NGC7023

Weather

The QSO Team records weather conditions every night of the year. To properly interpret and use the information presented below, the reader should be aware that:

  1. Each CFHT instrument has its own observing limits. For example, an hour during which the extinction due to clouds is 0.7 magnitude will be calssified as "lost to weather" if we are using MegaCam but will result in no time lost if ESPaDOnS is used instead. A run or month or semester during which the time lost to weather is high may be due to bad weather conditions or the heavy use of an instrument that is more sensitive to sky conditions.
  2. The percentages of time lost presented below are calculated based on each night's length for the instrument in use. The "bright" time instruments WIRCam, ESPaDOnS, and SPIRou are used between the 8-degree twilights. The "dark" time instruments MegaCam and SITELLE are used between the 12-degree twilights.
  3. Recording time lost to weather is done before recording time lost to technical issues. If a technical issue prevents observing during a night that is not useable anyway bcause of sky conditions, the loss is attributed to the weather and not to the technical issue.
  4. WIRCam observations started in 2005B. ESPaDOnS observations started in QSO Mode in 2008A. SITELLE observations started in 2015A. SPIRou observations started in 2019A.

The following table and histogram present the average time lost to weather for semester 2001A to 2025A. Weather losses have been as low as 8 or 10% (2010A and 2000B) and as high as 50% (2008B and 2018A).

Our logs for 19A to 23B indicate that:

  • Averaged over semesters 19A to 23B, each instrument lost between 24% and 30% of the time to weather. The loss was generally due to high extinction (clouds), high humidity or fog, or high winds. The loss for a given run can be as low as 0% or as high as 40-50%.
  • Over those semesters, between 18% and 37% of the time was lost each semester for all instruments.
  • Averaged over those semesters, for all instruments, 26% of the time was lost to weather.

The general sky conditions from January 2022 to December 2024 are presented in the following pie charts. Note that these pie charts are using a different log and are based on hourly entries. Each entry reflects the conditions at that moment and not necessarily the conditions during the full hour. The category "Likely observing" includes the entries "Photometric" and "Thin Cirrus".

Hours per night

Taking into account the average time lost to weather and time used by overheads (e.g. slewing the telescope and initiating the guiding), the effective average number of hours per night for each instrument is:

  • MegaCam: 5.0 hours/night
  • Wenaokeao: approximately 7.0 hours/night

Image quality

The following plot compares the Image Quality (seeing) as measured by MegaCam in the r and the u band with the seeing measured by the Mauna Kea Atmospheric Monitor (MKAM) located on a tower between the CFHT and Gemini osbervatories. MegaCam measurements were corrected for the airmass and wavelength. The wavelength corrected applied follows a lambda^0.6 relationship. The seeing is generally well below 1 arcsec but does vary over time.

In the plot below, the u band data has been offset by 0.25", which represents the offset measured with MegaCam between the r and the u bands. The r band and u band data now overlap.