Projection method

Paweł Rudawy

DISCLAIMER. This method causes heating of telescope's optics and therefore may lead to melting of plastic elements in telescope's optical train, including lenses, diaphraghms, eypieces and secondary mirror holders. Before pointing your telescope at the sun, make sure that all optical and construction elements of your telescope is made of heat-resistant materials. F-Chroma team is not responsible for any damage to instruments caused by improper use.

To carry out successful observations of the solar disk with majestic sunspots, brilliant photospheric plagues and amazing solar limb darkening one does not need any sophisticated observing instruments or tools.

Below we present a detailed and clear description of the projection method of the observations of the Sun. The projection method is simple and very safe method of observations of the structures on the image of the solar disk projected from the telescope onto a screen. The method enables simultaneous observations of the big and bright image of the solar disk by numerous participants, thus it is perfectly applicable during presentations, lectures and shows attended by numerous persons, for example during the lectures in schools. The projection method was already applied by numerous very famous astronomers, like Christopher Scheiner and Jan Heweliusz.

Necessary instruments

• a telescope of any size or binoculars;

• photographic stand with solid tilted head;

• a sheet of cardboard;

• a sheet of white paper;

• scissors, clips, glue, sticky tape etc.

In case of construction of a more sophisticated observing system, one will need some planks, cardboard sheets, a table, small auxiliary mirror etc.

NOTE. conducting observations of the Sun one should always keep in mind that solar observations of any kind are potentially dangerous and could cause a high risk of severe inures. One should applied extraordinary safety measures and precautions in order to avoid eye damages leading even to complete and unrecoverable blindness! All observers of the Sun in visual domain should strictly follow general safety rules.

On the beginning one should read carefully an user manual of the telescope or binoculars intended to be used in observations (or should consult the manufacturer of the instrument) if the instrument could be applied for solar observations using the projection method. In case if the telescope is not prepared for this kind of the solar observations, a very intense focused beam of the solar light could damage some fragile optical or mechanical parts of the instrument. Special precautions should be applied in case of Newton-type telescopes, where the secondary mirror could be overheated and damaged and also in case of catadioptic telescopes, for example Maksutov-type or Schmidt-Cassegrain-type instruments.

An eyepiece of the telescope (a small lens in a plastic or metal barrel at the end of the telescope) applied for solar observations should be selected with a great care. Numerous older constructions of the eyepieces, having its optical components glued with a natural resins, as well as oculars covered with anti-reflection coatings may be over-heated by a condensed beam of the solar light and may break or be permanently damaged. The experienced solar observers sometimes recommend a tiny loose of the mounting rings in the eyepiece, in order to reduce mechanical stresses caused by thermal expansion of the lenses.

On the beginning of the observations the telescope should be set roughly toward the Sun. The main objective of the telescope (i.e. usually the biggest lens in the telescope, installed on the very end of the tube or the main mirror in mirror telescopes) should be aligned in the overall direction toward the Sun. Note: align the telescope roughly and by hand only, without looking through the eyepiece!

Next, making various gently movements of the telescope, one should adjust the position of the telescope in such a way that the objective will point directly toward the Sun. If this is done, the eyepiece will be automatically aligned down the light-beam and its lens will be filled with a very bright solar light. Under no circumstances look into the eyepiece, if you want something to see more! It is absolutely crucial to keep in mind, to remember and to apply the rule that all adjustments of the telescope should be made by trails and errors only, without looking through the eyepiece! It should be remembered that even the very brief look on the Sun by typical astronomical telescope will cause severe damage of the eye or even a complete blindness!

When the telescope is aligned directly toward the Sun and the bright light beam exits the telescope through the eyepiece, remains to be done one simple operation: the light rays passing through the telescope should be focused on a sheet of paper (it is on the screen) held perpendicular to the axis of the beam.

Probably the first image on the screen will be very fuzzy (“unsharp”) and detail less. The image could be easily sharpened by simple changing the distance between the eyepiece and the screen as well as by changing the position of the eyepiece in the telescope (by “refocusing” of the eyepiece), one can get sharp, round the Sun's disk image, whose brightness decreases slightly towards the edge (which is not caused by any imperfection of the observing system but is a real solar event), sunspots (more or less round dark spots surrounded by dim rings of penumbras) and bright plagues.

The binoculars are nothing but two small parallel telescopes mounted in a common frame. Because a single image of the Sun is sufficient, being formed by a single telescope, so one of the two objective of the binoculars should be covered (or closed). The resulting instrument is a little bizarre telescope, so our previous description is fully applicable also for the binoculars. Unfortunately, the quality of the image formed by the binoculars may be noticeably worse than of the image formed by an average telescope, but it will be still good enough to allow observations of the sunspots, plagues and limb darkening!

