Chandra and XMM.

We are now in the era of CCD X-ray telescopes. Chandra and XMM mark a considerable advancement in X-ray astronomy. Chandra was launched and deployed by Space Shuttle Columbia on July 23, 1999. It has an unprecedented spatial resolution of $\sim 1$ arcsecond with good spectral resolution, $\frac{\Delta E}{E}=0.02$-0.05 (for the ACIS cameras which are the most relevant in terms of studies of clusters), at 6keV or 1keV. XMM does not have as good spatial resolution, but is still capable of a total angular resolution of $\sim 15$ arcseconds. However, it has three large telescopes, each comprising 58 mirrors, each telescope providing a collecting area of 1430cm$^{2}$ at 1.5 keV or 610cm$^{2}$ each at 8.0 keV. It thus provides extremely good sensitivity, which coupled with its spectral resolution, $\frac{\Delta E}{E}=0.02$-0.05 (for the EPIC instrument, again most relevant regarding clusters), makes it an ideal observatory for studies of high redshift clusters.