Building an effective Point Spread Function (ePSF)

The ePSF

The instrumental PSF is a combination of many factors that are generally difficult to model. Anderson and King (2000; PASP 112, 1360) showed that accurate stellar photometry and astrometry can be derived by modeling the net PSF, which they call the effective PSF (ePSF). The ePSF is an empirical model describing what fraction of a star’s light will land in a particular pixel. The constructed ePSF is typically oversampled with respect to the detector pixels.

Building an ePSF

Photutils provides tools for building an ePSF following the prescription of Anderson and King (2000; PASP 112, 1360) and subsequent enhancements detailed mainly in Anderson (2016; WFC3 ISR 2016-12). The process involves iterating between the ePSF itself and the stars used to build it.

To begin, we must first define a sample of stars used to build the ePSF. Ideally these stars should be bright (high S/N) and isolated to prevent contamination from nearby stars. One may use the star-finding tools in Photutils (e.g., DAOStarFinder or IRAFStarFinder) to identify an initial sample of stars. However, the step of creating a good sample of stars generally requires visual inspection and manual selection to ensure stars are sufficiently isolated and of good quality (e.g., no cosmic rays, detector artifacts, etc.).

Let’s start by loading a simulated HST/WFC3 image in the F160W band:

>>> from photutils.datasets import load_simulated_hst_star_image
>>> hdu = load_simulated_hst_star_image()  
>>> data = hdu.data

The simulated image does not contain any background or noise, so let’s add those to the image:

>>> from photutils.datasets import make_noise_image
>>> data += make_noise_image(data.shape, distribution='gaussian',
...                          mean=10.0, stddev=5.0, seed=123)

Let’s show the image:

import matplotlib.pyplot as plt
from astropy.visualization import simple_norm
from photutils.datasets import (load_simulated_hst_star_image,
                                make_noise_image)

hdu = load_simulated_hst_star_image()
data = hdu.data
data += make_noise_image(data.shape, distribution='gaussian', mean=10.0,
                         stddev=5.0, seed=123)
norm = simple_norm(data, 'sqrt', percent=99.0)
plt.imshow(data, norm=norm, origin='lower', cmap='viridis')

(Source code, png, hires.png, pdf, svg)

_images/epsf-1.png

For this example we’ll use the find_peaks() function to identify the stars and their initial positions. We will not use the centroiding option in find_peaks() to simulate the effect of having imperfect initial guesses for the positions of the stars. Here we set the detection threshold value to 500.0 to select only the brightest stars:

>>> from photutils.detection import find_peaks
>>> peaks_tbl = find_peaks(data, threshold=500.0)  
>>> peaks_tbl['peak_value'].info.format = '%.8g'  # for consistent table output  
>>> print(peaks_tbl)  
 id x_peak y_peak peak_value
--- ------ ------ ----------
  1    849      2  1076.7026
  2    182      4  1709.5671
  3    324      4  3006.0086
  4    100      9  1142.9915
  5    824      9  1302.8604
...    ...    ...        ...
427    751    992  801.23834
428    114    994  1595.2804
429    299    994  648.18539
430    207    998  2810.6503
431    691    999  2611.0464
Length = 431 rows

Note that the stars are sufficiently separated in the simulated image that we do not need to exclude any stars due to crowding. In practice this step will require some manual inspection and selection.

Next, we need to extract cutouts of the stars using the extract_stars() function. This function requires a table of star positions either in pixel or sky coordinates. For this example we are using the pixel coordinates, which need to be in table columns called simply x and y.

