Access to FITS files

Direct reading of data

The simplest way to read some data in a FITS file is to call readfits:

data = readfits(filename, args...; kwds...)

with filename the name of the file.

By default, the data of the first FITS extension in filename is read. Keyword ext may however be set with a number, a name, or a symbol to select another extension. Another possibility is to specify the keyword extended = true to open the file using the extended file name syntax implemented by the CFITSIO library.

The default is to read all the data part of the selected FITS extension but optional arguments args... and keywords kwds... may be specified to restrict the read contents:

• If the FITS extension is an image, args... specifies the ranges of pixels to read along the dimensions. For example:

  data = readfits(filename, :, :, 5)

  would read the 5th slice in a 3-dimensional image. This is equivalent but more efficient than:

  data = readfits(filename)[:, :, 5]

  which amounts to reading all the data and then only keep the 5th slice.

• If the FITS extension is a table, args... may be up to 2 arguments, cols and rows, to respectively select a subset of columns and rows. For example:

  A = readfits(filename, ("Speed", "Height"))
  B = readfits(filename, :, 11:40)

  respectively yield the columns named Speed and Height of the table and all the columns of the table but only for rows in the range 11:40. Keyword case may be used to indicate whether letter case does matter in the column names. Note that, in EasyFITS the rows of a table correspond to the last dimension of arrays. This is to have the same storage order in memory and in the FITS file. Method permutedims can be used is this convention does not suit you.

The type of the object returned by readfits depends on the kind of the FITS extension and may also depend on the optional arguments args... and on the keywords kwds...:

• If the FITS extension is an image, the data is read as a Julia Array.

• If the FITS extension is a table, then, if cols is a single column name or number, an array of the columns values is returned, otherwise, a dictionary indexed by the column names is returned. Note that, a column range like 4:4 would yield a dictionary with a single column (the 4th one) in that context. Keywords case and rename are available to indicate how to search the columns by name in the table and how to translate these names into dictionary keys.

To avoid ambiguities or for improved type-stability, a leading type argument can be specified in readfits to indicate the expected type for the returned data:

data = readfits(R::Type, filename, args...; kwds...)::R

Array type parameters such as element type and number of dimensions may be specified in R. For example:

data = readfits(Array{Float32,3}, filename)

ensures that the result be a single precision floating-point 3-dimensional array; while:

data = readfits(Dict, filename, cols; ext=2)

ensures that the table in 2nd FITS Header Data Unit is returned as a dictionary even though cols specifies a single column.

Direct writing of data

In principle, directly writing data in a FITS file is as simple as direct reading of data.