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Astron. Astrophys. 345, 760-768 (1999)

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2. Observations and data reduction

We have observed OJ 287 using the 2.56 m Nordic Optical Telescope (NOT) at Observatorio del Roque de los Muchachos (La Palma, Canary Islands), the 40 cm Automatic Imaging Telescope (AIT) at the Perugia University Observatory (Perugia, Italy), the 1.5 m Telescope at the Observatorio Astronómico Nacional (San Pedro Mártir, Baja California, México), the 1.05 m REOSC Telescope at the Torino Observatory (Torino, Italy), and the 1.03 m Telescope at the Tuorla Observatory (Piikkiö, Finland). The radio observations were made at the Metsähovi Radio Research Station using the 13.7 m radio telescope. The number of observations obtained at each observatory are listed in Table 1.


Table 1. The number of observations from each observatory from Fall 1993 to Spring 1998.

2.1. Observations at the Nordic Optical Telescope

2.1.1. Photometric observations

We have made photometric observations of OJ 287 using three different CCD cameras: the StanCam camera, the Andalucia Faint Object Spectrograph (ALFOSC) camera, and the High Resolution Adaptive Camera (HiRAC) camera. The StanCam CCD has a 1k SiTe (TEK1024) chip with 0.176"/pixel and 24µm pixels. The ALFOSC CCD has a 2k Loral (W11-3AC) chip with 0.189"/pixel and 15 µm pixels. The HiRAC CCD has a 2k Loral (W14-2AC) chip with 0.110"/pixel and 15 µm pixels.

Most of the observations have been made using UBVRI filters with standard Johnson/Kron-Cousins responses. Some observations (27 February 1998, NOT) have been made with Gunn R filter, because the Kron-Cousins R filter was temporarily unavailable. The CCD frames were reduced (de-biased and flat-fielded) using standard techniques with the IRAF-package; the magnitudes were determined from relative photometry using the standards by Smith et al. (1985). In most cases we used star 10 as the comparison star, but sometimes it was saturated so we used star 11 instead (Smith et al. 1985). The integration times varied from 30 to 120 seconds. We invited observers at the NOT telescope to monitor OJ 287 during January and February 1998, in addition to our three observing runs.

2.1.2. Polarimetric observations

The polarization measurements were made using the Turku photopolarimeter (5 January 1998) and the ALFOSC CCD camera (20 January 1998).

The Turku photopolarimeter is a double beam chopping photopolarimeter that uses four dichroic mirrors and five photomultipliers to achieve simultaneous measurements in the UBVRI bands. The filters give Johnson (UBV) and Kron-Cousins (RI) responses. A more detailed description of the instrument can be found in Korhonen et al. (1984), and the observation and data reduction methods have been described in Takalo et al. (1992).

We made a total of 32 single observations with a 10 s integration time (for each of the eight positions of the calcite crystal). This gives us a total of 43 min of observation time of OJ 287. The observations were averaged due to poor conditions. The R and I channels were unavailable during our observations.

With the ALFOSC CCD camera, we used imaging polarizing optics consisting of a calcite crystal and a half-wave retarder plate to get two images of the object on the same frame. We used the calcite crystal in [FORMULA] and [FORMULA] orientations to obtain two of the normalized Stokes parameters (Q/I and U/I) using differential photometry, and from them we calculated the degree of linear polarization and the polarization position angle. The CCD frames were reduced using standard reduction techniques with the IRAF package. The filters used were the same as in the photometric observations. The integration times varied from 60 to 300 s depending on the band.

We have also observed zero and high polarization standard stars to correct for instrumental polarization and to get the zero point correction for the polarization position angle (Schmidt et al. 1992).

2.2. Observations at the Perugia Observatory

The Perugia photometric observations were carried out with the 0.4 m Automatic Imaging Telescope (AIT) (Tosti et al. 1996). The integration times during the exposures varied from 4 to 6 min depending on the brightness of the object. Data reduction was performed with locally-developed software packages, which apply all the standard corrections required (bias and dark subtraction, flat field correction) and the instrumental magnitudes computation. The [FORMULA] magnitudes of the comparison stars in the field of OJ 287 have been published by Fiorucci & Tosti (1996).

2.3. Observations at San Pedro Martir (SPM)

Observations were carried out from January 26 to 31 (UT) with the 1.5m (f13.5) Telescope at SPM. The detector was a Tektronix TK1024AB CCD. This chip has [FORMULA] pixels, each [FORMULA] µm, (the scale of plate of the system is [FORMULA]/pix).

Broad band B,V (Johnson) and R,I (Cousins) filters were used in this run. The reductions were made using IRAF in the standard way (bias subtraction, flat-fielding and cosmic ray correction). The photometry was done using IRAF/APPHOT with an aperture of [FORMULA]. Details will be found in Dultzin-Hacyan et al. (1999).

2.4. Observations at the Torino Observatory

The observations were made with the 1.05 m REOSC telescope equipped with a [FORMULA] pixel CCD camera with a [FORMULA] per pixel scale. Standard BV (Johnson) and R (Cousins) filters were used with integration times varying from 240 to 420 s depending on the band. Data were reduced by using the locally-developed Robin procedure, including bias subtraction, flat field correction and circular Gaussian fit after background subtraction. For more details see Villata et al. (1997).

2.5. Observations at the Tuorla Observatory

The observations were made using the 1.03 m telescope at the Tuorla Observatory equipped with a [FORMULA] pixels ST-8 CCD camera. Only a V filter with standard Johnson response was used. For more details, see Katajainen et al. (in preparation).

2.6. Observations at the Metsähovi Radio Research Station

The radio observations were made at the Metsähovi Radio Research Station, as part of an ongoing long-term project to monitor extragalactic radio sources. The monitoring started in 1980 and now includes over 80 sources, which are mostly flat-spectrum northern sources. The observations have been made at two frequencies, 22 and 37 GHz, using a dual-beam method on the 13.7 m Metsähovi Radio Telescope. The fluxes are relative fluxes calibrated against DR21 according to the flux-scales in Baars et al. (1977). The data presented here are weekly averages. The observational techniques are described in more detail in Teräsranta et al. (1992).

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© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999