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Ed. Note: The FITS Basics and Information is revised from 
time to time.  This version was last updated on 
February 1, 1996.  The current version is available on 
the World Wide Web at the following URL: 

This document was provided with permission from the
FITS Support Office, which is sponsored by the Office of 
Space Science at NASA Headquarters and operated under the 
guidance of the Astrophysics Data Facility at the NASA 
Goddard Space Flight Center.


FITS Basics and Information


This Flexible Image Transport System (FITS)
information is provided by the FITS Support Office,
operated under the guidance of the Astrophysics
Data Facility at the NASA Goddard Space Flight
Center. It provides a brief description of FITS
and information on software and documentation,
discusses some topics that have appeared on the
Usenet news group sci.astro.fits, and answers some
questions on FITS frequently received by the FITS
Support Office. 

      What FITS Is 
      How FITS Evolves 
      What FITS Is Not 
   FITS Documents 
      Published Papers 
      Binary Tables 
      User's Guide 
      NOST Definition of FITS 
         Version 1.1 
         Changes from Version 1.0 
      Floating Point Agreement 
      Blocking Agreement 
      List of Registered Extensions 
      World Coordinates 
      Proposed Conventions 
   Software and Sample Data 
      FITS Support Office 
         FITS Product Conformance Tester with
         Header Lister 
         Error Test Files 
      ADC FITS Table Browser 
      FITS I/O software in IDL 
      Display of FITS Image Files 
         Major Astronomical Image Analysis
         IMDISP (IBM/PC) 
         Applications with XV 
         FITS and the Macintosh 
         FITSview (Windows) 
      World Coordinates 
   On-line Information Sources 
      FITS Support Office 
      HEAFITS exploder 
   FITS Support Services from GSFC 


What FITS Is

FITS (Flexible Image Transport System) is a data
format designed to provide a means for convenient
exchange of astronomical data between
installations whose standard internal formats and
hardware differ. A FITS file is composed of a
sequence of Header Data Units (HDUs). The header
consists of keyword=value statements, which
describe the format and organization of the data
in the HDU and may also provide additional
information, for example, about instrument status
or the history of the data. The data follow,
structured as the header specifies. The data
section of the HDU may contain a digital image,
but, except for the first HDU, it doesn't have to. Other
possible formats include tables and
multidimensional matrices that are not images. The
first HDU must contain a multidimensional matrix
or no data at all; the data in subsequent HDUs,
called extensions, may be of any type, consistent
with certain rules. The "Image" in FITS comes from
the original use of the format to transport
digital images, but it's not just for images any

FITS supports 5 data types in primary or IMAGE
data arrays: 8-bit unsigned binary integers,
16-bit twos-complement signed binary integers,
32-bit twos-complement signed binary integers,
32-bit IEEE-754 standard floating point numbers,
and 64-bit IEEE-754 floating point numbers. For
signed integers, the byte that includes the sign
bit is first and the byte that has the 1-bit as
its least significant bit is last.

FITS does not support the 16-bit unsigned integer
data type generated by many analog/digital
converters. Conforming FITS files can be produced
from such data by subtracting 32768 (decimal) from
the converter output before writing to the file,
while setting the BZERO keyword in the FITS header
equal to 32768 and the BSCALE keyword equal to 1. A
FITS reader will then add 32768 to the value in
the file, restoring the original value, before
interpreting it. Whether a 16-bit unsigned data
type should be added, and if so, how, is
controversial and under discussion, especially in 

How FITS Evolves

The International Astronomical Union FITS Working
Group (IAUFWG) was given authority over FITS
matters by the 1988 (IAU) General Assembly. This
Group is associated with the Working Group on
Astronomical Data. The current chair is D. Wells (
NRAO) and the vice-chair is E. Raimond (NFRA).
When the developer of a data structure finds that
it does not fit well into an existing standard
FITS format, a new design may be developed. No
change can be made that would cause existing FITS
files to be out of conformance -- the "once FITS,
always FITS" rule. A unique name for any new
extension type must be registered with the IAU
FITS Working Group, optionally through the FITS
Support Office. After astronomical community
discussion, most of which will be electronic, a
formal proposal is distributed. This proposal is
discussed by the community and may be further
modified. Tests are run using the new format to
confirm that it can be practically used for data
transport. If the astronomical community reaches a
consensus that the proposal should be adopted as
standard FITS, and if successful data transfer
using the proposed extension can be demonstrated,
it is submitted for ratification to the regional
committees--the European FITS Committee, the
Japanese FITS Committee, and the American
Astronomical Society Working Group on Astronomical
Software (WGAS) FITS Committee. Following approval
by the regional committees, it is submitted to the
IAU FITS Working Group. Approval by the Working
Group establishes it as a standard extension. 

