CCDPROC¶

Welcome to the ccdproc documentation! Ccdproc is is an affiliated package for the AstroPy package for basic data reductions of CCD images. The ccdproc package provides many of the necessary tools for processing of ccd images built on a framework to provide error propogation and bad pixel tracking throughout the reduction process.

Documentation¶

The documentation for this package is here:

Installation¶

Requirements¶

Ccdproc has the following requirements:

One easy way to get these dependencies is to install a python distribution like anaconda.

Installing ccdproc¶

Using pip¶

To install ccdproc with pip, simply run:

pip install --no-deps ccdproc

Note

The --no-deps flag is optional, but highly recommended if you already have Numpy installed, since otherwise pip will sometimes try to “help” you by upgrading your Numpy installation, which may not always be desired.

Using conda¶

To install ccdproc with anaconda, simple run:

conda install -c astropy ccdproc

Building from source¶

Obtaining the source packages¶

Source packages¶

The latest stable source package for ccdproc can be downloaded here.

Development repository¶

The latest development version of ccdproc can be cloned from github using this command:

git clone git://github.com/astropy/ccdproc.git

Building and Installing¶

To build ccdproc (from the root of the source tree):

python setup.py build

To install ccdproc (from the root of the source tree):

python setup.py install

Testing a source code build of ccdproc¶

The easiest way to test that your ccdproc built correctly (without installing ccdproc) is to run this from the root of the source tree:

python setup.py test

CCD Data reduction (`ccdproc`)¶

Introduction¶

Note

ccdproc works only with astropy version 1.0 or later.

The ccdproc package provides:

An image class, CCDData, that includes an uncertainty for the data, units and methods for performing arithmetic with images including the propagation of uncertainties.
A set of functions performing common CCD data reduction steps (e.g. dark subtraction, flat field correction) with a flexible mechanism for logging reduction steps in the image metadata.
A function for reprojecting an image onto another WCS, useful for stacking science images. The actual reprojection is done by the reproject package.
A class for combining and/or clipping images, Combiner, and associated functions.
A class, ImageFileCollection, for working with a directory of images.

Getting Started¶

A CCDData object can be created from a numpy array (masked or not) or from a FITS file:

>>> import numpy as np
>>> from astropy import units as u
>>> import ccdproc
>>> image_1 = ccdproc.CCDData(np.ones((10, 10)), unit="adu")

An example of reading from a FITS file is image_2 = ccdproc.CCDData.read('my_image.fits', unit="electron") (the electron unit is defined as part of ccdproc).

The metadata of a CCDData object may be any dictionary-like object, including a FITS header. When a CCDData object is initialized from FITS file its metadata is a FITS header.

The data is accessible either by indexing directly or through the data attribute:

>>> sub_image = image_1[:, 1:-3]  # a CCDData object
>>> sub_data =  image_1.data[:, 1:-3]  # a numpy array

See the documentation for CCDData for a complete list of attributes.

Most operations are performed by functions in ccdproc:

>>> dark = ccdproc.CCDData(np.random.normal(size=(10, 10)), unit="adu")
>>> dark_sub = ccdproc.subtract_dark(image_1, dark,
...                                  dark_exposure=30*u.second,
...                                  data_exposure=15*u.second,
...                                  scale=True)

See the documentation for subtract_dark for more compact ways of providing exposure times.

Every function returns a copy of the data with the operation performed.

Every function in ccdproc supports logging through the addition of information to the image metadata.

Logging can be simple – add a string to the metadata:

>>> dark_sub_gained = ccdproc.gain_correct(dark_sub, 1.5 * u.photon/u.adu, add_keyword='gain_corrected')

Logging can be more complicated – add several keyword/value pairs by passing a dictionary to add_keyword:

>>> my_log = {'gain_correct': 'Gain value was 1.5',
...           'calstat': 'G'}
>>> dark_sub_gained = ccdproc.gain_correct(dark_sub,
...                                        1.5 * u.photon/u.adu,
...                                        add_keyword=my_log)

You might wonder why there is a gain_correct at all, since the implemented gain correction simple multiplies by a constant. There are two things you get with gain_correct that you do not get with multiplication:

Appropriate scaling of uncertainties.
Units

The same advantages apply to operations that are more complex, like flat correction, in which one image is divided by another:

>>> flat = ccdproc.CCDData(np.random.normal(1.0, scale=0.1, size=(10, 10)),
...                        unit='adu')
>>> image_1_flat = ccdproc.flat_correct(image_1, flat)

In addition to doing the necessary division, flat_correct propagates uncertainties (if they are set).

