The monument at Antietam to Company F of the 1st US Sharpshooters (Vermont) is in Sharpsburg on the west side of the National Cemetery. It was dedicated in 1900. (National Cemetery tour map) The granite monument is topped with the Maltese Cross symbol of the Union 5th Corps, although the symbol was not yet in use at the time of the Battle of Antietam. Company F was commanded at Antietam by Sergeant Henry Kinsman.
There is also a monument to Company F at Gettysburg, and a monument at Antietam to two other Vermonth Sharpshooter companies, E & H of the Second Regiment.
The United States Sharpshooters
Two regiments of U.S. Sharpshooters were recruited in 1861 by engineer, inventor and champion marksman Hiram Berdan. Berdan’s regiments were unique to the Army of the Potomac, using different uniforms, weapons, equipment and tactics from the “line units”. Although operating as United States Army units, each company within the regiments was formed of recruits from a single state, with eight different states contributing companies. Company F was one of two companies recruited from Vermont.
Each recruit had to qualify by passing a shooting test. Each had to place ten shots in a ten inch circle from 200 yards away. The sharpshooters wore a distinctive green uniform with no brass in order to blend into the terrain, and used light infantry skirmishing tactics that were very different than the rest of the army. 
Text from the monument to Company F:
1861 1864
Company F
First Regiment
United States Sharpshooters
First Sergt. Henry E. Kinsman
—–
Enlisted 177
Killed and died of wounds 32
Died of disease 17
Wounded 45
VERMONT
Location of the monument to Company F of the 1st US Sharpshooters (Vermont)
The monument is on the west side of the National Cemetery on the east side of the town of Sharpsburg, Maryland (39°27’31.6″N 77°44’31.1″W).