What FITS Is Not

FITS is not principally a graphics format designed
for the transfer of pictures; it does not
incorporate "FITS viewers," packages for decoding
the data into an image. Users must develop or
obtain separate software to read and display the
data from the FITS file. There are a number of
different packages for particular applications and
hardware, but there is no single standard package
for all applications. 

FITS Documents

Published Papers

The fundamental references on FITS are the
following five papers. The first four have often
been referred to collectively as the Four FITS Papers.
These five papers, along with the Floating Point Agreement 
and the binary tables definition, are the formal
standard for FITS, endorsed by the IAU.

Wells, D. C., Greisen, E. W., and Harten, R. H., 
FITS: A Flexible Image Transport System, Astronomy and Astrophysics Supplement
Series, 44, 363-370, 1981.

Greisen, E. W. and Harten, R. H., An Extension of FITS for
Small Arrays of Data, Astronomy and Astrophysics Supplement Series, 44,
371-374, 1981. (NOTE: The format described in this
paper has been used almost exclusively to
transport radio interferometry and is likely to be
replaced by other formats in the future. Writing
data other than radio interferometry data using
this format is not recommended.)

Grosbol, P., Harten, R. H., Greisen, E. W., and
Wells, D. C., Generalized Extensions and Blocking Factors for FITS, 
Astronomy and Astrophysics Supplement Series, 73, 359-364, 1988.

Harten, R. H., Grosbol. P., Greisen, E. W., and
Wells, D. C., The FITS Tables Extension, Astronomy and Astrophysics
Supplement Series, 73, 365-372, 1988.

Ponz, J. D., Thompson, R. W., and Munoz, J. R., The
FITS Image Extension, Astronomy and Astrophysics Supplement Series, 105,
53-55, 1994.

Binary Tables

On June 15, 1994, the IAU FITS Working Group
announced the acceptance of BINTABLE, the binary
table extension, as a standard extension. The
description of the extension, to be published in
the Astronomy and Astrophysics Supplement Series,
is available in PostScript form from the National
Radio Astronomy Observatory.

User's Guide

A User's Guide for FITS, commissioned by NASA Headquarters,
is maintained by the FITS Support Office. It is
available in uncompressed Postscript, Z-compressed
Postscript, and LaTeX forms, with a style file for
the LaTeX version. This Guide is intended to be a
tutorial for new FITS users. In addition to
presenting the rules of FITS, it provides some of
the history and reasoning behind the choice of the
rules, adds recommendations on good practices, and
discusses current developments in FITS. The
current version, 3.1, was issued in May 1994. 

NOST Definition of FITS

Version 1.1

The NASA/Science Office of Standards and
Technology (NOST) has codified FITS as endorsed by
the IAU into a formal standard, the Definition of FITS.
In developing this Standard, some contradictions
and ambiguities in the original FITS papers have
been eliminated. The current standard, version 1.1
was approved as a NOST standard on September 29,
1995 by a NOST Accreditation Panel, replacing
version 1.0. The NOST Standard has been developed
by Technical Panels chaired by R. J. Hanisch (
STScI), with review by the astronomical community:
the first panel drafted version 1.0, and a new
panel drafted version 1.1, which incorporated new
text on units. The Accreditation Panel consisted
of the NOST Executive Board and an astronomical
community representative; it reviewed the process
of development to confirm that the community had
been given a satisfactory opportunity to review
the standard and that the Technical Panel had
properly considered and responded to all comments.

The NOST Definition of FITS is available in LaTeX form,
with style and index files, in uncompressed
PostScript, compressed PostScript, and in ASCII
text. The NOST standard has been submitted to the 
IAUFWG for endorsement as the international FITS
standard. Version 1.1 will replace the original
endorsed standard -- the four FITS papers, with
the addition of the Floating Point Agreement. While
oversights in non-controversial areas may be
rectified as a result of the review by the IAUFWG,
significant changes are unlikely because members
of this committee were active in the process of
reviewing the standard and their comments were
given significant weight in the deliberations of
the Technical Panel.

