Spanish Version

Catch a Star Logo

Catch a Star 2004



Authors: Diego Castellano Sánchez, Daniel Cantero González (15), Irene Llucia López (16), José María Rosales Crespo (16)

Secondary School: IES "La Algaida", C/ Jamaica, 2, Río San Pedro, Puerto Real, 11519


European Southern Observatory

Astronomic Group "Astroalgaida" EAAE


Home   Catch a Star


This work intends to make an approach to the study of the most near and visible celestial bodies, by means of their mechanical analysis and the construction of simple observation devices from recycled and reused materials.

As an example of this procedure, the study of the Venus transit by the sun disc is proposed. For the study of the Venus transit, the construction of a plastic adapter for a telescope was planed. The image of the sun disc was projected over it once the image passed through the telescope optical system. This device was coupled to a conventional telescope so that the projection of the sun disc over a white screen was allowed. The sun could be observed without any risk. PVC tube was used to couple the pot that supports the screen to the telescope. Screws and rubber disks allowed to align the telescope optical system with the screen.

Due to the difficulty of following the transit during the seven hours that this phenomenon last, a motorisation system for the equatorial mounting of the telescope was designed. The sun disc continuously remains in sight over the screen. This was possible by means of a motor coupled to a screw for the slow height movement, a step-by-step motor from a unused printer, and the construction of a control system by using a power amplifier and a programmable microcontroller. This development stage implied the collaboration from the Technology Department of the Secondary School “San Isidoro” (Cartagena, Spain). The most complicated work in this step was the programation of the device to follow the sun with an adequate synchronicity. This was a complex work and we had to be assessed by our Cartagena colleagues. We are in a debt with them and with their professor Mariano Castellano for the motor that allowed us to observe the transit easily.




Este trabajo pretende hacer una aproximación al estudio de los cuerpos celestes más próximos y visibles, mediante el análisis de su mecánica y la construcción de instrumentos de observación sencillos a partir de materiales reciclados y reutilizados.

Como ejemplo de esta mecánica se pone de manifiesto el estudio del tránsito de Venus por el disco Solar. Para el estudio del tránsito se ha planteado la construcción de un adaptador plástico sobre el que proyectar la luz del Sol , una vez que ésta ha atravesado el ocular del telescopio. Se ha utilizado un dispositivo acoplable a un telescopio convencional que permita la proyección del disco solar sobre una pantalla blanca. De esta manera podemos observar el Sol sin riesgo. Para acoplar la maceta que soporta la pantalla se ha utilizado tubo de PVC unido al tubo del telescopio mediante palometas y discos de goma que permiten graduar la sujeción y alinear el objetivo con la pantalla.

Dada la complejidad del seguimiento del tránsito durante las siete horas que dura el fenómeno se plantea la incorporación de un sistema de motorización de la montura ecuatorial. El disco solar permanece continuamente dentro del campo de visión gracias a un motor acoplando al tornillo de movimiento lento en altitud. Para ello ha sido necesario la utilización de un motor paso a paso de una impresora en desuso, y la construcción del sistema de control mediante un microcontrolador programable y una etapa de potencia. Lo más complicado era programar este dispositivo para que siguiera al Sol continuamente, sin adelantarse o atrasarse.Esta fase implicó la colaboración del Departamento de Tecnología del IES “San Isidoro” de Cartagena. A Mariano Castellano, Jefe del Departamento, le debemos el motor con el que observar comodamente el tránsito.




1- Introduction
2- Methodology


a) First Prototypes


b) Helioscope Construction


- Telescope-dark chamber adapter


- Screen purchase


- Motorization of the equatorial mounting
3- Observation and Results
4- Discussion of Results
5- Conclusions
6- Acknowlegements



1- Introduction

On 8th June, Venus crossed the solar disc in a spectacular and amazing phenomenon. The last one happened on 6th December 1882, 122 years ago. Therefore, none alive person has seen this event called transit. In this last time, the approximated duration of the phenomenon was 6 hours time, being visible form Asia (except its most Oriental part), Africa (except the West region), Europe and most of the IndicOcean (Figure 1).

