The 100-inch telescope at Mount Wilson

Why it is on the list

The 100-inch was the largest telescope in the world from 1917 to 1949, and Edwin Hubble and Milton Humason used it in the late 1920s to measure the expansion of the universe.¹ Today the observatory rents it to one group at a time, for half a night or a whole one, and it is the largest telescope in the world that can be rented.¹ The observatory itself was founded by George Ellery Hale in 1904, on a 5,710-foot peak in the San Gabriel Mountains near Pasadena.²

It is the wrong night for children under 12, who are not allowed at sessions, and for anyone who cannot climb stairs: the 1917 dome has no step-free access, and the observatory advises against the visit for heart or breathing problems at 5,700 feet.¹ For a party of up to 25, the 60-inch next door, Hale's 1908 telescope and the largest in the world until the 100-inch, is booked the same way.³

How to get it

Request a date on the observatory's reservation page at least two weeks ahead; an open date on the calendar is not a promise.¹ A half night runs from the end of civil twilight to 1 am and is always the first half; a full night runs until shortly before dawn.¹ The fee is $2,500 for a half night and $4,000 for a full night, for up to 20 people.¹ Ask at the same time for a private pre-session tour of both big telescopes and the solar tower: $170 for up to ten guests and $17 for each guest beyond.¹

A session director calls a week before to set the meeting time, usually around sunset, and meets the party outside the main gate; a party that drives on to the public lots inside the gate misses the escort.¹ Up to ten cars or a 30-passenger bus can park at the dome.¹ Weather can cancel a night at short notice, with a full refund or a new date; a client who cancels three weeks or more ahead is refunded in full, two weeks ahead half the telescope fee, and later nothing of the telescope fee.¹ The dome is unheated: bring warm layers and closed shoes.¹ For general questions, call +1 626-214-8030.⁴

Discretion

A night is booked by one group only; the observatory does not split a night between two parties or pair groups to share one.¹ The session director escorts the party through the gate to park by the dome.¹ Non-flash photographs through the eyepiece are allowed with the operator's approval; no smoking or alcohol on the mountain.¹