Telescope Performance

Pointing and Tracking

1. All sky blind pointing: 1.15″ RMS
2. Offset pointing from a bright setup star for faint targets: 0.08″ if target is within 20′ of setup star
3. Sidereal tracking error with no influence of the wind: 0.07″ RMS

Wind Conditions

The telescope is affected by the wind, and, although our elevation and azimuth servos include wind disturbance rejection, it is impossible to completely eliminate the impact of the wind. Once the wind hits certain limits it can help your observing to point away from the wind. Rough guidelines to keep in mind are:

1. When using the f/9, point away from the wind when it is 30 mph or stronger.
2. When using the f/5, point away from the wind when it is 25 mph or stronger.
3. When using the f/15 we must point away from the wind in 30 mph winds and close the chamber at 35 mph winds to protect the adaptive secondary mirror.

If observing with the f/9 or f/5 the front shutters must be closed if the wind has sustained speeds of 45mph or gusting above 50mph. If the f/15 is mounted the wind constraints are tighter to protect the deformable secondary mirror. If the wind is sustained at 35 mph or gusting above 40 mph we have to shut the front shutters when using the f/15. Additionally, if the wind is coming out of the North-East or East then it arrives at the telescope over Mt Wrightson and the ridge. This causes major turbulence above the telescope and will degrade the seeing and make it highly variable. In this case there is nothing we can do to improve the conditions other than wait for the wind to change direction.


Median seeing, as measured via the wavefront sensors, is in the range 0.77″ – 0.85″ based on data from 2003 to the present. For more details, there is a seeing analysis repository on GitHub, or contact a staff scientist.

Weather Conditions

The MMT typically loses one-third of its time to poor weather conditions. See a plot of average weather losses by month.

Slew Speeds

The slew speeds of the telescope are 1 degree per second in elevation and 1.5 degrees per second in azimuth.

Instrument Rotator

Learn all about using the instrument rotator during observing.