2 Photometry with the APT

The observations were carried out with one of the two 0.75 m APTs located at Washington Camp in Arizona, USA. Since 1996, the twin telescopes named "Wolfgang" and "Amadeus" are the property of the University of Vienna. A detailed description of these telescopes can be found in the paper by Strassmeier et al. (1997). In this paper it was reported that for variable stars, a photometric accuracy of $\pm$4 mmag in b and y had been achieved. This suggests that with some additional improvement of the accuracy, the telescopes could be used for the study of $\delta$ Scuti stars.

The configuration used for the present observations with the Wolfgang telescope is a Cassegrain system with a focal ratio of f/8. The detector system consists of a 800 $\times$ 490 pixel CCD used for centering the object. The actual data aquisition is performed with a blue-sensitive bi-alkali EMI-9124/QB PMT. The water-cooled tube is operated at a temperature of 3$^\circ$C and has a typical dark count of 20 cs^-1.

The outstanding advantage of using an APT for our three-star method is the telescope's speed. The acquisition of the navigation star, which is needed for finding the actual target group, is performed with a slew speed of 10$^\circ$ s^-1 at a general pointing accuracy of 30 arcsec. Once this star has been found, the time needed to slew between the stars of the group and to center the star is about 1 s, which is fast compared to the time the human observer needs for finding and centering the next object. For this reason the APT can be regarded as ideal instrument for time-series photometry, making it possible to obtain more than 300 separate measurements during a good night.

On the other hand, some problems specific to the APT were found to exist:

• Difficulties in finding and centering the objects, especially if the weather conditions are not ideal. The APT recognizes objects by their brightness. An average brightness value is given and the telescope measures and compares the objects in question with the preset value. If cloudy or hazy weather influences the brightness of a star, misidentifications may occur. Such problems are, of course, recognized from the data.
• No other stellar object within 0.5 mag (in Johnson B) of the target's brightness may be in the finder field (about 9 arcmin), as this may make it difficult for the centering algorithm to choose the correct target.
• Another problem concerns the lack of the clear error messages coming from the telescopes. This makes it hard for the observer to pinpoint the source of possible problems. For example, the message "Object not found" could be the result of bad weather, wrong input data to the telescope, technical problems of the telescope or its steering, or a number of other possible reasons.
• The present operating procedure of the APT limits the maximum hour angle to 4.2 hours. This means that the longest set of observations of 4 CVn could only cover slightly more than 8 hours.

These difficulties can be attributed to the operation program ATIS (Automatic Telescope Instruction Set) controlling the telescope. A new, improved version has been developed by NASA engineers and is currently undergoing the first tests at the APT.

The current limitations of the telescope render it unusable for certain applications such as observations in crowded fields, since the centering algorithm of the instrument might become confused. This does not present a problem for the present program reported in this paper. We also note that APT measurements should not be obtained within 30$^\circ$ of the Moon.

Between 1997 March 2 and May 8, more than 40 nights of photometric data of 4 CVn were collected with the Wolfgang APT telescope. 4 CVn and two comparison stars were measured through the Stromgren y and v filters. The data were reduced in the standard photometric manner. The comparison stars used were HR 4728 (G9III) and HR 4843 (F6IV), for which no variability had been detected during previous campaigns. Each observation consisted of three single 10-s integrations.

  

Figure 1: Extinction diagrams for the night of 1997 March 19. The diagram demonstrates the high accuracy with which the extinction coefficients could be determined due to the large number of measurements

A difference between measurements obtained automatically and manually by an observer lies in the handling of measurements obtained during marginal weather conditions. While an observer might stop or attach notes to marginal measurements, the quality of APT measurements need to be judged afterwards. This quality control is not available for the present program: instead we rejected all measurements for which the standard deviation of the three single 10 s integrations was $\geq$ 10 mmag. Furthermore, all nights or longer fraction of nights during which the standard deviation of the C1-C2 measurements was larger than 4.0 mmag were rejected. We were left with high-quality data covering 204 hours during 32 nights (see Table 1). Most of the observing time, which could not be used or kept after reduction, was due to unfavorable weather conditions rather than equipment problems.

  

Table 1: Journal of the APT measurements of 4 CVn

Table 1 also lists the extinction coefficients which were derived from the two comparison stars for each night. The agreement between the coefficients derived from the two comparison stars is demonstrated in Fig. 1 for the night of 1997 March 19. From the residuals of C1-C2 listed in Table 1, it can be seen that the night is of average quality. The data also show a small, systematic variation in extinction between the Eastern and Western hemispheres. This effect can also be seen on a few other nights, but with different signs. Consequently, a systematic East/West extinction difference cannot be demonstrated at this stage. These differences are too small to produce any detectable effect on the differential photometry of 4 CVn. We have also checked the extinction coefficients derived in this standard way by computing differential extinction coefficients between C1-C2. The agreement with the values determined in the standard way was excellent.

It might be interesting to look at the average extinction coefficients during the campaign. If we restrict ourselves to nights with $\geq$ 3.0 hours of observation and exclude the night of 970425, which clearly has higher extinction values, the extinction coefficients for the remaining 29 nights show a Gaussian distribution. The following average values are obtained:

k_y = 0.163 $\pm$ 0.021, and k_v = 0.353 $\pm$ 0.037.

These values are typical for the site. Further measurements are needed to judge whether or not the observed variations of the coefficients are significant.

  

Figure 2: Variation of brightness difference between the two comparison stars (normalized to zero). The error bars are based on the scatter of the individual measurements within a night. The slow variation is probably a consequence of instrumental variations of the APT and not due to small variability of one of the comparison stars (see text)

  

Figure 3: APT photometry of 4 CVn obtained during the 1997 APT campaign. $\Delta y$ and $\Delta v$ are defined to be the magnitude differences (variable-comparison stars) normalized to zero. The fit of the 19-frequency solutions derived in this paper is shown as a solid curve

Figure 2 shows that the magnitude difference between C1 and C2 exhibits a small, systematic drift over the nine weeks. The drift in v is about twice the amount in y. The most natural explanation would be small variability of one of the two comparison stars. Since the third star observed, 4 CVn, is not expected to show such slow variability, it should be possible to determine which of the two comparison stars might be variable. An examination of the zero-point residuals of 4 CVn (after the fits in the next section) shows similar drifts in both V-C1 and V-C2, but with different amplitudes. This suggests an instrumental origin of the drifts.

We note the possible existence of low-frequency stability problems of the APT in the millimag range, which do not seriously affect the reductions of the much more rapidly varying star 4 CVn. For the reduction of 4 CVn, we have represented the drift between the two comparison stars by two different values of (C1-C2) in each filter (shown as straight lines in the figure.) The two-week gap in the middle of the observations were caused by poor weather conditions, rather than equipment problems. Consequently, the agreement between the observing gap and the drift maximum is probably accidental. The power spectrum of C1-C2 shows a peak of 0.7 mmag at 0.036 cd^-1 in v and 0.7 mmag at 0.79 cd^-1 in y. Consequently, any detected low frequencies in program stars observed with the APT should be checked whether they are shown in all filters and relative to all comparison stars.

The following residuals were found for C1-C2: $\pm$2.7 mmag for y and $\pm$3.0 mmag for v per single measurement.

The light curves of 4 CVn are shown in Fig. 3.