Atlantis' Stargate

"The Atlantis Stargate was the one and only link back to Earth from this galaxy..."

- Melia hologram

The City-ship of Atlantis possesses its own Stargate and Starbase Atlantis has its own Stargate which is currently controlled by the Federation. In most respects it is similar to every other Stargate in the Pegasus galaxy, however Atlantis's Stargate is the only one with the ability to dial its eighth chevron, allowing it to reach other galaxies. Atlantis's Stargate also features a shield, which serves a role similar to the Iris protecting Earth's primary Stargate.

Dialing
The Dial Home Device for Atlantis' Stargate consists of a series of crystal panels, similar to the dialing device featured on Gateships, and has the ability to block specific incoming gate addresses. It also features a specific Control crystal allowing it to activate its eighth Chevron to dial extragalactic addresses, making it the only Stargate in the Pegasus galaxy capable of dialing out of the galaxy.

Power
Starbase Atlantis' Stargate is typically powered by at least one Zero Point Module, however when ZPMs are not available Naquadah generators have proven sufficient. Although Naquadah generators have proven to provide more than enough power to dial any gate in the Pegasus galaxy, a power source on the magnitude of a ZPM is needed to sustain a wormhole to another galaxy for any meaningful amount of time. However, Dr. Meredith Rodney McKay, a member of the First Atlantis Reconnaissance Team and the Chief Science Officer of the Atlantis Expedition was able to connect the power core of a Gateship to Atlantis' power grid, allowing for a brief intergalactic wormhole to be established.

Shield
The shield featured in Starbase Atlantis' and Atlantis Stargate was designed by the Ancients to prevent objects traveling through the Stargate from re-materializing, and was likely developed during the war with the Wraith. The Stargate's shield has never been breached, due to the fact that it is so close to the event horizon that not even subatomic particles can reintegrate. McKay programmed a "back door" into the code of the shield that allowed him to deactivate the Stargate shield, which he later put to good use to prevent the Asurans (who were in control of Starbase Atlantis at the time) from reactivating it, allowing the starbase to be retaken.

The shield has been used several times to prevent dangers from entering Starbase Atlantis, such as the Genii reinforcements, lava from the erupting supervolcano on Taranis, Wraith Darts pursuing a Gateship, and a flood on M44-5YN.

Mechanical Iris
After Doctor Rodney McKay got turned into a Wraith and was able to disable Atlantis' defenses, a mechanical iris was installed similar to the one on the Earth Stagate. Just like the Earth iris, this one was very effective against the Wraith.

History
Atlantis' Stargate was built millions years ago by the Ancients while in the Milky Way, and was the first of a new design of Stargates that used the Pegasus coordinate system designed for their relocation to that galaxy. The gate remained under their control until they abandoned the Pegasus galaxy and returned to Earth, ironically using Atlantis' gate itself for their exodus. The Stargate remained unused for thousands of years, and blocked every incoming wormhole with an address from anywhere but Earth. Atlantis' Stargate has remained under Federation control since their arrival, excluding the short times when the Genii, Asurans and once, a group of Ancients, controlled Starbase Atlantis.

In 2383, Atlantis' Stargate was destroyed due to the activation of the Attero device. The explosion was equivalent to twelve Naquadah-enhanced Nuclear warheads and not even the full power of the Atlantis' shield could fully contain the explosion because it was covering such a small area. The inevitable explosion destroyed not only the gate, but also a large part of the wall behind it and much of Stargate Operations. The Stargate was eventually replaced with one of the gates from the McKay/Carter Intergalactic Gate Bridge. The gate shield still operates with the new gate, suggesting that the shield generator for the Stargate shield is either not actually inside the Stargate itself or it is somehow an extension of Atlantis' primary shield generators. Also, because dialing another galaxy is also not dependent upon the actual gate, that function remains as well.