Visibility map of the Venus transit 2004 Figure 1 .Visibility map of the Venus transit 2004.

Transits happen when a planet crosses ahead the Sun and they are nearer the Sun than the Earth. That is why only the interior planets, Mercury and Venus can provoke transits.

Mercury transits are more frequents, approximately every seven years, because the turning period of Mercury around the Sun is only of 88 days time. The last one happened on last 7th May 2003. The next one will be on 8th November 2.006, but it also will not be able to be seen from Spain.

Venus is the second planet from the Sun and turns around it in a different speed that the Earth. This makes that from time to time Venus is between the Sun and the Earth, in what’s denominated Inferior Conjunction? It is here where the transit is produced.

A Venus transit is similar to a solar eclipse, in which the solar disc is hidden by the Moon. But in the same way, that we do not see a solar eclipse every New Moon? (lunar phase in which the Moon is between the Sun and the Earth), neither a transit happens every time Venus is between the Sun and the earth. (fact that happens every 584 days or 1.6 years).

This is so because Venus orbit and the Earth's one are not in the same plane, but the first one is 3.4 degrees inclined with regard to the second one, cutting it in two points, called nodes. Only in those points, the Sun, Venus, and the Earth are aligned (Figure 2).

Sun, Venus and Earth alignement in nodes Figure 2 . Sun, Venus and Earth alignement in nodes.


Summing up, a transit need two requirements:

        • An inferior conjunction of Venus.
        • This conjunction must be produced near one of the nodes.

    The inferior conjunction of the 8th June carried outthis two requirements, and thanks to the meteorological conditions, it was possible to see the Venus transit by the southern part of the Sun.


2- Methodology

To the Venus transit by the solar disc observation, the following modalities were studied:

        • Direct observation.
        • Projection.

The first of the options was rejected by several motives. Firstly, it is one of the most insecure methods of observation, since it demands the direct exposition of the eye to the sunlight, despite using adequate protective filters (Mylar or another kind). Any imperfection in the fabrication of the of the filter or a due-to-temperature rupture leads to possible irreversible damages in vision. Secondly, providing contact time data were expected, the little size of the Sun directly observed would produce, inevitably, huge measure errors. Finally, since photos and video pictures were wanted to be taken, this method was not suitable unless very expensive cameras were available.

Consequently, it was chosen the observation in a screen of the Sun shape projection. This method is well known by astronomy fans to the observation of solar phenomenon as, e.g. solar strains or eclipses (Figure 3).

Schematic drawing of used sun ligth projection method. Figure 3 . Schematic drawing of used sun ligth projection method.

However, one of the objectives was achieving one solar shape as big as possible. In fact, the first idea was performing a public observation of the eclipse in the High School "Algaida" courtyard, where a large meeting of pupils and teachers was expected. So that the all were able to see the transit without difficulty, a big solar disc image was indispensable. After an investigation period, it was decided how to get an improvement in the method.

The solution basically consists of the adaptation of a big object (pot of flowers, bucket) to the back of a telescope provided with a projection screen where the enlarged Sun shape is projected.


a) First Prototypes

At first meetings, some aspects were clear: we had an 80 mm opening telescope handed over by Raul Fernandez Velasco, so it was jumbled up getting a diameter for the enlarged solar disc of, at least, 20 cm. On the other hand, having an equatorial mounting allowed to follow the sun during the seven hours of the phenomenon automatically, what was an advantage to gain accuracy and commodity in the measures.

Before building any adapter to the 80 mm telescope, it was necessary to validate our hypotheses respect to the projection, so several initial tests were made. The first Astroalgaida prototype was a 50 mm diameter refractor telescope, to which a plastic funnel in its back was incorporated (Figure 4). To the projection, a transparency screen was used. This was able to be approached or moved away to get a sharp image, and study the optic phenomenon that made possible the calculation of the ocular distance to the screen so as to achieve the required solar disc size.

First prototype funnel fitted to a 50 mm telescope Figure 4 . First prototype funnel fitted to a 50 mm telescope.