The function wcs_project allows you to reproject an image onto a different WCS. For more details see

To make applying the same operations to a set of files in a directory easier, use an ImageFileCollection. It constructs, given a directory, an Table containing the values of user-selected keywords in the directory. It also provides methods for iterating over the files. The example below was used to find an image in which the sky background was high for use in a talk:

>>> from __future__ import division, print_function
>>> from ccdproc import ImageFileCollection
>>> import numpy as np
>>> from glob import glob
>>> dirs = glob('/Users/mcraig/Documents/Data/feder-images/fixed_headers/20*-??-??')

>>> for d in dirs:
...     print(d)
...     ic = ImageFileCollection(d, keywords='*')
...     for data, fname in ic.data(imagetyp='LIGHT', return_fname=True):
...         if data.mean() > 4000.:
...             print(fname)

Using `ccdproc`¶

CCDData class¶

Getting started¶

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
>>> import ccdproc
>>> ccd = ccdproc.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 = ccdproc.CCDData([1, 2, 3], unit=u.photon)
>>> ccd_electron = ccdproc.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 = ccdproc.CCDData(np.zeros((10, 10)),
...                                unit=u.dimensionless_unscaled)

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

>>> ccd = ccdproc.CCDData.read('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 = ccdproc.CCDData.read('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.

Metadata¶

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 = ccdproc.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.

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 = ccdproc.CCDData([1, 2, 3], unit="adu", mask=[0, 0, 1])
>>> 2 * np.ones(3) * ccd_masked   # one return value will be masked
masked_array(data = [2.0 4.0 --],
             mask = [False False  True],
       fill_value = 1e+20)

>>> 2 * np.ones(3) * ccd_masked.data   # ignores the mask
array([ 2.,  4.,  6.])

You can force conversion to a numpy array with:

>>> np.asarray(ccd_masked)
array([1, 2, 3])
>>> np.ma.array(ccd_masked.data, mask=ccd_masked.mask)
masked_array(data = [1 2 --],
             mask = [False False  True],
       fill_value = 999999)

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')

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.

WCS¶

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.

Uncertainty¶

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

>>> data = np.random.normal(size=(10, 10), loc=1.0, scale=0.1)
>>> ccd = ccdproc.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 * ccd.data  # 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 * ccd.data  
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  
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]
>>> round(uncertainty_ratio, 5)   
0.2
>>> result.unit
Unit("adu electron")
>>> result.header
OrderedDict()

Note

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.

Note

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

Combining images and generating masks from clipping¶

Note

No attempt has been made yet to optimize memory usage in Combiner. A copy is made, and a mask array constructed, for each input image.

The first step in combining a set of images is creating a Combiner instance:

>>> from astropy import units as u
>>> from ccdproc import CCDData, Combiner
>>> import numpy as np
>>> ccd1 = CCDData(np.random.normal(size=(10,10)),
...                unit=u.adu)
>>> ccd2 = ccd1.copy()
>>> ccd3 = ccd1.copy()
>>> combiner = Combiner([ccd1, ccd2, ccd3])

The combiner task really combines two things: generation of masks for individual images via several clipping techniques and combination of images.

Image masks/clipping¶

There are currently two methods of clipping. Neither affects the data directly; instead each constructs a mask that is applied when images are combined.

Masking done by clipping operations is combined with the image mask provided when the Combiner is created.

Min/max clipping¶

minmax_clipping masks all pixels above or below user-specified levels. For example, to mask all values above the value 0.1 and below the value -0.3:

>>> combiner.minmax_clipping(min_clip=-0.3, max_clip=0.1)

Either min_clip or max_clip can be omitted.

Sigma clipping¶

For each pixel of an image in the combiner, sigma_clipping masks the pixel if is more than a user-specified number of deviations from the central value of that pixel in the list of images.

The sigma_clipping method is very flexible: you can specify both the function for calculating the central value and the function for calculating the deviation. The default is to use the mean (ignoring any masked pixels) for the central value and the standard deviation (again ignoring any masked values) for the deviation.

