Plate Scale, Wavelength and Geometric Calibration of Interface Region Imaging Spectrograph (IRIS) Data

NASA's newest solar observing mission, the Interface Region Imaging Spectrograph (IRIS) contains both a spectrograph and a slit-jaw imager (SJI) that observe at the near-UV and far-UV wavelengths. The plate scale for the two SJI cameras was determined by comparing IRIS 1330Å and 2834Å images to SDO/HMI intensity data and SDO/AIA 1600Å images taken at the same time. The SDO images were then scaled so that the prominent sunspot found in both data sets was at the same size. The amount of scaling needed in the SDO data was then used in conjunction with the known SDO instrument plate scales to calculate the plate scale of the IRIS SJI images. Plate scale appears constant over the orbit to within 0.1%. A slight distortion of the spectral line fiducials is present in the near-UV and far-UV spectrograph channels. A static correction is determined based on the locations of neutral lines in each spectral range, and is applied simultaneously in the Level 1.5 IRIS data using a bilinear interpolation. Shifts of a few pixels due to thermal variations on the timescale of an orbit are observed in both spectra and slit-jaw images. The spectral line positions change in the wavelength dimension by about 3 pixels in one orbit in both the FUV and NUV channels. These shifts of neutral line wavelengths are correlated with measurements of the spectrograph temperature; however, the correlation may not be simple and consistent enough to allow correction of line position using instrument temperature measurements. We plan to use the strong neutral line Mn I at 2801.081Å (which is believed to have zero velocity on average) to correct the orbital variation of line positions. The fiducial marks move by approximately 2 pixels over the orbit in the spatial dimension of the spectra, and in both dimensions on the slit-jaw images. This variation was then compared to various instrument temperatures to determine if there was any correlation.