Was pretty fortunate to have the opportunity to work up at the observatory for a fortnight, so got to see quite a bit of it. Was able to do a bit of the tourist stuff too, so thought I'd leave a few quick tips…
The visitor centre is near Hale Pohaku, the residence for scientists/engineers working at the summit, and is easily accessible by road, a ~45 min drive from Hilo. It is at about 10000 feet, and hosts a gift shop, space for seeing an educational film, and several amateur telescopes, that they set up on clear nights, from about 7.30pm-10pm. On one of my nights off I went to one of the astronomy 'talks' which was an excellent interactive tour of the sky, using a laser pointer and the telescopes, to point out lots of amazing sights. Mars and Jupiter look cracking through a 16 inch Meade LX200 GPS telescope, and they have several similar large telescopes which they point at various different objects, such as Supernova SN2014J, the Orion nebula, the Pleiedes, etc. (For a sense of scale, the 'amateur' telescopes cost in excess of ~$10000.) It was all very well organised, but ultimately, weather dependent, so try to be flexible if you want the sky tour.
Anyway. Beyond the visitor centre, the road is unpaved, and pretty dangerous, due to inclement weather, an aggressively low sun in the morning and evening, and steep drops at various points. People are often killed up there. Most hire car companies specifically exclude people from driving up there (even if they hire 4x4/SUVs), and fine heavily for disobedience. Moreover, people often break down, either by burning their brakes out coming down, or oxygen-starving the engine on the way up, due to dust and low air pressure, and towing fees are large. So if you do want to go up to the summit, at 14000 feet, either book a tour, or hire from a company that loans out cars specifically for that purpose (with low range 4x4 gear box).
The summit itself offers incredible views of sunset/sunrise and is where you can see the professional telescopes, like Keck, NASA's IRTF, Gemini North, JCMT, Subaru etc. At night, the active volcano can be seen glowing in the distance. The night sky is phenomenally clear, but as you're quite oxygen starved, doesn't look as spectacular as at the visitor centre. It's often at freezing temperature up there, and the altitude can affect people heavily. But as long as you consider these factors, you shouldn't have a problem.
All in all, well worth a visit!
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