You can mask pixels more than 5 standard deviations above or 2 standard deviations below the median with

>>> combiner.sigma_clipping(low_thresh=2, high_thresh=5, func=np.ma.median)

Note

Numpy masked median can be very slow in exactly the situation typically encountered in reducing ccd data: a cube of data in which one dimension (in the case the number of frames in the combiner) is much smaller than the number of pixels.

A

Iterative clipping¶

To clip iteratively, continuing the clipping process until no more pixels are rejected, loop in the code calling the clipping method:

>>> old_n_masked = 0  # dummy value to make loop execute at least once
>>> new_n_masked = combiner.data_arr.mask.sum()
>>> while (new_n_masked > old_n_masked):
...     combiner.sigma_clipping(func=np.ma.median)
...     old_n_masked = new_n_masked
...     new_n_masked = combiner.data_arr.mask.sum()

Note that the default values for the high and low thresholds for rejection are 3 standard deviations.

Image combination¶

Image combination is straightforward; to combine by taking the average, excluding any pixels mapped by clipping:

>>> combined_average = combiner.average_combine()

Performing a median combination is also straightforward,

>>> combined_median = combiner.median_combine()  # can be slow, see below

With image scaling¶

In some circumstances it may be convenient to scale all images to some value before combining them. Do so by setting scaling:

>>> scaling_func = lambda arr: 1/np.ma.average(arr)
>>> combiner.scaling = scaling_func
>>> combined_average_scaled = combiner.average_combine()

This will normalize each image by its mean before combining (note that the underlying images are not scaled; scaling is only done as part of combining using average_combine or median_combine).

With image transformation¶

Note

Flux conservation Whether flux is conserved in performing the reprojection depends on the method you use for reprojecting and the extent to which pixel area varies across the image. wcs_project rescales counts by the ratio of pixel area of the pixel indicated by the keywords CRPIX of the input and output images.

The reprojection methods available are described in detail in the documentation for the reproject project; consult those documents for details.

You should carefully check whether flux conservation provided in CCDPROC is adequate for your needs. Suggestions for improvement are welcome!

Align and then combine images based on World Coordinate System (WCS) information in the image headers in two steps.

First, reproject each image onto the same footprint using wcs_project. The example below assumes you have an image with WCS information and another image (or WCS) onto which you want to project your images:

>>> from ccdproc import wcs_project
>>> reprojected_image = wcs_project(input_image, target_wcs)

Repeat this for each of the images you want to combine, building up a list of reprojected images:

>>> reprojected = []
>>> for img in my_list_of_images:
...     new_image = wcs_project(img, target_wcs)
...     reprojected.append(new_image)

Then, combine the images as described above for any set of images:

>>> combiner = Combiner(reprojected)
>>> stacked_image = combiner.average_combine()

Reduction toolbox¶

Note

This is not intended to be an introduction to image reduction. While performing the steps presented here may be the correct way to reduce data in some cases, it is not correct in all cases.

Logging in `ccdproc`¶

All logging in ccdproc is done in the sense of recording the steps performed in image metadata. if you want to do logging in the python sense of the word please see those docs.

There are basically three logging options:

Implicit logging: No setup or keywords needed, each of the functions below adds a note to the metadata when it is performed.
Explicit logging: You can specify what information is added to the metadata using the add_keyword argument for any of the functions below.
No logging: If you prefer no logging be done you can “opt-out” by calling each function with add_keyword=None.

Gain correct and create deviation image¶

Uncertainty¶

An uncertainty can be calculated from your data with create_deviation:

>>> from astropy import units as u
>>> import numpy as np
>>> import ccdproc
>>> img = np.random.normal(loc=10, scale=0.5, size=(100, 232))
>>> data = ccdproc.CCDData(img, unit=u.adu)
>>> data_with_deviation = ccdproc.create_deviation(
...                           data, gain=1.5 * u.electron/u.adu,
...                           readnoise=5 * u.electron)
>>> data_with_deviation.header['exposure'] = 30.0  # for dark subtraction

The uncertainty, \(u_{ij}\), at pixel \((i,~j)\) with value \(p_{ij}\) is calculated as

\[u_{ij} = \left(g * p_{ij} + \sigma_{rn}^2\right)^{\frac{1}{2}},\]

where \(\sigma_{rn}\) is the read noise. Gain is only necessary when the image units are different than the units of the read noise, and is used only to calculate the uncertainty. The data itself is not scaled by this function.

