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# Images

## `ImageHDU`

class `astropy.io.fits.``ImageHDU`(data=None, header=None, name=None, do_not_scale_image_data=False, uint=True, scale_back=None)

  Bases: `astropy.io.fits.hdu.image._ImageBaseHDU`, `astropy.io.fits.hdu.base.ExtensionHDU`

  FITS image extension HDU class.

  Construct an image HDU.

  Parameters:

  data : array

  > The data in the HDU.

  header : Header instance

  > The header to be used (as a template). If `header` is `None`, a minimal header will be provided.

  name : str, optional

  > The name of the HDU, will be the value of the keyword `EXTNAME`.

  do_not_scale_image_data : bool, optional

  > If `True`, image data is not scaled using BSCALE/BZERO values when read. (default: False)

  uint : bool, optional

  > Interpret signed integer data where `BZERO` is the central value and `BSCALE == 1` as unsigned integer data. For example, `int16` data with `BZERO = 32768` and `BSCALE = 1` would be treated as `uint16` data. (default: True)

  scale_back : bool, optional

  > If `True`, when saving changes to a file that contained scaled image data, restore the data to the original type and reapply the original BSCALE/BZERO values. This could lead to loss of accuracy if scaling back to integer values after performing floating point operations on the data. Pseudo-unsigned integers are automatically rescaled unless scale_back is explicitly set to `False`. (default: None)

  `ImgCode`

    Deprecated since version 1.1.0: The ImgCode class attribute is deprecated and may be removed in a future version. Use the module level constant DTYPE2BITPIX instead.

  `NumCode`

    Deprecated since version 1.1.0: The NumCode class attribute is deprecated and may be removed in a future version. Use the module level constant BITPIX2DTYPE instead.

  `add_checksum`(when=None, override_datasum=False, blocking='standard', checksum_keyword='CHECKSUM', datasum_keyword='DATASUM')

    Add the `CHECKSUM` and `DATASUM` cards to this HDU with the values set to the checksum calculated for the HDU and the data respectively. The addition of the `DATASUM` card may be overridden.

    Parameters:

    when : str, optional

    > comment string for the cards; by default the comments will represent the time when the checksum was calculated

    override_datasum : bool, optional

    > add the `CHECKSUM` card only

    blocking : str, optional

    > “standard” or “nonstandard”, compute sum 2880 bytes at a time, or not

    checksum_keyword : str, optional

    > The name of the header keyword to store the checksum value in; this is typically ‘CHECKSUM’ per convention, but there exist use cases in which a different keyword should be used

    datasum_keyword : str, optional

    > See `checksum_keyword`

    Notes

    For testing purposes, first call `add_datasum` with a `when` argument, then call `add_checksum` with a `when` argument and `override_datasum` set to `True`. This will provide consistent comments for both cards and enable the generation of a `CHECKSUM` card with a consistent value.

  `add_datasum`(when=None, blocking='standard', datasum_keyword='DATASUM')

    Add the `DATASUM` card to this HDU with the value set to the checksum calculated for the data.

    Parameters:

    when : str, optional

    > Comment string for the card that by default represents the time when the checksum was calculated

    blocking : str, optional

    > “standard” or “nonstandard”, compute sum 2880 bytes at a time, or not

    datasum_keyword : str, optional

    > The name of the header keyword to store the datasum value in; this is typically ‘DATASUM’ per convention, but there exist use cases in which a different keyword should be used

    Returns:

    checksum : int

    > The calculated datasum

    Notes

    For testing purposes, provide a `when` argument to enable the comment value in the card to remain consistent. This will enable the generation of a `CHECKSUM` card with a consistent value.

  `copy`()

    Make a copy of the HDU, both header and data are copied.

  `data`

    Image/array data as a `ndarray`.

    Please remember that the order of axes on an Numpy array are opposite of the order specified in the FITS file. For example for a 2D image the “rows” or y-axis are the first dimension, and the “columns” or x-axis are the second dimension.

    If the data is scaled using the BZERO and BSCALE parameters, this attribute returns the data scaled to its physical values unless the file was opened with `do_not_scale_image_data=True`.

  `filebytes`()

    Calculates and returns the number of bytes that this HDU will write to a file.

  `fileinfo`()

    Returns a dictionary detailing information about the locations of this HDU within any associated file. The values are only valid after a read or write of the associated file with no intervening changes to the `HDUList`.

    Returns:

    dict or None

    > The dictionary details information about the locations of this HDU within an associated file. Returns `None` when the HDU is not associated with a file.

    > Dictionary contents:

    > Key | Value
    > --- | ---
    > file | File object associated with the HDU
    > filemode | Mode in which the file was opened (readonly, copyonwrite, update, append, ostream)
    > hdrLoc | Starting byte location of header in file
    > datLoc | Starting byte location of data block in file
    > datSpan | Data size including padding

  `fromstring`(data, checksum=False, ignore_missing_end=False, **kwargs)

    Creates a new HDU object of the appropriate type from a string containing the HDU’s entire header and, optionally, its data.

    Note: When creating a new HDU from a string without a backing file object, the data of that HDU may be read-only. It depends on whether the underlying string was an immutable Python str/bytes object, or some kind of read-write memory buffer such as a `memoryview`.

