Spectra obtained using the EXOSAT transmission grating spectrometer, TGS. There are two databases TGS and TGS2 as follows: <pre> * TGS - the grating spectra averaged over the positive and negative orders * TGS2 - the postive and negative orders kept separate </pre> TGS provides a better overview of the spectrum, and is quicker to use with a spectral fitting program. This is the default that most users will want to use. Once a user has become more expert and want to see, for example, if a subtle feature is present in both halves of the grating spectra, the user can access TGS2. This is a service provided by NASA HEASARC .