|Class Name :
Starbase manufacturing and support facility|
|Unit Run :
40 built in total. 3 have been lost in all.
2345 - 2365, class remains in service
||Diameter : 2,842 m
Height (overall) : 3,445 m 
Decks : 397 in m
23,000,000 metric tons
5,700 Starfleet, typically 4 - 6,000 civilian
50 x Type XII phaser arrays, total output 300,000 TeraWatts
4 x Type 3 burst fire photon torpedo tube
|Defence Systems :
||High Capacity shield system, total capacity 1,215,000 TeraJoules
Light Duranium / Tritanium Double hull.
Low level Structural Integrity Field
|Docking Facilities :
||Internal docking bay capable of holding up to fifteen starships, depending on type
Starbase 173 :
||Not capable of independent movement; attitude control and/or orbital adjustment only.
|Strength Indices :
(Galaxy class = 1,000)
|Beam Firepower : 6,000
Torpedo Firepower : 2,000
Weapon Range and Accuracy : 1,200
Shield Strength : 450
Hull Armour : 6.25
Speed : -
Combat Manoeuvrability : -
|Overall Strength Index :
|Diplomatic Capability :
|Expected Hull Life :
|Refit Cycle :
||Minor : 5 year
Standard : 5 years
Major : 50 years
One of two Starbase designs fielded in the 2320's, Starbase 173 was the more science oriented of the two. Both designs were based on the old Regula design, but significantly scaled up 
to provide the room for enhanced capabilities. Starbase 173 was designed to provide the usual functions of starship support, but was also intended to provide a deep space platform for scientific research of unparalleled quality. Although the base was primarily a scientific establishment, it was also designed with a significant combat capability of its own. This was underlined when Starfleet chose to site the first of these designs in Sector 23, near the Romulan Neutral Zone. 
The scientific facilities are impressive, and include a dome which mounts a large telescope. The original design included a 70 metre diameter optical instrument, but different stations have mounted various instruments since then. Starbase 173 was the site of an important Starfleet legal decision, when in 2364 Lieutenant Commander Data objected to Commander Maddox's plans to disassemble him as part of the ongoing research into Dr. Soongs work. The JAG officer for the sector, Phillipa Louvois, conducted a hearing in which Data defended his right to choose whether to undergo the procedure. With the base still not fully operational and few of the JAG crew assigned, Captain Picard and Commander Riker were forced to act as defence and prosecution council respectively. Picard was able to convince Louvois to rule in Data's favour, a landmark decision in the rights of artificial life forms.  Whilst at the station the Enterprise took several crew members aboard, including Ensign Sonia Gomez, a newly graduated officer on her first assignment. 
Seen in TNG's first season episode "The Measure of a Man", Starbase 173 was a re-use of the Orbital Office model which was first seen in Star Trek : The motion Picture and has formed the basis of several stations since. As with Regula 1, Starbase 173 was upside down compared to the original office. In fact, the only external difference between Starbase 173 and Regula is in the mysterious "dome object" location on top of one of the circular modules attached to the station. Many have claimed this as some kind of weapon, usually a super-sized phaser cannon, but this makes little sense. The location and design of the dome means it can only cover about a third of the sky without changing the orientation of the station - if it were a weapon you would expect either a location that gives a greater field of fire, two or three of the weapons installed to give all-round coverage, or a combination of the two.
My take on the object is that it is a telescope of some description.
It's tempting to scale Starbase 173 to the same size as Regula, but "The Measure of a Man" indicates otherwise. The first shot on the images page shows the Enterprise-D in front of the station; the hangar bay behind the ship is clearly almost half the length of the Enterprise, even if we ignore the effects of perspective. This scales the whole station to almost three and a half kilometres tall, far larger than Regula. Happily, it makes the stations upper section virtually identical in size to Starbase 375 even though the two figures are arrived at from totally different effects sequences made over a decade apart (in real time as well as on the Trek timeline). As a result, I've made the upper sections of the two stations exactly the same size, so I can say one design is a direct development of the other.
So far as I know, we've never seen a 173-type Starbase in any other episode. I've pitched the specs so that it's roughly in proportion to the size difference between this station and Spacedock.