    Parameters:

    data : str, bytearray, memoryview, ndarray

    > A byte string containing the HDU’s header and data.

    checksum : bool, optional

    > Check the HDU’s checksum and/or datasum.

    ignore_missing_end : bool, optional

    > Ignore a missing end card in the header data. Note that without the end card the end of the header may be ambiguous and resulted in a corrupt HDU. In this case the assumption is that the first 2880 block that does not begin with valid FITS header data is the beginning of the data.

    kwargs : optional

    > May consist of additional keyword arguments specific to an HDU type–these correspond to keywords recognized by the constructors of different HDU classes such as `PrimaryHDU`, `ImageHDU`, or `BinTableHDU`. Any unrecognized keyword arguments are simply ignored.

  `readfrom`(fileobj, checksum=False, ignore_missing_end=False, **kwargs)

    Read the HDU from a file. Normally an HDU should be opened with `open()` which reads the entire HDU list in a FITS file. But this method is still provided for symmetry with `writeto()`.

    Parameters:

    fileobj : file object or file-like object

    > Input FITS file. The file’s seek pointer is assumed to be at the beginning of the HDU.

    checksum : bool

    > If `True`, verifies that both `DATASUM` and `CHECKSUM` card values (when present in the HDU header) match the header and data of all HDU’s in the file.

    ignore_missing_end : bool

    > Do not issue an exception when opening a file that is missing an `END` card in the last header.

  `req_cards`(keyword, pos, test, fix_value, option, errlist)

    Check the existence, location, and value of a required `Card`.

    Parameters:

    keyword : str

    > The keyword to validate

    pos : int, callable

    > If an `int`, this specifies the exact location this card should have in the header. Remember that Python is zero-indexed, so this means `pos=0` requires the card to be the first card in the header. If given a callable, it should take one argument–the actual position of the keyword–and return `True` or `False`. This can be used for custom evaluation. For example if `pos=lambda idx: idx > 10` this will check that the keyword’s index is greater than 10.

    test : callable

    > This should be a callable (generally a function) that is passed the value of the given keyword and returns `True` or `False`. This can be used to validate the value associated with the given keyword.

    fix_value : str, int, float, complex, bool, None

    > A valid value for a FITS keyword to to use if the given `test` fails to replace an invalid value. In other words, this provides a default value to use as a replacement if the keyword’s current value is invalid. If `None`, there is no replacement value and the keyword is unfixable.

    option : str

    > Output verification option. Must be one of `"fix"`, `"silentfix"`, `"ignore"`, `"warn"`, or `"exception"`. May also be any combination of `"fix"` or `"silentfix"` with `"+ignore"`, `+warn`, or `+exception" (e.g. ``"fix+warn"`). See Verification options for more info.

    errlist : list

    > A list of validation errors already found in the FITS file; this is used primarily for the validation system to collect errors across multiple HDUs and multiple calls to `req_cards`.

    Notes

    If `pos=None`, the card can be anywhere in the header. If the card does not exist, the new card will have the `fix_value` as its value when created. Also check the card’s value by using the `test` argument.

  `run_option`(option=u'warn', err_text=u'', fix_text=u'Fixed.', fix=None, fixable=True)

    Execute the verification with selected option.

  `scale`(type=None, option='old', bscale=1, bzero=0)

    Scale image data by using `BSCALE`/`BZERO`.

    Call to this method will scale `data` and update the keywords of `BSCALE` and `BZERO` in the HDU’s header. This method should only be used right before writing to the output file, as the data will be scaled and is therefore not very usable after the call.

    Parameters:

    type : str, optional

    > destination data type, use a string representing a numpy dtype name, (e.g. `'uint8'`, `'int16'`, `'float32'` etc.). If is `None`, use the current data type.

    option : str

    > How to scale the data: if `"old"`, use the original `BSCALE` and `BZERO` values when the data was read/created. If `"minmax"`, use the minimum and maximum of the data to scale. The option will be overwritten by any user specified `bscale`/`bzero` values.

    bscale, bzero : int, optional

    > User-specified `BSCALE` and `BZERO` values

  `section`

    Access a section of the image array without loading the entire array into memory. The `Section` object returned by this attribute is not meant to be used directly by itself. Rather, slices of the section return the appropriate slice of the data, and loads only that section into memory.

    Sections are mostly obsoleted by memmap support, but should still be used to deal with very large scaled images. See the Data Sections section of the PyFITS documentation for more details.

  `shape`

    Shape of the image array–should be equivalent to `self.data.shape`.

  `size`

    Size (in bytes) of the data portion of the HDU.

  `update_ext_name`(*args, **kwargs)

    Deprecated since version 0.3: The update_ext_name function will be deprecated in a future version. Use the `.name` attribute or `Header.set` instead.

    Update the extension name associated with the HDU.

    If the keyword already exists in the Header, it’s value and/or comment will be updated. If it does not exist, a new card will be created and it will be placed before or after the specified location. If no `before` or `after` is specified, it will be appended at the end.

    Parameters:

    value : str

    > Value to be used for the new extension name

    comment : str, optional

    > To be used for updating, default=None.

    before : str or int, optional

    > Name of the keyword, or index of the `Card` before which the new card will be placed in the Header. The argument `before` takes precedence over `after` if both are specified.

    after : str or int, optional

    > Name of the keyword, or index of the `Card` after which the new card will be placed in the Header

    savecomment : bool, optional

    > When `True`, preserve the current comment for an existing keyword. The argument `savecomment` takes precedence over `comment` if both specified. If `comment` is not specified then the current comment will automatically be preserved.

