Question: What optical device is the Dutch scientist, after whom the lunar crater Jansen is named, credited for? 

Context paragraphs:
Kao (crater): Kao is a small lunar crater that is located near the eastern limb of the Moon.  It lies near the southern edge of the Mare Smythii, a lunar mare that continues onto the far side of the surface.  This crater lies to the east-southeast of the crater Widmannstätten.  Less than a crater diameter to the north-northeast is the small crater Tucker.

Marius (crater): Marius is a lunar crater located on the Oceanus Procellarum.  The surface to the west and north of this crater contains a large number of lunar domes spread across an area over a hundred kilometers in diameter that may be of volcanic origin dubbed the Marius Hills.  These domes, if volcanic, may have been formed by magma that is fairly more viscous than the volcanic material that formed the basaltic lunar maria.  The nearest named crater feature is Reiner to the southwest.  Kepler is located to the east-southeast, and rays from that formation reach the rim of Marius.

Chamberlin (lunar crater): Chamberlin is a lunar crater that is located on the far side of the Moon, just past the southeastern limb.  It lies to the southeast of the crater Jeans, and Moulton is attached to the southeastern rim of Chamberlin.  This crater is located in a part of the lunar surface that has undergone resurfacing of crater interiors, producing dark-hued crater floors.

Zacharias Janssen: Zacharias Janssen (also Zacharias Jansen or Sacharias Jansen) (1585 – pre-1632) was a Dutch spectacle-maker from Middelburg associated with the invention of the first optical telescope.  Janssen is sometimes also credited for inventing the first truly compound microscope.  However, the origin of the microscope, just like the origin of the telescope, is a matter of debate.

Cajal (crater): Cajal is a tiny lunar impact crater on the northern part of the Mare Tranquilitatis, in memory of the Spanish doctor and Nobel laureate Santiago Ramón y Cajal.  It is a circular, cup-shaped formation that lies about 60 km southeast of the lava-inundated crater Jansen.  Cajal was formerly designated Jansen F, prior to renaming by the IAU.  Also to the northwest is a system of wrinkle ridges designated the Dorsa Barlow.

LCROSS: The Lunar Crater Observation and Sensing Satellite (LCROSS) was a robotic spacecraft operated by NASA.  The mission was conceived as a low-cost means of determining the nature of hydrogen detected at the polar regions of the moon.  Launched immediately after discovery of lunar water by Chandrayaan-1, the main LCROSS mission objective was to further explore the presence of water ice in a permanently shadowed crater near a lunar polar region.  It was successful in confirming water in the southern lunar crater Cabeus.

Humboldt (crater): Humboldt is a large lunar crater that is located near the eastern limb of the Moon.  Due to foreshortening this formation has an extremely oblong appearance.  The actual shape of the crater is an irregular circle, with a significant indentation along the southeastern rim where the prominent crater Barnard intrudes.  To the north-northwest of Humboldt is the large crater Hecataeus.  Phillips is attached to the western rim.  The rim of Humboldt is low, worn, and irregular in outline.  The central peak forms a range on the crater floor.  The floor surface contains a network of rilles forming a pattern of radial spokes and concentric arcs.  There are also some dark patches located near the walls to the northeast, northwest, and southeast.  There is a chain of craters leading from the northwest crater rim to a distance almost as long as the crater is wide.  This formation is designated Catena Humboldt.  Due to its location near the lunar limb, little detail was known about this crater until it was photographed by orbiting spacecraft (mainly Lunar Orbiter 4).

Jansen (crater): Jansen is a lunar crater in the north part of the Mare Tranquillitatis, named after Zacharias Janssen.  It is located to the east-southeast of the crater Plinius.  The rim of Jansen is low and narrow, with a notch along the western edge.  The interior is relatively level, which may indicate it has been covered by lava.  To the south-southwest a small but prominent crater lies on the crater floor, halfway between the center and the rim, further southeast is a small but prominent crater named Sinas.

Beketov (crater): Beketov is a small lunar crater that lies in the northern reaches of the Mare Tranquillitatis and is named after Nikolay Beketov.  To the south is the ghost crater Jansen R. Northeast of Beketov, along the edge of the mare, is the crater Vitruvius located about 53 km.  Beketov was previously designated Jansen C before being named by the IAU.  The flooded crater Jansen itself lies nearly 70 km to the south.  Other nearby large craters are Dawes located about 75 km west and Fabbroni to the north.

Plana (crater): Plana is a lunar crater that lies on the boundary between two small lunar mare areas, with Lacus Mortis to the north and the larger Lacus Somniorum on the southern side.  At this location, on Earth, the crater appears oval due to foreshortening, but the crater is actually nearly circular.  It is joined to the crater Mason to the east by a short stretch of rugged ground.  Due north of Plana in the midst of the Lacus Mortis is the prominent crater Bürg.

Supporting paragraphs: Jansen (crater) Zacharias Janssen
Answer: optical telescope