Go on their website on the 1st wednesday of each month at 9am for advance passes,or 6:30 am on on the day you want to go for walk in passes. Good luck to you.
If you can get an early reservation time it's best. They will let you in at any time after your reservation time. If you can't get tix, and are coming in spring it's very possible there will be a student group with extras if you only need a couple.
You have to go to the website.
Available on line in advance and in person day of visit.
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