The first essay allowed to, in this way, compare the results on a second one outcomes. This second prototype was the foresaid 80 mm diameter refractor telescope with a 20 mm diameter ocular (60X), to which a 15 mm diameter plastic pot of flowers was fitted. The first rudimentary screen was a round-shape piece of white sheet held on the pot thanks to a metallic bracket auto adjustable (Figure 5). The obtained solar disc had only a 6 cm diameter.

Second Prototype: pot of flowers fitted to 80 mm telescope Figure 5 . Second Prototype: pot of flowers fitted to 80 mm telescope.

At this stage, it was clear that the key factor on which the enlarged-solar-disc size depended was the height of the recipient, more than its diameter. Therefore, having a suitable recipient was vital to the wanted goals. The first recipient search was carried out by the Algaida components in all the nurseries and shopping centres of Bahia de Cadiz.

The solution was a dissolvent bottle donated by the Cadiz University Science Faculty, which was 63 x 33 cm. This one seemed to be ideal to the definitive helioscope construction. It was with this piece that we began to work. After having drilled the bottom of the bottle to adapt it to the telescope, a first test was made in the High school roof with the rudimentary telescope (Figure 6). After focusing, eureka!, it was possible to observe a 13 cm diameter solar disc. The general happiness was even bigger when a solar strain was observed. That was the confirmation of, of meteorology helped, nobody in the High school was going to miss the Venus transit.

Third prototype: dissolvent bottle fitted to a 80 mm telescope Figure 6 .Third prototype: dissolvent bottle fitted to a 80 mm telescope


b) Helioscope Construction

- Telescope-dark chamber adapter

The chosen material was PVC Ease to work and quite strong. The acquired items were:

        • A 90 mm diameter PVC pipe.
        • A 90 mm diameter horizontal drainpipe.
        • PVC glue.
        • 4 x 8 mm screws.
        • Wings screws.
        • Plastic joints.
        • Pipe brackets.
Over the telescope tube three plastic brackets were placed, opened and displayed at different heights. Several drill holes were made in the PVC so that they were aligned with the bracket threads. To a better stability, the drill holes were triangle shaped. Wings screws with screws were used in the adjustment of the tube to the brackets so as to align the tube and the eye axis quick and easily.

At the pot of flowers bottom a 5 cm hole was made so as to introduce the telescope focus tube completely. The pot was joined with the drainpipe thanks to four screws in its bottom, so that both pieces were solidly united.

To the final assembly, firstly, part of the telescope tube was completely introduced into the drainpipe-waste-pipe system, having previously unscrewed the plastic wheels that move the ocular. The telescope tube was so perfectly fitted to the waste-pipe and the focusing tube totally inside the bottle. In this way, it was possible to assemble again the focusing wheels, staying the device completely hidden inside the bottle . The focusing of the telescope was made indirectly, setting or removing the screen, what was slower and harder but made the system stronger.

Secondly, the PVC tube was introduced by the top of the telescope and fitted into the bottom of the wastepipe, in a way that the holes coincided with the brackets. Both the telescope and the PVC tubes were joined by double screws. The adapter whole allowed a strong adjustment between the bottle and the telescope since the three elements were an only piece.

Telescope-dark chamber whole adapter Figure 7 . Telescope-dark chamber whole adapter.

At every moment the use of glues was avoided so as to make easier the dismantlement of the system, provided that the adapter had to be an accessory, not a definitive element.


- Screen purchase .

After several test, different materials (cotton, paper, etc), the final screen fitted to the dark chamber was an Indoor/Outdoor Backlite . It is a material used in expositors, placards, posters and segnalizacion vials , installed both indoors and outdoors. The advantages of this material were a lot: waterproof, resistant to be torn, and it provides a sharp and good-quality image. This screen was donated unselfishly by Printings "Martinez" of Puerto Real.

The screen was cut out in a 32 cm diameter round shape, tautened and fixed with glue to a cardboard circumference, to which two holes were practised to fix it to the pot of flowers (Figure 8).

Indoor/Outdoor Backlite Figure 8 . Indoor/Outdoor Backlite.


