Telescopes

In contrast to most other areas of astronomy, there is no problem with lack of light when observing the Sun. For most purposes a 75-100 mm (3-4 inches) or smaller aperture will be sufficient. Owners of larger telescopes will therefore need to reduce the effective size in order to undertake solar work. This is most simply done by stopping the telescope down.

Stopping down just involves placing an opaque screen of cardboard, thin plywood or metal, which has a hole 75-100 mm across cut in it, over the telescope's objective (Fig. 2.5). (inly the light passing through the hole is then received by the telescope. For reflectors with a secondary mirror, the hole in the screen will need to be placed off-axis so that it is not obscured by the secondary mirror. Care should be taken to ensure thai the screen is firmly attached for if it were to fall off or blow away, the full aperture of the telescope would suddenly be gathering sunlight with possibly disastrous results to the observer or instrument.

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Figure 2.5» A 180 mm (7 inch) Maksutov telescope stopped down to 100 mm (4 inches) aperture For eyepiece projection leffeetive aperture is 80 mm. because oF the secondary mirror obstruction). N6 See the warning oboul the use of foam for mounting filters with Fig. 2.3,

First stop your telescope down to an aperture ot 75-100 mm (3-4 inches} or so+ Tlien securely cover the finder telescope if there is one attached to the main telescope. Put an inexpensive* low power eyepiece into the main telescope. Such an eyepiece is needed because even stopped down, a lot of energy will be gathered by the telescope. This could heat the eyepiece to the point where it might be damaged. It is therefore wise not to use an expensive eyepiece for eyepiece projection. Furthermore the more expensive eyepieces are likely to have more lenses, and to be more complex in construction than simpler eyepieces; they are therefore more likely to be vulnerable to heat damage. NB As an additional point, do not use an eyepiece which has cross-wires within it for guiding; the energy from the Sun may well distort them or even melt or burn them.

Next point the telescope at the Sun - but you must do this without looking through the telescope. The simplest method of finding the Sun without looking through the telescope is lo circularise the shadow of the

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Figure

Circuiof iiiiiig the shadow of The telescope in order to Find the Sun For projection (see dso Fig. 6,4).

telescope (Fig. 2.6). First point the telescope roughly towards the Sun, and then look at its shadow on the wall, the ground, or on a suitable board placed behind the telescope. Then move the telescope, without looking through it, so that the shadow decreases in size. When the telescope is pointing at the Sun, the shadow will be at its minimum size, and square-on to the telescope wilt be circular (or whatever may be the shape of the cross-section of the telescope).

Probably while setting the telescope on the Sun through circularising its shadow, you will have had a brief glimpse of the solar image zipping through the telescope's shadow, With luck, when you have finished setting the telescope, some of the solar image will be visible within the shadow as part of a brighter circle. If no part of the solar image can be seen, then the telescope should be moved by small amounts, until it does appear. Do not be tempted to look through the telescope if you have difficulty locating the solar image

Lighl from Sur>

When circular it is pointing of the Sun

Shodow of telescope

Light from Sun

Lighl from Sur>

When circular it is pointing of the Sun

Shodow of telescope

Light from Sun

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- recircularise the telescope's shadow and persist with nudging the telescope. The image will appear eventually, Once a part of the image is visible, then it may be centred on the projection screen using the telescope's slow motions (Fig. 2.8). The solar image is then ready for projection.

Owners of Cassegrain, Schmidt-Cassegrain and Maksutov telescopes, or any other design in which the primary mirror has a small focal ratio, need to take extra care whilst finding the Sun (NB See aiso the warning above about invalidation of some manufacturer's guarantees). The primary mirror of such telescopes can form an interna) image which has sufficiently high intensity to cause damage to the structure of the telescope, or even to cause it to catch fire, if that image falls onto parts of the telescope that can absorb heat, during the process of finding the Sun. The telescope should therefore be stopped down to no more than 50 mm (2 inches), and preferably to 25-30 mm (1-1.2 inches), while the image of the Sun is acquired. Once the image has been centred, the larger stop can be substituted for the smaller one. However, take care not to allow the full aperture of the telescope to be exposed to the Sun while swapping the stops. A smaller stop which may be inserted inside the larger one (Fig. 2.7) is thus probably better than two separate stops.

Care needs to continue to be taken when using the larger stop to ensure that the solar image remains centred. If the telescope is not driven at the solar rate23 or does not have a drive at all, then the image will drift and can soon overlap onto and damage the internal structure of the telescope. This can be a worse problem than that which occurs during finding the Sun, since the image may drift slowly and so remain on one part of the telescope for some time.

^ The Sun mnves from day to day around and also up and down in the sky The normal drive for a telescope therefore docs not allow the telescope to track the Sun correctly. The Sun's movement around the sky ranges from 0.A96 /day to 1.113 /day from west to east. The telescope drive must therefore be ad fit sled to be between 0.25% and 0.31% slower than normal if it is to track the Sun. The Sun's vertical move menu however* is not corrected by most telescope drives and this can be up to 0.395 /day,

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Figure 2.7, A 25 mrri

(1 inch] finding stop in place on a 1 80 mm (7 inch} Maksutov telescope NB See the warning about tfie use oF foam far mounting filters with Fig. 2.3.

Any design of telescope may have internal stops and/ or shade tubes which are intended to cut down background light during normal observing. Since the Sun is 0.5 across, parts of its image may fall onto these stops even when that image is centred in the telescope (see the warning above). If your telescope includes stops or shade tubes, check with its manufacturer before using it to project a solar image. Even if the telescope is not damaged, its guarantee may be invalidated if it is used against the manufacturer's guidelines.

Having found the solar image as described above, hold a sheet of white cardboard 0,2-0,5 m (8-20 in) behind the eyepiece. You should then see at least a part of the projected image of the Sun (Fig, 2.8). Move the telescope until the whole disk of the Sun can be seen, and adjust the eyepiece (while looking at the screen -do not look through the eyepiece) until the image is focused. Features such as sunspots, limb darkening and possibly faculae and granulation (Chapters 1 and 3) should then easily be visible on the projected image.

If you intend observing the Sun frequently, then it is worth constructing a framework to attach to the telescope to hold the screen onto which the image is projected (Figs, 2,8 and 2,9), A simple cardboard shield to prevent direct solar light from falling onto the screen will also help to improve the image by increasing its contrast.

Figure 2.7, A 25 mrri

(1 inch] finding stop in place on a 1 80 mm (7 inch} Maksutov telescope NB See the warning about tfie use oF foam far mounting filters with Fig. 2.3.

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Solar Observing Techniques

The image of the Sun obtained by eyepiece projec- Figore 2 g Eyepiece tion can easily be sketched. The outlines of the solar pr , , . solar disk and its features can be traced directly from the image onto a piece of paper placed onto the projection screen.24 Fine detail can then be added when the paper is removed from the screen. The image is also usually easily bright enough to be photographed (see also Chapter 4), If the camera» however, is held to one side of the screen, then the image will be slightly distorted by foreshortening. With care, however, the screen can be tilted slightly without the image going out of focus. If the camera is held at an equal angle on the opposite side of the screen to the telescope, then an undistorted image will be obtained (Fig. 2.9). With electronic images, simple image processing (Chapter 4) will restore a circular image.

The size of the image of the Sun obtained by eyepiece projection depends upon the focal length of the telescope, that of the eyepiece and the projection

24 This is the same principle as the way in which the camera obstura was used in the eighteenth and nineteenth centuries io sketch the outlines for landscape paintings.

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Protected solar image

Titled screen

Telescope

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