Optical abberations

The JKT mirrors have abberations (mainly astigmatism) such that the best seeing you can measure on the detector is about 0.7 arcsec. Contrary to what many astronomers think, this 0.7 arcsec seeing is achieved regularly. Due to flexure the quality and focus may degrade a little when observing at large zenith distance.

Usually, best seeing occurs when the difference between mirror and outside temperature is less than 2 degrees centigrade. The JKT is equipped with 4 dome fans to enhance temperature exchange. However, many observers open the doors to the dome and the catwalk to improve the airflow in the dome.