We have tried to generalize the FITS keywords requirements across a number of spectra types, therefore some of these keywords may not be applicable to your data, especially for composite spectra. We suggest you consult a MAST staff member prior to delivery, to be sure your data abides by the FITS and MAST delivery standards.

MAST has put together the following information because it is essential for data deliveries. If the archive cannot extract the information they need from dataset headers, MAST will attempt to calculate it based on original exposures when possible. If MAST is not able to extract or calculate what it needs based on the delivered dataset headers, then some of the data will not be searchable and will not be displayed across multi-mission search results.

• Relevant ID information (program ID, proposal ID, etc) if appropriate
• Time coverage, in order of preference (more than one may be present):
  • Flux-weighted midpoint of integration.
  • Mid-point of integration.
  • Start time of integration + total length of integration.
• List of exposures (origin of the spectra, only needed for MAST Missions)
• Pointing location (RA, DEC)
• World Coordinate System (WCS, for wavelengths)

The following list enumerates what MAST will accept in order of preference; the links provided show the mandatory keys for that extension based on the FITS Standard v3.0 (Pence et al. 2010) paper:

1. FITS Binary Table Extension - rows and columns of data in binary representation
2. FITS ASCII Table Extension - rows and columns of data in ASCII character format
3. FITS 1-D "Image"/Spectra - a 1-dimensional array of pixels, like in a primary array
4. FITS N-D Image/Spectra - a N-dimensional array of pixels, like in a primary array

The FITS paper also provides a summary of the mandatory keywords for the above file types.

Another option is to use non-FITS, ASCII tables of 1-D spectra, which are also accepted by MAST. This format equates to the FITS Binary or ASCII Table Extension formats, but in straight ASCII files. More information is provided below.

The data format (columns) must be documented within the file itself and/or the supporting README file. For 1-D tabular spectra, we suggest three data columns: wavelength, flux and flux_error. All columns and rows must contain data values; blanks are not allowed. For blank values, please use value 'NaN' or 'NULL' in the ASCII file, but not both within the same file. Within a data line, fields are separated by one or more whitespace characters (space or tab). A field is either a sequence of non-whitespace characters or a sequence of characters between two matching quote characters (single (') or double (") quotes) - spaces are therefore allowed in quoted fields.

Specification of keywords and description of the column names in the ASCII file should be done as follows:

• Use the '#' symbol to precede all keyword text lines (non-data text lines)
• The next 8 characters are used for FITS keyword name; buffer the 8-chars name with spaces (see "NAXIS" keyword in example below)
• The 9th character is an equal sign '=' followed by a at least a single space
• The keyword value can be a number or string; strings must be in single quotes
• Keyword comments are written after a "/" symbol
• The FITS 80-character line length limit applies here: no more than 79 characters in total can be used on any given line of text; the 80^th char is the end of line/return char
• The FITS "COMMENT = " keyword should be used to describe any additional information
• The "COMMENT = " keyword value is always a string within single quotes

The following is an ASCII header example of keyword, value pairs, including the 4 data column descriptions for wavelength, flux, error, and data quality. Note that these map directly onto FITS header keywords so that MAST can build FITS files from ASCII tabular spectra deliveries.

                                                           
#TELESCOP= 'HST'                / telescope used to acquire data                 
#INSTRUME= 'STIS'               / instrument used to acquire data                              
#RA_TARG =            82.586460 / right ascension of target (deg) (J2000)        
#DEC_TARG=            -7.434805 / declination of target (deg) (J2000)               
#DATE-OBS= '2003-04-28'         / UT date of start of first exposure             
#TIME-OBS= '09:20:38'           / UT start time of first exposure                
#
#XTENSION= 'BINTABLE'           /Written by IDL:  Mon May 25 12:26:32 2009       
#BITPIX  =                    8 /                                                
#NAXIS   =                    2 /Binary table                                    
#NAXIS1  =              1152000 /Number of bytes per row                         
#NAXIS2  =                    1 /Number of rows                                  
#PCOUNT  =                    0 /Random parameter count                          
#GCOUNT  =                    1 /Group count                                     
#TFIELDS =                    4 /Number of columns                               
#EXTNAME = 'E230H-2013_310X005N_52757' /Extension name                           
#EXTNO   =                    1 /Extension number                                
#TFORM1  = '64000D  '           /Real*8 (double precision)                       
#TTYPE1  = 'WAVE    '           /Column 1: Wavelength                            
#TUNIT1  = 'Angstroms'          /Units of column 1                               
#TFORM2  = '64000E  '           /Real*4 (floating point)                         
#TTYPE2  = 'FLUX    '           /Column 2: Flux Density                          
#TUNIT2  = 'erg/s/cm^2/A'       /Units of column 2                               
#TFORM3  = '64000E  '           /Real*4 (floating point)                         
#TTYPE3  = 'ERROR   '           /Column 3: Photometric Error                     
#TUNIT3  = 'erg/s/cm^2/A'       /Units of column 3                               
#TFORM4  = '64000I  '           /Integer*2 (short integer)                       
#TTYPE4  = 'DQ      '           /Column 4: Data Quality                          
#TUNIT4  = 'unitless'           /Units of column 4  
#
#COMMENT = 'Delivered to MAST from the StarCat HLSP project'
#END                                                                             

Composite data are defined as spectra which were constructed from other spectra (e.g., spectra from multiple instruments stitched together). The most common examples of composite data are highly processed science products made from other spectra, or 1-D spectra extracted from 2-D or 3-D datasets.