As with all of the functions in ccdproc, the input image is not modified.

In the example above the new image data_with_deviation has its uncertainty set.

Gain¶

To apply a gain to an image, do:

>>> gain_corrected = ccdproc.gain_correct(data_with_deviation, 1.5*u.electron/u.adu)

The result gain_corrected has its data and uncertainty scaled by the gain and its unit updated.

There are several ways to provide the gain, among them as an astropy.units.Quantity, as in the example above, as a ccdproc.Keyword. See to documentation for gain_correct for details.

Clean image¶

There are two ways to clean an image of cosmic rays. One is to use clipping to create a mask for a stack of images, as described in Image masks/clipping.

The other is to replace, in a single image, each pixel that is several standard deviations from a central value in a region surrounding that pixel. The methods below describe how to do that.

LACosmic¶

The lacosmic technique identifies cosmic rays by identifying pixels based on a variation of the Laplacian edge detection. The algorithm is an implementation of the code describe in van Dokkum (2001) [1] as implemented in [astroscrappy](https://github.com/astropy/astroscrappy) [2].

Use this technique with cosmicray_lacosmic:

>>> cr_cleaned = ccdproc.cosmicray_lacosmic(gain_corrected, sigclip=5)

median¶

Another cosmic ray cleaning algorithm available in ccdproc is cosmicray_median that is analogous to iraf.imred.crutil.crmedian. This technique can be used with ccdproc.cosmicray_median:

>>> cr_cleaned = ccdproc.cosmicray_median(gain_corrected, mbox=11,
...                                       rbox=11, gbox=5)

Although ccdproc provides functions for identifying outlying pixels and for calculating the deviation of the background you are free to provide your own error image instead.

There is one additional argument, gbox, that specifies the size of the box, centered on a outlying pixel, in which pixel should be grown. The argument rbox specifies the size of the box used to calculate a median value if values for bad pixels should be replaced.

Subtract overscan and trim images¶

Note

Images reduced with ccdproc do NOT have to come from FITS files. The discussion below is intended to ease the transition from the indexing conventions used in FITS and IRAF to python indexing.
No bounds checking is done when trimming arrays, so indexes that are too large are silently set to the upper bound of the array. This is because numpy, which provides the infrastructure for the arrays in ccdproc has this behavior.

Indexing: python and FITS¶

Overscan subtraction and image trimming are done with two separate functions. Both are straightforward to use once you are familiar with python’s rules for array indexing; both have arguments that allow you to specify the part of the image you want in the FITS standard way. The difference between python and FITS indexing is that python starts indexes at 0, FITS starts at 1, and the order of the indexes is switched (FITS follows the FORTRAN convention for array ordering, python follows the C convention).

The examples below include both python-centric versions and FITS-centric versions to help illustrate the differences between the two.

Consider an image from a FITS file in which NAXIS1=232 and NAXIS2=100, in which the last 32 columns along NAXIS1 are overscan.

In FITS parlance, the overscan is described by the region [201:232, 1:100].

If that image has been read into a python array img by astropy.io.fits then the overscan is img[0:100, 200:232] (or, more compactly img[:, 200:]), the starting value of the first index implicitly being zero, and the ending value for both indices implicitly the last index).

One aspect of python indexing may particularly surprising to newcomers: indexing goes up to but not including the end value. In img[0:100, 200:232] the end value of the first index is 99 and the second index is 231, both what you would expect given that python indexing starts at zero, not one.

Those transitioning from IRAF to ccdproc do not need to worry about this too much because the functions for overscan subtraction and image trimming both allow you to use the familiar BIASSEC and TRIMSEC conventions for specifying the overscan and region to be retained in a trim.