  `update_ext_version`(*args, **kwargs)

    Deprecated since version 0.3: The update_ext_version function will be deprecated in a future version. Use the `.ver` attribute or `Header.set` instead.

    Update the extension version associated with the HDU.

    If the keyword already exists in the Header, it’s value and/or comment will be updated. If it does not exist, a new card will be created and it will be placed before or after the specified location. If no `before` or `after` is specified, it will be appended at the end.

    Parameters:

    value : str

    > Value to be used for the new extension version

    comment : str, optional

    > To be used for updating; default=None.

    before : str or int, optional

    > Name of the keyword, or index of the `Card` before which the new card will be placed in the Header. The argument `before` takes precedence over `after` if both are specified.

    after : str or int, optional

    > Name of the keyword, or index of the `Card` after which the new card will be placed in the Header.

    savecomment : bool, optional

    > When `True`, preserve the current comment for an existing keyword. The argument `savecomment` takes precedence over `comment` if both specified. If `comment` is not specified then the current comment will automatically be preserved.

  `update_header`()

    Update the header keywords to agree with the data.

  `verify`(option=u'warn')

    Verify all values in the instance.

    Parameters:

    option : str

    > Output verification option. Must be one of `"fix"`, `"silentfix"`, `"ignore"`, `"warn"`, or `"exception"`. May also be any combination of `"fix"` or `"silentfix"` with `"+ignore"`, `+warn`, or `+exception" (e.g. ``"fix+warn"`). See Verification options for more info.

  `verify_checksum`(blocking='standard')

    Verify that the value in the `CHECKSUM` keyword matches the value calculated for the current HDU CHECKSUM.

    blocking : str, optional
      “standard” or “nonstandard”, compute sum 2880 bytes at a time, or not

    Returns:

    valid : int

    > - 0 - failure
    > - 1 - success
    > - 2 - no `CHECKSUM` keyword present

  `verify_datasum`(blocking='standard')

    Verify that the value in the `DATASUM` keyword matches the value calculated for the `DATASUM` of the current HDU data.

    blocking : str, optional
      “standard” or “nonstandard”, compute sum 2880 bytes at a time, or not

    Returns:

    valid : int

    > - 0 - failure
    > - 1 - success
    > - 2 - no `DATASUM` keyword present

  `writeto`(name, output_verify='exception', clobber=False, checksum=False)

    Works similarly to the normal writeto(), but prepends a default `PrimaryHDU` are required by extension HDUs (which cannot stand on their own).

## `CompImageHDU`

class `astropy.io.fits.``CompImageHDU`(data=None, header=None, name=None, compression_type='RICE_1', tile_size=None, hcomp_scale=0, hcomp_smooth=0, quantize_level=16.0, quantize_method=-1, dither_seed=0, do_not_scale_image_data=False, uint=False, scale_back=False, **kwargs)

  Bases: `astropy.io.fits.BinTableHDU`

  Compressed Image HDU class.

  Parameters:

  data : array, optional

  > Uncompressed image data

  header : Header instance, optional

  > Header to be associated with the image; when reading the HDU from a file (data=DELAYED), the header read from the file

  name : str, optional

  > The `EXTNAME` value; if this value is `None`, then the name from the input image header will be used; if there is no name in the input image header then the default name `COMPRESSED_IMAGE` is used.

  compression_type : str, optional

  > Compression algorithm: one of `'RICE_1'`, `'RICE_ONE'`, `'PLIO_1'`, `'GZIP_1'`, `'GZIP_2'`, `'HCOMPRESS_1'`

  tile_size : int, optional

  > Compression tile sizes. Default treats each row of image as a tile.

  hcomp_scale : float, optional

  > HCOMPRESS scale parameter

  hcomp_smooth : float, optional

  > HCOMPRESS smooth parameter

  quantize_level : float, optional

  > Floating point quantization level; see note below

  quantize_method : int, optional

  > Floating point quantization dithering method; can be either `NO_DITHER` (-1), `SUBTRACTIVE_DITHER_1` (1; default), or `SUBTRACTIVE_DITHER_2` (2); see note below

  dither_seed : int, optional

  > Random seed to use for dithering; can be either an integer in the range 1 to 1000 (inclusive), `DITHER_SEED_CLOCK` (0; default), or `DITHER_SEED_CHECKSUM` (-1); see note below

  Notes

  The astropy.io.fits package supports 2 methods of image compression:

  > 1. The entire FITS file may be externally compressed with the gzip or pkzip utility programs, producing a `*.gz` or `*.zip` file, respectively. When reading compressed files of this type, Astropy first uncompresses the entire file into a temporary file before performing the requested read operations. The astropy.io.fits package does not support writing to these types of compressed files. This type of compression is supported in the `_File` class, not in the `CompImageHDU` class. The file compression type is recognized by the `.gz` or `.zip` file name extension.
  > 2. The `CompImageHDU` class supports the FITS tiled image compression convention in which the image is subdivided into a grid of rectangular tiles, and each tile of pixels is individually compressed. The details of this FITS compression convention are described at the FITS Support Office web site. Basically, the compressed image tiles are stored in rows of a variable length array column in a FITS binary table. The astropy.io.fits recognizes that this binary table extension contains an image and treats it as if it were an image extension. Under this tile-compression format, FITS header keywords remain uncompressed. At this time, Astropy does not support the ability to extract and uncompress sections of the image without having to uncompress the entire image.

