It is possible to extract a spectrum without passing from a catalog. The system will take care of making a mosaic image and extract sources from a catalog.
However, this is not recommended, as there could be NEW fake sources that appear and slow down or even degrade the spectral quality.
We give this example:
We look for the Crab nebula and pulsar, so we type "Crab" and click on "resolve". Then, we select a date in which we know there are Crab observations (here we limit ourselves to one hour). Then, we select "Spectrum", put a minimum detection significance of 7 to reduce the number of fake detected sources in the field of a bright object and use an energy range from 30 to 50 that we know is less noisy. Finally, we click on "submit".