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The Telescope

The 12-inch Meade Schmidt Cassegrain telescope

The 12-inch Meade Schmidt Cassegrain telescope

The Orion telescope is a refractor and so has a limited focal length (400mm), but has a very wide view angle. This means that the focused image is that of the whole sun so only the larger, more prominent, details of the solar disk are observable. When a higher resolution is needed, greater detail of the surface and solar events can be achieved by using the large focal length and large aperture of the Meade telescope.

The Meade is a Schmidt Cassegrain telescope, so it has a spherical primary mirror that focuses the image viewed by sliding up and down the tube to alter the focal length, and a secondary mirror by the way of an aluminised cell placed on the convex part of a correcting lens. The correcting lens is at the opening of the telescope tube and is there to correct for spherical aberration caused by the primary mirror being curved and not parabolic in shape. Spherical aberration is a blurring at the focus of the light rays caused by the different angles and respective distances that incident light is reflected off the primary mirror. Reflected light from the edges of a spherical mirror will converge slightly before light from further towards the centre.

Diagram depicting spherical aberration

Diagram depicting spherical aberration.

The corrector lens achieves this by having spherical aberration itself but to such an extent that it is exactly the opposite of that at the primary mirror. This creates a cancellation. The lens also stops coma effects, which is where stars at the edge of the field of view are elongated but that doesn't really have any relevance to this project. With the lens in place at the beginning of the tube, the telescope system becomes closed which reduces the seeing effects caused by turbulent air currents (that can be set up in other open telescopes). The closed system provides an isolated environment, free of corrosion and dust, prolonging the use of the telescope by years. One of the few problems caused by this type of telescope design is that there is a deliberate obstruction to the light beam before the primary mirror reflects it. It is caused by the secondary mirror being placed where it is, on the correcting lens. This can have an effect such that up to 20% of the incoming photons never get to be processed and observed. This doesn't, however, degrade the image to a major extent.

To use the Meade for this project the H-alpha filter has to be attached to it. This complication (the aperture is too wide to directly attach the filter to it) was by-passed by a hole that was cut into the lens cap of the telescope to accommodate the filter. The hole was placed so that it would intercept parallel beams of light, which are incident on the lens, so that they would conveniently miss the secondary mirror obstruction. With the filter affixed to the telescope the back lens cap can be removed and observing can commence. One of the benefits of having the Meade is that it comes on its own mount and can be moved both in right ascension and declination very smoothly by hand, with the flick of a couple of switches, or by using the motor function. The motor function proves useful when tracking the sun over a period of hours as it is always easy to keep the sun in the centre of view. The telescope can be directed at a variety of speeds so that accuracy is kept to a maximum. The telescope has an f-ratio of f/10, which means that the light comes to a focus quickly, and so has a focal length of 3048mm (3.048m) and it achieves this, in the space of about 75cm, by `folding' or `bouncing' the light beam off the two mirrors and it comes to a focus behind the telescope. Having the focus at the back of the telescope enables the attachment of extra equipment, such as CCD cameras, to be mounted on the back and not obstruct the incoming beam.

Schematic diagram of the components of the Meade Schmidt Cassegrain

Schematic diagram of the components of the Meade Schmidt Cassegrain

With the equipment as such then the television and video-recorder can be linked up to a CCD camera that replaces the eye at the focus point. The CCD camera is attached to the lens at the back of the telescope by another adapter so that a `snug' fit is obtained to retain all of the light that is being sent to the area of pixels. The adapter also acts as an extension to make sure that the focus point is on the pixels and the resultant is not a blurred image.

The video footage is composed of images from a black and white CCD camera and from a colour one. The major differences in the recorded information is that the black and white camera has a 1/4-inch lens which has a smaller plate scale than the 1/2-inch colour camera lens. This means that the intensity of the image is much greater in the black and white footage, so there is a more noticeable effect from saturation. CCD cameras record information from the light that is incident on them. The light (`packets' of electromagnetic radiation or photons) strikes a cell of minute pieces of silicon called pixels. The elements absorb specific photons, which are incident on them, and transfer this to conduction electrons. This gives them the energy they need to become free of the atomic nucleus and hence create a piece of stored charge where the photon struck. Over a designated integration time the charge will build up to be finally read off, line by line, by a computer and processed to form the final image. Saturation is a consequence of using the CCD cameras on a very luminous source. The pixels are inundated with so much light that conduction electrons are created only to exceed their pixels capabilities and so spill out into neighbouring pixels. For a very bright object, such as the sun, this happens for the majority of the pixels, hence the inability to see the chromosphere in visible light. Saturation is a greater problem with the black and white CCD camera because of its sensitivity. The images received by the camera are projected on to the television so that the solar activities and their progressions are easily followed. It also means that they can be recorded immediately if need be.

Once the session in the observatory has finished and all of the equipment has been put away, the telescope secured in an appropriate position and the dome closed, then the images and recordings can be taken to the lab and replayed on the computer using a package called `Motion Picture.' The package enables the recordings, which are done in real-time, to be looked at in individual frames and picked to provide the best of the days observations. The computer also enables the user to enhance the image to bring out the main features that were being studied. These images are then saved to file and disk (in j-peg or html format to keep them compatible for the transfer onto a web page) and taken to a computer that has a separate package, either `Paint-Shop Pro' or `Coral Photo Paint,' and enhanced further by techniques called unmasking and sharpening to really bring out the detail that is sometimes drowned by patches of higher intensities.