  The astropy.io.fits package supports 3 general-purpose compression algorithms plus one other special-purpose compression technique that is designed for data masks with positive integer pixel values. The 3 general purpose algorithms are GZIP, Rice, and HCOMPRESS, and the special-purpose technique is the IRAF pixel list compression technique (PLIO). The `compression_type` parameter defines the compression algorithm to be used.

  The FITS image can be subdivided into any desired rectangular grid of compression tiles. With the GZIP, Rice, and PLIO algorithms, the default is to take each row of the image as a tile. The HCOMPRESS algorithm is inherently 2-dimensional in nature, so the default in this case is to take 16 rows of the image per tile. In most cases, it makes little difference what tiling pattern is used, so the default tiles are usually adequate. In the case of very small images, it could be more efficient to compress the whole image as a single tile. Note that the image dimensions are not required to be an integer multiple of the tile dimensions; if not, then the tiles at the edges of the image will be smaller than the other tiles. The `tile_size` parameter may be provided as a list of tile sizes, one for each dimension in the image. For example a `tile_size` value of `[100,100]` would divide a 300 X 300 image into 9 100 X 100 tiles.

  The 4 supported image compression algorithms are all ‘lossless’ when applied to integer FITS images; the pixel values are preserved exactly with no loss of information during the compression and uncompression process. In addition, the HCOMPRESS algorithm supports a ‘lossy’ compression mode that will produce larger amount of image compression. This is achieved by specifying a non-zero value for the `hcomp_scale` parameter. Since the amount of compression that is achieved depends directly on the RMS noise in the image, it is usually more convenient to specify the `hcomp_scale` factor relative to the RMS noise. Setting `hcomp_scale = 2.5` means use a scale factor that is 2.5 times the calculated RMS noise in the image tile. In some cases it may be desirable to specify the exact scaling to be used, instead of specifying it relative to the calculated noise value. This may be done by specifying the negative of the desired scale value (typically in the range -2 to -100).

  Very high compression factors (of 100 or more) can be achieved by using large `hcomp_scale` values, however, this can produce undesirable ‘blocky’ artifacts in the compressed image. A variation of the HCOMPRESS algorithm (called HSCOMPRESS) can be used in this case to apply a small amount of smoothing of the image when it is uncompressed to help cover up these artifacts. This smoothing is purely cosmetic and does not cause any significant change to the image pixel values. Setting the `hcomp_smooth` parameter to 1 will engage the smoothing algorithm.

  Floating point FITS images (which have `BITPIX` = -32 or -64) usually contain too much ‘noise’ in the least significant bits of the mantissa of the pixel values to be effectively compressed with any lossless algorithm. Consequently, floating point images are first quantized into scaled integer pixel values (and thus throwing away much of the noise) before being compressed with the specified algorithm (either GZIP, RICE, or HCOMPRESS). This technique produces much higher compression factors than simply using the GZIP utility to externally compress the whole FITS file, but it also means that the original floating point value pixel values are not exactly preserved. When done properly, this integer scaling technique will only discard the insignificant noise while still preserving all the real information in the image. The amount of precision that is retained in the pixel values is controlled by the `quantize_level` parameter. Larger values will result in compressed images whose pixels more closely match the floating point pixel values, but at the same time the amount of compression that is achieved will be reduced. Users should experiment with different values for this parameter to determine the optimal value that preserves all the useful information in the image, without needlessly preserving all the ‘noise’ which will hurt the compression efficiency.

  The default value for the `quantize_level` scale factor is 16, which means that scaled integer pixel values will be quantized such that the difference between adjacent integer values will be 1/16th of the noise level in the image background. An optimized algorithm is used to accurately estimate the noise in the image. As an example, if the RMS noise in the background pixels of an image = 32.0, then the spacing between adjacent scaled integer pixel values will equal 2.0 by default. Note that the RMS noise is independently calculated for each tile of the image, so the resulting integer scaling factor may fluctuate slightly for each tile. In some cases, it may be desirable to specify the exact quantization level to be used, instead of specifying it relative to the calculated noise value. This may be done by specifying the negative of desired quantization level for the value of `quantize_level`. In the previous example, one could specify `quantize_level = -2.0` so that the quantized integer levels differ by 2.0. Larger negative values for `quantize_level` means that the levels are more coarsely-spaced, and will produce higher compression factors.

  The quantization algorithm can also apply one of two random dithering methods in order to reduce bias in the measured intensity of background regions. The default method, specified with the constant `SUBTRACTIVE_DITHER_1` adds dithering to the zero-point of the quantization array itself rather than adding noise to the actual image. The random noise is added on a pixel-by-pixel basis, so in order restore each pixel from its integer value to its floating point value it is necessary to replay the same sequence of random numbers for each pixel (see below). The other method, `SUBTRACTIVE_DITHER_2`, is exactly like the first except that before dithering any pixel with a floating point value of `0.0` is replaced with the special integer value `-2147483647`. When the image is uncompressed, pixels with this value are restored back to `0.0` exactly. Finally, a value of `NO_DITHER` disables dithering entirely.

