What is an image strip?
The GALEX spectroscopic mode employs slitless spectroscopy. This provides an image of the sky in which every object is spread out into a spectrum similar to what one sees when viewing the sky directly through a prism or transmission grating. GALEX uses a transmission grism, a ruled prism. This has the advantage of providing spectra for all the objects in the large GALEX field of view but the disadvantage of overlapping spectra in crowded fields. For this reason we take spectra of each part of the sky at different spectral position angles on the sky. This permits us to remove the confusion caused by overlapping spectra. An image strip is a two-dimensional spectrum. As in slit spectroscopy, one dimension is the spectral dimension and the other is the spatial dimension. In slitless spectroscopy however, the spectral dimension is also a spatial dimension, thus a single point on a GALEX grism-mode image represents various wavelengths depending on the source position in the sky, along the sp
