3 The observations

The STACC 1998 campaign on BN and BV Cnc is the first large-scale photometric campaign based (mainly) on the differential CCD photometry aimed at $\delta$ Scuti stars. Using CCD cameras one can observe both stars simultaneously.

Ten sites participated in the photometric observations, which took place from January to April 1998. A variety of instrumentation was used and the number of nights allocated varying from a few nights to several weeks.

The number of useful nights was considerably lower than expected due to an unusual bad observing season at several sites. In Fig. 1 the nights with data are shown schematically for the participating sites (photometry only). In addition, spectroscopic data were obtained at five observatories. The main part of observing took place in February 1998. The distribution of sites did not provide a good 24-hour coverage. In Table 1 the instrumentation and observations are described in detail.

Figure 1: Distribution of the photometric observations of BN Cnc carried out during the 1998 STACC campaign. Five consecutive nights are shown in each row.

 

 

Table 1: Details on the observations and instrumentation.

	Telescope		Filter(s)	Field of	Nights	Hours
Observatory	diam. [cm]	Detector	used	view	observed	obtained	Remarks
Uttar Pradesh (India)	100	CCD	V	6.6$^\prime$ $\times$6.6$^\prime$	6	15.5	guiding problems
Konkoly (Hungary)	100	CCD	V	5.3$^\prime$ $\times$4.6$^\prime$	12	57.0
	60	CCD	V	27$^\prime$ $\times$ 18$^\prime$	5	17.8
Bia ków (Poland)	60	CCD	V	6$^\prime$ $\times$4$^\prime$	8	32.0
Teide (Spain)	80	CCD	V	7.3$^\prime$ $\times$ 7.3$^\prime$	9	59.0
Sutherland (S. Africa)	100	CCD	$v,b,y,I_{\rm C}$	3.4$^\prime$$\times$ 3.1$^\prime$	10	22.3
ESO, La Silla (Chile)	90	CCD	y	3.9$^\prime$ $\times$3.9$^\prime$	12	11.5
Vienna APT (AZ, USA)	75	PM	v,y	--	7	46.4
La Palma (Spain)	100	CCD	V	5.6$^\prime$ $\times$5.6$^\prime$	3	7.2
Haute Provence (France)	80	CCD	V	6.4$^\prime$ $\times$6.4$^\prime$	2	5.2	overexposed
Odessa (Ukraine)	50	CCD	V	20$^\prime$ $\times$ 20$^\prime$	2	2.6	high noise

All sites except the Vienna APT in Arizona were using CCD cameras, but with quite a range in field of view and filters. A Johnson V filter was used at the majority of sites. At some sites (Sutherland, La Silla, Arizona) the observations were carried out in other filters (Strömgren vby, Cousins $I_{\rm C}$). The differences in fields of view are best illustrated by displaying the two extreme cases. Figure 2 shows a CCD frame obtained at Sutherland, while Fig. 3 features a much larger field representing a frame from the 60-cm Schmidt telescope at Piszkésteto in Hungary. The latter clearly supplies a much larger set of reference stars for the two variable stars. The two variables both have close fainter stars, which in most cases are used as reference stars. These fainter stars are uncomfortably close, as a certain defocusing was needed to permit exposure times that give a decent duty cycle.

Figure 2: A sample CCD frame of BN and BV Cnc field observed with the 1.0-m telescope at Sutherland Observatory. The two variables are labeled. North is up, east to the left.

Figure 3: A sample CCD frame observed with the 60-cm Schmidt telescope at Konkoly Observatory.

Some additional details on the observations at different sites are given here in separate paragraphs.

Uttar Pradesh State Observatory (UPSO).

The data were obtained on 6 nights out of 20 allocated, which represent 15.5 hours of data. The longest set for one night covers 4.5 hours. Unfortunately, the frames do not carry a lot of weight in the total data set due to one failure in meeting the requirements for the campaign. The guiding failed to keep the two stars within a few pixels, and large flat-field effects are seen due to a drift of up to 30 pixels on a single night. This is particularly disappointing due to the few observatories joining the campaign at non-European longitudes.

Konkoly Observatory, Piszkésteto.

Data from this observatory come from the 1.0-m RCC telescope, where around 6 weeks of observing time was allocated, and from the 0.6-m Schmidt telescope, where a smaller allocation was obtained late in the campaign. In terms of the climate this is not an outstanding site, but data were collected on 12 nights with the 1.0-m telescope and 5 nights with the 0.6-m telescope.