  As mentioned above, when using the subtractive dithering algorithm it is necessary to be able to generate a (pseudo-)random sequence of noise for each pixel, and replay that same sequence upon decompressing. To facilitate this, a random seed between 1 and 10000 (inclusive) is used to seed a random number generator, and that seed is stored in the `ZDITHER0` keyword in the header of the compressed HDU. In order to use that seed to generate the same sequence of random numbers the same random number generator must be used at compression and decompression time; for that reason the tiled image convention provides an implementation of a very simple pseudo-random number generator. The seed itself can be provided in one of three ways, controllable by the `dither_seed` argument: It may be specified manually, or it may be generated arbitrarily based on the system’s clock (`DITHER_SEED_CLOCK`) or based on a checksum of the pixels in the image’s first tile (`DITHER_SEED_CHECKSUM`). The clock-based method is the default, and is sufficient to ensure that the value is reasonably “arbitrary” and that the same seed is unlikely to be generated sequentially. The checksum method, on the other hand, ensures that the same seed is used every time for a specific image. This is particularly useful for software testing as it ensures that the same image will always use the same seed.

  `add_checksum`(when=None, override_datasum=False, blocking='standard', checksum_keyword='CHECKSUM', datasum_keyword='DATASUM')

    Add the `CHECKSUM` and `DATASUM` cards to this HDU with the values set to the checksum calculated for the HDU and the data respectively. The addition of the `DATASUM` card may be overridden.

    Parameters:

    when : str, optional

    > comment string for the cards; by default the comments will represent the time when the checksum was calculated

    override_datasum : bool, optional

    > add the `CHECKSUM` card only

    blocking : str, optional

    > “standard” or “nonstandard”, compute sum 2880 bytes at a time, or not

    checksum_keyword : str, optional

    > The name of the header keyword to store the checksum value in; this is typically ‘CHECKSUM’ per convention, but there exist use cases in which a different keyword should be used

    datasum_keyword : str, optional

    > See `checksum_keyword`

    Notes

    For testing purposes, first call `add_datasum` with a `when` argument, then call `add_checksum` with a `when` argument and `override_datasum` set to `True`. This will provide consistent comments for both cards and enable the generation of a `CHECKSUM` card with a consistent value.

  `add_datasum`(when=None, blocking='standard', datasum_keyword='DATASUM')

    Add the `DATASUM` card to this HDU with the value set to the checksum calculated for the data.

    Parameters:

    when : str, optional

    > Comment string for the card that by default represents the time when the checksum was calculated

    blocking : str, optional

    > “standard” or “nonstandard”, compute sum 2880 bytes at a time, or not

    datasum_keyword : str, optional

    > The name of the header keyword to store the datasum value in; this is typically ‘DATASUM’ per convention, but there exist use cases in which a different keyword should be used

    Returns:

    checksum : int

    > The calculated datasum

    Notes

    For testing purposes, provide a `when` argument to enable the comment value in the card to remain consistent. This will enable the generation of a `CHECKSUM` card with a consistent value.

  `columns`

    The `ColDefs` objects describing the columns in this table.

  `compData`

    Deprecated since version 0.3: The compData function will be deprecated in a future version. Use the `compressed_data` attribute instead.

  `copy`()

    Make a copy of the table HDU, both header and data are copied.

  `dump`(datafile=None, cdfile=None, hfile=None, clobber=False)

    Dump the table HDU to a file in ASCII format. The table may be dumped in three separate files, one containing column definitions, one containing header parameters, and one for table data.

    Parameters:

    datafile : file path, file object or file-like object, optional

    > Output data file. The default is the root name of the fits file associated with this HDU appended with the extension `.txt`.

    cdfile : file path, file object or file-like object, optional

    > Output column definitions file. The default is `None`, no column definitions output is produced.

    hfile : file path, file object or file-like object, optional

    > Output header parameters file. The default is `None`, no header parameters output is produced.

    clobber : bool

    > Overwrite the output files if they exist.

    Notes

    The primary use for the `dump` method is to allow viewing and editing the table data and parameters in a standard text editor. The `load` method can be used to create a new table from the three plain text (ASCII) files.

    - datafile: Each line of the data file represents one row of table data. The data is output one column at a time in column order. If a column contains an array, each element of the column array in the current row is output before moving on to the next column. Each row ends with a new line.

      Integer data is output right-justified in a 21-character field followed by a blank. Floating point data is output right justified using ‘g’ format in a 21-character field with 15 digits of precision, followed by a blank. String data that does not contain whitespace is output left-justified in a field whose width matches the width specified in the `TFORM` header parameter for the column, followed by a blank. When the string data contains whitespace characters, the string is enclosed in quotation marks (`""`). For the last data element in a row, the trailing blank in the field is replaced by a new line character.

      For column data containing variable length arrays (‘P’ format), the array data is preceded by the string `'VLA_Length= '` and the integer length of the array for that row, left-justified in a 21-character field, followed by a blank.

      Note

      This format does not support variable length arrays using the (‘Q’ format) due to difficult to overcome ambiguities. What this means is that this file format cannot support VLA columns in tables stored in files that are over 2 GB in size.

