What are fits files

Why do we use it, and how can you convert it to something your normal image processing application can understand? The current standard as of this article was defined and finalized in , FITS version 3. FITS is not just an image format! Although, images are what is stored most often in these types of files, you can perceivably store anything you want in a FITS file. Data is data. S System: I think they added a fourth letter to make it a naughty word. You pick. Think of FITS as a document, a very accurate written description of an image or other data.

One of the really cool things about FITS files is also the fact that there is an ASCII human-readable file header the first few bytes of the file that can contain pretty much whatever you want. Want to tell us what camera, temperature, wind speed, GPS coordinates, astronomical coordinates, height, weight, eye color, and middle name of the photographer, or virtually any other fact about the data contained there is to know?

FITS can handle that. The files are big, but the data is very accurate. One of the reasons it was created in the first place was for archival purposes. The standard used for FITS is important, because each newer version has a mandate to support all older versions. FITS can handle it because there is no real theoretical limit. It just stores a bigger chunk of data for each pixel as you increase the BPP.

Yes, the file sizes dramatically increase, but so what, space is cheap …another bad pun. While there surely is a BPP limit that our weak human eyes can discern, when teasing out the faintest details with some local contrast, the more the merrier. So, bring it on and use the highest quality image format your CPU can handle without releasing the magic smoke that makes it go. Co-founder of PhotographingSpace. An avid astrophotographer for timelapse, deep-space imaging, lunar, planetary, and star trail imagery, he is an all-around jack-of-most-trades for night-sky photography.

This section provides a quick introduction of using astropy. Once the astropy. The open function has several optional arguments which will be discussed in a later chapter.

After the above open call, hdulist[0] is the primary HDU, hdulist[1] is the first extension HDU, etc if there are any extensions , and so on. After you are done with the opened file, close it with the HDUList. The headers will still be accessible after the HDUList is closed. The data may or may not be accessible depending on whether the data are touched and if they are memory-mapped, see later chapters for detail.

This is particularly useful for working with very large arrays that cannot fit entirely into physical memory. This has minimal impact on smaller files as well, though some operations, such as reading the array data sequentially, may incur some additional overhead. This means that even after calling hdul. In order to force the mmap to close either wait for the containing HDUList object to go out of scope, or manually call del hdul[0].

Keywords are usually unique within a header, except in a few special cases. The header attribute is a Header instance, another Astropy object.

To get the value associated with a header keyword, simply do a la Python dicts :. If the specified keyword name does not exist, it will raise a KeyError exception. Please note however that almost all application code should update header values via their keyword name and not via their positional index.

This is because most FITS keywords may appear at any position in the header. It is also possible to update both the value and comment associated with a keyword by assigning them as a tuple:. Another way to either update an existing card or append a new one is to use the Header. To see the entire header as it appears in the FITS file with the END card and padding stripped , simply enter the header object by itself, or print repr header :.

Entering simply print header will also work, but may not be very legible on most displays, as this displays the header as it is written in the FITS file itself, which means there are no linebreaks between cards.

This is a common confusion in new users. To get a list of all keywords, use the Header. Refer to the numpy documentation for details on manipulating these numerical arrays. The returned numpy object has many attributes and methods for a user to get information about the array, e. Since image data is a numpy object, we can slice it, view it, and perform mathematical operations on it.

This example changes the values of both the pixel [1, 4] and the sub-section [, ] to the new value of See the Numpy documentation for more details on Python-style array indexing and slicing.

Note that performing an operation like this on an entire image requires holding the entire image in memory.

This example performs the multiplication in-place so that no copies are made, but the original image must first be able to fit in main memory. For most observations this should not be an issue on modern personal computers. If at this point you want to preserve all the changes you made and write it to a new file, you can use the HDUList.

This section describes reading and writing table data in the FITS format using the fits package directly. Although originally conceived as simply a standard interchange format for digital images, FITS files are now often used as a working data format, and can be used to store ASCII or binary tabular data, in addition to images and spectra.

Once again, there are sometimes duplicate procedures to do nearly the same operation. Each header and data array of every extension is placed into a separate structure tag.