The diameter of the solar image projected onto the screen depends on two basic technical parameters of the applied telescope (or binoculars): the optical magnification of the optical system and the distance between the eyepiece and the screen. Of course, the greater the image, the dimmer the projected solar disk. So, the observer should select the best compromise between the size and the brightness and contrast of the image, in order to make a comfortable observations of the solar surface.

The magnification of the optical system of the telescope (we denote it with the letter P) is equal to the ratio of the focal lengths of the objective and the eyepiece (we denote them with the symbols F1 and F2, respectively). Thus, the magnification of the telescope P is equal to:

P = F₁ / F₂

In many small optical instruments having fixed optical magnification (for example binoculars and small telescopes), the value of their optical magnification is given in the form of the inscription on the casing, where “8x” symbol means that the optical magnification is equal to 8 times.

If the distance between the eyepiece of the telescope and the screen is equal to L centimetres, the discussed optical system forms an image of the visible solar image having a diameter of D centimetres, equal to:

D = L * (P - 1) / 107

(note: L and D should be expressed in the same units of length, such as centimetres).

The table below gives examples of the diameters of the solar image, which can be obtained using a typical binoculars and small telescopes.

telescope	F1	F2	distance between the eyepiece and the screen L	diameter of the solar image D
average binoculars	-	-	100 cm	6.5 cm
small telescope	75 cm	3 cm	70 cm	16 cm
small telescope	150 cm	3 cm	50 cm	23 cm

Of course, instead of keeping the screen (actually a sheet of white paper) in hand, it is much more convenient to stick it to a stiff cardboard, attached to the telescope by a few slats or thin rods on the extension of the axis of the eyepiece (high quality sticky tape can give invaluable services during the construction of the instrument...). In this situation any corrections of the telescope position which should follows gradually the Sun, does not require additional corrections of the screen position. More experienced tinkerers may be tempted to build - instead of the simple screen – a light-tight housing with a suitable length, terminated with a "milky screen", where the solar image will be well seen with a great comfort.

All objects visible against the solar disk, which maintain a fixed position relative to the Sun's disk image despite moving a screen or telescope, are real solar structures (sunspots and plagues).

Apart from them, some other stains or dots could be visible, moving with respect to the image of the Sun for each movement of the telescope. These stains are caused by defects or dirt on lenses and have nothing to do with real astronomical objects.

Observation of the projected image of the solar disk is usually disturbed by a scattered and straight solar light, which passes next to the telescope and illuminate the screen, substantially lowering a contrast of the image. Thus, much better results of the observations could be achieved in well darkened room, where the objective of the telescope alone is lighted through a hole in a tight curtain. The amount of the scattered light could be also substantially lowered using a stiff piece of a cardboard with punched hole, top-mounted on a housing of the main objective of the telescope. Thus, the screened image of the Sun will be placed inside a column of a shadow projected by the cardboard, which greatly improves image contrast.

The diagram above shows how one can build more advanced "solar observatory", where in addition to the binoculars (or the telescope) mounted on a tripod an auxiliary mirror and a cardboard box were used. The mirror breaks a beam of light from the telescope and allows one to direct it easily on the screen, allowing to mount a screen almost "back" to the sunlight. Additional card-box can further shade the screen, improving the comfort of observation.

Note: If after performing all procedures described above no sunspots could be spotted, three various causes should be considered:

1. The unlucky observer examines the Sun just at the time when in fact there are no sunspots on it. This situation is, however, extremely rare.

2. A more likely reason for the lack of the visible sunspots can be bad sharpness of the image.

3. It may also happen that so-called theatre binoculars was used for the observation. The theatre binoculars is built on the principle of Galilean telescope, where the objective is a convex lens, while the eyepiece is a concave lens. Using theatre binoculars one can not get the image of the Sun on a screen (while the light is not focused by the system). The good news is that almost all contemporary binoculars and astronomical telescopes (refractors) are build according the principle of Kepler telescope (in very simple terms their main objectives and oculars are equivalents of convex lenses). They could project the solar image onto a screen. Heweliusz and Scheiner also used Kepler-type telescopes.

Johannes Kepler have made observations of the sunspots using... a cathedral in Regensburg. A small opening in a roof of this huge temple projected a beam of the solar light into a relatively dark interior of the building and thus converted it into a giant “camera obscura”. “Camera obscura”, as it is well known for everybody from scholar lectures in physics, is a lens-less optical device forming an image of the distant objects by geometrical projection of the light by a small opening only. Despite no problems with preparation of the appropriate instrument, Kepler had the advantage of any other co-temporal observers holding telescopes in hand or in primitive stands, that the cathedral is not shaking. Indeed, one can immediately see much more details on the image the solar disk, if the applied telescope is mounted on a rigid stand.