Non-Periodic Phenomena in Variable Stars
				      IAU Colloquium, Budapest, 1968

   SYNCHRONOUS THREE COLOUR STELLAR PHOTOMETRY AT THE
	    CATANIA ASTROPHYSICAL OBSERVATORY


	     S. CRISTALDI and L. PATERNO
 Astrophysical Observatory of Catania, Italy


		     SUMMARY

A synchronous three colour stellar photometer using a single 
photomultiplier has been constructed at Catania. In this communication 
the characteristics and the efficiency of this photometer are briefly 
described. At present the instrument is used for simultaneous UBV 
photometry of flare stars. A graph of simultaneous measurements in the 
UBV system of a flare of EV Lac is shown. A more detailed description of 
the instrument had been published elsewhere (Cristaldi and Paterno 1968).


		    INTRODUCTION 

During the last few years various astronomers have emphasized the 
importance of simultaneous observations in different colours in stellar 
photometry. Besides, multicolour and simultaneous automatic photometry 
is indispensable in the study of fast phenomena, in general, and of 
stellar flares in particular. Finally, the use of this kind of 
photometry makes the observer's work easier and permits a uniform 
presentation of the data to equipments for digital measurements. For 
these reasons we constructed a synchronous photometer which executes 
simultaneous measurements within the UBV system. Unlike other 
multichannel photometers it uses only one photomultiplier. Fig. 1 shows 
the block diagram of the apparatus.
The light beam from the telescope, after having crossed the focal-plane 
diaphragm D is choppered by a filter-carrying disc, rotating at 900 
revs. per minute. The disc has three round windows 120 deg apart, in which 
there are three filters: one UG1 (1 mm), one BG12 (1 mm) + GG13 (2 mm) 
and one OG4 (2 mm). The photomultiplier is of type EMI 6256 S.

  

    Fig. 1. Block diagram of the photometer.

The output of the photomulitiplier is connected by means of an impedance 
matcher (I.M.) to a d. c. amplifier which branches into two outputs; one 
is connected to an oscilloscope (CRT) for monitoring signals, the other 
is connected to a gate which acts in synchronism with three filters 
through the transducer S. The signals obtained through a given filter 
always exit through the same gate channel.
In our case we have three channels, the output signals of which are the 
UBV signals of our system. Each output gate is connected to an integrator 
circuit (I_1, I_2, I_3) whose output is measured in one of the three channels 
of a recording potentiometer (R) which records every six seconds.
Regularity in filter rotation and therefore in the synchronous signals for 
the gate is assured by a stepping motor (SM), powered by an amplifier (PA) and 
a regulating oscillator (OSC).


	    OPTICS OF THE PHOTOMETER

The optical part of the photometer is shown in Fig. 2. The focal-plane 
diaphragm D consists of a slide with three holes 1, 2, 3 mm in diameter.
The field lens is of fused quartz: diameter 20 mm and focus 70 mm. 
The lens is located approximately 70 mm from the photocathode of the 
photomultiplier (F), so that with a f/10 reflector the diameter of the 
luminous disc which is formed on it is approximately 7 mm in size. The 
rotating disc with the three filters is located between the field lens 
and the photocathode.
Two small optical devices permit monitoring, respectively the pointing 
of the instrument and the inserted filter, if one desires to make continuous 
measurements using still filters.

  

       Fig. 2. Optics of the photometer.

The device (S) consists of a small cylinder mounted on the same shaft as 
the filters' carrier-disc. This cylinder is internally lighted and its 
inside walls are reflective; besides, it is cut transversely with three 
slots 0.7 mm wide and of such a length that each subtends, with respect 
to the axis of the cylinder, the same angle at the center as the 
corresponding filter in the disc. The light paths that emerge from the 
three slots are intercepted, at a fixed position, by three photodiodes 
F_1, F_2, F_3, one for each window opening for the entire time that the 
corresponding filter passes under the diaphragm opening. The signals 
from the photodiodes pilot the gate.