Overscan subtraction¶

To subtract the overscan in our image from a FITS file in which NAXIS1=232 and NAXIS2=100, in which the last 32 columns along NAXIS1 are overscan, use subtract_overscan:

>>> # python-style indexing first
>>> oscan_subtracted = ccdproc.subtract_overscan(cr_cleaned,
...                                              overscan=cr_cleaned[:, 200:],
...                                              overscan_axis=1)
>>> # FITS/IRAF-style indexing to accomplish the same thing
>>> oscan_subtracted = ccdproc.subtract_overscan(cr_cleaned,
...                                              fits_section='[201:232,1:100]',
...                                              overscan_axis=1)

Note well that the argument overscan_axis always follows the python convention for axis ordering. Since the order of the indexes in the fits_section get switched in the (internal) conversion to a python index, the overscan axis ends up being the second axis, which is numbered 1 in python zero-based numbering.

With the arguments in this example the overscan is averaged over the overscan columns (i.e. 2000 through 2031) and then subtracted row-by-row from the image. The median argument can be used to median combine instead.

This example is not very realistic: typically one wants to fit a low-order polynomial to the overscan region and subtract that fit:

>>> from astropy.modeling import models
>>> poly_model = models.Polynomial1D(1)  # one-term, i.e. constant
>>> oscan_subtracted = ccdproc.subtract_overscan(cr_cleaned,
...                                              overscan=cr_cleaned[:, 200:],
...                                              overscan_axis=1,
...                                              model=poly_model)

See the documentation for astropy.modeling.polynomial for more examples of the available models and for a description of creating your own model.

Trim an image¶

The overscan-subtracted image constructed above still contains the overscan portion. We are assuming came from a FITS file in which NAXIS1=2032 and NAXIS2=1000, in which the last 32 columns along NAXIS1 are overscan.

Trim it using trim_image,shown below in both python- style and FITS-style indexing:

>>> # FITS-style:
>>> trimmed = ccdproc.trim_image(oscan_subtracted,
...                              fits_section='[1:200, 1:100]')
>>> # python-style:
>>> trimmed = ccdproc.trim_image(oscan_subtracted[:, :200])

Note again that in python the order of indices is opposite that assumed in FITS format, that the last value in an index means “up to, but not including”, and that a missing value implies either first or last value.

Those familiar with python may wonder what the point of trim_image is; it looks like simply indexing oscan_subtracted would accomplish the same thing. The only additional thing trim_image does is to make a copy of the image before trimming it.

Note

By default, python automatically reduces array indices that extend beyond the actual length of the array to the actual length. In practice, this means you can supply an invalid shape for, e.g. trimming, and an error will not be raised. To make this concrete, ccdproc.trim_image(oscan_subtracted[:, :200000000]) will be treated as if you had put in the correct upper bound, 200.

Subtract bias and dark¶

Both of the functions below propagate the uncertainties in the science and calibration images if either or both is defined.

Assume in this section that you have created a master bias image called master_bias and a master dark image called master_dark that has been bias-subtracted so that it can be scaled by exposure time if necessary.

Subtract the bias with subtract_bias:

>>> fake_bias_data = np.random.normal(size=trimmed.shape)  # just for illustration
>>> master_bias = ccdproc.CCDData(fake_bias_data,
...                               unit=u.electron,
...                               mask=np.zeros(trimmed.shape))
>>> bias_subtracted = ccdproc.subtract_bias(trimmed, master_bias)

There are several ways you can specify the exposure times of the dark and science images; see subtract_dark for a full description.

In the example below we assume there is a keyword exposure in the metadata of the trimmed image and the master dark and that the units of the exposure are seconds (note that you can instead explicitly provide these times).

To perform the dark subtraction use subtract_dark:

>>> master_dark = master_bias.multiply(0.1)  # just for illustration
>>> master_dark.header['exposure'] = 15.0
>>> dark_subtracted = ccdproc.subtract_dark(bias_subtracted, master_dark,
...                                         exposure_time='exposure',
...                                         exposure_unit=u.second,
...                                         scale=True)

Note that scaling of the dark is not done by default; use scale=True to scale.

Correct flat¶

Given a flat frame called master_flat, use flat_correct to perform this calibration:

>>> fake_flat_data = np.random.normal(loc=1.0, scale=0.05, size=trimmed.shape)
>>> master_flat = ccdproc.CCDData(fake_flat_data, unit=u.electron)
>>> reduced_image = ccdproc.flat_correct(dark_subtracted, master_flat)

As with the additive calibrations, uncertainty is propagated in the division.

The flat is scaled by the mean of master_flat before dividing.