The IAUFWG has since endorsed the IMAGE and
BINTABLE extensions, and the agreement on physical
blocking. The Technical Panel that drafted the
changes between versions 1.0 and 1.1 will draft a
revised NOST Standard incorporating these
additions and also rectifying any oversights or
omissions that may be brought to its attention by
the community. Dr. Hanisch, the chair of the panel
that developed version 1.0 of the Definition of FITS, is
chairing this new panel.

Changes from Version 1.0

Version 1.1 was developed to add text covering
recommendations and requirements on units to be
used in FITS files. For the reference of holders
of version 1.0, the text of the substantive
revisions is available in flat ASCII text form.
The NSSDC Coordinated Request and User Support
Office (CRUSO) can provide printed copies and
electronic copies for those without ftp access.

Floating Point Agreement

Originally, FITS permitted only integers in the
data array following the first, or primary header.
The IAU has since endorsed the Floating Point Agreement,
which specifies the use of IEEE-754 floating point
and describes its use in FITS. The basic agreement
appears verbatim in the User's Guide, and the substance
is incorporated in the NOST Standard. 

Blocking Agreement

The IAU FITS Working Group has endorsed a set of 
rules for physical blocking of FITS files,
designed to address the requirement that many
controllers and devices for high density storage
media can access data only in blocks of fixed
length. These rules prescribe the number of
2880-byte FITS logical records in a physical block
and how to proceed when the block size is not an
integral multiple of 2880 bytes. They have been
formulated for fixed-block sequential media,
variable block sequential media, and bitstream

List of Registered Extensions

The rules for generalized extensions in FITS
require a unique name for every extension type, to
permit software readers to identify whether or not
an extension is of a type that the software can
handle. In order to ensure uniqueness, all
extension type names, even for local extensions
used only at one installation, must be registered
with the IAU FITS Working Group. The FITS Support
Office maintains the List of Registered Extensions.
This list includes a brief description of the
extension each type name identifies, the developer
or responsible organization, and the status of the
extension (e. g., standard, under discussion,
local). Also described is the procedure for
registration of extension type names. This list is
updated as new extension type names are proposed
and extensions with reserved names progress
through the process required for IAU FITS Working
Group approval. Consequently, it should be used as
the primary reference on registered extensions
rather than the appendix in the Definition of FITS or the
corresponding section in the User's Guide. 

World Coordinates

A draft text of conventions for World Coordinates
is currently under community review. It proposes
rules for describing the physical coordinate
values attached to each member of a FITS data
array, with detailed discussion of projections
from the celestial sphere to the array plane. This
draft and related documents are available from

Proposed Conventions

R. Seaman and W. Pence have proposed a scheme for
embedding a checksum within a FITS header. This
checksum could be used to verify that the data in
a file were transported without errors. 
Documentation is available by anonymous ftp from
the National Optical Astronomy Observatories
(NOAO). IAU FITS Working Group Chair D. Wells has
recommended that this proposal be considered by
the regional FITS committees.

D. Jennings, W. Pence, M. Folk, and B. Schlesinger
have proposed a convention for logically grouping
together FITS HDUs that are physically separated
in a given file or are located in different files.
This convention would facilitate HDU-FITS
conversion. It may be viewed with a Web browser or
downloaded in LaTeX or Postscript form. 

Software and Sample Data

FITS Support Office

FITS Product Conformance Tester with Instructions

The FITS Product Conformance Tester (FPCT), under
development by the FITS Support Office, is a
software package designed to validate FITS files.
The programs are coded in C. The available
prototype validates required keywords in the
primary header and, at the user's option, prints
selected values from the primary data array. Even
after finding an error in a required keyword, it
will continue to evaluate the file, looking for
additional errors in required keywords. If the
user specifies, and if major header syntax errors
do not prevent retrieval of the array dimensions,
the FPCT will try to read the primary data farray.
The separate instructions should be read before running the software. In
addition to providing directions for use, they
also discuss the capabilities and limitations of
the prototype. 