- Motorization of Equatorial Mounting .

To the construction of the equatorial mounting motor, we resorted to the collaboration of the Cartagena San Isidoro High School Technology Department Director, Mariano Castellano Sanchez. Provided that our purpose was that the telescope-dark chamber system were able to be orientated to the Sun constantly, we needed a precise motorization system. From the two possible options of incorporating a servomotor system or a step-to-step motor one, we finally opted for the step-to-step motor, since the first one was rejected by its high economic cost to Secondary Level students, in addition to the precise control difficulties that the conventional motors present. The step-to-step motor were built by the unselfish collaboration of Rafael Cambronero Canovas, form the enterprise “SECA Electricidad”, of Cartagena.

Bipolar stepper Figure 9 . Bipolar stepper.

In the first phase, the design consisted of a system totally dependent of a portable computer, although once made, it was thought to improve it so that the system were more autonomous, being the dependent system accessible to the participant High Schools.

As one of the mottos of Astroalgaida is “ Reuse and recycle ”, the step-to-step motor were obtained of a refused printed purchased. From three motors available, it was opted for the 6V one (2.5 A, 15 W), since it was the one able to move the telescope-dark chamber without difficulty.

The motorization system consists of two phases (Figure10):

- Power Phase: It has an external 6 V battery that provides electricity to an integrated circuit L298current amplifier able to provide such current up to 3 A to the four bipolar motor outputs.

- Control Phase : It was designed in such a way that the motor movement were regulated by a Basic language programmed microcontroller compiled with assembly software, the program was downloaded to the ROM memory (resident program similar to the used in robotics) and inserted in a control box. This also could be programmed by a portable computer with COM2 communication and a serial wire used to download the program and modify its parameters. The box has an infrared (IR) receptor so as to receive orders form a TV controller, compatible with Sony models. The connection between the motor and the control box was carried out by a DIN 5 connector.

Motorization system Figure 10 Motorization system.

As it has been said before, the box controls 4 power outputs up to 3 A, it is also possible to control a project that implicates 2 continuous current motors and make them turn in both hourly and antihourly directions, one of the which needed 4 continuous-current motors that turned in an only direction, or another that had 4 electromechanic or electronic pins controlling 4 devices such as continuous current motors. In the same way, although the control box was designed exclusively to this project, it is enlargeable and configurable to other educative purposes with the possibility to add another power pate that activated 4 outputs. It would even accept analogical control signals such as temperature, pressure or speed values.

The basic working program it is the following: The basic program is downloaded form the computer to microcontroller by a serial wire connected to the control box,that also includes integrate and compactly the power phase. Once made this step, the serial cable is disconnected and the microcontroller runs the program.

What does the program do? It generates four different sequences at via-software-configurable intervals, generating a step-to-step bipolar motor thatturns with the desired speed. The calibration is very easy and it allows an automatic and convenient pursuit of the Sun.

Why to generate 4 outputs? The bipolar motor consists of 2 bobbins and 2 terminals per bobbin, what sums 4 terminals to control. By physic beginnings it is necessary to apply them a determined sequence of positive and negative tensions, so that the motor follows the magnetic field of the bobbin (Figure 11).

Schematic functionation of stepper Figure 11 . Scheme of stepper functions.

In this way, it was possible to adjust the motor spin to the required speed, 15 degrees per hour, apparent speed with which the Sun crosses the sky, only by controlling the pauses between step and step. The advantage of this system is that any mistake in orientation to the Polar could be compensated reprogramming the motor.

The motor was placed in one of the telescope legs by a metallic sheet . The connection of the motor the ascension axis was fitted by the own slow control of the telescope, once unscrewed the plastic steering wheel (Figure 12).

Stepper and coupling system Figure 12 . Stepper and coupling system.

Although the final outcome has been a success, the way to achieve this goal has been arduous since a lot of problems has been to be solved, such as:

- System stability and movement: To improve the stability of the telescope and-dark chamber system and ensure a continuous and harmonic movement, it was decided to substitute the disolvent bottle by a lighter pot of flowers, sacrificing the solar disc amplification.

