Seeing

We appreciate the cooperation by the observers in performing the "AG Focus Check" three times during a night. It takes from 5 to 10 minutes to complete each AG Focus Check. This procedure provides the best focus as well as a historical record of image quality of the telescope and atmosphere above Mauna Kea.

All the seeing values shown here are for the zenith. The values have been converted from those sampled at various elevations mainly around 70 degrees with the auto guider camera at wavelength of 0.73 ± 0.08 μm.

Updates

Seeing statistics

Overall median seeing from May 2000 through April 2008 is 0.639 arcsec. Following table shows median seeing sizes for recent months.

Month	Median(arcsec)	Average* for 2000-2008
July 2008	0.558	0.634
August 2008	0.557	0.605
September 2008	0.565	0.544
October 2008	0.542	0.603

Clear sky ratios

Following table shows monthly clear sky ratios.

Month	Clear sky ratio (%)	Average* for 2000-2008
July 2008	91	80
August 2008	86	79
September 2008	86	80
October 2008	62	63

^*The values are average of the median or ratio for respective months within the years indicated.

Distribution of seeing

The seeing FWHM measured from the Auto Guider during focus checks has been compiled for the period of May 2000-October 2006. Measurements were made three times ( before 21:00; Orange, 21:00-26:00; Green, after 26:00; Blue ) each night.

There is a trend that the seeing improves during the night. There are several possible explanations for this behavior:

1. Some heat from the daytime remains inside the enclosure
2. Instabilities in the upper atmosphere after sunset
3. The primary mirror temperature control is optimized for 26:00 and it might be over-cooled in the early evening

Overall distribution of seeing

The plot below shows distribution of seeing from all the samples we have taken since May 2000.

Seeing size since January 1999

Monthly Average Seeing Size

Clear sky ratio since May 2000

Monthly Average Clear sky ratio