      For column data representing a bit field (‘X’ format), each bit value in the field is output right-justified in a 21-character field as 1 (for true) or 0 (for false).

    - cdfile: Each line of the column definitions file provides the definitions for one column in the table. The line is broken up into 8, sixteen-character fields. The first field provides the column name (`TTYPEn`). The second field provides the column format (`TFORMn`). The third field provides the display format (`TDISPn`). The fourth field provides the physical units (`TUNITn`). The fifth field provides the dimensions for a multidimensional array (`TDIMn`). The sixth field provides the value that signifies an undefined value (`TNULLn`). The seventh field provides the scale factor (`TSCALn`). The eighth field provides the offset value (`TZEROn`). A field value of `""` is used to represent the case where no value is provided.

    - hfile: Each line of the header parameters file provides the definition of a single HDU header card as represented by the card image.

  `filebytes`()

    Calculates and returns the number of bytes that this HDU will write to a file.

  `fileinfo`()

    Returns a dictionary detailing information about the locations of this HDU within any associated file. The values are only valid after a read or write of the associated file with no intervening changes to the `HDUList`.

    Returns:

    dict or None

    > The dictionary details information about the locations of this HDU within an associated file. Returns `None` when the HDU is not associated with a file.

    > Dictionary contents:

    > Key | Value
    > --- | ---
    > file | File object associated with the HDU
    > filemode | Mode in which the file was opened (readonly, copyonwrite, update, append, ostream)
    > hdrLoc | Starting byte location of header in file
    > datLoc | Starting byte location of data block in file
    > datSpan | Data size including padding

  `from_columns`(columns, header=None, nrows=0, fill=False, **kwargs)

    Given either a `ColDefs` object, a sequence of `Column` objects, or another table HDU or table data (a `FITS_rec` or multi-field `numpy.ndarray` or `numpy.recarray` object, return a new table HDU of the class this method was called on using the column definition from the input.

    This is an alternative to the now deprecated `new_table` function, and otherwise accepts the same arguments. See also `FITS_rec.from_columns`.

    Parameters:

    columns : sequence of `Column`, `ColDefs`, or other

    > The columns from which to create the table data, or an object with a column-like structure from which a `ColDefs` can be instantiated. This includes an existing `BinTableHDU` or `TableHDU`, or a `numpy.recarray` to give some examples.

    > If these columns have data arrays attached that data may be used in initializing the new table. Otherwise the input columns will be used as a template for a new table with the requested number of rows.

    header : `Header`

    > An optional `Header` object to instantiate the new HDU yet. Header keywords specifically related to defining the table structure (such as the “TXXXn” keywords like TTYPEn) will be overridden by the supplied column definitions, but all other informational and data model-specific keywords are kept.

    nrows : int

    > Number of rows in the new table. If the input columns have data associated with them, the size of the largest input column is used. Otherwise the default is 0.

    fill : bool

    > If `True`, will fill all cells with zeros or blanks. If `False`, copy the data from input, undefined cells will still be filled with zeros/blanks.

    Notes

    Any additional keyword arguments accepted by the HDU class’s `__init__` may also be passed in as keyword arguments.

  `fromstring`(data, checksum=False, ignore_missing_end=False, **kwargs)

    Creates a new HDU object of the appropriate type from a string containing the HDU’s entire header and, optionally, its data.

    Note: When creating a new HDU from a string without a backing file object, the data of that HDU may be read-only. It depends on whether the underlying string was an immutable Python str/bytes object, or some kind of read-write memory buffer such as a `memoryview`.

    Parameters:

    data : str, bytearray, memoryview, ndarray

    > A byte string containing the HDU’s header and data.

    checksum : bool, optional

    > Check the HDU’s checksum and/or datasum.

    ignore_missing_end : bool, optional

    > Ignore a missing end card in the header data. Note that without the end card the end of the header may be ambiguous and resulted in a corrupt HDU. In this case the assumption is that the first 2880 block that does not begin with valid FITS header data is the beginning of the data.

    kwargs : optional

    > May consist of additional keyword arguments specific to an HDU type–these correspond to keywords recognized by the constructors of different HDU classes such as `PrimaryHDU`, `ImageHDU`, or `BinTableHDU`. Any unrecognized keyword arguments are simply ignored.

  `load`(datafile, cdfile=None, hfile=None, replace=False, header=None)

    Create a table from the input ASCII files. The input is from up to three separate files, one containing column definitions, one containing header parameters, and one containing column data.

    The column definition and header parameters files are not required. When absent the column definitions and/or header parameters are taken from the header object given in the header argument; otherwise sensible defaults are inferred (though this mode is not recommended).

    Parameters:

    datafile : file path, file object or file-like object

    > Input data file containing the table data in ASCII format.

    cdfile : file path, file object, file-like object, optional

    > Input column definition file containing the names, formats, display formats, physical units, multidimensional array dimensions, undefined values, scale factors, and offsets associated with the columns in the table. If `None`, the column definitions are taken from the current values in this object.

    hfile : file path, file object, file-like object, optional

    > Input parameter definition file containing the header parameter definitions to be associated with the table. If `None`, the header parameter definitions are taken from the current values in this objects header.

    replace : bool

    > When `True`, indicates that the entire header should be replaced with the contents of the ASCII file instead of just updating the current header.

    header : Header object

    > When the cdfile and hfile are missing, use this Header object in the creation of the new table and HDU. Otherwise this Header supercedes the keywords from hfile, which is only used to update values not present in this Header, unless `replace=True` in which this Header’s values are completely replaced with the values from hfile.

