional astronomers' equipment, and is easier to understand diagrammatically than from photographs, or from actually seeing it. The telescope is based on an 11 cm (4.5 in.) f15 oil-spaced doublet lens. This was originally made by the famous English optician Horace Dall. The flint glass element of it is adjustable, by screws, with respect to the crown glass element, to compensate for atmospheric dispersion (the differential refraction of colours, most apparent when an object is low in the sky). Both elements were re-figured by Es to be optimal for the wavelength of hydrogen alpha light. The rest of the telescope is a plastic drain-pipe, which has no function other than to shade the objective. The objective is moved back and forth to focus the telescope; the cell is mounted on a block moved by a threaded-rod mechanism, turned by a cardboard tube that just acts as a long handle. A short way along from the objective, there is a side-tube connected to the main telescope tube,
containing a 45° flat mirror and a negative lens. This acts, with the objective, as a Galilean telescope. It gives a wide-field view of the Heliostat mirror, which has sometimes been used for locating Venus in the daytime. This, however, is incidental (Fig. 9.53).
There are two main ways in which the telescope can be used on the Sun: for projection onto the far wall of the shed, or as a feed to the spectrohelioscope. Projection along the length of the shed produces a large solar image - 75 cm (30 in.) across. A piece of paper is hung from the roof in such a way that it can be swung like a pendulum. Swinging it helps to remove the grain of the paper from visual perception, making the solar details more obvious. A circle is drawn on the paper. The atmospheric refractive flattening of the Sun's disk in the winter is very obvious against this circle. However, dispersion can be almost removed by adjustment of the objective, so keeping the white-light image sharp.
The main point of the observatory, however, is the spectrohelioscope, which occupies an area of the bench alongside the telescope light path. The word spectrohelioscpe is confusing, as it is not a type of spectroscope, though it has much in common with one. It is a device for imaging the sun in any desired single wavelength. It is thus an alternative to the etalon-type solar filters such as the Daystar, used by Dave Tyler, or the popular Coronado solar telescopes. These are fixed to work in only one wavelength, normally hydrogen alpha or calcium K. To examine another wavelength, you have to buy a different filter or another complete solar telescope. This is very expensive. Es Reid's shoestring spectrohelioscope, however, cost something like £400 to make, and can be used to isolate any solar wavelength and image in it.
The design was due to Brian Manning, and those interested can follow up the original reference.6 It was further refined by Norman Groom, who previously had this spectrohelioscope set up in his roof, and by Es. Fig. 9.54 shows in essence how it works. A flat mirror at 45° to the light path of the telescope directs light towards a prism, with two aluminised faces at 90° to one another. The first of these faces deflects the light along a path parallel to the telescope, and of almost the same length, to a diffraction unit. This unit consists of a collimating lens, figured by Es, and a reflective diffraction grating 60 mm (2.4 in.) square, made by Brian Manning. The lens is necessary because the grating must receive and transmit parallel light for the system to work. The diffraction grating, set at not quite a right-angle to the main axis of the instrument, reflects a spectrum of the solar radiation back to the second face of the prism, where it is again deflected through 90 into either an eyepiece or a camera.
In fact, a spectrum is not observed, because of the pair of connected slits, one on either side of the prism (Fig. 9.55). These isolate one colour, or spectral line, and produce a vertical line of light at the detector. If the first slit were not present, there would not be a sharp spectrum, because of the angular size of the solar disk, and if the second were not present, the whole spectrum would be observed at once. But because they are vertical slits at the focus of a telescope, and because the lines of the diffraction grating are also orientated vertically, it should be realised that what is actually observed is a narrow, vertical section of an image of the solar disk in one colour. Now, the slits are connected to each other and to a moving coil system, similar to that of a loudspeaker, which is controlled with electronics to cause the slits to oscillate laterally. When the slits are moved laterally though a small distance, the colour that is isolated remains constant, because angles of incidence, reflection and diffraction remain
6Journal of the British Astronomical Association 1982, Vol. 92, No. 3, p 112.
almost constant, but the vertical slice of the solar image they transmit to the detector moves across the solar disk, as the slits move across the focus of the objective.
If the slits are made to oscillate fast enough, and the eye is placed at an eyepiece at the final focus, persistence of vision will make the travelling slice of the monochromatic solar image appear as a complete disk. Note that no other filtration of the solar radiation is required in this system, because the slits only transmit a tiny proportion of the light from the objective. Es has been adapting this system to webcam imaging. A relay lens is used to amplify the image for a Philips ToUcam, used at 320 x 240 resolution. The complication with this initially was found to be that the webcam scanning rate could interfere with the scanning rate of the slits to produce image artefacts. This problem has now been overcome, by finding the optimum slit scanning rate, and some results are shown in Fig. 9.56 for both H alpha and Ca K light. The ToUcam is a colour camera, which produces a red image for H alpha and a purple one for Ca K. An improvement could be obtained by using a monochrome camera, which would be more sensitive, lacking the Bayer colour matrix on the chip, not needed for monochromatic imaging.
The slits are actually razor-blades, and their distance apart is adjustable to suit different viewing conditions. The slits also have to be kept very clean. The frequency of oscillation is about 20 Hz, but the system is effectively a pendulum, whose frequency can be modified by changing its mass, which Es does by adding bits of Blu-Tack and coins. Blu-Tack was also found to be useful for preventing a resonant response of the telescope tube to this oscillation. (All round, Es finds
it an invaluable substance for the optical engineer.) The frequency of light to be examined is changed by adjusting the angle of the diffraction unit. This shifts the whole spectrum slightly with respect to the slits, and thus changes the colour of the virtual image. The adjustment is precise, and is done with the slow-motion rods shown in Fig 9.53. The collimating lens is a singlet, and needs re-focusing for each colour.
Attached to his house, Es has a garage, which, like many astronomers' garages, is not used for storing a car. It has been converted into an optical test-tunnel 6 m (20 ft) long, lined with rock-wool to prevent thermal variations along its length. This he uses for testing mirrors, lenses and optical windows by doublepass against a 62 cm (25 in.) mirror, which is flat on one side and concave on the other. To me, one of the pleasures of visiting amateur astronomers' homes is coming across such extraordinary setups completely hidden behind bland, normal house-fronts.
Was this article helpful?