# GAIA DR2

### The second GAIA data release is now available and can be used with Skychart

The second Gaia data release, Gaia DR2, consist of astrometry, photometry, radial velocities, astrophysical parameters and variability information for about 1.7 billion sources brighter than magnitude 21.

DR2 sky coverage is now very good and this catalog can be use for general star chart. But many bright stars are missing so you must use it along with the default Hipparcos catalog.

You can read more detail about GAIA DR2 in the release documentation

### Use with Skychart

#### Online catalog

You can use the GAIA DR2 data with Skychart by using the VO catalog feature.

Make a chart with a FOV of less than one degree in the area of interest.
Open the Catalog settings.

Click Add in the VO catalog tab and search for GAIA, select the catalog I/345, click Select catalog.

Select the first tab I/345/gaia2, enter the default magnitude: 21, click Download catalog.

Make sure the GAIA DR2 row is green and click the OK button.
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.
Beware that the one degree FOV around M13 take 28 MB of data to download, so do not do it on large area.

#### 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 DR2 1328053954366555648
The Vizier link at the bottom of the detailed information window give you access to all the other Gaia data for the selected star.

This catalog format require you use a version of Skychart higher than 4.1.1-3736. Do not try with a previous version, this result in totally wrong name, proper motion and parallax.

The files are split in three parts of increasing magnitude so you can limit your download size.
You can download each part using Bittorent, open the torrent files in your bittorrent client software to download the real data:

- gaia-dr2-1.torrent : The stars up to magnitude 15, without Hipparcos stars, download size 1.1GB
- gaia-dr2-2.torrent : The stars from magnitude 15 to 18, download size 7.8GB
- gaia-dr2-3.torrent : The remaining stars from magnitude 18 to about 21, download size 36.4GB

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/GaiaDR2/

Extract all the 3 parts on the same directory, preferably in your Documents folder. On Windows you can use 7-zip, on Linux use “`unrar x gaia*.rar`”. This make the following folders:
…/gaia/gaia1
…/gaia/gaia2
…/gaia/gaia3

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

As the first part do not include the know Hipparcos stars to avoid duplicate plot of the bright stars you must also check Hipparcos at the same time as Gaia to add the bright stars.

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.

#### Use a database prepared for HNSKY

The 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 three versions of different limiting magnitude: 16 for visual use, 17 or 18 for photographic use.
They contain the star position, the magnitude and for the G16 the star color.
The bright stars are completed using Tycho-2 data so you can use only this database if you want.

They not include the star name, the proper motion, the parallax and the radial velocity. This is the price for the compactness but this values are not mandatory to make a nice star chart.

The download size is much smaller, between 300MB and 1GB, see:
https://sourceforge.net/projects/hnsky/files/star_databases/

Extract the files in your HNSKY directory or in any other folder.
From Skychart (version 4.1.1-3707 and up), open the menu “Setup/Catalog/Other software data”.
Indicate the path where you extract the files, check to use the HNSKY database, enter the mini and maxi FOV number as for the other catalog, and select the database to use if you have more than one at the same location.

#### Note about the star magnitudes

GAIA use its own 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 GAIA documentation. The star diameter on the chart is relative to the G magnitude.
The HNSKY database G16 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.

Do you get enough stars now?