Last Revision: 5-July-2005
This document describes a minimal set of keyword definitions which permit application of SSW software tools to existing and new solar data sets. While the definitions are described as FITS Keywords, these are equivilent to the IDL structure tag definitions. Many SSW utilities operate on either FITS files or IDL structures and there are SSW routines which interconvert between the two (fitshead2struct.pro and struct2fitshead.pro). One very important implication of this interconvertability is that existing FITS files which do not include one or more of these tags are easily brought into the fold by definition of a function which adds the missing tags or adjusts incorrectly defined tags to the structure interpretation. This type of function is easily written using SSW structure manipulation SW. You do not have to regenerate any FITS files! An additional implication is that there is NO file format imposed or implied by SSW - it is an IDL based system which just happens to have alot of FITS users and includes extensive FITS support utilities. For example, the routine mreadfits.pro reads multiple 2D FITS files into a vector of IDL header structures and a corresponding 3D data cube which allows vectorized treatment of the large FITS image data bases, simplifies movie making, and offers many other advantages over traditional FITS processing techniques. The routine struct2ssw.pro is available to add or modify "partially conforming" data bases - for example, if any one of a few possible time or pointing standards is present, the others are derived according to the SSW definitions described below and included in the output structures. This facilitates integration of new data bases and maximizes the use of downline software available to the data base.
Use of these suggested standards permit immediate use of many utilities available under SSW including:
Many thanks to Dominic Zarro, Markus Aschwanden, Jean-Pierre Wuelser, Bill Thompson, Craig DeForrest, Richard Schwartz, Mons Morrison, Jeff Newmark, Russ Howard, and other SSW contributors for participating in discussions or providing documentation which led to distillation of these minimal SSW standards. Special thanks to Craig DeForrest for the use of his ztools WWW documentation which was used as the starting point for this document.
|FITS std||The width and height of the image, respectively, in pixels.|
|FITS std||The reference pixel coordinates.|
May be zero
The reference data coordinates corresponding to |
|FITS std||The width and heighth of a pixel in data units. For square pixels, |
|FITS std||Definition of the data units - ie., the interpretation of |
|FITS std|| Rotation angle of the image about the axis perpendicular to the plane
of the image, through the point |
|Suggested||World Coordinate System||Units along the data axes, e.g. "arcsec", "deg". See Greisen and Calabretta below for more information.|
|Suggested (if ||World Coordinate System||Transformation matrix, used in place of |
|Suggested||De Facto Solar Convention||East-West FOV center of image relative to sun center in
|Suggested||De Facto Solar Convention||North-South FOV center of image relative to sun center in
|FITS std||The time and date of the start of the observation, in
UT at the observatory or spacecraft, in CCSDS ASCII Calendar Segmented (ISO 8601).
At least one of
|W.Thompson, R.Howard||The time and date of the start of the observation, in Earth-adjusted UT in CCSDS ASCII Calendar Segmented (ISO 8601). Example: 1988-01-18T17:20:43.123|
|FITS Std||The duration of the observation, in seconds (floating point).
Permits calculation the "mean time" of an observation with the
agreed standard for |
|Yohkoh / SOHO||Milliseconds of day (long integer)|
|SOHO||Modified Julian Day (long integer). Used in conjunction with TIME to provide efficient binary time representation.|
|SMM / Yohkoh||Integer number of days since 1-1-79 (short integer). Used in conjunction with TIME to provide efficient binary time representation.|
|Common usage||The time at which the start of the observation occurred, UT. This
tag is not explicitly defined in the FITS standard, but has been adopted
into common usage. (To access this tag in an IDL structure, remember to use
|FITS Std, |
|The name of the telescope, observatory, or mission (string) .|
For example,"Yohkoh", "SOHO", "GOES-M", "TRACE". Important for space based observatories which do not have an earth-like view (L1 for example) since transformations will depend on satellite and 'apparent' solar ephemeris.
|FITS Std, |
|The name of the specific instrument (string).|
For example, "SXT", "EIT", "SXI-M", "TRACE"
|W.Thompson, R.Howard||Wavelength of observation. Some SSW instruments have chosen
to use a string descriptor, particularily for broadband observations |
Pointing registration is accomplished in 2-D by defining a Cartesian coordinate system that lays upon the natural pixel coordinate system of the image. This "unitted" coordinate system is defined by a single datum of correspondence between the two systems, a scale for the unitted system, and a rotation angle.
