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Astron. Astrophys. 338, 479-490 (1998)

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2. Observations

The vast majority of the observations has been performed with CCD-equipped telescopes with sizes ranging from 0.6m to 1.2m. Although the techniques for high precision photometric work using photo multipliers have become very refined (e.g. Young et al. 1991), such work has concentrated mainly on objects significantly brighter than CM Dra, which is magnitude 11.07 in R-band. With one - or two - channel photometers requiring frequent sky or standard-star observations, their duty cycles are relatively low. The use of CCD cameras allows the simultaneous observation of several reference stars in the same field as CM Dra, and the duty cycle is only limited by the time to read out the CCD and to save the image to disk, both together being on the order of one minute. This allows tracing the lightcurve with measurements spaced 2-4 minutes apart. The close spacing of measurements is important in order to recognize observed brightness variations as potential planetary transits. Kjeldsen & Frandsen (1992) have demonstrated the feasibility of the use of CCD's in high-precision time-resolved photometry, and emphasized their usefulness for the study of low-amplitude variables (see also Gilliland & Brown 1988). In addition, occasional small-scale variations in atmospheric transmission properties (for example, atmospheric density waves) that could appear as planetary transit events in conventional photometry can be isolated with a wide-field CCD photometric system, using a significant number of comparison stars within the field. We would also like to emphasize CCD's relative ease of use, as the simultaneous observations of reference stars allows one to leave the telescope pointed towards CM Dra throughout a whole observing night, which is a tremendous advantage on simpler telescopes without computer control.

A list of the properties of the CCD systems used at the various telescopes is given in Table 2. Only at Kourovka Observatory a two-star photometer was employed. In all cases - except as noted - observations were taken through a standard R filter. The CCDs had a field of view of at least 5' side-length, which allowed us to observe within the images of CM Dra at least 5 reference stars simultaneously - these were normally the stars numbered 1, 4, 15, 16 and 17 in Fig. 2. Exposure times at the various telescopes ranged from 30 to 300 sec, depending on the telescope's size and the dynamic range of the CCD (see Table 2). A maximum exposure time of 300 sec was set to acquire lightcurves with sufficient temporal resolution. At the two-star photometer at Kourovka Observatory, star HD 150172 was observed simultaneously as a reference. This star is very similar in brightness to CM Dra, and photometric counts were recorded every 128 seconds.


Table 2. CCD systems [FORMULA] is the typical exposure time used for a CCD image of CM Dra 2 The Duty fraction is the fraction of time the camera is collecting light while observing the object. It is given by: [FORMULA], where [FORMULA] is the CCD read-out time, and [FORMULA] is the time to save an image to the disk. 3 Different cameras or settings were used at these telescopes through the years 4 A nonlinearity not exceeding 140 e- was corrected from the CCD's linearity calibration curve. 5 Nonlinearity was corrected using precints by Deeg and Ninkov (1996) 6 Used CCD camera in [FORMULA] bin mode 7 This CCD system is described in Dunham (1995). Duty fraction of 1.0 results from use of CCD frame-transfer mode

[FIGURE] Fig. 2. Field of CM Dra with the most commonly used reference stars marked. N is up and E is left. The side length of the field is 7.2 arcmin

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© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998