If desired, you can specify a minimum value the flat can have (e.g. to prevent division by zero). Any pixels in the flat whose value is less than min_value are replaced with min_value):

>>> reduced_image = ccdproc.flat_correct(dark_subtracted, master_flat,
...                                      min_value=0.9)

Basic Processing¶

All of the basic processing steps can be accomplished in a single step using ccd_process. This step will call overscan correct, trim, gain correct, add a bad pixel mask, create an uncertainty frame, subtract the master bias, and flat-field the image. These can be run together as:

>>> ccd = ccdproc.CCDData(img, unit=u.adu)
>>> nccd = ccdproc.ccd_process(ccd, oscan='[1:10,1:100]',
...                            trim='[10:100, 1:100]',
...                            error=True, gain=2.0*u.electron/u.adu,
...                            readnoise = 5*u.electron)

Reprojecting onto a different image footprint¶

An image with coordinate information (WCS) can be reprojected onto a different image footprint. The underlying functionality is proved by the reproject project. Please see :ref:reprojection for more details.

[1]	van Dokkum, P; 2001, “Cosmic-Ray Rejection by Laplacian Edge Detection”. The Publications of the Astronomical Society of the Pacific, Volume 113, Issue 789, pp. 1420-1427. doi: 10.1086/323894

[2]	McCully, C., 2014, “Astro-SCRAPPY”, https://github.com/astropy/astroscrappy

Image Management¶

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.

The class 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 ccdproc 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, a Table of the value of each keyword for each file in the collection:

>>> ic1.summary.colnames
['file', 'imagetyp', 'object', 'filter', 'exposure']
>>> ic_all.summary.colnames 
# long list of keyword names omitted

Note that the name of the file is automatically added to the table as a column named file.

Selecting files¶

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 = ic1.summary['file'][matches]

The column 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 .files_filtered:

>>> my_files = ic1.files_filtered(filter='I', exposure=30)

The optional arguments to files_filtered are used to filter the list of files.

Sorting files¶

Sometimes it is useful to bring the files into a specific order, e.g. if you make a plot for each object you probably want all images of the same object next to each other. To do this, the images in a collection can be sorted with the sort method using the fits header keys in the same way you would sort a Table:

>>> ic1.sort(['object', 'filter'])

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 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 headers and data.

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 saving 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 (almost) 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 images with 30 second exposure from the original location to other_dir:

>>> 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 same location with the overwrite argument; use it carefully because it preserves no backup. The example below replaces each of the I band images 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()

Note

This functionality is not currently available on Windows.

Reduction examples¶

Mostly still TBD, hopefully filled in with examples from users. There is one example ipython notebook.

ccdproc Package¶

The ccdproc package is a collection of code that will be helpful in basic CCD processing. These steps will allow reduction of basic CCD data as either a stand-alone processing or as part of a pipeline.

Functions¶

`background_deviation_box`(data, bbox)	Determine the background deviation with a box size of bbox.
`background_deviation_filter`(data, bbox)	Determine the background deviation for each pixel from a box with size of bbox.
`ccd_process`(ccd[, oscan, trim, error, ...])	Perform basic processing on ccd data.
`combine`(img_list[, output_file, method, ...])	Convenience function for combining multiple images
`cosmicray_lacosmic`(ccd[, sigclip, sigfrac, ...])	Identify cosmic rays through the lacosmic technique.
`cosmicray_median`(ccd[, error_image, thresh, ...])	Identify cosmic rays through median technique.
`create_deviation`(ccd_data[, gain, ...])	Create a uncertainty frame.
`fits_ccddata_reader`(filename[, hdu, unit, ...])	Generate a CCDData object from a FITS file.
`fits_ccddata_writer`(ccd_data, filename[, ...])	Write CCDData object to FITS file.
`flat_correct`(ccd, flat[, min_value, add_keyword])	Correct the image for flat fielding.
`gain_correct`(ccd, gain[, gain_unit, add_keyword])	Correct the gain in the image.
`rebin`(ccd, newshape)	Rebin an array to have a new shape.
`sigma_func`(arr[, axis])	Robust method for calculating the deviation of an array.
`subtract_bias`(ccd, master[, add_keyword])	Subtract master bias from image.
`subtract_dark`(ccd, master[, dark_exposure, ...])	Subtract dark current from an image.
`subtract_overscan`(ccd[, overscan, ...])	Subtract the overscan region from an image.
`test`([package, test_path, args, plugins, ...])	Run the tests using py.test.
`transform_image`(ccd, transform_func[, ...])	Transform the image
`trim_image`(ccd[, fits_section, add_keyword])	Trim the image to the dimensions indicated.
`wcs_project`(ccd, target_wcs[, target_shape, ...])	Given a CCDData image with WCS, project it onto a target WCS and return the reprojected data as a new CCDData image.