Pixellized images are assumed to be arranged in horizontal rasters running left to right, which in turn are stacked from bottom to top. X coordinates run left to right and are the fastest-running index; Y coordinates bottom to top and are the slower index.
There is an additional complication: FITS uses a different pixel coordinate system than does IDL. IDL takes (0,0) to be the lower left corner of the image, while FITS takes (1,1) to be the lower left corner.
Each pixel is taken to be centered on the appropriate coordinate:
the extreme lower left corner of the lower left pixel is (-0.5,-0.5) in
IDL pixel coordinates; or (0.5,0.5) in FITS pixel coordinates. This is
important, for example, when keeping track of
The "unitted" coordinates in the image are themselves the subject of some ambiguity. The SOHO and Yohkoh communities seem to use "Solar" coordinates a lot; these coordinates have the names "solar-x" and "solar-y", and are angular coordinates defined only for small angles away from Sun center, from the vantage point of the Earth.
Images are usually maps of some scalar quantity against 2-D angle; however, from a particular vantage point and for small angles (such as solar images as seen from Earth), they may be converted to maps versus position. The conversion requires knowledge of the observer's position relative to the Sun . This is accomplished with by keeping track of the observer's B-angle ('colatitude'), longitude, and radius from the Sun
Many generic solar SW utilities are available under SSW for things like solar ephemeris, grid overlay, NOAA AR overlay, image co-alignment, solar feature tracking and subfield extraction from full disk data sets, etc. Major contributors in these areas include T. Metcalf, J.P.Wuelser, G.Slater, L.Wang, B.Handy, M.Morrison, C.DeForrest, D.Zarro, W.Thompson, and S.Freeland.
Dominic Zarro has recently developed a powerful, object oriented suite of image mapping, rotation, reprojection, and overlay tools which can interface to solar data sets which provide the pointing information and time references keywords described in this document.
The ztools package by Craig DeForrest provide passing support for some coordinate systems that are not semi-cartesian. In particular, heliographic coordinates and image-plane radial coordinates are supported. Currently, there is no universally accepted way to specify an image's coordinate system explicitly; Craig has worked around the problem by naming the coordinates themselves. For example, a map of the solar magnetic field in heliographic coordinates would have CTYPE1="degrees-longitude" and CTYPE2="degrees-latitude". Craig has made a start at implementing a general coordinate-system naming regimen, with the V4 package of IDL routines; if you have a suggestion for how to incorporate coordinate system naming into FITS images, please let him know.
It's important to specify the time at which an observation occured. Unfortunately, the standard ways to do this are complex and have been rendered more complex by some of the historical accidents of FITS interpretation with IDL in the solar community. In addition, the time given in the header is not well defined by either the FITS standard or the SOHO documentation.
The de facto (and now formal) standard for
is to use the start time of the observation. (Some
instruments, eg MDI, use the middle of the observation, as that
represents a better "mean time" for the observation as a whole;
caveat emptor!) For timing critical investigations, where the mean time of
the observation is needed, people can use
EXPTIME/2 as the time of the observations.
As instruments get spread around the solar system, speed-of-light effects
become important to observation timing. The SOHO standard for the
time in the
DATE_OBS stamp is "Earth-adjusted UT" --
that is, the UT time at Earth when an observer would have seen the same
feature as is pictured.
One should be aware that the solar radius is 2.3 light seconds. This leads to some small temporal effects for spacecraft (such as STEREO) that will subtend significant elongation angles. For low-altitude coronal studies that require less precision than, say 10 seconds this should not be a problem. However, angle-induced timing errors of up to 1 minute will be seen in wide-field spectrographs in other parts of the solar system. If STEREO does indeed include LASCO-type instruments, we will probably need to deal with this.
On the other hand, the FITS standard keyword
defined to be the time at the observatory, i.e. unadjusted for speed-of-light
effects. This puts the onus on the analysis software, but avoids such
viewpoint ambiguities. The expectation is that the SOHO
DATE_OBS keyword will be superceded by the new standard for