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I.Kulyk, K.Jockers Photometric magnitudes of the small inner Jovian moons.

Description:
The observations of the small inner Jovian satellites Thebe, Amalthea, Metis were obtained with the 2m Zeiss RCC telescope at Terskol Observatory (Pik Terskol, Northern Caucausus, long=42.50083 degree,lat=43.27427deg, h=3100 m) between October 1999 and January 2002. The Two-Channel Focal Reducer of the Max-Planck Institute for Aeronomy (MPAe, Germany) was used for the acquisition of the images.

The device has two CCD detectors. The beam from the Cassegrain focal plane is recollimated and splitted by the color divider. The long-wave part of light is transmitted into the "red" channel and the short-wave part is reflected into the "blue" channel. 576x385 array of the "red" channel covers a sky projected area of 7.8 x5.2 arcmin. The plate-scale is about 0.81" per pixel. 512x512 array of the "blue" channel covers an area of 7.8x7.8 arcmin, the plate scale is about 1.01" per pixel. Both channels allow interference filters. A methane band filter centered at 887 nm with FWHM=28.5 nm was employed. Although Jupiter's disk is comparatively dark at methane band,it was still necessary to block the light from the planet and the Galilean moons. We put masks containing black occulting areas in the focal plane of the telescope. The width of a mask covering Jupiter was about 77 arcsec. We placed Jupiter's image near the western or eastern edge of the glass to observe the satellites at the corresponding elongation. A Lyot stop suppressed Jupiter's diffraction pattern.

Twilight sky was used for flat-field frames. The integration time was from 180 to 300 s for the scientific images. Every clear night, the spectrophotometric standards HR718,HR1544,and HR8634 were observed at different air masses in order to make an absolute photometric calibration as well as to measure an extinction coefficient. Although the telescope was locked on guided stars, the images of the satellites were not trailed for the nominal integration time about 240 sec. The standard reduction procedures we applied to each frame consisted of the electronic bias subtraction and the flat-field correction. We produced a mean flat-field image for each observational period summing appropriate frames taken from all over the set.

The problem of scattered light from the planet was paid much attention due to the proximity of the small inner moons to Jupiter. Special routines were developed for effective background subtraction. Aperture photometry with a circular aperture was performed on each image of the standard stars and the satellite. The sky background was estimated based on counts within a concentric annulus centered on the star image. Since the difference between the standard stars and the satellites is about 10 magnitudes, we had to use the different aperture sizes. 20-pixel size aperture was selected for the standard stars while the 5-pixel, 5-pixel and 3-pixel radii were chosen for Amalthea, Thebe and Metis respectively. To take into account the difference in the instrumental magnitudes due to the aperture sizes we used the field secondary stars. We integrated the secondary stars' fluxes over the different aperture sizes and corrected the fluxes of the satellites to find their large-aperture instrumental magnitudes.

The extinction coefficient was calculated for each observational set. The observations of the spectrophotometric standard stars agree well within each observational set and indicate that our calibration uncertainty is about 4 %. To perform absolute photometry of the satellites we compared the integrated brightness of each satellite with that of the standard stars. The satellite magnitudes were reduced to the magnitudes at unit distances from the Earth and the Sun.

    
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 File Summary:

 File Name         Lrec      Records      Explanations
 c5_des.htm        106         .          This file
 File c5_dat.htm:    57        231        Photometric magnitudes
                                          of Amalthea
 File c14_dat.htm:   57        217        Photometric magnitudes
                                          of Thebe
 File c16_dat.htm:   57         98        Photometric magnitudes
                                          of Metis
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   Bytes       Format  Units   Label            Explanations
   1-5         I5      year     year        Date of observation  (year)
   6-8         I3      month     m          Date of observation  (month)
   9-11        I3      day       d          Day of  observation  (day)
   12-14       I3      hour      h          Time of observation in UT (hour)
   15-17       I3      min       m          Time of observation in UT (min)
   18-20       I3      sec       s          Time of observation in UT (sec)
   21-28       F8.1    sec      exp         Exposition time (sec)
   29-36       F8.3   airmass   air         Air mass (airmass)
   37-43       F7.2    mag      mag         Mag reduced to unit distances from the Earth and the Sun (mag)
   44-51       F8.2   degree    long        Orbital longitude of the satellite (deg)
   52-57       F6.2   degree    phas        Solar phase angle (deg)
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Photographic Survey of the Northern Sky (Фотографічний огляд Північного неба - ФОН)

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L.Pakuliak. Questions, remarks and propositions are welcome.