Lowell Observatory

LMI

In last week's in-depth feature we looked at the infrared spectrograph NIHTS. This week we discuss our proposed Large Monolithic Imager (LMI).

In the figure at right, NIHTS is the red instrument with the gray cylinder projecting from it. The LMI is at the bottom, at what we call the "straight through" position of the instrument cube. This means light coming through the telescope arrives directly at the imager, without having to be diverted to one side or the other by a fold mirror. This provides the maximum field of view, as is desirable for an imaging device. Unlike spectrographs, which disperse light into its component wavelengths, imagers obtain pictures of the region of sky at which the telescope is pointed, providing not only data fpr scientific analysis, but also the beautiful images of the heavens that grace Web pages and magazine covers.

Pictured at right is a prototype of the device we plan to use for the LMI. It is a charge coupled device (CCD) with 6,000 pixels on each side. Each pixel is sensitive to the intensity of light falling on it, so over a timed exposure it stores an image of the sky; this can be retrieved from the CCD at the end of the exposure and stored on a computer.

The LMI will be the workhorse instrument of the DCT during its early science operations. Lowell astronomers will use it to conduct a variety of programs, including observations of eclipsing massive stars in Local Group galaxies, studies of the physical properties of comets, and measurements of the faint, diffuse outer regions of dwarf galaxies. An important advantage of the LMI for all these studies is that it employs just one large CCD rather than an array of smaller ones. While its field of view is not as large as that of the larger CCD arrays, the LMI avoids the calibration complexities of merging data from multiple CCDs. Our astronomers will carry out high precision studies of many types of objects – and while they're at it, they'll provide some beautiful pictures for us to post here on the site.

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The charge-coupled device, or CCD, is the detector of choice in modern astrophysics, and the LMI will be a high-precision example of the state of the art in CCD manufacturing.

As light shines on a CCD, it creates an accumulating electric charge in each pixel that is proportional to the intensity of the light falling on it. At the end of an exposure, the pixels are "read out," or shifted sequentially into electronics that convert the amount of charge into a digital datum. At the end, you have a huge array of numbers representing the image collected by the CCD. These digital data form the bedrock of modern astrophysical analysis, and it not overstated to say that CCDs have revolutionized the field.

We were pleased to note that just this month the Royal Swedish Academy of Sciences awarded the Nobel Prize in Physics to Willard Boyle and George Smith, inventors of the CCD. This is most appropriate, and congratulations to them.

And if CCD's seem strange or exotic...if you have a digital camera, you have a CCD!