First Light

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First Light Information

The "First Light" image at LBT was obtained on the night of 12 October 2005 (UT). The target was an edge-on spiral galaxy (type Sb) in the constellation of Andromeda known as NGC891 (from the New General Catalog). This galaxy lies at a distance of 24 million light years -- pretty much in our backyard in terms of the galaxies that LBT will eventually study. We admit that we picked NGC891 because we find it to be a very attractive galaxy. However, NGC891 is of particular scientific interest because the galaxy-wide burst of star formation inferred from X-ray emission is stirring up the gas and dust in its disk, resulting in filaments of obscuring dust extending vertically for hundreds of light-years. Note that there are numerous smaller and more distant galaxies in the background of the NGC891 field. These are more typical of what a large telescope like LBT will study.

The First Light observation was made through a blue filter (B-Bessel) as a series of ten 30-second exposures. The images were captured through a state-of-the-art camera known as the "Large Binocular Camera" (LBC) which is mounted high above the primary mirror at the prime focus of the left primary mirror. The camera has four CCD (charge-coupled devices) chips in the focal plane - each with dimensions of 2048x4608 pixels - for a total of 36 megapixels. In front of the CCD array is a set of 6 fused silica corrector lenses that correct the comatic aberration of the fast primary mirror to make an extended field-of-view. The ten exposures were each offset slightly on the sky so that the seams in the CCD array do not appear in the final combined image. After the images were calibrated and registered, they have been combined in the computer to make a single blue image of the galaxy with an exposure time of 300 seconds. The image quality of the individual exposures ranges from 2.9 pixels to 3.6 pixels FWHM where each pixel corresponds to 0.227 arcseconds on the sky. Thus the final stacked image has a resolution of 0.8 arcseconds which is typical of the wide field imaging which will be obtained with this camera. The angular size of the final image on the sky is about 30 arcminutes across -- similar to the angular size of the full moon.

The final reduced digital image will be (has been) displayed for viewing in several different ways:

1. a "night-sky" view with white stars on a black sky background
2. a "photographic negative" view with black stars on a white background
3. a "cool blue" view which somewhat resembles how the galaxy would look to your eye when viewed through a blue filter.

The First Light observations were obtained by: A. Baruffolo (Padua), R. Bertram (Tucson), E. Diolaiti (Bologna), A. DiPaola (Rome), J. Hill (Tucson), F. Pedichini (Rome), R. Speziali (Rome) and D. Thompson (Tucson). The image reduction and analysis was done by D. Thompson, and V. Testa (Rome). And of course there have been hundreds of astronomers, engineers, technicians, programmers and other folks across the LBT partnership and in industry who have worked for many years to get the LBT to the present state of operation.

 

Additional technical details

The individual 30-second exposures were each taken with the two axes of the telescope (elevation and azimuth) and the rotator of the camera tracking open-loop according to a pre-calculated trajectory. The telescope trajectory was corrected with a pointing map (T-point) which compensates the gravitational flexure of the telescope structure. (The first longer guided image with stars providing feedback to the telescope drives was taken later on the same night.)

The telescope was focused and collimated before each set of 5 images that were taken. The primary mirror cell has a look-up table to make tiny corrections to the focus as a function of telescope elevation angle before each individual exposure. While the primary mirror cell has the ability to make active force corrections to adjust the shape of the primary mirror, these force adjustments were not used for the First Light images. The forces supporting the primary mirror were those calibrated in the Mirror Lab 2-years earlier.

The mirror ventilation system which holds the honeycomb primary mirror near ambient nighttime temperature was running during these exposures.

The individual images were each bias subtracted (to remove an electronic background level), flat fielded (to remove sensitivity variations across the CCD detector) using some images of the twilight sky, sky subtracted (to remove the uniform sky background from airglow, light pollution, etc.), filtered for cosmic rays (to remove excess signal in individual pixels.). Then an astrometric solution was fit to faint stars in the individual images in order to bring them all to a common coordinate system before stacking the images (this is needed to remove the distortion in the camera optics and the deliberate image offsets). The images were rectified for stacking with the "swarp" program. Stars as faint as B-magnitude 25 are visible in the original image.

The Large Binocular Camera was built in Italy by a collaboration of Italian astronomers centered mainly in Rome and Florence. See their web page for additional details about the camera: http://lbc.mporzio.astro.it.