    Notes

    The primary use for the `load` method is to allow the input of ASCII data that was edited in a standard text editor of the table data and parameters. The `dump` method can be used to create the initial ASCII files.

    - datafile: Each line of the data file represents one row of table data. The data is output one column at a time in column order. If a column contains an array, each element of the column array in the current row is output before moving on to the next column. Each row ends with a new line.

      Integer data is output right-justified in a 21-character field followed by a blank. Floating point data is output right justified using ‘g’ format in a 21-character field with 15 digits of precision, followed by a blank. String data that does not contain whitespace is output left-justified in a field whose width matches the width specified in the `TFORM` header parameter for the column, followed by a blank. When the string data contains whitespace characters, the string is enclosed in quotation marks (`""`). For the last data element in a row, the trailing blank in the field is replaced by a new line character.

      For column data containing variable length arrays (‘P’ format), the array data is preceded by the string `'VLA_Length= '` and the integer length of the array for that row, left-justified in a 21-character field, followed by a blank.

      Note

      This format does not support variable length arrays using the (‘Q’ format) due to difficult to overcome ambiguities. What this means is that this file format cannot support VLA columns in tables stored in files that are over 2 GB in size.

      For column data representing a bit field (‘X’ format), each bit value in the field is output right-justified in a 21-character field as 1 (for true) or 0 (for false).

    - cdfile: Each line of the column definitions file provides the definitions for one column in the table. The line is broken up into 8, sixteen-character fields. The first field provides the column name (`TTYPEn`). The second field provides the column format (`TFORMn`). The third field provides the display format (`TDISPn`). The fourth field provides the physical units (`TUNITn`). The fifth field provides the dimensions for a multidimensional array (`TDIMn`). The sixth field provides the value that signifies an undefined value (`TNULLn`). The seventh field provides the scale factor (`TSCALn`). The eighth field provides the offset value (`TZEROn`). A field value of `""` is used to represent the case where no value is provided.

    - hfile: Each line of the header parameters file provides the definition of a single HDU header card as represented by the card image.

  `readfrom`(fileobj, checksum=False, ignore_missing_end=False, **kwargs)

    Read the HDU from a file. Normally an HDU should be opened with `open()` which reads the entire HDU list in a FITS file. But this method is still provided for symmetry with `writeto()`.

    Parameters:

    fileobj : file object or file-like object

    > Input FITS file. The file’s seek pointer is assumed to be at the beginning of the HDU.

    checksum : bool

    > If `True`, verifies that both `DATASUM` and `CHECKSUM` card values (when present in the HDU header) match the header and data of all HDU’s in the file.

    ignore_missing_end : bool

    > Do not issue an exception when opening a file that is missing an `END` card in the last header.

  `req_cards`(keyword, pos, test, fix_value, option, errlist)

    Check the existence, location, and value of a required `Card`.

    Parameters:

    keyword : str

    > The keyword to validate

    pos : int, callable

    > If an `int`, this specifies the exact location this card should have in the header. Remember that Python is zero-indexed, so this means `pos=0` requires the card to be the first card in the header. If given a callable, it should take one argument–the actual position of the keyword–and return `True` or `False`. This can be used for custom evaluation. For example if `pos=lambda idx: idx > 10` this will check that the keyword’s index is greater than 10.

    test : callable

    > This should be a callable (generally a function) that is passed the value of the given keyword and returns `True` or `False`. This can be used to validate the value associated with the given keyword.

    fix_value : str, int, float, complex, bool, None

    > A valid value for a FITS keyword to to use if the given `test` fails to replace an invalid value. In other words, this provides a default value to use as a replacement if the keyword’s current value is invalid. If `None`, there is no replacement value and the keyword is unfixable.

    option : str

    > Output verification option. Must be one of `"fix"`, `"silentfix"`, `"ignore"`, `"warn"`, or `"exception"`. May also be any combination of `"fix"` or `"silentfix"` with `"+ignore"`, `+warn`, or `+exception" (e.g. ``"fix+warn"`). See Verification options for more info.

    errlist : list

    > A list of validation errors already found in the FITS file; this is used primarily for the validation system to collect errors across multiple HDUs and multiple calls to `req_cards`.

    Notes

    If `pos=None`, the card can be anywhere in the header. If the card does not exist, the new card will have the `fix_value` as its value when created. Also check the card’s value by using the `test` argument.

  `run_option`(option=u'warn', err_text=u'', fix_text=u'Fixed.', fix=None, fixable=True)

    Execute the verification with selected option.

  `scale`(type=None, option='old', bscale=1, bzero=0)

    Scale image data by using `BSCALE` and `BZERO`.

    Calling this method will scale `self.data` and update the keywords of `BSCALE` and `BZERO` in `self._header` and `self._image_header`. This method should only be used right before writing to the output file, as the data will be scaled and is therefore not very usable after the call.

