Using the CCDData image class: I/O, properties and arithmetic

Input and output

Getting data in

The tools in ccdproc accept only CCDData objects, a subclass of NDData.

Creating a CCDData object from any array-like data is easy:

>>> import numpy as np
>>> from astropy.nddata import CCDData
>>> import ccdproc
>>> ccd = CCDData(np.arange(10), unit="adu")

Note that behind the scenes, NDData creates references to (not copies of) your data when possible, so modifying the data in ccd will modify the underlying data.

You are required to provide a unit for your data. The most frequently used units for these objects are likely to be adu, photon and electron, which can be set either by providing the string name of the unit (as in the example above) or from unit objects:

>>> from astropy import units as u
>>> ccd_photon = CCDData([1, 2, 3], unit=u.photon)
>>> ccd_electron = CCDData([1, 2, 3], unit="electron")

If you prefer not to use the unit functionality then use the special unit u.dimensionless_unscaled when you create your CCDData images:

>>> ccd_unitless = CCDData(np.zeros((10, 10)),
...                        unit=u.dimensionless_unscaled)

A CCDData object can also be initialized from a FITS file:

>>> ccd ='my_file.fits', unit="adu")  

If there is a unit in the FITS file (in the BUNIT keyword), that will be used, but a unit explicitly provided in read will override any unit in the FITS file.

There is no restriction at all on what the unit can be – any unit in astropy.units or that you create yourself will work.

In addition, the user can specify the extension in a FITS file to use:

>>> ccd ='my_file.fits', hdu=1, unit="adu")  

If hdu is not specified, it will assume the data is in the primary extension. If there is no data in the primary extension, the first extension with data will be used.

Getting data out

A CCDData object behaves like a numpy array (masked if the CCDData mask is set) in expressions, and the underlying data (ignoring any mask) is accessed through data attribute:

>>> ccd_masked = CCDData([1, 2, 3], unit="adu", mask=[0, 0, 1])
>>> res = 2 * np.ones(3) * ccd_masked
>>> res.mask   # one return value will be masked
array([False, False,  True]...)
>>> 2 * np.ones(3) *   
array([ 2.,  4.,  6.])

You can force conversion to a numpy array with:

>>> np.asarray(ccd_masked)
array([1, 2, 3])
>>>, mask=ccd_masked.mask)  

A method for converting a CCDData object to a FITS HDU list is also available. It converts the metadata to a FITS header:

>>> hdulist = ccd_masked.to_hdu()

You can also write directly to a FITS file:

>>> ccd_masked.write('my_image.fits')

Essential properties


When initializing from a FITS file, the header property is initialized using the header of the FITS file. Metadata is optional, and can be provided by any dictionary or dict-like object:

>>> ccd_simple = CCDData(np.arange(10), unit="adu")
>>> my_meta = {'observer': 'Edwin Hubble', 'exposure': 30.0}
>>> ccd_simple.header = my_meta  # or use ccd_simple.meta = my_meta

Whether the metadata is case sensitive or not depends on how it is initialized. A FITS header, for example, is not case sensitive, but a python dictionary is.

Masks and flags

Although not required when a CCDData image is created you can also specify a mask and/or flags.

A mask is a boolean array the same size as the data in which a value of True indicates that a particular pixel should be masked, i.e. not be included in arithmetic operations or aggregation.

Flags are one or more additional arrays (of any type) whose shape matches the shape of the data. For more details on setting flags see astropy.nddata.NDData.


The wcs attribute of CCDData object can be set two ways.

  • If the CCDData object is created from a FITS file that has WCS keywords in the header, the wcs attribute is set to a astropy.wcs.WCS object using the information in the FITS header.

  • The WCS can also be provided when the CCDData object is constructed with the wcs argument.

Either way, the wcs attribute is kept up to date if the CCDData image is trimmed.


Pixel-by-pixel uncertainty can be calculated for you:

>>> data = np.random.normal(size=(10, 10), loc=1.0, scale=0.1)
>>> ccd = CCDData(data, unit="electron")
>>> ccd_new = ccdproc.create_deviation(ccd, readnoise=5 * u.electron)

See Gain correct and create deviation image for more details.

You can also set the uncertainty directly, either by creating a StdDevUncertainty object first:

>>> from astropy.nddata.nduncertainty import StdDevUncertainty
>>> uncertainty = 0.1 *  # can be any array whose shape matches the data
>>> my_uncertainty = StdDevUncertainty(uncertainty)
>>> ccd.uncertainty = my_uncertainty

or by providing a ndarray with the same shape as the data:

>>> ccd.uncertainty = 0.1 *  
INFO: array provided for uncertainty; assuming it is a StdDevUncertainty. [...]

In this case the uncertainty is assumed to be StdDevUncertainty. Using StdDevUncertainty is required to enable error propagation in CCDData

If you want access to the underlying uncertainty use its .array attribute:

>>> ccd.uncertainty.array  

Arithmetic with images

Methods are provided to perform arithmetic operations with a CCDData image and a number, an astropy Quantity (a number with units) or another CCDData image.

Using these methods propagates errors correctly (if the errors are uncorrelated), take care of any necessary unit conversions, and apply masks appropriately. Note that the metadata of the result is not set if the operation is between two CCDData objects.

>>> result = ccd.multiply(0.2 * u.adu)
>>> uncertainty_ratio = result.uncertainty.array[0, 0]/ccd.uncertainty.array[0, 0]
>>> float(round(uncertainty_ratio, 5))   
>>> result.unit
Unit("adu electron")


In most cases you should use the functions described in Reduction toolbox to perform common operations like scaling by gain or doing dark or sky subtraction. Those functions try to construct a sensible header for the result and provide a mechanism for logging the action of the function in the header.

The arithmetic operators *, /, + and - are not overridden.


If two images have different WCS values, the wcs on the first CCDData object will be used for the resultant object.