- Implementing a totally autonomous and portable system, similar to a microcomputer able to move a telescope-dark chamber system, similar to an automaton.

- Estimate the energy consumption: Given that the motor consumption is 2.5 A per hour and the estimated transit time was seven hours, it was necessary to use two 12 A batteries to ensure the motor working during all the observation. It was required to have a fast and easy-to-change system of batteries, solved by using Faston connectors.

- Adapting the signals between control and power phases. The control part worked at a low-active level and the power one worked with high active signals. To solve this incapability several solderings were made in an integrated circuit and its functions annulled.

- Programming a microcontroller able to receive IR signals. To solve the problems of reception of the signals sent by the TV controller an intense revision of the existent bibliography was carried out.


3- Observation and Results

One of the posed problems in the coordination meetings of Astroalgaida was the necessity of finding a suitable place from where observe the phenomenon due to the fact that the ephemerids of the transit to Cadiz forecasted that it was going to be produced at 5h 20m 31.6 s UT, just some minutes after the sunrise and with the solar disc at a 2.3 degrees height over the horizon (Table 1).

Tabla 1 . Ephemerids of the transit of Venus for Puerto Real (Cádiz)

Time (UT)


5h 20 m 31.6 s

Primary Inmmersion T1 PA=117.6º; Altitude= 2.3º

5h 40m 15.0 s

Secondary Inmmersion T2 PA=120.8º; Altitude=5.8º

8h 24 m 13.2 s

Maximum of the transit PA=166.8º; Altitude=37.5º

11h 06m 06.0 s

Primary Emersion T3 PA=212.6º; Altitude=68.7º

11h 25m 19.1 s

Secondary Emersion T4 PA=215.9º; Altitude=71.8º

That is why it was necessary to find a place clear enough to observe the first contacts. The chosen place was Las Aletas marshes of Puerto Real, Cádiz (36º 32´ 53.7´´N; 6º 10´ 3.3´´W) (Figura 13).

"Las Aletas" marshes. Puerto Real Figure 13 . "Las Aletas" marshes. Puerto Real, Cádiz.

The observation program was the following:

      • 19 h UT (june 7th) . Observation and participant tents assembly.
      • 22 h UT (june 7th) . Colimationn of the seeker and alignement with Polar. First test of the stepper motor.
      • 01 h UT (june 8th) . Observers break.
      • 04 h UT (june 8th) . Breakfast. Helioscope assembly. Watches synchronization with regard to GPS (Garmin Etrex)
      • 5h 10m UT (june 8th) . Sunrise 61º degrees in azimut.
      • 5 h 20 m UT ( june 8th) . Start of the Venus transit observation.

Venus transit observation was carried out by direct observation at the beginning with a 50 mm diameter refractor telescope with a 12.5 mm ocular (48 X) because the low sunshine intensity prevented the solar disc projection in the helioscope screen. In addition to the transit pursuit with the helioscope, some photos of the event were taken (Figure 14).

Images of transit Images of transit Images of transit Images of transit Images of transit Images of transit Images of transit

Figure 14 . Images of transit (click to see) .

Moreover, the participants drew pictures of the transit every 30 minutes (Figure 15).

Schematic drawings of transit Figure 15 . Schematic drawings of transit.

The contact times are the following (Table 2).

Table 2. Astronomical unit measures results (AU)

Instant (U.T.)

AU (Km)

P (”)

D (AU) (Km)

D (P) (”)


5h 16m 9.00 s






5 h 36 m 13.00 s





1.317 %

11 h 2 m 42.00 s





1.095 %

11 h 22 m 40.00 s





0.846 %


Mean AU = 149305961 km

Mean P = 8.8126 ''

Mean Error = 0.195 %


AU, astronomical unit, is the value of the distance from the Earth to the Sun, calculated using our measures.

P, Solar parallax, is the angle under which the Earth radius is seen from the Sun centre.

D (AU) (Km) and D (P) ( ”) , are the corresponding increments in the measures.

Error , is the imperfection committed measured in %.

