Astron. Astrophys. 348, 1035-1039 (1999)

3. Observational data collected with the UH 8k CCD Mosaic Camera

An observing program was conducted with the 3.60 m CFH telescope, located in Hawaii, using its prime focus and the UH 8k CCD Mosaic Camera. One and a half nights had been allocated, on October 25th and 26th, 1997. These observations permitted to observe three different fields in using the "standard" method (i.e. observations near opposition), during the second night.

For each of these fields eight images, corresponding to the eight CCD composing the UH 8k CCD mosaic camera, had to be processed separately. Each original image had 40962048 pixels. Table 1 summarizes the coordinates of the field centers. These coordinates were chosen to avoid the already known TNOs, to be as close as possible to the ecliptic, and to encompass an area as empty of stars as possible. For each field the size was 29´29´, corresponding to the whole area covered by the UH 8k CCD Mosaic.

Table 1. Fields observed by using the "standard method" with the UH 8k CCD Mosaic Camera. They were obtained near opposition, with 5 images each obtained on October 27th with 15 mn of integration time and a red filter.

These data were processed, taking into account the relatively bad seeing that occured during the observations (from 0.8" to 1.6") and the large field of the CCD Mosaic camera. The bad seeing permitted us to bin two by two the images (one pixel covering, then, 0.40.4"), greatly decreasing the amount of data to process. After binning each image, the data processing was as follows: (i) Subtraction of the BIAS, using the overscan area of each CCD and through a close fit with a polynomial in the y-axis; (ii) subtraction of the darks; (iii) division by the corresponding flat-field for each CCD and (iv) absolute calibration in units of ph/m²/s/µm using standard stars.

The flat-fields were a combination of those obtained inside the dome with a tungsten lamp (small scale sensitivity variations) and on the sky (large scale sensitivity variations). With flat A designating the average dome flat, flat B the same flat but smoothed (with a square of 2121 pixels), and flat C the flat obtained in using scientific frames, then: flat-field = (flat A / flat B) flat C. This method allows a good correction both for the small scale variations of sensitivity of the pixels and for the large scale variations.

© European Southern Observatory (ESO) 1999

Online publication: August 13, 199
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