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The best site for the new 2 m telescope is searched with the help of G1-0300
 The new telescope with 2 m mirror requires site with both dark sky and good seeing. The best device to measure seeing is the Differential Image Motion Monitor (DIMM). DIMM consists of a telescope, fast CCD camera and computer running image processing software. Camera takes very short exposures of a bright star through a telescope with an aperture mask containing two round holes. Star image, slightly out of focus, is split to two images by the aperture mask. Mutual movement of both images indicates the current seeing.

Seeing, caused by different light refraction in hotter and colder air, distorts images created by all ground-based telescopes. Because seeing is caused by atmosphere, it depends on current weather and also differs from site to site. While light pollution can be measured relatively easily by taking several exposures and measuring sky background levels, measuring seeing is more complicated.

While the seeing is often expressed as the angular diameter of the star image profile in the half of its maximum (FWHM—Full Width Half Maximum), estimating it from the single star image is quite inaccurate—star image profile is affected not only by seeing, but also by mount tracking errors etc.

Much more reliable and accurate is measuring of differential motion of two (or more) images of single star. Aperture mask with two (or more) holes must be inserted in front of the telescope. Slight de-focusing of the telescope causes splitting of the star image to two parts. Each part of the image is created by light, which passed through one hole in the aperture mask. Because each light beam passed through different portion of the atmosphere, it moves slightly due to seeing. Measuring relative distance of images instead of their absolute position on the frame eliminates mount tracking errors, wind gusts etc.

Image of Polaris, split by telescope aperture mask, shows seeing

Image of Polaris, split by telescope aperture mask, shows seeing

Astronomers from the Astronomical Institute of Academy of Sciences of the Czech Republic (http://www.asu.cas.cz/) started searching for the best place for the new telescope with 2 m mirror. They built their own DIMM setup from commercial 9.25 inch (235 mm) Schmidt-Cassegrain telescope (STC), G1-0300 CCD camera and laptop computer running SIMS (Simple Image Manipulation System) software package.

G1-0300 camera on the Celestron 9.25 inch telescope (left) and whole DIMM setup on the candidate site (right)

The G1 camera takes very short exposures (0.005 sec) of Polaris (Alpha UMi) and stores a series of several hundreds of images on the computer. SIMS then determines centroids of each Polaris image, which allows calculation of the current seeing in the particular time interval. The measurement is then repeated for the whole night—this allows evaluation of the seeing on the particular site and its development through the night. G1 camera takes many tens of thousands of frames per night.

G1-0300 on the back side of the Celestron CPC 925

All images courtesy of Kamil Hornoch, AsU CAS Czech Republic.

 
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