Classes¶

`CCDData`(args, *kwd)	A class describing basic CCD data
`Combiner`(ccd_list[, dtype])	A class for combining CCDData objects.
`ImageFileCollection`([location, keywords, ...])	Representation of a collection of image files.
`Keyword`(name[, unit, value])

Class Inheritance Diagram¶

Contributors¶

Authors and Credits¶

ccdproc Project Contributors¶

Project Coordinators¶

Matt Craig (@mwcraig)
Steve Crawford (@crawfordsm)

Alphabetical list of contributors¶

Christoph Deil (@cdeil)
Carlos Gomez (@carlgogo)
Hans Moritz Günther (@hamogu)
Forrest Gasdia (@EP-Guy)
Nathan Heidt (@heidtha)
Anthony Horton (@AnthonyHorton)
Jennifer Karr (@JenniferKarr)
Stefan Nelson (@pulsestaysconstant)
Joe Philip Ninan (@indiajoe)
Punyaslok Pattnaik (@Punyaslok)
Evert Rol (@evertrol)
William Schoenell (@wschoenell)
Michael Seifert (@MSeifert04)
Sourav Singh (@souravsingh)
Brigitta Sipocz (@bsipocz)
Connor Stotts (@stottsco)
Ole Streicher (@olebole)
Erik Tollerud (@eteq)
Nathan Walker (@walkerna22)

(If you have contributed to the ccdproc project and your name is missing, please send an email to the coordinators, or open a pull request for this page in the ccdproc repository)

Full Changelog¶

0.4.0 (2016-03-15)¶

General¶

ccdproc has now the following requirements:
- Python 2.7 or 3.4 or later.
- astropy 1.0 or later
- numpy 1.9 or later
- scipy
- astroscrappy
- reproject

New Features¶

Add a WCS setter for CCDData. [#256]
Allow user to set the function used for uncertainty calculation in average_combine and median_combine. [#258]
Add a new keyword to ImageFileCollection.files_filtered to return the full path to a file [#275]
Added ccd_process for handling multiple steps. [#211]
CCDData.write now writes multi-extension-FITS files. The mask and uncertainty are saved as extensions if these attributes were set. The name of the extensions can be altered with the parameters hdu_mask (default extension name 'MASK') and hdu_uncertainty (default 'UNCERT'). CCDData.read can read these files and has the same optional parameters. [#302]

Other Changes and Additions¶

Issue warning if there are no FITS images in an ImageFileCollection. [#246]
The overscan_axis argument in subtract_overscan can now be set to None, to let subtract_overscan provide a best guess for the axis. [#263]
Add support for wildcard and reversed FITS style slicing. [#265]
When reading a FITS file with CCDData.read, if no data exists in the primary hdu, the resultant header object is a combination of the header information in the primary hdu and the first hdu with data. [#271]
Changed cosmicray_lacosmic to use astroscrappy for cleaning cosmic rays. [#272]
CCDData arithmetic with number/Quantity now preserves any existing WCS. [#278]
Update astropy_helpers to 1.1.1. [#287]
Drop support for Python 2.6. [#300]
The add_keyword parameter now has a default of True, to be more explicit. [#310]
Return name of file instead of full path in ImageFileCollection generators. [#315]

Bug Fixes¶

Adding/Subtracting a CCDData instance with a Quantity with a different unit produced wrong results. [#291]
The uncertainty resulting when combining CCDData will be divided by the square root of the number of combined pixel [#309]
Improve documentation for read/write methods on CCDData [#320]
Add correct path separator when returning full path from ImageFileCollection.files_filtered. [#325]

0.3.3 (2015-10-24)¶

New Features¶

add a sort method to ImageFileCollection [#274]

Other Changes and Additions¶

Opt in to new container-based builds on travis. [#227]
Update astropy_helpers to 1.0.5. [#245]

Bug Fixes¶

Ensure that creating a WCS from a header that contains list-like keywords (e.g. BLANK or HISTORY) succeeds. [#229, #231]

0.3.2 (never released)¶

There was no 0.3.2 release because of a packaging error.

