Working with a directory of images¶
For the sake of argument all of the examples below assume you are working in a directory that contains FITS images.
ImageFileCollection is meant to
make working with a directory of FITS images easier by allowing you select the
files you act on based on the values of FITS keywords in their headers.
It is initialized with the name of a directory containing FITS images and a
list of FITS keywords you want the
ImageFileCollection to be aware of. An
example initialization looks like:
>>> from msumastro import ImageFileCollection >>> keys = ['imagetyp', 'object', 'filter', 'exposure'] >>> ic1 = ImageFileCollection('.', keywords=keys) # only keep track of keys
You can use the wildcard
* in place of a list to indicate you want the
collection to use all keywords in the headers:
>>> ic_all = ImageFileCollection('.', keywords='*')
Most of the useful interaction with the image collection is via its
.summary property, an `~astropy.table.Table`_ of the value of each keyword for each
file in the collection:
>>> ic1.summary.colnames ['imagetyp', 'object', 'filter'] >>> ic_all.summary.colnames # long list of keyword names omitted
Selecting the files that match a set of criteria, for example all images in the I band with exposure time less than 60 seconds you could do:
>>> matches = (ic1.summary['filter'] == 'I' & ic1.summary['exposure'] < 60) >>> my_files = summary['file'][matches]
file is added automatically when the image collection is created.
For more simple selection, when you just want files whose keywords exactly
match particular values, say all ‘I’ band images with exposure time of 30
seconds, there is a convenience method
>>> my_files = ic1.files_filtered(filter='I', exposure=30)
The optional arguments to
files_filtered are used to filter the list of
Iterating over hdus, headers or data¶
Three methods are provided for iterating over the images in the collection, optionally filtered by keyword values.
For example, to iterate over all of the I band light images with exposure of 30 seconds, performing some basic operation on the data (very contrived example):
>>> for hdu in ic1.hdus(imagetyp='LiGhT', filter='I', exposure=30): ... hdu.header['exposure'] ... new_data = hdu.data - hdu.data.mean()
Note that the names of the arguments to
hdus here are the names of FITS
keywords in the collection and the values are the values of those keywords you
want to select. Note also that string comparisons are not case sensitive.
The other iterators are
All of them have the option to also provide the file name in addition to the hdu (or header or data):
>>> for hdu, fname in ic1.hdus(return_fname=True, ... imagetyp='LiGhT', filter='I', exposure=30): ... hdu.header['meansub'] = True ... hdu.data = hdu.data - hdu.data.mean() ... hdu.writeto(fname + '.new')
That last use case, doing something to several files and wanting to same them somewhere afterwards, is common enough that the iterators provide arguments to automate it.
Automatic saving from the iterators¶
There are three ways of triggering automatic saving.
1. One is with the argument
save_with_name; it adds the value of the
argument to the file name between the original base name and extension. The
example below has (almsot) the same effect of the example above, subtracting
the mean from each image and saving to a new file:
>>> for hdu in ic1.hdus(save_with_name='_new', ... imagetyp='LiGhT', filter='I', exposure=30): ... hdu.header['meansub'] = True ... hdu.data = hdu.data - hdu.data.mean() It saves, in the ``location`` of the image collection, a new FITS file with the mean subtracted from the data, with ``_new`` added to the name; as an example, if one of the files iterated over was ``intput001.fit`` then a new file, in the same directory, called ``input001_new.fit`` would be created.
2. You can also provide the directory to which you want to save the files with
save_location; note that you do not need to actually do anything to the
hdu (or header or data) to cause the copy to be made. The example below copies
all of the I-band light files with 30 second exposure from the original
>>> for hdu in ic1.hdus(save_location='other_dir', ... imagetyp='LiGhT', filter='I', exposure=30): ... pass This option can be combined with the previous one to also give the files a new name.
3. Finally, if you want to live dangerously, you can overwrite the files in
the location with the
overwrite argument; use it carefully because it
preserves no backup. The example below replaces each of the I-band light files
with 30 second exposure with a file that has had the mean subtracted:
>>> for hdu in ic1.hdus(overwrite=True, ... imagetyp='LiGhT', filter='I', exposure=30): ... hdu.header['meansub'] = True ... hdu.data = hdu.data - hdu.data.mean()