SIMPLE*•=••••••••••••••••••••!•/»this file conforms to the FITS standard**»

BITPIX*••••••••••16*/*it consists of 16-bit 2s complement integers*••

NAXIS***=••••••••••••••••••••2*/«the data is two-dimensional*••*•••*

NAXIS1* *••••••••****378*/*width of the image in pixels**

NAXIS2*•=*••••• •**•• **242*/«depth of the image in pixels *

BZERO* •••••••••••••0.0* / *pixel_value==BZERO+BSCAI<E*array_value* * * * * *

BSCALE ••«••••••••••••••••••l.o* / *pixel_value«BZERO+BSCALE*array_value

OBJECT*'M101' •••••••••••••••••••••/• subject of this image****

DATE_OBS=*'2003-05-07'*•••••••• /*UT date of integration*

TIME_OBS=* '05:19:30.10' * * * * •••••/•UT time, start of integration *

EXPOSURE= ••••••••••••••••300.0*/*integration time in seconds *

TELESCOP=* ' 6«inch*f/5*reflector' /»telescope used to take this image*

OBSERVER=* ' Richard Berry' •••••• /«the name of the observer* ••••••••••••••• *

INSTRUME=* ' CB245 »CCD «camera' •••••••••/• the device used to capture the image

COMMENT»exceptional night, very clear, no clouds anywhere in the sky*•••••••••••

Figure 3.1 This is a Basic FITS header. The number bar at the top is intended to help you follow the columns more clearly. Keywords occupy columns 1 through 8, value indicators columns 9 and 10, and values columns 11 and higher. Integer and floating-point numbers are right-justified to column 30.

line is called a "card image," and contains one keyword followed by a specified type of data. (The term "card image" dates back to earlier days of computing, when data was stored on punched cards, each of which held 80 characters.) Depending on how much information is stored in the header, there may be 36, 72, 108, or even more card images—but the number must always be a multiple of 36.

Some keywords are mandatory and some are optional. The mandatory keywords tell both humans and computers how the binary data that contain the image are structured—how many bits, lines, and samples make up the image. The optional parts of the header describe when and where the image was taken, what sort of camera took it, what the image shows, and who took it.

The binary array contains the image. Basic FITS supports five types of data: 8-bit unsigned data, 16-bit signed integers, 32-bit signed integers, and 16- and 32bit floating-point data. The exact format for the data is defined rigidly because different types of computers and operating systems write binary data differently. Because the data format is rigorously defined, no matter what type of computer wrote the FITS file, your computer should be able to read it.

Finally, the tailer consists of the number of ASCII 0 (zero) characters necessary to pad the file length to a multiple of 2,880 bytes. The tailer is a throwback to the old days of computing when data stored on tape had to contain an integral number of records, but it is necessary today so that every FITS file conforms to the FITS standard.

The FITS standard admittedly makes tedious reading, but its fussiness means that your computer will be able to read images stored in the FITS format.

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