0.3.1 (2015-05-12)¶

New Features¶

Other Changes and Additions¶

Add extensive tests to ensure ccdproc functions do not modify the input data. [#208]
Remove red-box warning about API stability from docs. [#210]
Support astropy 1.0.5, which made changes to NDData. [#242]

Bug Fixes¶

Make subtract_overscan act on a copy of the input data. [#206]
Overscan subtraction failed on non-square images if the overscan axis was the first index, 0. [#240, #244]

0.3.0 (2015-03-17)¶

New Features¶

When reading in a FITS file, the extension to be used can be specified. If it is not and there is no data in the primary extension, the first extension with data will be used.
Set wcs attribute when reading from a FITS file that contains WCS keywords and write WCS keywords to header when converting to an HDU. [#195]

Other Changes and Additions¶

Updated CCDData to use the new version of NDDATA in astropy v1.0. This breaks backward compatibility with earlier versions of astropy.

Bug Fixes¶

Ensure dtype of combined images matches the dtype of the Combiner object. [#189]

0.2.2 (2014-11-05)¶

New Features¶

Add dtype argument to ccdproc.Combiner to help control memory use [#178]

Other Changes and Additions¶

Added Changes to the docs [#183]

Bug Fixes¶

Allow the unit string “adu” to be upper or lower case in a FIS header [#182]

0.2.1 (2014-09-09)¶

New Features¶

Add a unit directly from BUNIT if it is available in the FITS header [#169]

Other Changes and Additions¶

Relaxed the requirements on what the metadata must be. It can be anything dict-like, e.g. an astropy.io.fits.Header, a python dict, an OrderedDict or some custom object created by the user. [#167]

Bug Fixes¶

Fixed a new-style formating issue in the logging [#170]

0.2 (2014-07-28)¶

Initial release.

Navigation

CCDPROC¶

Documentation¶

Installation¶

Requirements¶

Installing ccdproc¶

Using pip¶

Using conda¶

Building from source¶

Obtaining the source packages¶

Source packages¶

Development repository¶

Building and Installing¶

Testing a source code build of ccdproc¶

CCD Data reduction (ccdproc)¶

Introduction¶

Getting Started¶

Using ccdproc¶

CCDData class¶

Getting started¶

Uncertainty¶

Arithmetic with images¶

Combining images and generating masks from clipping¶

Image masks/clipping¶

Image combination¶

With image scaling¶

With image transformation¶

Reduction toolbox¶

Logging in ccdproc¶

Gain correct and create deviation image¶

Clean image¶

Subtract overscan and trim images¶

Subtract bias and dark¶

Correct flat¶

Basic Processing¶

Reprojecting onto a different image footprint¶

Image Management¶

Working with a directory of images¶

Selecting files¶

Sorting files¶

Iterating over hdus, headers or data¶

Automatic saving from the iterators¶

Reduction examples¶

ccdproc Package¶

Functions¶

Classes¶

Class Inheritance Diagram¶

Contributors¶

Authors and Credits¶

ccdproc Project Contributors¶

Project Coordinators¶

Alphabetical list of contributors¶

Full Changelog¶

0.4.0 (2016-03-15)¶

General¶

New Features¶

Other Changes and Additions¶

Bug Fixes¶

0.3.3 (2015-10-24)¶

New Features¶

Other Changes and Additions¶

Bug Fixes¶

0.3.2 (never released)¶

0.3.1 (2015-05-12)¶

New Features¶

Other Changes and Additions¶

Bug Fixes¶

0.3.0 (2015-03-17)¶

New Features¶

Other Changes and Additions¶

Bug Fixes¶

0.2.2 (2014-11-05)¶

New Features¶

Other Changes and Additions¶

Bug Fixes¶

0.2.1 (2014-09-09)¶

New Features¶

Other Changes and Additions¶

Bug Fixes¶

0.2 (2014-07-28)¶

CCD Data reduction (`ccdproc`)¶

Using `ccdproc`¶

Logging in `ccdproc`¶