Weather Limits

3-meter observers should be aware of the following conditions which limit or prevent operations of the telescope.

Current Limits

Note that these limits are only guidelines.
The decision to open or close the dome is entirely at the discretion of the telescope operator.

Wind Limits

Average (MPH) Persistent Gusts (MPH)

Into Wind 35 40

Crosswind 40 45

Downwind 45 55

Exercise caution using windscreen as windspeeds approach limits.

Humidity

94% or at the telescope operator's discretion based on blowing fog, condensation on dome and handrails, etc.. Note that at freezing temperatures the humidity detectors can become unreliable (see below for information on temperature limits)

Outdew

Outdew alarms in the control room will begin sounding at 20% outdew. At this point the telescope operator will begin closing the mirror cover and telescope dome. The telescope operator may exercise their discretion if they find dew forming on exterior surfaces, despite what the outdew sensors report.

Dewpoint differential

If the difference between the outside temperature and the dewpoint is less than 4 degrees F the telescope operator will close the dome to protect the telescope optics.
The secondary mirror can be significantly cooler than the ambient temperature in the dome due to its location at the top of the dome and e.g. wind chill, and dew can thus begin forming there while the primary mirror remains above the dewpoint.

Temperature

As the temperature approaches freezing the humidity detectors can become unreliable. We therefore do not operate when the temperature is close to 32F unless the telescope operator has sufficient reason to be confident that humidity will remain low and stable.
The telescope operator may consider opening again if the temperature is either sufficiently above freezing, or falls below 26F.

Airbourne Particles/Ash

Enclosures must be closed if airborne particles (e.g. ash from forest fires) are deemed a threat to the optics. A particle counter resides inside the Shane enclosure and measures particles 0.3 microns in size and 0.5 microns in size.
A second particle counter resides outside the APF dome and measures particles 0.3, 0.5, 0.7, 1.0, 2.0, 3.0, 5.0, and 10.0 microns in size.
The interior particle counter responds to outside changes, even when the enclosure is closed. The following threshold values (adopted 2013-08-05, updated 2021-03-04) apply (even when the Shane enclosure is closed):

0.3 micron 0.5 micron 0.7 micron 1.0 micron 2.0 micron 3.0 micron 5.0 micron 10.0 micron

Warning 21400 1750 307 192 77 19 7 2

Closure 30600 2500 - - - - - 3

(Note that in 2023 our outside particle detector returned from calibration with a significant increase in sensitivity. To prevent excess closures the particle thresholds were increased by 80% to match readings from the inside counters).

Particle counts must remain consistently below the closure thresholds for at least 30 minutes before (re-)opening enclosure can be considered.

If in doubt, contact the Shane telescope operator (8-0652).

Snow on Dome

If snow has accumulated on the dome and not been shoveled, the telescope operator will keep the dome closed to prevent snow from falling or blowing onto the telescope.

Lightning

If distant lightning can be seen but no thunder heard, there is no immediate threat and observing may continue provided there is no danger of rain.

If lightning is 10 miles away or less (about 50 seconds from lightning to thunderclap), there may be a danger of a strike. Telescope operator can stop observations to shutdown critical electrical systems as his or her discretion.

Wait at least 30 minutes after the last audibule thunderclap before declaring the storm passed and resuming operations.

Last modified: Wed Mar 10 15:11:44 PDT 2021

User's Guide to the Shane 3-m Telescope

Table of Contents