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M101: The Big Picture
(a.k.a. "Mind the Gap" II)

Our good friends at the European Coordinating Facility of the Hubble Space Telescope in Garching, Germany have created a wonderful mosaic of M101. The Heritage image is but a small portion of the overall massive dataset that went into creating the multipointing Hubble image of M101.

The awesome Hubble mosaic of M101 is created mostly from data from Hubble's Advanced Camera for Survey's Wide Field Channel detector (ACS/WFC). This detector is an array of two 4000-pixel-wide by 2000-pixel-high chips that sit close to, but not exactly next to each other. When a single image is taken with this Wide Field chip, a small "chip gap" of several hundred pixels is missing across the center of the image, and runs the full width of the 4000 pixel detector.

To compensate for this section of missing data on the ACS/WFC, astronomers sometimes take more than one image and move the second (and possibly third and fourth images) in position such that the gap falls on a slightly different place in the image. By stacking all of these images together, the chip gap can be minimized. In theory, it could be said that the chip gap is removed by taking more than one image of the object, however, in reality, the chip gap portion of all the stacked images will always have slightly less signal than adjoining areas on the detector.

There are times when the goals of a Hubble science proposal can be obtained with out the need to "dither" exposures over the chip gap of the ACS/WFC camera. As this was the case with the extensive study of M101, the preliminary draft image of multiple pointings of the ACS/WFC detector had chip gaps as well as rather sharp and incomplete edges around the entire mosaic. A method of dealing with Hubble images where no Hubble data is available to fill in the chip gaps and corners with ground-based data. Upon close inspection, it is evident from the resolution changes which areas are Hubble data and which are ground-based.

Even if similar filter ground-based data can be used to fill in areas of missing data, there are times when all of the Hubble data include different filters for various areas. In other cases, more exposures of a particular filter may exist than in other areas. This may make for a rather patchwork-style image prior to all levels being normalized.

Nonetheless, M101 is a spectacular nearby galaxy with much information to share. By studying this spiral galaxy we may be able to learn more about our own Milky Way galaxy.

Thanks to our friends and collaborators at the Hubble ST-ECF for their work on the full mosaic image of M101 and for allowing Heritage and STScI to co-release with them: Lars Christensen, Martin Kornmesser, and Davide de Martin. For more information on these image processors, visit their bios.

Click on images to enlarge. Image credits (top to bottom): NASA and ESA; NASA, M. Mutchler (STScI) and K.D. Kuntz (GSFC/JHU); NASA, Z. Levay (STScI), K.D. Kuntz (GSFC/JHU) and F. Bresolin (Institute for Astronomy, University of Hawaii). For more information on the M101 Hubble mosaic release, visit the STScI News Release Website. For more information on other gap features related to instruments on Hubble, visit Hubble's High Resolution Camera - "Mind the Gap."