Introduction

What amateur astronomer did not dream to see solar flares or impressive prominences standing out the solar limb?

Actually, there are filters which allow to see and to photograph these phenomena but it was not always so. In 1927, Bernard Lyot invents the monochromatic filter which allows to acquire the picture of the Sun in a very thin part of the spectrum. This kind of filter, giving instantaneously a 2D image, was inaccessible to the amateurs for a long time. © Observatoire de Paris

Before this revolutionary filter, another instrument allowed to explore the surface and the immediate environment of the Sun. Invented independently in 1980 by Henri Deslandres and George E. Hale, the spectroheliograph (SHG) constist mainly of a spectrograph with entrance slit and a mechanism recording a thin part of the spectrum.

Concept is simple: Recording the linear image of the Sun bounded by the entrance slit of the spectrograph at the wanted wavelength, and cycling this process until the whole disk be scanned. Final image of the solar disk will be reconstituted by juxtaposition of all the linear records.

Originally, the photographic plate was at the time the means to record and to display a spectrohéliogram. This had for consequence the necessity of complex, cumbersome and heavy mechanisms for the realization of a spectroheliograph. Typically, An "old" is equipped with an entrance slit of the monochromateur scanning the solar disk and an exit slit scanning the film in a synchronous way and acting as shutter.

The dissociation of the means of recording and display (that we will nammed in a generic way "synthesizer") allows to considerably simplify the structure of the instrument, and to make it completely static during the capture of the images. In this case, the acquisition is done by a CCD sensor, the collected information is stored in the memory of a microcomputer and the synthesis of the image appear on a screen or, for example, on a paper via a printer.

A spectrohéliograph with linear CCD

What means "CCD"? CCD is "Charge Coupled Device". It is an optoelectronic component made up of many photoelements able to produce electric charges under the effect of the light and to preserve them in a well of potential. The dimension of a photoelement is about 10 µm and now "chips" integrate millions of them. The reading of the tension of each element of the CCD is done on an unique exit. Having done an integration (the equivalent of an exposure in photograph), it is necessary to transfer charges accumulated from a photoelement to the other one until the exit by making vary the wells of potential, where from the name of the device.

What is a linear CCD (or linear array)? Photoelements can be arranged according to 1 dimension (line or bar) or according to 2 dimensions (matrix). In the case which interests us, recording a linear image, a linear CCD is the ideal sensor. The number of photoelements is usually from 512 to 4000. Such a device appears like an integrated circuit with on its superior face a window through which we can see the sensor. These circuits equip for example scanners and telefaxes.

Capture and image synthesis

We form a spectrum with spatial resolution of a linear portion of the solar image (source) and we select its monochromatic image in the desired wavelength. This selection is usually done by means of an exit slit of the spectrograph (monochromatic image). Here, it's the linear CCD which do this function. You can see, at left, the positionning of linear CCD in SHG mode.

The choice of the wavelength is realized by drifting the spectrum onto the sensor placed in the axis of the lines. The received signal is the photometric profile of the cutting of the solar image in the chosen wavelength.   The essential principle of the spectroheliography with linear CCD consists in making move the image of the Sun on the entrance slit of a spectrograph while microcomputer collects in regular intervals the signal delivered by the CCD placed in coincidence with a spectral line. Each "line" is digitized then memorized by a microcomputer. The synthesis of images is made by juxtaposing lines to form 1 image in 2D, for example on the screen of the computer. There is a lot of advantages to use this method : All the system is static. One can use the diurnal movement as means of scanning (there is not more regular). Given the dimension of photoelements - and so that of the sensor - a solar image of rather small diameter is sufficient. The objective having to supplied this image will have a focal length about 1 metre (even less than a metre + Barlow lens) and so reduced overall dimensions. It ensures from 2 previous points that SHG, if it is rigid enough, can be a directional instrument on equatorial mount, as well as a telescope, and does not require necessarily a heavy installation fed by a coelostat. Naturally, if you have place and means... The focusing of the telescope providing the solar image and that of the spectroscope can be controlled on the screen of the computer. Images appear during their acquisition. Phenomena with fast evolution are perceived immediately. It does not need to wait that the film is developed to evaluate the result and not of boring work in a photo lab. (Let us note however that Devices cameras contributed to minimize the importance of the last 2 points.) In the other hand, the rate of obtaining images is weak compared with photograph or with CCD camera (the acquisition of an image is about 2 minutes) and turbulence can thus make devastation on the image quality.

Visual version : le spectrohélioscope

It is possible to realize a version intended for the visual observation. We use rotating or vibrating slits to scan the solar disk and isolate the monochromatic lines. It is the retinal persistence which is the key of the synthesis of the image. The realization is delicate but completely accessible for a motivated amateur.

To learn more about the spectrohelioscope (SHS), I advise you to have a look attentive on the online document of the pioneer of the subject - Fredrick Veio - and that you can download here (4.2 Mo).

Few optical layouts of spectrohelioscopes

• Réfractor + Arcetri

Primary solar image is provided by an achromat which focal length is one meter at least and with F/D ratio is rather high (10 or more). It is necessary to consider residual chromatic aberrations and build a focuser to adjust the image of the Sun on the slit for red light (H-alpha) as well as violet light (Ca-K). This arrangement does not give a lot of aberrations (mainly sphericity). Primary beam is folded by 2 flat mirrors to get a short instrument.

• Maksutov + Hale (or Ebert)

This assembly is simplified by the use of a very short telescope which makes useless the flat mirrors (for folded beam). But, the risk - if the secondary mirror of the telescope is stuck on the meniscus - is to see the adhesive melting or softening under the effect of heat. Better is to carry out the test with an old telecope !. Aberrations of astigmatism are to be envisaged with this monochromator if mirrors have short f.l.

• Réfractor + Littrow

Primary solar image is provided by an achromat which focal length is one meter at least and with F/D ratio is rather high (10 or more). It is necessary to consider residual chromatic aberrations and build a focuser to adjust the image of the Sun on the slit for red light (H-alpha) as well as violet light (Ca-K). This assembly finds its simplification in the monochromator where there is only one lens serving at the same time as collimating and camera (autocollimator). Its focal length should be long and a simple meniscus could make the deal. One will take care to minimize the angle between slit and sensor. Linear sensor can be placed in the vicinity and in the prolongation of the slit.