With these times it is possible to calculate the Earth-Sun distance by parallax method. We call parallax of a star to the angle which it is shown under this star one length situated to a distance choosen in the Earth. For the Solar System it is used the Earth radius (Figure 16).

Sun parallax calculation method Figure 16 . Sun parallax calculation method

Although this distance lacks of global relevance, since there are more precise methods, it means such a challenge to us building ourselves un reliable system of astronomic distances calculation, applicable to other phenomenon. The results of solar distance are the shown in Table 2.


4- Discussion of Results

In the obtained results it is observed that the error made is of 0.195 %/. In the following (Figure 17) the different participant observers values are shown, as well as the real value. These measures could be consulted at the web site of the prize catch the transit .

Gaussian distribution of participant observers measures Figure 17. Gaussian distribution of participant observers measures.

It is observed how our measure value is next to the real value, as well as it is under the maximum of most of the participants distribution . Observing Table 2, it is possible to see how the errors are decreasing along the transit, what indicates that provided the unusual of this phenomenon, we have not been able to test the contacts with the solar disc before and only the acquired experience during the event has made us lessen the errors in it.

On the other hand, it is also observed that the main errors were made in the two first contacts, actually the ones where the observation was direct, as it is foresaid. This fact corroborates that the accuracy of our projection methodology opposite to the direct one, since the error between the first and the last contact is reduced almost to a half (1.405% to 0.846%).


5 - Conclusions

The culmination of these project objectives has allowed finishing successfully not only the observation of an unusual astronomic phenomenon such the Venus transit, but also an educative experience between two distant educative centres thanks to a common objective which has implicated two despair departments as Natural Science and Technology are.

On the one hand, Astronomy introduction as curricular content of Third Course of Secondary Level has eventually been a very suitable resource to these ages students motivation to science study. That is how in our centre an Astronomic Observation Group has been consolidated ( Astroalgaida ) after having started during the current course with the work titled “observing an eclipse”, presented during the Technology And Science Week 2003.

On the other hand, the equatorial mounting motorization is totally achievable by any Secondary Level Technology Department and it is a non-complicated and novel field to the students, as the educative experience held in San Isidoro High School after finishing the project shows. The students were able to control themselves with a simple TV controller a telescope aligning it in the desired position.

Finally, the realization of this projects represents a multidisciplinary activity of team work in Secondary High Schools, since in this project some specialities such as Physics, Mathematics, Astronomy, Optics, Mechanic, Electronics or Computer Science has been conjugated.


6- Acknowlegements

To all of you, thanks a lot:

- To Raúl , for his patience, dedication and good mood. Without him this work would be impossible.
- To Mariano , for his inventiveness and effectiveness with the motorization.
- To Rafa , for the good work made with the motorization.
- To Ramón , for his support in the public publication of the results.
- To Ceferino y Nuria , for their time and good work in the video editing.
- To David M. , for his work as Webmaster.
- To Chema y David Ch. , for their opinions.
- To Roberto , for the translation and his enthusiasm.
- To Televisión de Puerto Real , for their images.
- To Radio Sol Puerto Real , for their sounds.
- To Jonathan , for his kindness.
- To Gonzalo , for his cooperation.
- To the Secondary School , for the facilities.
- To our fellows , for their encouragement.
- To our families , for their understanding.




- click to see videosequences-


Preparation Phase (1 m 36 s)


Observation Phase (2 m 44s)

Obtanined Images of transit

Obtained Images (55 s)

Photographic Sequences

Photographic Sequence (44 s)


Timings (30 s)


Acknowlegements (24 s)




  • “Astronomía”. Martín Asín, F. 1990. Editorial Paraninfo. Madrid.
  • “Astronomía amateur”.1991.Newton, J. y Teece, P. Editorial Omega. Barcelona.
  • “Estrellas y telescopios”. 1995. Arranz García, P. y García Martín, J. Equipo Sirius. Madrid.
  • “Guía de de las estrellas y los planetas”. 1989. Editorial Omega. Barcelona.

Web Pages.

- General:

- About Venus Transit:

- About Steppers:

- About Sun observation:



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