We plan to extract 25 x 25 pixel cutouts of our selected stars, so let’s explicitly exclude stars that are too close to the image boundaries (because they cannot be extracted):

>>> size = 25
>>> hsize = (size - 1) / 2
>>> x = peaks_tbl['x_peak']  
>>> y = peaks_tbl['y_peak']  
>>> mask = ((x > hsize) & (x < (data.shape[1] -1 - hsize)) &
...         (y > hsize) & (y < (data.shape[0] -1 - hsize)))

Now let’s create the table of good star positions:

>>> from astropy.table import Table
>>> stars_tbl = Table()
>>> stars_tbl['x'] = x[mask]  
>>> stars_tbl['y'] = y[mask]

The star cutouts from which we build the ePSF must have the background subtracted. Here we’ll use the sigma-clipped median value as the background level. If the background in the image varies across the image, one should use more sophisticated methods (e.g., Background2D).

Let’s subtract the background from the image:

>>> from astropy.stats import sigma_clipped_stats
>>> mean_val, median_val, std_val = sigma_clipped_stats(data, sigma=2.0)  
>>> data -= median_val

The extract_stars() function requires the input data as an NDData object. An NDData object is easy to create from our data array:

>>> from astropy.nddata import NDData
>>> nddata = NDData(data=data)

We are now ready to create our star cutouts using the extract_stars() function. For this simple example we are extracting stars from a single image using a single catalog. The extract_stars() can also extract stars from multiple images using a separate catalog for each image or a single catalog. When using a single catalog, the star positions must be in sky coordinates (as SkyCoord objects) and the NDData objects must contain valid WCS objects. In the case of using multiple images (i.e., dithered images) and a single catalog, the same physical star will be “linked” across images, meaning it will be constrained to have the same sky coordinate in each input image.

Let’s extract the 25 x 25 pixel cutouts of our selected stars:

>>> from photutils.psf import extract_stars
>>> stars = extract_stars(nddata, stars_tbl, size=25)

The function returns a EPSFStars object containing the cutouts of our selected stars. The function extracted 403 stars, from which we’ll build our ePSF. Let’s show the first 25 of them:

>>> import matplotlib.pyplot as plt
>>> from astropy.visualization import simple_norm
>>> nrows = 5
>>> ncols = 5
>>> fig, ax = plt.subplots(nrows=nrows, ncols=ncols, figsize=(20, 20),
...                        squeeze=True)
>>> ax = ax.ravel()
>>> for i in range(nrows * ncols):
...     norm = simple_norm(stars[i], 'log', percent=99.0)
...     ax[i].imshow(stars[i], norm=norm, origin='lower', cmap='viridis')

(Source code, png, hires.png, pdf, svg)

_images/epsf-2.png

With the star cutouts in hand, we are ready to construct the ePSF with the EPSFBuilder class. We’ll create an ePSF with an oversampling factor of 4.0. Here we limit the maximum number of iterations to 3 (to limit its run time), but in practice one should use about 10 or more iterations. The EPSFBuilder class has many other options to control the ePSF build process, including changing the centering function, the smoothing kernel, and the centering accuracy. Please see the EPSFBuilder documentation for further details.

We first initialize an EPSFBuilder instance with our desired parameters and then input the cutouts of our selected stars to the instance:

>>> from photutils.psf import EPSFBuilder
>>> epsf_builder = EPSFBuilder(oversampling=4, maxiters=3,
...                            progress_bar=False)  
>>> epsf, fitted_stars = epsf_builder(stars)

The returned values are the ePSF, as an EPSFModel object, and our input stars fitted with the constructed ePSF, as a new EPSFStars object with fitted star positions and fluxes.

Finally, let’s show the constructed ePSF:

>>> import matplotlib.pyplot as plt
>>> from astropy.visualization import simple_norm
>>> norm = simple_norm(epsf.data, 'log', percent=99.0)
>>> plt.imshow(epsf.data, norm=norm, origin='lower', cmap='viridis')
>>> plt.colorbar()

(Source code, png, hires.png, pdf, svg)

_images/epsf-3.png

The EPSFModel object is a subclass of FittableImageModel, thus it can be used as a PSF model for the PSF-fitting machinery in Photutils (i.e., PSFPhotometry or IterativePSFPhotometry).