F555W fits image, the above image is the correct N up image. Armin suggests the best angle for the strip mask is angle
90 (horizontal)... therefore, the light echos are not in the same places as
the HII regions (which seems to be brightest towards the east)
FORS1_polarization.pdf Standard Stars for Linear Polarization Observed with FORS1 (pdf)
link to SM page showing light echoes