One, back in the 80's, called for using the sun as the primary 'lens' and had the eyepiece out in the asteriod belt (or maybe further out). The claim was that this arrangement would be capable of resolving objects the size of continents on worlds some dozens of lightyears off.
Yes, but the sun is kinda bright to have in your field of view (even if occulted). Besides, the sun's gravity is relatively puny for gravitational lensing. Sure it works, but isn't much of a magnifier.
One of the great missions for the 21st century could be FOCAL — a space probe sent to the Sun’s gravity lens some 550 AU out. Gravitational lensing is becoming a major tool for astronomers, and we’ve even seen planetary detections using microlensing, looking at targets in the direction of galactic center and the faint changes in light that indicate a planet’s passage. The gravity lens concept, harking back to a 1936 Einstein paper, came to the fore in 1978, when Dennis Walsh and team spotted a twin quasar image, the result of the lensing caused by an intervening galaxy as it bends light around it.
But no one has put more thought into a FOCAL mission than Claudio Maccone. The Italian physicist led a 1992 conference that investigated mission concepts, and submitted a proposal to the European Space Agency the following year. Since then, he has followed up this work with a series of papers in Acta Astronautica and the Journal of the British Interplanetary Society, investigating among other things the uses of the gravity lens for cosmology (detailed imaging of a small slice of sky to study the cosmic microwave background), communications (using gravity lenses around nearby stars to boost signals from interstellar probes) and astronomy. His 1997 book The Sun As a Gravitational Lens: Proposed Space Missions (Colorado Springs: IPI Press) is an exhaustive analysis of the topic now in its 3rd edition.