       3. THE ELECTRONICS OF THE PHOTOMETER

The output of the photomultiplier is connected to an impedance matcher 
constructed with field effect transistors (Paterno 1967). At the matcher 
output, the voltage-pulses are amplified by a D.C. amplifier with 
variable gain from 1 to 1000. The amplifier consists of a cascade system 
of 5 identical operational amplifiers having strong negative feedback.
The UBV pulses at the amplifier output are selected and led to the 
proper channel by means of three photodiodes operated gates. The gates 
consist of a mercury relais type Clare HGSM 51111LOO.
All signals relative to each channel are detected and then, via integrator 
and differential amplifier, activate the recorder.


       4. CHARACTERISTICS OF THE PHOTOMETER

It must be noted, above all, that the chopping of the radiation does not 
change the signal-to-noise ratio, so taking into consideration that the 
system of the amplification practically eliminates all the noise of the 
electronic apparatus, it is possible to reach, integrating on convenient 
time intervals, the same limit magnitude which is reached utilizing all 
the incident radiation.
The accuracy of the measurements, as known, is proportional to the total 
sum of the available radiation for each measure. Neglecting the radiation 
coming from the sky with respect to the signal, the average error (a.e.) of a 
measurement is given by the formula:

		 a.e. = +-(1/nqt)^1/2

where n is the number of the photons due to irradiance of the star collected 
for a unit of time, q is the effective efficiency of the receiver and t is 
the exposure time.
At present the photometer is placed at the f/10 quasi-Cassegrain arrangement 
of the universal 61 cm reflector.
Considering a star of 10m, the number of photoelectrons obtained from the 
collected photons is about 10^4 sec^-1 (Allen 1963). Therefore, with an 
exposure of 1 sec the average error is about + - 0.01m for one measurement; 
averaging 10 measurements the error is reduced to + - 0.001m. In our photometer 
the exposure time of each measurement in one band is 0.5 sec. However, as it 
was noted, the gate circuit is very prompt in action, it seems possible not 
only to increase the exposure time up to 1 sec, but even to increase it, gaining
in the efficiency of the photometer which at present utilizes only 25 per cent 
of the incident radiation.
We have calculated that the magnitude limit which can be reached with 
our photometer, applied to a telescope of 61 cm, is 12.5m; this result 
was verified by the observations. At present, the photometer works with 
the 61 cm reflector and is used for a research programme on flare stars.
Fig. 3 shows three photograms of oscilloscope tracings. They show the 
different responses of the photometer in the UBV for three stars of 
spectral types B6, G2 and K5 respectively (from the top to the bottom).
Finally, Fig. 4 shows a flare of the star EV Lacertae, observed with our 
apparatus on 8 August 1968 at 23h23m, Universal Time.

  

Fig. 3. The response of the photometer for B6, G2 and K5 spectral type stars 
	  (from the top to the bottom).

  

Fig. 4. Simultaneous light curves for a flare of EV Lac on 18th August 1968 
		      in the UBV system


		     REFERENCES

Allen, C. M., 1963, Astrophysical Quantities, 2nd edit. Univ. of London, 
London. p. 191-192.
Cristaldi, S. and Paterno, L., 1968, Synchronous three colour stellar 
photometer (in press) Mem. Soc. astr. Ital., 39.
Paterno, L., 1967, Mem. Soc. astr, Ital., 38, 555.


		   DISCUSSION

Detre: I should like to hear something about your observing conditions, 
the location of your observatory, number of clear nights, etc.
Cristaldi: The Catania Astrophysical Observatory has its stellar station 
on the Etna at about 1700 m of altitude. (We have a conventional 
reflector of 91 cm d ; 1 universal telescope Schmidt-Quasi-Cassegrain 
61/41 cm f/3 and two others telescopes 30 cm d in Cassegrain 
arrangement.) In the summer all our instruments work for photoelectric 
photometry, but in the winter the Universal telescope in Schmidt 
combination is used for photography, its limit is about 17.5m. In the 
summer we have more than 60-70 clear nights for photoelectric work.