The list of original datasets from which the dataset was made can be a FITS table as a separate HDU in the composite dataset, or an ASCII list of exposure names (e.g. IPPPSSOOT convention for HST, etc.) per composite dataset.

An example composite map from the CLASH HLSP project can be viewed here where the 2nd column is the ipppssoot name for HST; for other missions, the data ID is required, see example for the FUSE Magellanic Cloud HLSP project.

The data provider can include values in the header which are needed to do that correction (barycentric correction, observed Doppler shifts). In addition, spectra corrected to the rest-frame may be submitted along with the uncorrected spectra, but should never be provided as the sole data product.

For example, a single spectrum can have FITS keyword 'INSTRUME' = "COS". For a composite spectrum made by stitching COS and STIS spectra, the keys would be as follows:

• INSTRUME = "MULTI"
• INSTRU01 = "COS"
• INSTRU02 = "STIS"

To designate multiple keywords, we denote this as [nn].

1. KEYWORD + KEYWORD_UNIT pair

You can specify the keyword units using a second, similar keyword where the keyword name contains the string "_UNIT" or "UNIT". Please remember to stay within the FITS 8-character keyword name limit. E.g.:

RADVEL = 1688 / The observed Radial Velocity of the object in velocity units.
RVUNIT = 'km/s' / The units of RADVEL, one of {m/s, km/s}.

2. KEYWORD = value / [unit] in brackets within the FITS comment

Comments can contain the keyword unit within brackets; this should be the first text following the standard fits comment delimiter " / ", the single slash. Please remember to stay within the FITS 80-character line length limit, which includes the comments. E.g.:

RA_TARG = 2.0952854139 / [deg] right ascension of target

• The cross-correlation function (CCF) should be provided if a radial velocity (RADVEL) is measured with such a technique. Other similar products should be provided if using broadening functions, etc. (This is to be able to assess (a)symmetry of the function's shape, search for signals from additional sources, etc.)
• If high spectral resolution, line bisectors would be another good product to provide if an RV measurement is provided.
• IRAF-style wavelength solution parameters (e.g., best-fit coefficients in functional fit to identified reference lines) can be supplied, along with the linelist and/or calibration spectra used to determine the wavelength solution.
• If masks or corrections used to remove contaminating lines are not in the FITS files already, we encourage users to include these files.

MAST has been ingesting and distributing HLSP data products for over 10 years. During this period, the requirements for HLSP data deliveries have expanded in order to help unify all datasets housed at MAST for ease of multi-mission searching. The example HLSP headers may not abide by all the requirements listed above because they were delivered prior to some requirements being written. We encourage the data delivery teams to provide data sample so that all header and data issues can be worked out prior to the actual delivery for ingestion into the archive.

The following projects contain many examples of spectra file headers, which can be used as a guideline for your datasets:

• FITS BINARY Table, 1-D Spectra Examples with WAVE, FLUX, ERROR columns
  From the FUSE Magellanic Clouds Legacy Project:
• FUSE MC Spectra Header (plain text h_z91220_all1alif.fits header)
• FUSE MC Spectra FITS File (h_z91220_all1alif.fits from plain text header example)
From the StarCat Project:
• StarCat Spectra Header (plain text h_ad-leo_e140m-1425_020x020_51613-52427_spc.fits header)
• StarCat Spectra FITS File (h_ad-leo_e140m-1425_020x020_51613-52427_spc.fits from plain text header example)


• FITS ASCII Table, Catalog Example (not spectra) with photometric data stored in this format
  From the Ultraviolet Imaging Telescope (UIT) Project holdings at MAST:
• UIT Catalog Header (plain text fuv2019aph.fits header)
• UIT Catalog FITS File with 21 (photometry) columns (fuv2019aph.fits from plain text header example)


• ASCII Table, Non-FITS 1-D Spectra Example with WAVE, FLUX columns
  From the HUT White Dwarf Project:
• HUT Spectra Header + Data (plain text hlsp_hut-wds_astro_hut_g191-b2b_fuv_v2_cal.txt)


• FITS 1-D Spectra, 1-D and N-D Spectra Examples ("Image")
  Note, MAST does not have example HLSP 1-D or N-D spectra at this time.