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I am writing a python code to calculate the background of an astronomical image of globular cluster M15 (M15 reduced). My code can calculate the background and plot it using plt.imshow(). To save the background subtracted image I have to convert it to a str from a numpy.nparray. I have tried many things including the np.array2string used here. The file just stays as an array, which can't be saved as I need it to save as a .fits file. Any ideas how to get this to a str? The code:

#sigma clip is the number of standard deviations from centre value that value can be before being rejected
sigma_clip = SigmaClip(sigma=2.)
#used to estimate the background in each of the meshes
bkg_estimator = MedianBackground()
#define path for reading in images
M15red_path = Path('.', 'ObservingData/M15normalised/')
M15red_images = ccdp.ImageFileCollection(M15red_path)
M15reduced = M15red_images.files_filtered(imagetyp='Light Frame', include_path=True)
M15backsub_path = Path('.', 'ObservingData/M15backsub/')
for n in range (0,59):
    bkg = Background2D(CCDData.read(M15reduced[n]).data, box_size=(20,20), 
                   filter_size=(3, 3), 
                   edge_method='pad', 
                   sigma_clip=sigma_clip, 
                   bkg_estimator=bkg_estimator)
    M15subback = CCDData.read(M15reduced[n]).data - bkg.background
    np.array2string(M15subback)
    #M15subback.write(M15backsub_path / 'M15backsub{}.fits'.format(n))

    print(type(M15subback[1]))
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    Possible duplicate of Write 3d Numpy array to FITS file with Astropy Commented Oct 28, 2019 at 16:12
  • Looks like M15subback is an array, which doesn't have a write method. Usually it's a file like object that has a write method, not an array, list, or string. Commented Oct 28, 2019 at 16:46
  • And array2string does not operate in place; it returns a string. It is difficult, if not impossible, to recreate an array from the result of array2string. Don't use to save an array! Commented Oct 28, 2019 at 16:51
  • What's a fits file? I see you use fits in the name of the file, but that doesn't define the save method. If this is an astropy format, then tag the question appropriately, and use an astropy function. There's nothing in numpy that uses this format. Commented Oct 28, 2019 at 17:41

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You could try using [numpy.save][1] (but it saves a '.npy' file). In your case, 

import numpy as np
...
for n in range (0,59):
    ...
    np.save('M15backsub{}.npy'.format(n), M15backsub)

Since you need to store a numpy array, this should work.

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I need the file to save as a fits file for the analysis.