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====== GAIA EDR3 ======
==== GAIA EDR3 is now available and can be used with Skychart  ====
{{ :en:news:cdc_gaiaedr3.jpg?350|M13 with GAIA EDR3}}

The Gaia Early Data Release 3, consist of astrometry, photometry, radial velocities, astrophysical parameters and variability information for about 1.8 billion sources brighter than magnitude 21. 

EDR3 completness as improved over DR2 but it still miss many bright stars so you must use it along with the default Hipparcos catalog. 

You can read more detail about GAIA EDR3 in the [[https://www.cosmos.esa.int/web/gaia-users/archive|release documentation]]

**What to do if I already install the Gaia DR2 catalog?**\\
Upgrading to EDR3 will hardly make a difference in the chart display.\\
Reason to upgrade is if your are sensible to one of the improvement made by EDR3, principally better precision for astrometry and photometry, see the documentation link above.

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====Use with Skychart====

===Online catalog===

You can use the GAIA EDR3 data with Skychart without installing anything by using the [[en:documentation:catalog#vo_catalog|VO catalog]] feature.

Make a chart in Eq projection with a FOV of 10-15 arc minutes in the area of interest.\\
Open the Catalog settings.

Click Add in the VO catalog tab and search for EDR3, select the catalog I/350, click Select catalog.

Select the first tab I/350/gaiaedr3, enter the default magnitude: 21, for "Name" select EDR3Name, click Download catalog.

Make sure the GAIA EDR3 row is green and click the OK button.\\
If no star are show be sure in menu "Chart/Show Objects" that "Show stars" and "Show Virtual Observatory data" are checked.\\
You can return to the screen later to load the data for a new area with a click on the arrow in the Reload column.

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===Use a catalog prepared for Skychart===

You can download this full catalog for local offline use.\\
This catalog include the following fields: source_id, ra, dec, phot_g_mean_mag, phot_bp_mean_mag, phot_rp_mean_mag, pmra, pmdec, parallax, radial_velocity.\\
You can search by source_id using the format: Gaia EDR3 1328053954366555648.\\
The Vizier link at the bottom of the [[en:documentation:detailed information]] window give you access to all the other Gaia data for the selected star.

This catalog work with Skychart version 4.2 but searching by EDR3 name require a version of Skychart higher than 4.3-4238.

The files are split in three parts of increasing magnitude so you can limit your download size.\\
You can download each part using [[wp>BitTorrent_(protocol)|Bittorent]] for better reliability, open the torrent files in your [[wp>Comparison_of_BitTorrent_clients|bittorrent client software]] to download the real data:

  * [[https://sourceforge.net/projects/skychart/files/2-catalogs/Stars/GAIA_EDR3/GaiaEdr3_1.torrent/download|GaiaEdr3_1.torrent]] : The stars up to magnitude 15, download size 1.2 GB
  * [[https://sourceforge.net/projects/skychart/files/2-catalogs/Stars/GAIA_EDR3/GaiaEdr3_2.torrent/download|GaiaEdr3_2.torrent]] : The stars from magnitude 15 to 18, download size 7.8 GB 
  * [[https://sourceforge.net/projects/skychart/files/2-catalogs/Stars/GAIA_EDR3/GaiaEdr3_3.torrent/download|GaiaEdr3_3.torrent]] : The remaining stars from magnitude 18 to about 21, download size 39 GB

For convenience you can also download the files from your browser using http, but this is likely to take more time and is less reliable than the torrent: [[https://vega.ap-i.net/pub/GaiaEDR3/]]

Extract all the 3 zip file on the same directory, preferably in a "gaia" directory in your Documents folder, do not extract in "Program Files" or in /usr. This make the following directories:\\
Documents/gaia/gaia1\\
Documents/gaia/gaia2\\
Documents/gaia/gaia3

From Skychart, menu Setup/Catalog check Gaia and indicate the full path for Documents/gaia , not for any of the sub-directories.

{{:en:news:cat-gaia.png?350|}}

As many bright stars are still missing from the Gaia data you must also check Hipparcos at the same time as Gaia to add them.

It is mandatory to install the lower parts, you cannot install only gaia3 without gaia1 and gaia2. \\
But you can install only gaia1 if the magnitude 15 is sufficient for you.

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===Use a database prepared for HNSKY===

The [[http://www.hnsky.org/software.htm|HNSKY]] software use a very interesting compact format that is complimentary to the Skychart format. Many thanks to Han Kleijn for building this database and to help me to use them from Skychart.

You can make your choice between four versions of different limiting magnitude, with color or not. 
  * V16 COLOUR, Gaia eDR3 up to Johnson-V magnitude 16, containing 60 million stars, including star colour information, native 290-6 format, 346 MB, epoch 2025.
  * V17 COLOUR, Gaia eDR3 up to Johnson-V magnitude 17, containing 120 million stars, including star colour information, native 290-6 format, 695 MB, epoch 2025
  * G17 Gaia eDR3 up to BP-magnitude 17, containing 105 million stars, native 290-5 format, 503 MB, epoch 2025
  * G18 Gaia eDR3 up to BP-magnitude 18, containing 205 million stars, native 290-5 format, 996 MB, epoch 2025

You can download them from:
https://sourceforge.net/projects/hnsky/files/star_databases/

After installing the data, open the menu "Setup / Catalog / Other software data" to select the file path and the database you want.

{{:en:news:cdc_hnsky.png?350|}}

A big advantage of this files is the compactness, you can have very good star data to magnitude 16 in a download of only 346 MB.\\
You can share the same data with HNSKY, ASTAP and CdC, no additional download is required.\\
But this is at the cost of limited data for each star: coordinates, magnitude and color.

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===Note about the star magnitudes===

GAIA use its own [[https://www.cosmos.esa.int/web/gaia/edr3-passbands|specific passbands]] for the star magnitude.\\
On a first approximation you can imagine G as equivalent to unfiltered CCD, BP as the full visual range (B+V+R), and RP as the I near infrared.\\
The Skychart catalog show this three magnitudes and in addition it compute a V magnitude and a B-V color index using the relation given in the [[https://gea.esac.esa.int/archive/documentation/GEDR3/Data_processing/chap_cu5pho/cu5pho_sec_photSystem/cu5pho_ssec_photRelations.html|GAIA documentation]]. The star diameter on the chart is relative to the G magnitude.\\
The HNSKY database V16 and V17 use the V magnitude and show the BP-RP color index as well as B-V computed using the same relation. G17 and G18 use the BP magnitude.

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