Header Lister

HEADLIST prints out all the headers in a FITS
file, including the primary header and all
extension headers. It does not evaluate them for
errors. It is a useful tool for obtaining a quick
summary of the contents of a FITS file. It is
written in ANSI C; users with Sun and other
systems that normally use earlier C dialects will
need to devolve the function prototyping syntax or
use an ANSI C compiler. 

Error Test Files

The files consist of several versions of the same
FITS file, one in conformance with the FITS
standard and recommended practices, several with
different kinds of header errors, and two that are
in technical conformance but have features that
might cause problems for some readers. They are
useful for testing the ability of FITS reading
software to cope with erroneous or unusual files
and to identify correctly the errors encountered.
Users who download the files from direct anonymous
ftp should remember to use binary transfer. 


The NASA/Goddard High Energy Astrophysics Science
Archive Research Center (HEASARC) has developed
and maintains FITS software and utilities,
including the FITSIO package and the FTOOLS


The FITSIO package, maintained by W. D. Pence, is
a machine-independent subroutine interface for
reading or writing data files in FITS format.
FITSIO is written in portable Fortran-77 and runs
on most commonly used computers. In addition to
the Fortran subroutine interface, a set of C
macros, one for each FITSIO subroutine, has been
defined for convenience in calling FITSIO from C
programs. These macros replace the C wrapper
routines of previous versions. FITSIO supports all
the standard FITS extensions and contains world
coordinates subroutines for conversion between
pixel and celestial coordinates. In addition,
software support for the checksum proposal is


J. K. Blackburn and W. D. Pence document the 
FTOOLS collection of over 100 utility programs to
create, examine, or modify FITS data files. These
programs are useful for examining the contents of
FITS files and modifying them for input to more
involved analysis tasks; they cannot generally be
used for detailed data analysis or model fitting.
New versions are released about every 3 months.
Users have the option of installing the entire
FTOOLS package, which includes many routines
specific to high energy astrophysics, or a core
set that contains only the routines that perform
general operations on FITS files. FTOOLS can be
built as a package within IRAF or as a set of
stand-alone executable tasks. FTOOLS is supported
on the following platforms:


Questions or comments should be sent to
[email protected] 


W. Pence (GSFC/HEASARC) has announced the VERIFITS
program to verify the conformance of any FITS
format data file on magnetic disk to the standard,
checking keywords and data. At user option it will
list the total number of pixels, the number of
null pixels and the maximum and minimum data
values. If an error is found while evaluating the
header, validation ceases, the error is reported
and the first 72 keywords of the header are
listed. The VERIFITS program is a stand-alone
version of the fverify task that is included in
the IRAF or Host FTOOLS package. Both VERIFITS and
fverify perform the same verification checks, but
fverify has a nicer user interface, as provided by
the IRAF or Host environments. Several different
binary executable versions of VERIFITS are
available, for running on Sun workstations,
DECstations, DEC Alphas running OSF/1 or VAX/VMS
machines. The VERIFITS source code is also
provided and may be easily linked with the FITSIO
library to run on the other types of machine on
which FITSIO is supported. While VERIFITS has been
extensively tested, under some unusual
circumstances not covered by the tests it may
still fail to detect a FITS format error, or it
may issue an error message that does not
accurately describe the problem.

VERIFITS has broader applicability than the FITS
Product Conformance Tester (FPCT), being able to
handle all standard extensions, while the
currently available FPCT can handle only primary
HDUs. The FPCT however is more forgiving and more
thorough; VERIFITS will usually reject a file
after finding one error, while the FPCT will
attempt to retrieve the information needed to read
the data if doing so is possible even with errors
in the file, and will continue to check the header
even after an error has been found.

The VERIFITS program is available from HEASARC by 

ADC FITS Table Browser

The Astronomical Data Center has developed a FITS
Table Browser, which has been tailored
specifically for use with the ADC CD-ROMs but may
be used with other FITS ASCII Tables. It reads
standard FITS ASCII tables and allows the user to
browse through them interactively and selectively
display any field or record in a table. File
extraction facilities allow the writing of all or
part of the input table to disk in FITS or text
file format.

Copies of the program for both MS-DOS and Unix are
available by anonymous ftp. See the file
readme.ftb for instructions on downloading,
installation, and use. 

