Telescope Trends Telescope Performance (on planetary detail with 20% contrast) 10 12 14 16 18 20 2 4 6 8 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 The airflow provided by a fan makes the air mass throughFull aperture, unobstructed Full aperture, 40% obstruction Smaller (60%) aperture, unobstructed out the light path more thermally (and hence optically) homogeneous. Convection produced by heat exchange along the inner walls of a telescope's tube is also a source of turbulence in the light path, which a fan can eliminate. Additionally, ventilation improves the performance of closed-tube systems like SCTs and Maksutov-Cassegrains. Today, cooling fans are integral components of several commercial telescopes as well as popular aftermarket accessories. The thickness of the primary mirror plays a large part in its 0.8 Cycles per arcsecond (for 113-mm aperture) PLANETARY PERFORMANCE The graph above shows the image contrast of typical planetary features as seen in a telescope with a central obstruction 40% of the diameter of the primary mirror. The red dotted line depicts the contrast in a smaller, unobstructed telescope. The visibility of low-contrast features in the smaller instrument (in the region above the visual threshold) is nearly identical to the larger obstructed one. above its surface that can blur the image every bit as much as turbulence thousands of meters overhead. French master optician Jean Texereau determined that a thermal gradient of only 0.13°C along a one-meter light path degrades the wavefront error of a telescope by ¼-wave.