    Parameters:

    type : str, optional

    > destination data type, use a string representing a numpy dtype name, (e.g. `'uint8'`, `'int16'`, `'float32'` etc.). If is `None`, use the current data type.

    option : str, optional

    > how to scale the data: if `"old"`, use the original `BSCALE` and `BZERO` values when the data was read/created. If `"minmax"`, use the minimum and maximum of the data to scale. The option will be overwritten by any user-specified bscale/bzero values.

    bscale, bzero : int, optional

    > user specified `BSCALE` and `BZERO` values.

  `shape`

    Shape of the image array–should be equivalent to `self.data.shape`.

  `size`

    Size (in bytes) of the data portion of the HDU.

  `tcreate`(*args, **kwargs)

    Deprecated since version 0.1: The tcreate method is deprecated and may be removed in a future version. Use `load()` instead.

  `tdump`(*args, **kwargs)

    Deprecated since version 0.1: The tdump function is deprecated and may be removed in a future version. Use `dump()` instead.

  `update`()

    Update header keywords to reflect recent changes of columns.

  `updateCompressedData`(*args, **kwargs)

    Deprecated since version 0.3: The updateCompressedData function is deprecated and may be removed in a future version. Use (refactor your code) instead.

  `updateHeader`(*args, **kwargs)

    Deprecated since version 0.3: The updateHeader function is deprecated and may be removed in a future version. Use (refactor your code; this function no longer does anything) instead.

  `updateHeaderData`(*args, **kwargs)

    Deprecated since version 0.3: The updateHeaderData function will be deprecated in a future version. Use (refactor your code) instead.

  `update_ext_name`(*args, **kwargs)

    Deprecated since version 0.3: The update_ext_name function will be deprecated in a future version. Use the `.name` attribute or `Header.set` instead.

    Update the extension name associated with the HDU.

    If the keyword already exists in the Header, it’s value and/or comment will be updated. If it does not exist, a new card will be created and it will be placed before or after the specified location. If no `before` or `after` is specified, it will be appended at the end.

    Parameters:

    value : str

    > Value to be used for the new extension name

    comment : str, optional

    > To be used for updating, default=None.

    before : str or int, optional

    > Name of the keyword, or index of the `Card` before which the new card will be placed in the Header. The argument `before` takes precedence over `after` if both are specified.

    after : str or int, optional

    > Name of the keyword, or index of the `Card` after which the new card will be placed in the Header

    savecomment : bool, optional

    > When `True`, preserve the current comment for an existing keyword. The argument `savecomment` takes precedence over `comment` if both specified. If `comment` is not specified then the current comment will automatically be preserved.

  `update_ext_version`(*args, **kwargs)

    Deprecated since version 0.3: The update_ext_version function will be deprecated in a future version. Use the `.ver` attribute or `Header.set` instead.

    Update the extension version associated with the HDU.

    If the keyword already exists in the Header, it’s value and/or comment will be updated. If it does not exist, a new card will be created and it will be placed before or after the specified location. If no `before` or `after` is specified, it will be appended at the end.

    Parameters:

    value : str

    > Value to be used for the new extension version

    comment : str, optional

    > To be used for updating; default=None.

    before : str or int, optional

    > Name of the keyword, or index of the `Card` before which the new card will be placed in the Header. The argument `before` takes precedence over `after` if both are specified.

    after : str or int, optional

    > Name of the keyword, or index of the `Card` after which the new card will be placed in the Header.

    savecomment : bool, optional

    > When `True`, preserve the current comment for an existing keyword. The argument `savecomment` takes precedence over `comment` if both specified. If `comment` is not specified then the current comment will automatically be preserved.

  `verify`(option=u'warn')

    Verify all values in the instance.

    Parameters:

    option : str

    > Output verification option. Must be one of `"fix"`, `"silentfix"`, `"ignore"`, `"warn"`, or `"exception"`. May also be any combination of `"fix"` or `"silentfix"` with `"+ignore"`, `+warn`, or `+exception" (e.g. ``"fix+warn"`). See Verification options for more info.

  `verify_checksum`(blocking='standard')

    Verify that the value in the `CHECKSUM` keyword matches the value calculated for the current HDU CHECKSUM.

    blocking : str, optional
      “standard” or “nonstandard”, compute sum 2880 bytes at a time, or not

    Returns:

    valid : int

    > - 0 - failure
    > - 1 - success
    > - 2 - no `CHECKSUM` keyword present

  `verify_datasum`(blocking='standard')

    Verify that the value in the `DATASUM` keyword matches the value calculated for the `DATASUM` of the current HDU data.

    blocking : str, optional
      “standard” or “nonstandard”, compute sum 2880 bytes at a time, or not

    Returns:

    valid : int

    > - 0 - failure
    > - 1 - success
    > - 2 - no `DATASUM` keyword present

  `writeto`(name, output_verify='exception', clobber=False, checksum=False)

    Works similarly to the normal writeto(), but prepends a default `PrimaryHDU` are required by extension HDUs (which cannot stand on their own).

### `Section`

class `astropy.io.fits.``Section`(hdu)

  Bases: `object`

  Image section.

  Slices of this object load the corresponding section of an image array from the underlying FITS file on disk, and applies any BSCALE/BZERO factors.

  Section slices cannot be assigned to, and modifications to a section are not saved back to the underlying file.

  See the Data Sections section of the PyFITS documentation for more details.