[AstroPy] AstroPy Digest, Vol 161, Issue 5

[Harry Ferguson]

If you would like to record the mapping from pixels to angular size in your FITS
header, that’s what “world coordinate systems” are for. There are some instructions
at https://docs.astropy.org/en/stable/wcs/.

This can get very arcane, but for your simple application, ifthe third axis
is redshift, you could do something like:

from astropy.io<http://astropy.io> import fits
from astropy.wcs import WCS

myarray = np.ones((200,200,300),dtype=np.float64)   # Your 3D array

w = WCS(naxis=3)
w.wcs.crpix = (100,100,150) # Reference pixel at the center
w.wcs.crval = (30.,60.,1.0) # centered at RA=30 deg, Dec=60 deg, redshift z=1.0
w.wcs.ctype = ('RA---TAN','DEC--TAN','ZOPT')
w.wcs.cunit = ('deg','deg’,'')

# Increments 1 arc second per pixel in the spatial dimensions and delta_z = 0.1 in redshift
w.wcs.pc = [[1/3600.,0.,0.],
            [0,1/3600.,0.],
            [0,0,0.01]]

hdu = fits.PrimaryHDU(myarray, header=w.to_header())
hdu.writeto('myfile.fits')

If you display myfile.fits in ds9 you should get slider for the redshift axis, labeled
with redshift, and the coordinate info should show RA & Dec.


Dr. Henry C. Ferguson
Space Telescope Science Institute
3700 San Martin Drive, Baltimore, MD 21218
Phone: (410) 338-5098
www.stsci.edu/~ferguson<http://www.stsci.edu/~ferguson>



On Feb 13, 2020, at 12:00 PM, astropy-request at python.org<mailto:astropy-request at python.org> wrote:

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Today's Topics:

  1. Re: Making fits images with given angular size and pixel
     resolution (Mohammad Shameoni Niaei)


----------------------------------------------------------------------

Message: 1
Date: Thu, 13 Feb 2020 08:33:48 +0300
From: Mohammad Shameoni Niaei <m.shemuni at gmail.com<mailto:m.shemuni at gmail.com>>
To: Astronomical Python mailing list <astropy at python.org<mailto:astropy at python.org>>
Subject: Re: [AstroPy] Making fits images with given angular size and
       pixel resolution
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You're looking for "writeto" method. Here a simple script:

from astropy.io<http://astropy.io> import fits as fts

dest = "/Output/file"

fits_file = "/Input/file"

hdu = fts.open(fits_file, "readonly")

header = hdu[0].header
data = hdu[0].data

#
#Do something to data
#

fts.writeto(dest, data, header=header)


On Wed, Feb 12, 2020 at 5:50 PM AISHRILA MAZUMDER <aishri0208 at gmail.com<mailto:aishri0208 at gmail.com>>
wrote:

Hello,

I have a numpy 3D array of size (200,200,300). The third axis is the
redshift (frequency) axis, and the first two axes are coordinates in Mpc.
Now I have converted the Mpc values to angular separation using
cosmological calculations, and I have determined the pixel size in arcsec.
I want to know if its possible to make a fits image out of this 3D array
using the pixel size and angular size in Astropy. I could not find any
method of doing this hence I am writing this email.  The image (which is a
single slice along z axis) should look like image.png attached with this
email.

Thank you in advance,

best regards,
Aishrila Mazumder

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--
Mohammad SHAMEONI NIAEI
Astronomer / Software Specialist
Astrofizik Ara?t?rma ve Uygulama Merkezi (ATASAM),
Yakutiye, ERZURUM/T?RK?YE
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