------------------------  Technical Note n°4  PHEMU  November 2, 2002  ------------------------

 
1. Introduction

The goal of the observations of mutual events of the Galilean satellites is to record fast variation of the light received from these satellites. The receptors which are well adapted to this type of observation are the fast photoelectric photometers. However, the two-dimensional receptors such as CCD video cameras or TV tube cameras are interesting for the recording of these events. Especially for amateurs, they allow to record easily the events. The present note does not describe the methods to be used for the reduction of the observations but provides indications and help in order to make observations with this type of camera. The use of analogic-digital converters such as the one developed at IMCCE (system AVIA) will be necessary for the reduction.
 

2. The material

The making of the recording of a mutual event needs to be careful and to follow some regulations which will permit to obtain a useful record leading lately to useful data for the study of the dynamics of the Galilean satellites. The first thing is the camera which should produce a video composite signal black and white (CCIR norm) or a colour signal PAL, NTSC or SECAM. The second material is the VCR recorder. It is also necessary to have a microphone connected to the VCR allowing to record what happens during the observation and the signal of an audio clock (cf. figure 1).

a) The camera

Any video camera may be used but it is preferable that it is not equipped with an automatic gain not removable. This would make difficult the photometric analysis of the signal. The CCD cameras are now used world-wide. They are smaller than the old TV tube cameras and more reliable either for the photometry or for the geometric characteristics of the targets. We should mention here that we are speaking of uncooled CCD cameras. This is not a problem because of the fast lecture of the target thanks to the CCIR norm (50 half-images interlaced per second analogically recorded by the VCR). The video camera must but settled on the telescope directly at the focal plane. It is preferable to avoid any optical system between the telescope and the camera. However, if the field is too large due to a too short focal length of the instrument, it may useful to add lenses in order to increase the focal length and to have on the image well separated satellites and to avoid the presence of the planet Jupiter in the field. If the satellites are too bright, a filter may be used (cf. paragraph 3.a).

b) The timing of the events

As it has been recommended in previous technical notes, it is necessary that an observation be correctly related to the Universal Time within 0.1 to 0.2 second of time. The best way for that is the use of an image clock incrustator which will be carefully settled to the Universal Time and to verify at the end of the observation that it is always correct.

c) The VCR recorder

A VHS standard VCR (with clean heads and brand new video tapes) allows to make a useful record. Some improved video standards (S-VHS or Hi-8) or professional (U-Matic) allows to get a record of better quality. Short tapes (1 or 2 hours long) are less fragile. The black-andwhite position should be used when it exists, otherwise use the PAL mode. Note that a colour camera has no interest. The S-VHS, Hi-8 or U-matic formats may enhance the resolution using the black-and-white position.

d) The monitor

A TV screen monitor will be connected to the output of the VCR in order to watch the observation which will be recorded. It is useful to mark on the screen the position of the implied satellites in order to keep them always at the same place on the target by driving the telescope each time it is necessary as we do with a classical photometer in order to keep the satellites in the diaphragm. Then, the reduction will be more easy.
 

3. The observation

The technical notes PHEMU describing the photometric techniques of observation (note n°3) should be read carefully. The principles have to be applied with any receptor, even video cameras. It is necessary to follow the same regulations in any case.

a) Choice of the gain and/or the sensitivity

The recording must be made in a linear zone of the sensitivity which is normally the case with CCD cameras. However, the observed objects must never saturate the target.
If the camera is equipped with a gain controller, it is important to choose a level for which the saturation will never be reached for the satellites during the event. Be careful when Jupiter is after its rise and when the absorption is continuously decreasing. Be careful also to the sky background when the Sun is just before its rise. In the case where it is not possible to choose the sensitivity (this one should however be kept constant during all the observation) one will use filters in order to avoid the saturation. One should try the different
possibilities before the observation of the events. Simulations should be made several days before the events mainly for events occurring during twilight. Do not forget that the variation of the sky background is exponential. The use of a R or I filter is very interesting by decreasing the brightness of the sky.

b) The timing of the observation of a mutual event

- Reference objects :
During a mutual eclipse, only one satellite is implied : in the field, the eclipsing satellite will be visible most of time. It will be interesting to keep it in the field in order to have a reference object available in the field and allowing to correct for a variable absorption. In the case of an occultation (or if the eclipsing satellite is too close or too far from the eclipsed satellite) both occulting and occulted satellites will be observed together. Before and after the occultation, they will be observed separately but, during the occultation, it will be interesting to put in the field a third satellite which will be the reference object. If this configuration is not possible, two cases could occur : first, the event is a short one (less than 15 minutes). Then the reference object will be recorded only before and after the event. Second, the event is longer (more than 15 minutes) ; then it will be possible to move the telescope every 5 or 10 minutes towards the reference object if this one is not too far (it will be necessary to find the objects rapidly in order to loose as less event as possible).
- Duration of the observation :
It is important to record enough signal around the date of the event provided in the predictions. The analysis of the signal before and after the event allow to appreciate the quality of the signal and to proceed to a better reduction. If the duration of the event is « N » minutes and if the observing conditions are good (twilight, elevation above the horizon), then, the observation must be as follows :

c) Important items

Keep in mind that the Universal Time must be recorded during all the observation in order to date each image. An audio signal may be recorded through the microphone during all the observation. If the time signal is recorded from time to time (not continuously) be careful to do not stop the VCR even during the move of the telescope from one object to another, since the timing of the images will come from the VCR. If the VCR must be stopped for any reason, note it and record again the Universal Time.
 

4. The reduction of the observations

Even the stability of the observations made using a video receptor and recorded on VCR is not of a high level, the reduction of the observations may lead to very good and useful results. The main problem will come from the gain of the receptor which may be not linear. This could be checked: a reciprocity function may be built either with solar-type stars or with artificial sources well calibrated in a laboratory. Such a function should be available for the reduction in order to rebuild the true light curve of the event with the real magnitude drop. Then, the reduction will be made according to paragraph 3 of Technical Note n°5.
 

5. Other observations

A two-dimensional receptor allows to make another type of interesting observation as follows :
- close approaches (appulses) between asteroids and stars leading to occultations are observable anytime. Each year more than 30 occultations may be observed from a given site. The goal is to observe stellar occultations which, contrarily to mutual events, are observable only from restricted geographic areas since the prediction of such events is not accurate. These observations allow to get data providing information on the structure and the shape of the asteroids. These observations are performed simultaneously by several observers through a network such as EAON in Europe . Observations made using a video camera provide images and, if the magnitude of the observed objects is sufficient, they allow the interpretation of the observations made by the other observers of the network observing most of time with a photoelectric photometer or with the naked-eye.
 

6. Conclusion

The two-dimensional receptors such as the video cameras will provide very useful observational data of the mutual events. However, the observers must be sure that the recommendations provided in the present note are strictly applied. Then, the photometric accuracy of the data will have a good level.
The AVIA system developed at IMCCE/Bureau des longitudes is built for the reduction of video observations. The observers may contact us for more details on the reduction procedure.
Examples of light-curves obtained through video cameras are available on the server of IMCCE/Bureau des longitudes in the catalogues of the observations made during the previous campaigns and in the Technical Note n°8.