Caveats


Below gives a list of the known caveats for the current version of ESASky:

CAVEATS OF THE DATA

There are some caveats on the data currently provided by ESASky:

  • Data are regularly updated in the application, and some important datasets might still be missing. Data are complete up to dates described in the ESASky help pages.
  • The Herschel Data Distribution provides combined Standalone Browse Products per observation with either the three wavelengths for the SPIRE maps or two for the PACS maps. This makes it currently impossible to send a Herschel observation to Virtual Observatory tools like e.g. Aladin.

 

CAVEATS OF THE CATALOGUES

The current catalogues functionality on ESASky has the following caveats:

  • A limited number of columns are displayed for every catalogue. Currently it is not possible for the user to decide which columns to display.
  • For the XMM-Newton 3XMM-DR5 catalogue, only sources with QUALITY=GOOD are shown.
  • In the case of more than 2000 sources available in the Field of View, ESASky will order them by magnitude and show the brightest 2000. Users can read the number of sources displayed by hovering on the information icon in the catalogue tab. In the case of the Hubble Source Catalog (HSC), ESASky orders the catalogue by the number of sources (NumImages) and shows the 2000 most observed sources. In some rare cases this can lead to an uneven display of sources at a specific zoom level (a clustering of sources in crowded fields for example, where the top 2000 sources either have, or nearly all have, the same NumImages value). This pattern occurs because of the way ESASky selects the 2000 objects from the database (after applying the NumImages filter) and it is not a complete indication of where all the HSC sources are located. To correct for this, zoom to a smaller field of view and reload the HSC by clicking on the HSC tab's 'refresh data' button. If you want to be certain that you're looking at all the sources, zoom to a point where less than 2000 are displayed (hover over the HSC bar in the Source Catalogues histogram to see the number of sources in the displayed field of view).

    Below are two examples where this type of pattern can be seen (crowded fields where the displayed 2000 sources have ~all NumImages values the same):

    1) Target = IC 1613 and Field of View (FoV) ~ 16'
    Almost 50,000 HSC sources are in this FoV. The 2000 sources displayed are in a region to the North-West and nearly all NumImages = 21:




    Zooming in further to a FoV ~4.6' shows that there are sources centred on IC 1613 with NumImages ~ 10 and less (note there are still 20,000 HSC sources in this FoV):




    2) Target = M9 and FoV ~ 10'
    There are approximately 31,600 HSC sources in this FoV all with NumImages = 1. 2000 sources are visualised in the west of the ACS footprint:




    Zooming in further to FoV ~ 47'' (where the HSC number of sources are less than 2000) shows the sources at the centre of M9:


 

CAVEATS of the all-sky views (hips)

IMPORTANT The HiPS are intended for visualization only. They are not science-ready products and should not be used to perform scientific data analysis.

The all-sky views (HiPS) currently available on ESASky have some known caveats:

  • The current HiPS maps have fixed flux cuts and pixel intensity stretching for high and low intensities. As a consequence, if strong flux level differences exist between different observations, it is often not possible to find an automated way that would allow good static visualisation of all the data. For XMM-Newton and INTEGRAL, the scaling of the input data to the same background level provided a good representation of all the images in the HiPS. In other cases this approach did not work well and we proceeded with re-scaling of the standardised images based on the Planck flux scale (ISOCAM, AKARI, Herschel-SPIRE). This alternative processing was not successful for Herschel-PACS, where we implemented individual pixel cuts and stretching to each image before adding it to the HiPS all sky map.  There is still room for improvements and we are currently exploring more flexible scheme.

  • Herschel RGB HiPS maps: because of the way the Herschel images were standardised and stretched for the all sky HiPS maps, the pixel distribution does not have a physical meaning and as a consequence the RGB maps are not representative for the actual characteristics of the emitting regions (temperature). Even more difficult to produce is a representative RGB HiPS for PACS and SPIRE combined. That is why we do not provide such a HiPS map in this version. We are exploring alternative ways to produce meaningful RGB maps for Herschel for the next release.

  • HiPS from ongoing missions (HST, XMM-Newton, INTEGRAL) do not contain all the observational data available in the archives, but only the public data up to the HiPS map creation date indicated in the HiPS information page. These HiPS maps will be periodically replaced with newer ones adding more recent observations to the previous HiPS.