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SkyView : An All-Sky-Survey
Data Service
Thomas A. McGlynn (NASA GSFC), Keith Scollick (NASA GSFC),
Nicholas E. White (NASA GSFC) & Conrad A. Sturch (STScI)
Abstract
SkyView is a new network service that allows users to get digital images
of selected regions from all-sky and large area surveys. In recent years,
digital information from such surveys has become much more widely distributed
within the astronomical community, but it remains difficult to use. Data come
in different formats, projections, coordinate systems, equinoxes and scales.
This diversity makes it tedious for users to get at the data. SkyView
addresses these essentially geometric issues - providing the data to users in a
convenient form, and allowing astronomers to concentrate on research rather
than on geometry. SkyView is also a single source for many of the most
useful astronomical surveys. This paper describes the capabilities and
contents of the current SkyView system and our planned enhancements to
the system.
Introduction
SkyView is an on-line service that allows users to pick a region of the
sky and then obtain images of it at a number of wavelengths. The user chooses
the projection, coordinate system, scale, orientation, and size of the desired
image. The user selects these parameters to match exactly existing data from
ground- or space-based observations, or to meet their research needs. The data
is transformed from the available all-sky and wide-area surveys according to
the users specifications and the user is given the resulting FITS file or
PostScript image of the region.
SkyView can be used for many different applications. Users can generate
finding charts, look for optical counterparts to sources seen in other
wavelengths, or check for time variability of sources. SkyView images
can be used to look at large-scale features and see if there are
correspondences among different wavelengths, or to look at very small regions
where the user has some of his or her own data. SkyView can be used
anywhere that a user needs an image of the sky in the wavelengths of the
surveys that SkyView covers. While it is not intended to be the
facility where one pushes data to its extreme limits, SkyView provides a
convenient, single access point to all useful publicly-available large-scale
surveys.
SkyView can provide coordinate and contour overlays and locations of
objects from a number of catalogs. Future versions of SkyView will
provide capabilities to contour one image over another and to generate
multicolor images from several inputs.
SkyView is now available within the HEASARC xray account on
legacy. To use SkyView, type skyview once you have
logged into the xray account. Users with special needs can get
personal accounts on the SkyView system.
Using SkyView
When a user logs into the SkyView systems (either directly or through
the XRAY account), he or she is presented with a simple form interface to
request images of the sky. The user makes the request of the system to generate
images. The request can specify the following tasks:
- Selection of data from one or more surveys centered on any point in the
sky
- Data sampling in any of gnomonic (tangent), rectangular, and Aitoff projections
(Other projections will be available soon)
- Data sampling in any of Equatorial, Galactic or Ecliptic coordinates
- Coordinate system overlays
- Overlays of objects from SkyView catalogs
- Arbitrary equinox for equatorial or ecliptic coordinates
SkyView takes care of automatically mosaicking data when the survey data
is split into multiple regions, resampling of data in the desired coordinate
system and scale, and other geometric issues. The data from the surveys are
resampled into the users' desired frames so that users need not worry about how
the data is stored, but only about how it can be managed most conveniently for
their own needs.
The user may specify that the image is to be displayed immediately or can save
it as a FITS or PostScript file. Files are retrievable to the users' home
system using anonymous ftp.
SkyView has extensive on-line help, both on how to run the system and on
the included catalogs and surveys.
The SkyView GUI
The SkyView graphical user interface uses IDL widgets to generate a
simple forms interface. The current appearance of the GUI is illustrated in
Figure 1. The menu may be thought of having five areas from top to bottom.
Since the SkyView system is still undergoing changes you may see some
small changes when you start up the system.
The top area contains pull-down menus. The FILE menu allows the user to start
a request, to clear user inputs and to exit SkyView. The TOOLS menu
allows the user to start tools that manipulate the image, e.g., change the
color table, smooth the image, and the like. The entries in this menu are
generally IDL tasks that perform simple image manipulation or display tasks.
The comments button displays a window that tells users how to contact
the SkyView managers. The OPTIONS box, which is grayed out until a user
has made an image request, allows the user to add overlays to an image, e.g.,
catalog positions or a coordinate overlay. Finally the HELP button starts the
internal SkyView help.
The second area from the top is the image specification area. Here the user
specifies the attributes of the image that is to be created. The user may
select the Coordinate System box to switch among the available coordinate
systems. The user then enters the coordinates for the center of the image in
the boxes below whose labels will change according to the selected coordinate
system. Coordinates can be specified in both hh mm sss (or ddd mm
ss) and ddd.fff notation where the assumption is that if the
string the user specifies contains a space, then it is in the former format,
otherwise the user has entered decimal degrees. RA is assumed to be in hours
when a space is included, but in degrees if not.
The Map Projections button allows the user to change the projection used while
the user enters the Scale in arcseconds per pixel. The scale is given at the
special point in the projection, e.g., at the point of tangency for the gnomic
projection and at the 0,0 point in the Aitoff projection. Finally the user
specifies the number of pixels in the image using the X and Y boxes.
Shortly, the system will be enhanced to allow arbitrary rotations of the
coordinates. The user also will be allowed to specify the center point of the
data independently of the special point of the projection. For example, in a
tangent (gnomonic) projection, SkyView currently places the tangent
point at the center of the image. This enhancement will allow the tangent
point to be anywhere in the image (or even beyond its borders). For the Aitoff
projection, the projection is made around the coordinate origin, e.g., (RA,Dec)
= (0,0).
The third section of the interface consists of a number of selection boxes.
