In 2019, NASA's Astrophysics Division finished up two years assessing the feasibility of assembling a large-aperture observatory in-space. The In-Space Astronomical Telescope (iSAT) Assembly Design Study [1] concluded that In-Space Assembly (ISA) is the only option for building observatories with aperture diameters over 15 m and would still likely be strongly beneficial for smaller ones like the JWST (6.5 m aperture diameter). Efforts like Northrop Grumman’s successful Mission Extension Vehicles, the upcoming DARPA RSGS and NASA OSAM-1 missions, and the usage of Canadarm2 to install instruments with standardized interfaces on the outside of the ISS all demonstrate the increasing maturity of robotic servicing and assembly. The iSAT study describes a telescope composed of modules with standardized interfaces, launched with a spacecraft bus that has attached Canadarm2-like robotic arms that can assemble and deploy modules delivered by space tug from multiple launches. The benefits over launching monolithic spacecraft with hundreds of single points of failure (cough JWST cough) are clear: the mission won’t be limited by a single launch vehicle’s lift ability or fairing size; the same inchworming robotic arm that does initial ISA can later perform repairs and upgrades, either with freshly delivered replacement modules or by debugging malfunctioning parts (see Mars Insight); the final deployed structure doesn’t need to be designed to handle harsh launch conditions; and, design and development will be faster without needing to design and test super reliable deployment mechanisms—if a part fails during orbital checkout, launch a replacement. The primary challenge is designing hardware that today’s limited-dexterity robotics can manipulate, and figuring out supervised autonomy with fallback telerobotics for bringing humans into the loop when needed. There are definitely challenges, but this feels like the right approach. If you could do it near a crewed station for infrequent debugging EVAs, even better. After it's assembled, raise the orbit to L2 with solar electric propulsion.
[1] https://exoplanets.nasa.gov/exep/technology/in-space-assembl...
I'll be writing about this more in Orbital Index (https://orbitalindex.com) sometime soon.