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When we look at the sun in white light (although, of course never directly) we see the surface of the sun, called the photosphere. The chromosphere lies above this and is transparent in white light, but may be seen if we isolate the atomic lines. By combining a spectrograph and a solar telescope a spectrohelioscope allows us to tune to a specific wavelength without having to purchase several filters.

Being able to see the sun in white light is exciting and interesting. However, in certain wavelengths more features can be seen. For example, observing in the H-alpha line offers deeper understanding of the magnetic nature of sunspots, since filaments and plages are visible.

The Workings of the Spectrohelioscope

The sun is tracked using a heliostat and reflected down into the solar telescope. The incoming light is directed using a mirror and then focussed with a lens; from here a small strip of the image is passed through the primary slit where it is reflected into a diffraction grating, producing a spectrum.

If a screen is placed in front of the second slit, the spectrum and atomic lines on it can be seen clearly. Careful adjustment of the diffraction grating position alters the wavelength that will pass through the slit. The secondary slit can then be used to change the bandwidth. The original strip can now be seen in the selected wavelengths at the focal point of the lens. The movement of the nodding mirrors scans the whole image of the solar disc over the slits. If they move fast enough (about 30 images/sec) the eye will see the sun in the selected wavelength, in the same way a television screen appears to be a moving image rather than a number of static images.

Diagram showing how the spectrohelioscope works

The Design

The existing spectrograph uses the Hale design and the nodding mirror technique is known as the Young Image Synthesiser. Although there are many other designs available, students who have undertaken similar projects in previous years have selected these based on design factors such as size and minimisation of vibration. The mirrors will be oscillated by an arm attached to their fitting, which will then be displaced by a rotating cam with a small offset. This will hopefully be introduced in the coming weeks.