Users may check from which IMAGE SURVEYS they wish to extract images. An image
will be generated for each survey. At least one survey must be checked. Zero
or more CATALOGS may be checked. Catalog sources that lie within the image
will be marked on the image, and the a table of these catalog objects displayed
(and included in any FITS output). The user can select one or more output from
in the OUTPUTS box. The OUTPUT OPTIONS allows the user a few more
capabilities. The user may ask for contours of the image to be plotted, or for
a coordinate grid. The user also may ask that the data be resampled more
carefully. Users also may ask to be queried for the names of any files as they
are created.
When SkyView needs to resample data, the user has a choice of how the
resampling is to be done. By default SkyView uses the nearest-neighbor
method for resampling. Here the program finds the closest pixel in the archived
data to the one desired and uses that value. This works well when the
user-defined scale is close to the data being retrieved, or when the archived
data is reasonably smooth. In practice, nearest-neighbor works remarkably well
for most of the SkyView surveys. However, when the user wishes to
retrieve data with much larger pixels than in the original data,
nearest-neighbor resampling may miss significant features in the data.
In this case, the user may choose to rebin the data before it is sampled by the
nearest-neighbor technique. The data is rebinned to generate pixels of
approximately the same size as the users' desired pixels. The new pixels then
are sampled using the nearest-neighbor technique. With the rebinning, the
nearest neighbor technique will conserve flux reasonably well even in data with
large fluctuations.
The development version of SkyView also has the capability of
integrating the flux over the pixel boundaries of the new image. This is the
most accurate resampling technique but is currently quite slow. We anticipate
this technique will be sped up significantly and offered as an option in the
standard version. If a user has a need for this capability immediately, he or
she should get in contact with the SkyView managers using the addresses
below.
The final output option requires a little more explanation. Users may wish to
compare SkyView images with their own data where the data may be some
pixel array with NX and NY even, and the image center given not for the center
of the image, but for the center of one of the pixels nearest the center. The
'center at pixel center' option makes SkyView put the user specified
coordinates at the center of a pixel rather than at the center of an image.
For example, suppose we have a 500 by 500 image with a center at RA=r, and
DEC=d. The center of the image is the corner formed by pixels
(250,250),(250,251),(251,250) and (251,251). If the option is not checked, the
this corner corresponds to the (r,d) position. If the option is checked, then
(r,d) will correspond to the center of pixel (250,250).
The fourth area of the GUI is a set of accelerator buttons that include
features available from the pull down menus at the top.
The final area of the GUI is a status display line at the bottom. Any errors
encountered are displayed there. During the retrieval of data, the status
display will show the various stages of image retrieval.
Planned enhancements to SkyView
Several major enhancements are planned to the SkyView system. A command
line interface will be provided. This interface will allow users who do not
have X-compatible terminals to use SkyView. Perhaps more important,
SkyView users will be able to generate and run command scripts.
SkyView will soon have more generic capabilities for specifying the
users desired coordinate system. Users will be able to specify arbitrary
rotations in the coordinate system and also will be able to generate maps where
the special point in the coordinate system is not necessarily at the center of
the image.
SkyView also will soon have multi-image capabilities which will allow
the user to compare two or more images. Users will be able to generate
contours of one or more images on an image display of another. SkyView
also will support multi-color overlays, where one image is used to supply each
of the primary colors.
Another major feature will be a capability for the user to download their own
images (in FITS) to be used in the image comparisons.
We hope to begin integration of optical survey data into SkyView by the
end of the year, and many more surveys and catalogs will be made available.
SkyView's current holdings
Surveys currently available or scheduled for ingest in SkyView are shown
in the table below. We welcome any suggestions for additional surveys, and in
the next few months, we anticipate adding many small surveys not shown.
Name Regime Resolution Available Notes
Bonn 21cm Radio .5 deg Now Dec >-19 deg
IRAS Infrared 90" Now 100, 60, 25, 12 mu m
HEAO A-2 X-Ray 1 deg Now Contains glitches
Preliminary EUVE UV 1 deg Now 555, 405, 171, 83 A
Palomar SS Opt 1.4" Starting 1994 Compressed data
Southern SS Opt 1.4" Starting 1994 Compressed data
COS-B gamma-ray 1.5 deg 1994 Galactic plane
COMPTEL gamma-ray 2 deg 1994
EGRET gamma-ray 1 deg Early 1994 Several energies
ROSAT WFC UV 1995
Final EUVE UV When available
The current catalogs, shown in the second table, are intended to be
representative of the kinds of catalogs that will be available. Adding catalogs
to SkyView is very easy and with SkyView's integration into the
ADS and HEASARC, the vast catalog holdings of those systems also will be
available.
Catalogs currently available within SkyView
Catalog Regime
4C Radio
Veron-Cetty Radio
Dark Nebulae(Lynds) Optical-nebulae
Bright Nebulae(Lynds) Optical-nebulae
HII (Sharpless) Optical-nebulae
FK5 Optical-stars
Bright Star Optical-stars
POSS Plate centers Optical-observations
Seyfert (Weedman) Optical-galaxies
RNGC Optical-galaxies
Markarian Optical-galaxies
Uppsala Optical-galaxies
Abell Optical-clusters
4th Uhuru X-ray
How do I get started?
To start SkyView use please enter skyview in the HEASARC
xray account. Documentation for SkyView is available in the
/skyview/doc directory of the legacy anonymous ftp area. This
documentation includes a preliminary users guide and design document. If you
have questions or problems please get in touch with:
Keith Scollick, scollick@skview.gsfc.nasa.gov (301)-286-8143 or
Thomas McGlynn, mcglynn@grossc.gsfc.nasa.gov (301)-286-7743.
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