Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/hst/training/events/VO/specView-hands-on.pdf Дата изменения: Thu Mar 30 00:15:53 2006 Дата индексирования: Sun Dec 23 05:27:30 2007 Кодировка:

Specview VO exercise


The purpose of this exercise is to build a composite, wide band spectrum of
an astronomical source, retrieving from the VO, data from several
observatories and instruments and combining them into a single Spectral
Energy Distribution.

You can choose any object you like for this exercise, but a good candidate
is 3C273. You can also install the Specview desktop application, but an
easier way is to use its applet form, which can be run from here:

http://specview.stsci.edu/applet/specview_applet_run.html

You can also reach this link from the Specview home page, by clicking on
the "Run now" link, and then going to the "Run" link. Note that your
browser must be equipped with the Java 1.4 or later plug-in.

Before loading, the applet will ask you to accept a security certificate.
Answer "Yes", otherwise the applet will be prevented, by the Java security
mechanism, from accessing the network.

Invoking the VO interface

On the Specview main menu bar, select the "File" menu. Then select the
"Read from VO" menu item. This will cause Specview to retrieve from the VO
registry, information about all spectral services available at the moment.

Specview then uses the registry information to build its VO interface. It
displays two windows: one contains all that is required to access data from
the VO. The other, on the top right corner of your screen, is used to
display a list of the data already downloaded. It is obviously empty at
this point.

Accessing the VO spectral servers

The VO interface contains, at the top, fields that you use to type in
information about the object or objects for which you want to retrieve
spectral data. Once that information is properly entered, you click the
"Search" button. This causes the spectral servers to be queried about data
that fulfill your request.

Each server will return a table with a summary of its data holdings that
satisfy your query. These tables are displayed at the bottom half of the VO
interface window, one table per server. Each server gets a tabbed panel to
hold its table. If no data is found on a given server, a message is
displayed in the corresponding tabbed panel.

The easiest way to fill in the required information is to type the object
name, in this case 3C273, in the "Name" field at the top. Hit the
Enter/Return key on your keyboard, or click the "Resolve" button. This
causes the selected name resolver service to be queried, and the R.A. and
Dec. coordinates of the object automatically will fill in the appropriate
fields. Alternatively, you can type in the coordinates directly.

The "Radius" field can be left at its default value for this exercise.

Now click the "Search" button and observe the tabbed panels being filled
with the returned information from the servers. Sometimes servers misbehave
or are dead altogether, thus you may get one or more error dialog windows
that report what happened. Ignore them.

Selecting and downloading spectra

The table returned by each server is server-specific, thus we have no way
of predicting the exact way a table will look like. Each row contains a
description of a single piece of spectral data. One can scroll the table,
move columns around by dragging the column header, resize columns by
dragging the column header edges, or sort on a column by clicking on the
column header. Hierarchical sorts can be performed by holding the "Ctrl"
key down while clicking on successive column headers.

For this exercise, we are going to download one single spectrum from each
one of the servers that have 3C273 data. Click on the tab of each server
tabbed panel to select the server. If a table is present, click on the
first row of that table EXCEPT FOR THE ISO SERVER! On the ISO server,
select the third row. Once the row is selected, click the "Download" button
and observe the "Downloaded" column to see the downloading status. Each
successful download should result in a YES message in the Downloaded
column, and the spectrum name should show up on the Specview window at the
top right corner of your screen. No spectrum is plotted though. If ERROR is
displayed instead of YES, try again. Some servers can be quite stubborn.

Once all selected spectra are downloaded, the VO part of this exercise is
done!

Quick look

On the Specview window at upper right on your screen, select all spectra by
clicking on their names with the "Shift" key held down. Then click the
"Tile" button. This causes a "quick look" tiled plot to be displayed on a
floating window.

Combine spectra

With the same spectra selected, click now on the "Plot/Coplot" button. This
will cause the spectra to be co-plotted together on the main Specview
window.

A more appropriate plot for this kind of data would be a log(?F? ) against
log(?) plot. First, click on the "U" button at top right. Select e.g. Hz
and erg/cm^2/s to build a plot of ?F? against ?.

Then move the cursor to any one of the four corners of the plot surface,
until the cursor becomes a little hand. Click the mouse. Select log-log and
click the "OK" button.

Dragging the mouse zooms over selected regions on the plot. Left and right
clciking the mouse over pieces of spectral data brings up to screen more
information about the specific piece of data (FITS headers, data in tabular
form, and more). Specview has many features which just cannot be properly
demosntrated in the limited scope of this exercise. Take your time and play
with the controls.