# Virtual Observatory interface

From the menu: Setup → Catalog → VO Catalog → Add

This screen enables you to select and download data from more than 9000 catalogs available from the Vizier Virtual Observatory interface.

## Catalog selection

The first time you open this window you need to select a Vizier mirror near your location.

There are two ways to get a list of the catalogs:

- By name : type some words of the catalog name, an acronym, an object names, or the survey name.
- Around current position : this will search for catalogs with data around the current map center coordinates.

Scroll the list to select the line with the data you want and press Select catalog to go to the next screen.
For an example, here we select the Cepheids in M101 galaxy.

## Table and parameters

Be careful and take your time to examine this screen. There are some options that greatly affect the final result. Despite the program tries to determine the best settings from the data it receives, you probably want to alter a few for your display.

The next step is to click on the top tabs to select the data table you want from the catalog. Only the tables that contain the object coordinates are included because these are required to plot the objects on the map.
For example, here the table 2 lists the reference stars and table 3 the M101 Cepheids, so we select table 3.

The list contains all the fields of the table. With a “x” in the first column the field is selected to be included in our download.
The program will use the UCD field descriptions and the units to recognize the data. Review the list carefully to be sure the required data to plot the object is selected:

- identification (ucd = meta.id,meta.main)
- magnitude (ucd = phot.mag,em.opt.V)
- size (ucd = phys.angSize)

If the table contains more data of a similar type (same UCD), you can select below which column to use on the map for the magnitude, the size and the identification. You can also change the name prefix by another text than the column title.
The Units are also important, don't expect the program to plot the object at it's correct size if it is given in millimeters on the photographic plate!

At this stage it may helpful to have a look at the actual data. Click the Data preview button to load a sample in the screen described below.

Below the list are the table name, the total number of records, the table description and a check box to select if you want to download the full catalog or just the data at the current map position.

On the left you can select:

- The default magnitude to use if the magnitude data is missing.
- The object type or symbol to plot on the map
- The default size in arc seconds to use if this data is missing.

To override the default color for the type of object: Check “Force color” and click on the color square.

Then check the kind of drawing “as Star” or “as Deep sky object” in the middle box.
Sometimes the program makes a wrong choice. For example it wants to plot a star catalog as DSO because it includes some angular measurement wrongly taken as the object size.
Or as with the Cepheids in our example, we prefer to draw the star position with a symbol, so we select DSO.

When all is ready press the Download catalog button.

When it is completed, press the Close button to return to the catalog settings, or the Back button to return to the Catalog selection.

## Data preview

This is just a preview of a few rows of the data you selected in the previous screen.

Press the Back button to return.

## Result on the map

Our Cepheids example are the green squares.
We previously selected the M101 HII region as blue circles, the HyperLeda catalog as pink lozenges, and the stars from the NOMAD catalog.
Note that the VO button on the top bar is checked to plot this additional data.

If you need more powerful functions to select the data of interest you can use Topcat with the SAMP interface.