Astrophysics

8-INCH DIMM seeing monitor

The term seeing refers to the blurring of light from celestial objects caused by the atmosphere. Starlight is slightly dispersed in thin turbulent layers of the atmosphere, resulting in star images of about one arc second of size, while the optics of a two-meter class telescope enable it to resolve dimentions ten times smaller (as demostrated by the Hubble Space Telescope). As a consequence, the hability of producing good resolution astronomical images depends primarily on the seeing of the site. An important characteristic of a good astronomical site is a small value of the seeing.

The DIMM (Differential Image Motion Monitor) is an automatic seeing monitor designed and fabricated by the Instituto Astrofísico de Canarias and the University of Nice. It is described in detail in the article by Vernin & Muñoz-Tuñon (Publ. Astron. Soc. Pacific 107, 1, 1995) and its working principle is based on Sarazin & Roddier (Astron. & Astrophys. 227, 294, 1990). The instrument is basically an 8-inch telescope with a two-pupil mask at the aperture which forms two images of a same star in the field of view. An intensified CCD allows to register the relative position of both images tens of times per second and, after at least 200 images, provides an statistical value of the seeing based on the variations of this relative position.

The DIMM at the Observatorio Astrofísico Guillermo Haro (OAGH) was acquired, together with the 16-inch MEADE telescope, by the mining company Compañia Minera María and given to INAOE in "comodato", as part of the agreement between both parties and the National Institute of Ecology that conditions the work of the mining project Mariquita to the preservation of the observing conditions in the site of the OAGH. Apart from both telescopes, the Compañia Minera María operates a dust sampler and metereological station at the site of the Cananea Observatory.

You can browse the first measures of the seeing at the Cananea site here.