### 9 Retrieving Remote Images and Catalogues

When using remote facilities, such as those described in this recipe, you should ensure that you are both familiar with and comply with any obligations which they impose on you, such as correct acknowledgement of the use of data or services etc.

This recipe demonstrates how to retrieve an image of a specified region of sky from a remote archive and to overlay it with objects extracted from a remote catalogue. In both cases the data are retrieved via the Internet. As an example a region centered on the galaxy NGC 1275 will be used. Recall that the J2000 coordinates of this galaxy are:

$\alpha =$ $3h\phantom{\rule{0.3em}{0ex}}19m\phantom{\rule{0.3em}{0ex}}48\text{}s\phantom{\rule{0.3em}{0ex}}\phantom{\rule{0.3em}{0ex}}.\text{}16$,    $\delta =$ $+41\circ \phantom{\rule{0.3em}{0ex}}3{0}^{\prime }\phantom{\rule{0.3em}{0ex}}42\text{′′}1$

A region will be extracted from the on-line version of the DSS (Digitised Sky Survey)9 at ESO. The DSS is a photographic survey which covers the entire sky. It was constructed by the Space Telescope Science Institute (STScI) by digitising and combining surveys conducted with the Palomar and UK Schmidt telescopes (a UK mirror10 is available as part of the LEDAS data archive service at the Department of Physics and Astronomy, University of Leicester). The DSS image will then be overlaid with objects extracted from the USNO11 PMM astrometric catalogue[13].

The SuperCOSMOS image surveys12 also available from GAIA return images which have catalogues of objects detected in the images already attached and so combine the two stages into one operation. However, the SuperCOSMOS surveys currently only cover the sky south of Declination $+{3}^{\circ }$.

If the coordinates of your target region or object are not for epoch and equinox J2000 then you should convert them to this system. COCO (see SUN/56[18]) is available for this purpose. Though the target coordinates can be converted within GAIA it is probably better to convert them prior to starting it.

(1)
Start GAIA. Type:
%  gaia &

The ampersand (‘&’) is simply to run GAIA as a detached process, so that you can continue to issue Unix commands from the command line. After a few moments the main GAIA window should appear.

(2)
The first step is to retrieve a two-dimensional image of the region of sky containing your target region or object. Click on the Data-Servers menu, located towards the right of the menu-bar at the top of the main window. Choose the Image Servers option and then Digitized Sky at ESO from amongst the list of servers (it will probably be the only choice). If the Digitized Sky at ESO is not amongst the choices offered then see Section 9.1, below.

A window similar to Figure 3 and titled Digitized Sky at ESO (1) should appear; it allows you to specify the region of sky to be retrieved.

• You can simply enter the name of your target object in the Object Name: box, if you know it. GAIA will attempt to resolve the name of the object and look up its coordinates. Here simply enter ‘ngc 1275’ (any embedded spaces are ignored).
• However, often you will need to enter the coordinates directly. The units and formats required are:
$\alpha$
(Right Ascension): sexagesimal hours with a colon (‘:’) as a separator,
$\delta$
(Declination): sexagesimal degrees with a colon (‘:’) as a separator.

In the present example the values required are:

$\alpha$:
3:19:48
$\delta$:
41:30:42
• You also need to specify the height and width of the region to be retrieved. These values are specified in minutes of arc. In the present example values of 7${}^{\prime }$ should be specified, which is slightly larger than the field of view of the V band CCD image of NGC 1275 used in the previous recipe (Section 8).

When the values are set, click on the Get Image button. After a few moments the retrieved image should be displayed in the main GAIA window. A small window entitled ‘FITS HDUs (1)’ may also appear. Click on Close to close this window. Then click on Close to close the Digitized Sky at ESO (1) window.

(3)
You may wish to save the retrieved image as a file for future use. Click on the File menu (the leftmost item in the menu-bar along the top of the main window) and choose the Save as… item. A window allowing you to save the image as a file will appear. The file will be written in FITS format. It is usual to end the file-name with a file-type of ‘.fits’ or ‘.fit’.
(4)
You may wish to adjust the appearance of the displayed image. The most likely items to change are Colors… and Magnification in the View menu (second from the left in the menu-bar along the top of the window).

