Another way to access the HEASARC online data is by the World Wide Web (WWW).
The Web is a distributed hypermedia system that was initiated at CERN in
Switzerland. There are a number of WWW clients, such as the Mosaic software
developed at NCSA (National Center for Supercomputing Applications), or Lynx.
Looked at most simply, NCSA's Mosaic is a GUI that provides a way of unifying
numerous services, such as viewing structured documents written in HTML
(HyperText Markup Language), with anonymous ftp, WAIS (Wide Area Information
Server), ADS (Astrophysics Data System), Gopher services, and many others, by
wrapping them in one common interface. The Web system is so versatile that we
expect that it will soon be the dominant way in which most databases such as
ours are accessed. Its outstanding features include the following:
* Its hypermedia capabilities: the user can view text, figures, color images,
and movies, as well as listen to audio features. Many of these features do
require additional (usually public domain) software (for example, to get full
image display capability requires having something such JPEGView and /or
GIFConverter), but these are generally readily available.
* Its vast number of interconnections with information services all over the
globe.
* Its Mosaic interface, which generally requires a user only to
"point and click."
For example, on the Mosaic display, any item that is underlined is
an anchor, or hyperlink, either to the page describing that item, or, in some
cases, to another type of service. Clicking on the item ``HEASARC's Gopher
server" will connect the user to the Gopher server; clicking on ``HEASARC's
Online Service" will connect the user to our standard online service; clicking
on ``Images" moves the user to the ``Images from High-Energy Astrophysics" page
from which the user can select a particular mission such as ROSAT (again by
clicking), and move to the ROSAT Images page. From that page, the user is
presented with a list of images, small pictures which give an idea of what the
full image contains, and their captions; clicking on a caption will trigger a
request for the full image which can then be displayed using one of the above
mentioned image display programs.
The principal disadvantage of the WWW, if it can be so described, is that it
allows the user to access vast amounts of data very easily, and the data
transfer rate can, in consequence, be very slow. Also, the data storage of the
user's own computer (if it is a small Macintosh, for example) can be taxed.
Given the increasing availability of high data rate connections and the growing
memory and processing time capabilities of most computers, this disadvantage is
a minor one compared to the WWW's manifold advantages.
The software for the Mosaic interface to WWW is available from the NCSA. There
are versions of Mosaic for X11, Macintosh, and Microsoft Windows, all of which
can be obtained from
ftp.ncsa.uiuc.edu.
When Mosaic has been installed on your local computer, you can access the
HEASARC at
http://heasarc.gsfc.nasa.gov/
or by going to the NASA page, clicking on Goddard Space Flight Center (GSFC),
and then, on the GSFC page, clicking on the HEASARC.
From the HEASARC home page, the user can access a rapidly growing body
of information and services. The following section describes these links
in greater detail.
In addition, a WWW client called Lynx is available for users without the
graphics capabilities of X11, Macintosh or Windows. Lynx is a full-screen
browser allows text-only navigation of the WWW for users with ``dumb"
terminals. It has kept up with developments in Mosaic, and therefore recognizes
the same protocols as Mosaic (i.e. forms). Lynx can be obtained from:
The HEASARC now maintains an anonymous ftp facility from which HEASARC software
and data products are available.
ftp is a command-driven program in the traditional style with no graphical
capabilities. To list the contents of a directory, type the command ``ls" or
``dir." To change to another directory called fred, type ``cd fred." The
advantages of ftp are that most users are familiar with it and have access to
a local computer on which it is installed, and that its commands are similar to
standard Unix commands. The disadvantages are its lack of graphics capability,
as well as its limited capability to allow a user to view the contents of a
file without having to transfer it to his or her own computer.
The user must simply type ``ftp heasarc.gsfc.nasa.gov" and, when prompted, log on
as username = ``anonymous" (or ``ftp"), passowrd = email address. A short ASCII
text file called ftp_hints.asc in the top-level directory contains a brief
discussion of common problems and their solutions, and some hints that might
help an inexperienced user of anonymous ftp.
It is recommended that users accessing a directory for the first time read the
relevant README files. The short message that appears when entering directory
(the .message file) also often contains important information. The welcome
message to the anonymous ftp account that appears after log in is also ``must
reading" as it is used to announce major changes and/or enhancements to
databases and also to give advance notice of planned down-times and
configuration changes.
Access to legacy by Gopher is available.
Recently, the HEASARC installed a Gopher server on legacy that supports
access to the database through Gopher clients. Gopher was developed at the
University of Minnesota in 1991 and adds the attributes of a graphical user
interface to an anonymous ftp account. The following additional features also
make it much more convenient than ftp:
The ability to create bookmarks marking locations in a directory tree that a
user might frequently want to access. Once created, this enables the user to
move straight to this area without having to go through multiple steps.
The ability to spawn programs on the user's own computer.
The ability to display files immediately so that they can be previewed: the
user can route non-text files such as PostScript files through a display
program such as GhostScript so that whole documents including figures and
equations can be perused.
The ability to do searches in a directory for files with names containing the
specified alphanumeric string.
The ability to connect to other Internet services without having to know
details such as e-mail addresses or node names. For example, if one ``Gophers" to
the main NASA/Goddard Space Flight Center Gopher (gopher.gsfc.nasa.gov), one
can, by clicking on the appropriate option, transfer to the HEASARC Gopher
server.
To use the HEASARC Gopher utility, Gopher client software must be installed on
your home system. The Gopher client software is available either from
``boombox.micro.umn.edu" (138.84.132.2) or from ``heasarc.gsfc.nasa.gov". There
are Gopher clients for MS-DOS, Macintosh, NeXT, Unix, VMS, Windows, MVS, and
OS/2systems. Once Gopher is installed on your system, connect by Gopher to the
HEASARC Gopher by typing