The large field of view and the corresponding larger set of bright comparison stars obtained with the Schmidt telescope has the effect that better results are obtained with the 0.6-m than with the 1.0-m telescope (see later in Table 4). For the data coming from all other telescopes in the campaign equipped with CCD, stars fainter than the target stars BN and BV Cnc had to be used as comparisons.

Biaków Observatory.

The data were obtained on 8 nights. The results are of good quality and comparable to results obtained with larger telescopes proving that even a small telescope can make a significant contribution.

Observatorio del Teide.

Thirteen nights were allocated and data collected on 9 nights. The lost nights were all at the beginning of the observing run. The data are of good quality, but effects of bad seeing and drifting clouds on some nights are traceable in the time series deduced from the CCD frames.

Sutherland Observatory.

This site has one of the largest telescopes used in this campaign (1 m), but the field of view of its CCD camera is the smallest among all sites (Fig. 2). In consequence, only one comparison star could be used for these observations. Two weeks of time were granted and data collected on 10 nights. This illustrates a good climate at Sutherland, which is considered among the best photometric sites.

Observations were carried out in three Strömgren filters v, b, yplus the Cousins $I_{\rm C}$ filter.

The best signal-to-noise (S/N) was obtained in the v and b bands, where a larger amplitude more than compensates for the smaller number of photons. This is so much more true, because systematic effects dominate the noise budget making it almost colour independent.

European Southern Observatory (ESO).

The 0.9-m Dutch telescope with a 1k $\times$ 1k CCD was used. Time was available a few hours on selected nights during another observing program, and data were collected early in the campaign on 12 nights. The longest set is only 2 hours long.

Weather conditions were variable and the quality varies quite a lot from night to night. Observing took place at large air masses and the telescope did not produce nice ringformed images when defocused, making the photometry difficult.

Vienna APT, Arizona.

The observations were carried out with one of the two 0.75-m automatic photoelectric telescopes (Vienna APT's) located at Washington Camp in Arizona, USA. The data aquisition was performed with a blue-sensitive bi-alkali EMI-9124/QB photomultiplier and Strömgren v and yfilters. The water-cooled tube is operated at a temperature of 3 $^{\rm o}$C and has a typical dark current of 20 counts s^-1. A detailed description of the APTs and the attached equipment can be found in the paper by Strassmeier et al. (1997).

The telescopes have been used before in the campaigns on other $\delta$ Scuti stars with the three-star technique of alternating observations of the variable star and two comparison stars. This technique allows for a check of the accuracy of the measurements. For the star 4 CVn, Breger & Hiesberger (1999) found a precision of 3.0 mmag in y for all three stars. For a 9th magnitude star, BI CMi, the precision decreases to 3.8 mmag per single observation under good weather conditions.

The whole month of February was allocated, but due to the poor weather the data were taken on only 7 nights and only for a substantial part of the night on 5 nights. This is much below normal.

As this is one of a few sites off European longitudes this is again disappointing and to some extent makes the multisite character of the campaign less useful.

The quality of the data varies dramatically with weather conditions and is overall somewhat more noisy than the CCD photometry.

Observatorio del Roque de las Muchachos.

Time was allocated fairly late in the campaign on the JKT telescope at La Palma. Six partial nights (3 hours each) were at disposal, and useful data were obtained on half of them.

Observatoire de Haute Provence (OHP).

At OHP some test observations were spent on two nights observing BN and BV Cnc with a CCD camera using a new CCD controller. The controller is more advanced than the usual CCD controllers and permits to read out only windows around selected stars, which are then reorganized in a vertical stack. One example is shown in Fig. 4.

Figure 4: An example of an OHP image of the program stars. The stars are repositioned one above the other with the two target stars at the top and two reference stars at the bottom.

Unfortunately, due to the trial nature of the observations, the variable stars were slightly overexposed and although some decorrelation was attempted, it did not bring the noise down to levels, where the data contribute significantly to the final result.

The windowing technique is interesting, because the duty cycle is higher than for other sites, and there is no reason to believe the results had been less good than from full frame observations had the overexposure not happened.

Odessa Astronomical Observatory.

Another couple of nights were added from the Ukrainian site. The data are of lower quality than those from the sites providing the main part of the data.

3.1 Summary of observations

Only at a few sites was the number of clear nights a large fraction of the allocated time. This is often the case for northern observatories, especially in winter, except for the permanently good sites like the Canarian Islands or Hawaii. The Vienna APT site was considerably below normal owing to bad weather conditions caused by the El Niño.

It would clearly make this type of campaigns much more manageable, if telescopes and CCD cameras could be remotely operated in automatic or semi-automatic mode. In addition, telescopes should be situated at sites with a good photometric climate.