For example, click on the View menu and select the Colors… option. A panel will appear. Set:

• the colour scale algorithm to Linear,
• the colormap to ramp,
• the intensity to neg.

Then click on the Close button. Set the magnification by clicking on the Scale: button (in the bottom left of the control panel in the centre top of the window) and setting it to 2x.

The display should now look something like Figure 4, which is a a reasonable approximation to the appearance of the original photographic plate.

(5)
Various remote catalogues are accessible from GAIA. The present recipe extracts objects from the USNO13 PMM astrometric catalogue[13] and overlays them on the image.

First click on the Data-Servers menu, towards the right of the menu-bar at the top of the main window. Choose the Catalogs option and then USNO at ESO from amongst the list of catalogues (it will probably be towards the bottom of the list). If USNO at ESO is not amongst the choices offered then see Section 9.1, below.

A window similar to Figure 5 and titled USNO at ESO (1) should appear; it allows you to retrieve objects from the catalogue. The central position and minimum and maximum radius should be already filled in (the values have been obtained from the two-dimensional image). The ‘Brightest (min):’ and ‘Faintest (max):’ boxes allow these quantities to be set, if desired. However, in the present example they can be left blank, and all the objects in the PMM which overlay the image will be selected.

Simply click on the Search button (in the bottom left of the window). After a couple of moments the selected objects are listed in the Search Results box and overlaid on the image in the main window.

A useful feature for identifying objects in the list with the corresponding plotted symbol is that if you position the cursor over either a plot symbol or a row in the table and click with the left mouse button the corresponding row and plot symbol are highlighted.

To save the selected objects as a text file click on the File menu in the USNO at ESO (1) window (the leftmost item in its menu-bar) and choose either Save as… or Print…. In both cases a window will appear which allows you to save the list. Note that though both options produce text files they are in different formats.

If you choose the Save as… option the catalogue of objects will be saved as a file. The format in which this catalogue is saved depends on the file-type specified at the end of the file-name (see Section 4.2). Catalogues saved in the FITS tables, TST or STL formats can subsequently be imported into CURSA (see SUN/190[5]) which provides additional catalogue manipulation facilities.

When you have finished click the Close button to close the USNO at ESO (1) window.

(6)
To close GAIA click on the File menu (the leftmost item in the menu-bar along the top of the main window) and choose the Exit option.

#### 9.1 Restoring catalogue access

It is possible to configure the set of catalogues and sky surveys which GAIA can access, though the details are not germane here. However, if the Digitized Sky at ESO survey or USNO at ESO catalogue do not appear in the lists of Image Servers and Catalogs respectively then the most likely reason is that the default list of catalogues and surveys has been substituted with one which does not include them. The simplest way to restore access is to revert to using the default list. GAIA configuration files are kept in subdirectory .skycat of your top level directory. To restore the default catalogues and surveys you should delete (or rename) file:

~/.skycat/skycat.cfg

and then restart GAIA.

#### 9.2 Using a proxy server

Access to the World Wide Web at your site may be restricted (by a firewall or maybe just by policy), so that it is only available through a ‘web cache’ or proxy server. If you are in this situation you will need to configure GAIA so that its remote catalogue access will work. Proceed as follows.

(1)
Start GAIA, select the Data-Servers menu (in the menu-bar along the top of the main window) and choose any remote catalogue.
(2)
The catalogue window will appear (similar to Figure 5). Click on the Options menu in its menu-bar and choose the Proxies… item.
(3)
a Proxies window will appear. You should fill in the various fields (consult your system manager if you are unsure what to enter). Then click on OK.

9http://stdatu.stsci.edu/dss/

10http://ledas-www.star.le.ac.uk/DSSimage/

11http://www.nofs.navy.mil/

12http://www-wfau.roe.ac.uk/sss/

13http://www.nofs.navy.mil/