How to build ePSF with provided star fits

I tried to obtain the star PSF for a very large image of HST. The original image was huge, I tried to obtain the stars first and put them into individual fits. After subtracted the background and masked some contamination sources, I wanted to use the EPSFBuilder to combine these processed stars images and generate an effect native PSF. There were nearby sources around the original stars so I mask these regions with numpy.nan.

I followed the same method from the example document. I read the values of the stars and stored in a numpy array called “stars_sci” and then combined the data with np.hstack.

data=numpy.hstack(star_sci)

#Then I just follow the same code from the example document.

import matplotlib.pyplot as plt
from astropy.visualization import simple_norm
from photutils import datasets
from photutils.datasets import make_noise_image
peaks_tbl = find_peaks(data, threshold=5000)  
peaks_tbl['peak_value'].info.format = '%.8g'  # for consistent table output  
print(peaks_tbl)  
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)))  
from astropy.table import Table
stars_tbl = Table()
stars_tbl['x'] = x[mask]  
stars_tbl['y'] = y[mask]  

from astropy.stats import sigma_clipped_stats
mean_val, median_val, std_val = sigma_clipped_stats(data, sigma=2.)  
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)  

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

The stars looked ok However, the final EPSF was very strange. I didn’t know what happened and why the shape looked so irregular.

[cloud182]

Hi,

Yes, I’m not setup to use the development build, but as soon as the 0.7.2 comes out I’ll give it a try.

Thanks for your work, Enrico

Il giorno ven 15 nov 2019 alle ore 15:58 Erik Tollerud < [e-mail]> ha scritto:

@oxno2 https://github.com/oxno2 and @cloud182 https://github.com/cloud182 - we think #974 https://github.com/astropy/photutils/pull/974 should have solved this problem. Can you give the latest development build of photutils a try? (Or if you aren’t set up to install from source, try it when 0.7.2 comes out.) If that addresses your problems here, we can close this issue.

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[eteq]

@oxno2 and @cloud182 - we think #974 should have solved this problem. Can you give the latest development build of photutils a try? (Or if you aren’t set up to install from source, try it when 0.7.2 comes out.) If that addresses your problems here, we can close this issue.