FITS I/O software in IDL

W. Landsman (Hughes STX;
[email protected]) has announced that
FITS I/O software written in IDL is available as
part of the IDL Astronomy User's Library, a
central repository for general purpose astronomy
procedures written in IDL, including procedures to
convert between spherical coordinates and plan map
coordinates. The library is not meant to be an
integrated package, but rather is a collection of
procedures from which users can select. Submitted
procedures are given a cursory testing but are
basically stored in the library as submitted. The
IDL Astronomy User's Library is supported under
the NASA Astrophysics Software Aids program. 

Display of FITS Image Files

Disclaimer: The mention of particular software
packages is not intended as an endorsement of
those packages to the exclusion of others. Users
should obtain proper licensing for any proprietary
package or format mentioned. Information about
publicly available nonproprietary packages is
welcome and will be added to this Web site if the
package appears relevant and useful. Such
information should include how to obtain the
package and whom to contact with questions. It
should also describe any limits on the FITS files
that the package can handle (e. g., NAXIS must be
2; data array members must be integers). 

Major Astronomical Image Analysis Packages

The three major astronomical image analysis
packages -- the Astronomical Image Processing
System (AIPS), the European Southern Observatory
Munich Image and Data Analysis System (ESO-MIDAS),
and the Image Reduction and Analysis Facility (
IRAF) -- provide facilities for displaying images
stored in FITS files. These packages are large and
probably best installed on major systems. AIPS was
developed by the National Radio Astronomy
Observatory (NRAO), and IRAF by the National
Optical Astronomy Observatories (NOAO). 


The Extended Portable Bitmap Toolkit (pbm+) can be
used to convert many FITS files to image format.
However, support is not guaranteed for all FITS
files where the data are in the form of an image.
In particular, there may be problems when the data
array members are in IEEE floating point format
(BITPIX<0) or the array has more than two
dimensions (NAXIS>2). 


A. Warnock and R. Baalke have announced release of
version 7.9 of IMDISP, an interactive image
processing program that runs on an IBM PC computer
and supports FITS input. IMDISP 7.9 is available
via anonymous ftp. 


SAOimage is an X11 display tool which can be used
stand-alone or in conjunction with IRAF to display
FITS or IRAF images. It was developed at the 
Smithsonian Astrophysical Observatory. SAOimage
source code as well as compiled executables for
SUN, HP, DEC, and VMS workstations are available
by anonymous ftp.

A new version of SAOimage, called SAOimage: The Next
Generation (SAOtng) has been developed by SAO and NOAO.
It allows users to incorporate calls to SAOimage
into their own processing and analysis packages,
as an alternative to using it as a stand-alone
package. It can be retrieved as a compressed .tar
file. The SAO R&D software has been developed and
tested on Sun workstations; ports (with minimal
testing) have been done to the SGI, HP9000s700,
and Dec Alpha. See the BUILD, STARTING.ASSIST, and
STARTING.SAOTNG documents contained in the .tar
file for more information on build and use

Questions about the use of SAOimage may be sent to

[email protected]
[email protected]

Applications with XV

The XV package, as of version 3.10a, now includes
FITS support. It can handle more than 2 axes and
IEEE single and double precision regular floating
point values in the primary array. It can not
handle IMAGE extensions or IEEE special values
(NaN, Infinity, denormalized). An xv Web page
contains licensing and access information. 

FITS and the Macintosh

T. Lemke ([email protected]), at Peine in
Germany has added FITS support to GraphicConverter
for the Macintosh and provided the following
information. GraphicConverter 1.7.7 or later can
import FITS array files with all five permitted
data types (8, 16, 32 bit integer and 32, 64 bit
real). Every FITS file will be converted to 8 bit
gray scale on opening because this is the maximum
number of grays on a Macintosh. GraphicConverter
can convert a FITS files to the PICT, TIFF, GIF,
PCX, IFF, PPM, and other graphic file formats. The
package is shareware with a $35 registration cost.
Although our general rule is to list only software
that is available without cost, this package is
listed here because of the wide interest in FITS
packages for Macintosh.

D. Norton of Otter Solution has written a 
Photoshop plug-in called PhotoFITS which reads 8-,
16-, or 32-bit integer and 32- or 64- bit floating
point FITS images and converts them to 8-bit or
RGB images. It allows conversion of three-image
FITS files to RGB. It allows multiple image files
to be read in as a mosaic. Questions and problem
reports should be sent to [email protected]

The site at NRAO contains a collection of Usenet
postings and electronic mail messages about the
use of FITS on Macintosh hardware. 

