Richard Berry CCD Workshop

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Universe 2001: Results of Saturday Evening Imaging

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Here are some results from our outing to the patio of the science museum on Saturday evening. Although city lights and light cloud limited visibility to the summer Triangle and a handful of other first-magnitude stars, the CCD recorded stars down to about 8.5 magnitude, allowing us to obtain an astrometric position for the star cluster M11.

The imaging setup consisted of an SBIG ST237 CCD camera (loaned to us courtesy of SBIG) with a 28mm focal length camera lens stopped down to f/5.6 (for an aperture of 5 mm, the same as the dark-adapted pupil of a middle-aged adult) piggybacked on an LX200 (loaded to us for the evening by Scott Roberts) tracking in alt-azimuth mode. Field rotation was present in the image series, but not in individual images, and thus could be removed by registering the images and combining them using AIP's stack-and-track processing.

Our target was the star cluster M11 in Scutum, near (RA=19h, DEC=-6d). We obtained four exposures of 15 seconds each, one 60 second integration, and one track-and-accumulate exposure consisting of twenty 15-second integrations. Support data for calibrating the CCD consisted of eight bias frames and four 60-second dark frames. Unfortunately, because the CCD would not cool below -6.5 C, pixels in the dark frames reached saturation despite the releatively short integrations. Normal calibration therefore resulted in a scattering of blemishes across each image, and severe hot-pixel blemishes in the track-and-accumulate image.

In addition to not cooling enough, pixel values above 3000 ADU appeared dark, possibly because of a "stuck bit" in the camera electronics. A stuck bit occurs when a data bit generated by the cameras analog-to-digital converter does not change between 0 and 1. This caused the centers of the images of the brightest stars to appear black rather than white.

To rescue the images, we processed and registered the four 15-second integrations and the 60-second integration separately. The 15-second images were then combined using median combine, largely correcting the blemishes because they occurred in different locations in each image. The 60-second images were then registered to the composite of 15-second images to produce the image M11-COMBO.FTS. The resulting combination image is roughly equivalent to a single 60-second image.

Click here for the output of the astronometry tool.

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