Correct, normal optics would not work. X-ray telescopes use grazing incidence mirror arrangement (Wolter telescope  ).
I haven't kept track of the field. Has eROSITA reproduced most of XMM's data? I expect XMM will keep collecting data, but how useful/obsolete has it become?
XMM has a smaller field of view than eROSITA, but more light gathering capability (effective area), particularly at higher energies. It also has different types of instruments, such as the X-ray gratings.
eROSITA is really designed for doing surveys with its wide unobstructed field of view, and it will be doing just that for the next 3.5 yrs. We really need XMM to follow up in detail the most interesting sources we discover during the survey.
I'm interpreting this as a picture of all massive X-ray sources in the visible universe. And I'm kinda ... sad? that it looks so empty....? But at the same time I'm also thrilled by each and every point.
I was very impressed with the picture of the Shapley supercluster (see article), which is very overdense part of the local universe, containing dozens of clusters of galaxies, each of those with 100s to 1000s of galaxies. In addition, we took some performance verification observations at the start, and these were really nice: http://www.mpe.mpg.de/7362095/news20191022
The picture is a bit misleading about the number of sources. It's really dominated by our galaxy, as most sources are point-like. You cannot really see the majority of them individually in the full-sky map given its spatial resolution, particularly as we applied some smoothing to the data to help emphasise the galactic structure.
We're also going to get another 3.5 years more data in the survey, which is going to find a lot more sources.
Recently XMM looked at the centre of the galaxy, finding chimney-like structures which may link to the bubbles. This is on a much smaller region, however: http://www.esa.int/Science_Exploration/Space_Science/Giant_c...
Nothing since ROSAT has had the ability to map large areas like eROSITA/SRG. ABRIXAS was supposed to be the successor to ROSAT, but it failed shortly after launch.
Annotated and high res images can be found here: http://www.mpe.mpg.de/7461950/erass1-presskit
"Vela Junior was discovered just 20 years ago, although this object is so close to Earth that remains of this explosion were found in polar ice cores."
Aside from being some serious Hurcule Poirot meets Kip Thorne crossover stuff, I want to ask "How do we know?" but I'm pretty sure I wouldn't understand the answer.
Trotted out "hot gas", though.
Lev Landau is reputed to have said, "Cosmologists are often in error, never in doubt." Doubt is productive.