Info

2.2X10"308 to 1.8X10308

number of samples in a line and NAXIS2 is the number of lines. However, if the pixel values were stored by reading down each column and reading the columns in sequence, then NAXIS1 would be the number of lines and NAXIS2 would the number of samples.

EXTEND . (No associated value.) If the header contains more than 36 card images (totaling 2,880 bytes), the optional keyword EXTEND must appear among the first 36 card images, and must also appear in each subsequent block of 36 card images.

END. (No associated value.) This keyword is always the last keyword in the header. END has no associated value and no comment. Columns 4 through 80 must be filled with ASCII blanks (20 hex). END tells both computers and humans that they have reached the last keyword in the header.

3.4.2 Array Value Keywords

Array value keywords define the relationship between the contents of the data array and the real physical data. Although they are optional, they can be extremely useful and are often employed. However, if they appear in the header, they must be used as defined by the FITS standard.

BZERO and BSCALE . (Associated value is Floating Point.) These keywords were introduced before FITS allowed floating-point data arrays, with the original intention of allowing astronomers to convert values stored in an integer array into floating-point values. However, these keywords are now used to convert 16-bit unsigned integer CCD image data (i.e., values that range from 0 to 65535) into the twos-complement integer format (values that range from -32768 to +32767) mandated by FITS. BZERO and BSCALE are defined as:

pixel_value = BZERO + BSCALE * FITS_Value (Equ. 3.1)

To convert signed 16-bit array values into unsigned 16-bit pixel values, BZERO is set to 32768 . 0 and BSCALE should be set to 1. 0.

The values for BZERO and BSCALE must be floating-point numbers representing the offset and scaling terms in the equation above. The default value for the BZERO keyword is 0 . 0, and for BSCALE the default is 1. 0. If either keyword is omitted, the value of the missing keyword is assumed to be its default value.

DATAMIN and DATAMAX. (Associated value is Floating Point.) These optional keywords specify the lowest and highest pixel values stored in the data array. If these values are present, FITS-reading software can use them to check that scaled data lie between DATAMIN and DATAMAX.

BUNIT. (Associated value is Character.) This is a string giving the units of pixel value, such as electrons or magnitudes/pixel.

BLANK. (Associated value is Integer.) This optional keyword specifies an array value reserved for missing or bad data, such as a defective row or column in a CCD image. The value most commonly used is -32678. If the BLANK keyword appears, the reading software should flag that any pixel assigned this array value contains no data.

3.4.3 Observation Keywords

After an observing session, image files can become separated from other records of the observation, so it is desirable to store important information in the file itself. This is especially true in amateur astronomy, where image files may be copied and forwarded many times.

OBJECT . (Associated value is Character.) This is a string giving the name of the object observed, for example, Messier 1, Crab Nebula, NGC 4565.

TELESCOP. (Associated value is Character.) This is a string identifying the telescope used, such as 6-inch f/5 Newtonian reflector, 10-Meter Keck Telescope, or Hubble Space Telescope.

INSTRUME. (Associated value is Character.) This is a string identifying the instrument used, such as SXV-H9, or SBIG ST10.

OBSERVER. (Associated value is Character.) This is a string identifying the observer(s), for example, Richard Berry, A1 Kelly, or Dennis di Cicco.

DATE - OBS . (Associated value is Character.) This string identifies the date (and optionally the time) when the data contained in the array were acquired, and in the case of CCD integrations, the time of the beginning of the exposure. If possible, the date (and time) should be specified in Universal Time. The date must be specified using the YYYY-MM-DD format, where YYYY are the four digits of the year, MM is the number of the month, and DD is the day of the month. The string 2015-12-25 designates Christmas Day in 2015. The FITS standard encourages using DATE-OBS to hold the time as well as the date. Time is specified using the format hh: mm: ss . ss, where hh is hours, mm is minutes, and ss . ss is decimal seconds. The combined date/time format is: YYYY-MM-DDThh: mm: ss . ss, with the upper-case character "T" joining the date and time strings.

TIME-OBS. (Associated value is Character.) This string identifies the time at which the data contained in the array were acquired. The time is specified using the format hh: mm: ss . ss, as above. Although TIME-OBS is a legitimate FITS keyword, the FITS standard encourages combining date and time in the DATE-OBS keyword.

ORIGIN. (Associated value is Character.) This is a string identifying the observatory, institution or organization responsible for creating the FITS file, such as Rainbow Observatory, NCSA, or KPNO.

DATE . (Associated value is Character.) This is a string giving the date on which the FITS file was created. The date must be specified using the YYYY-MM-DDformat. July 4, 2010 would be written 2010-07-04.

AUTHOR. (Associated value is Character.) This string supplies the name of the person who compiled the data carried in the FITS file. This keyword applies less to images and more to the FITS files containing tabular data.

CREATOR. (Associated value is Character.) This is a string giving the name of the software package that created the FITS file, such as CBWinCam or AIP4Win_2 .1.0.

3.4.4 Comment Keywords

Any number of comment card images may appear as long as the header's total length is an integer multiple of 36 card images.

COMMENT . (Character data.) This keyword takes no associated value and therefore has no value indicator. Columns 9 through 80 may contain any ASCII text. Any number of COMMENT card images may appear in a header.

HISTORY. (Character data.) This keyword has no associated value and therefore has no value indicator. Columns 9 through 80 may contain any ASCII text. The text should contain a history of steps and procedures associated with the processing of the associated image. Any number of HISTORY card images may appear in a header.

Blank Card Images. If bytes 1 through 8 of a card image contain ASCII blanks, columns 9 through 80 may contain any ASCII text. Any number of card images with blank keywords may appear in a header. Header information from software that uses a different set of keywords could be padded with 8 ASCII blanks for inclusion in a FITS header.

3.4.5 Header Value Formats

Values in a FITS header can be Boolean, Integer, Floating Point, or Character. The correct value format is required for mandatory keywords.

Boolean Value. If the value is a logical constant, it must appear as an ASCII character T or F in byte 30 of the card image.

Integer Value. If the value is an integer, the ASCII representation of the integer must appear right-justified in bytes 11 through 30 of the card image. NAXIS, NAXIS1, and NAXISn take integer values. Some examples of valid integer values are: 0, 1, 2, 1000000, 12345678 9, -10000, and 32768.

Floating-Point Value. If a value is a floating-point number, the ASCII representation must appear right justified in bytes 11 through 30 of the card image. The decimal point must be shown, and letters indicating exponential notation must be upper case. Note that the commonly used keywords BZERO and BSCALE require floating-point values. The following are valid FITS floating-point values: 0 . 0, 1. 0, 3 .1415 9, 1. 0E6, -1. 2 34 5678E-0 9, and 3 . 2 76800000E004.

Character String Value. If the value is a character string, column 11 must contain a single quote (ASCII 27H) followed by the string starting in column 12. The end of the string is marked by a single quote that may not occur before column 20 and must occur in or before column 80. The character string must be composed solely of ASCII text. Proper interpretation of FITS data must not require reading more than the first eight characters of a character string.

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