Properties of EUV Imaging Spectrometer (EIS) Slot Observations

The Extreme ultraviolet Imaging Spectrometer (EIS) on board the Hinode spacecraft has
been operating since 2006, returning high-resolution data in the 170 – 212 and 246 – 292 Å wavelength regions. EIS has four slit options, with the narrow 1′′ and 2′′ slits used for spectroscopy and the wide 40′′ and 266′′ slits used for monochromatic imaging. In this article
several properties of the 40′′ slit (or slot) are measured using the Fe XII 195.12 Å line, which
is formed at 1.5 MK. The projected width of the slot on the detector shows a small variation
along the slit with an average value of 40.949′′. The slot image is tilted on the detector and
a quadratic formula is provided to describe the tilt. The tilt corresponds to four pixels on
the detector and the slot centroid is offset mostly to the right (longer wavelengths) of the
1′′ slit by up to four pixels. Measurement of the intensity decrease at the edge of the slot
leads to an estimate of the spatial resolution of the images in the x-direction. The resolution varies quadratically along the slot, with a minimum value of 2.9′′ close to the detector
center. Intensities measured from the slot images are found to be on average 14% higher
than those measured from the 1′′ slit at the same spatial location. Background subtraction is
necessary to derive accurate intensities in quiet-Sun and coronal-hole regions. Prescriptions
for deriving accurate slot intensities for different types of slot datasets are presented.