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.
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 |
| June 2008 | 0.617 | 0.624 |
| July 2008 | 0.558 | 0.634 |
Clear sky ratios
Following table shows monthly clear sky ratios.
| Month | Clear sky ratio (%) | Average* for 2000-2008 |
| June 2008 | 71 | 79 |
| July 2008 | 91 | 80 |
*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:
- Some heat from the daytime remains inside the enclosure
- Instabilities in the upper atmosphere after sunset
- 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