FITSview (Windows)

W. Cotton at NRAO has announced an updated beta
release, version 0.4.0, of FITSview, a FITS image
viewer for Windows. Celestial positions are
determined using world coordinate projections. All
defined FITS data types are recognized, as are
blanked pixels. Two and three dimensional simple
FITS images can be viewed. FITSview runs on
Windows 3.1 or later and uses any multicolor (or
multiple gray level) display. Extensive on-line
documentation is included. It is available by ftp.
Installation is described in the file
fitsv040.txt, and the software is in

World Coordinates

Two ANSI C functions, worldpos() and xypix(),
convert (RA, dec) <--> pixel location for 8 common
types of projective geometries where (RA,Dec) are
more generically (long,lat). These functions are
based on the World Coordinates implementation of
Classic AIPS. The software, is available by 
anonymous ftp in the worldpos.tar.gz file, and a
number of AIPS and more recent documents are
available in a separate location.

M. Calabretta (ATNF) has announced the general
release of WCSLIB 1.0 - a suite of routines which
implements the spherical projections proposed for
the World Coordinate System (WCS) convention in
FITS. The WCSLIB distribution kit contains
independent C and FORTRAN implementations of the
library. It is available via anonymous ftp in the
wcslib-1.0.tar.gz file. 

On-line Information Sources

FITS Support Office

This site provides the following material: 

   FITS documents and general FITS information 
   Software developed by the FITS Support Office 
   Primary HDUs designed for testing the ability
   of FITS readers to cope with a variety of


Two kinds of FITS resources are available from The
NASA/Goddard High Energy Astrophysics Science
Archive Research Center (HEASARC). HEASARC has
developed the FITSIO and FTOOLS software packages.
HEASARC also provides information on the
activities of the OGIP/HEASARC FITS Working Group
(OFWG), the internal legislative body on
FITS-related ma ithin the Office of Guest
Investigator Programs (OGIP) at NASA/GSFC. OFWG
has developed a number of FITS conventions that
are more specific than the requirements of the
FITS standards. Proposed OGIP conventions are
publicized to the FITS community as a whole, with
the goal of collaborative development of a set of
conventions that will be accepted throughout the
community as well as within OGIP/HEASARC. 


The National Radio Astronomy Observatory supports
a library of FITS material There is also support
for a WAIS server named nrao-fits which has an
index of all of the FITS-related text files in the

The archive contains a collection of FITS-related
documents, including the BINTABLE draft for
Astronomy and Astrophysics, detailed proposals
currently being considered by the FITS committees,
drafts of designs not yet submitted, and documents
on world coordinate system conventions. Some
support is provided for a variety of operating
systems -- utility software and pointers to other
code. There are sample FITS files and some
specially designed test files. There is also an
archive of FITS-related traffic from 
sci.astro.fits and other Usenet newsgroups, and
from a number of FITS-related electronic mail

HEAFITS exploder

An electronic mail listserver called HEAFITS has
been set up for discussion of High Energy
Astrophysics-specific FITS issues that would not
necessarily be of interest to the majority of
subscribers to the sci.astro.fits newsgroup and
fitsbits mailing list. To subscribe to the HEAFITS
group, send the following one-line e-mail message
to [email protected]:

subscribe heafits Your Name

where `Your Name' is your actual First and Last
names. Messages to the mailing list should be sent
to [email protected] There is an 
archive of messages. 

FITS Support Services from GSFC

The NSSDC Coordinated Request and User Support
Office (CRUSO) can provide printed copies of the
User's Guide and the NOST Standard. Because of
restrictions set by ESO, the copyright holder,
copies of the four FITS papers may be sent only to
non-profit organizations. The IEEE floating point
standard is copyrighted by IEEE, and must be
purchased from them.

Use the FITS Support Office electronic mail
address below for questions about this FITS
information. Other staff members monitor the FITS
electronic mail address when I am away from the
office, providing a greater certainty of rapid
response. Please provide your name, affiliation,
and location.

Barry M. Schlesinger,
FITS Support Office


[email protected]

Last revised 1 February 1996

Author: Barry M. Schlesinger/Hughes STX
([email protected])

Responsible NASA representative: David Leisawitz
([email protected])