[AstroPy] question on saving fits files

[Grigoris Maravelias]

Exactly! I forgot that what I was manipulating was just a variable and
that the object hdu remained in memory unchanged.
Now it works as it should be!

Thanks Marcio!

@Leticia : I have already tried that but it was not working for the
above reason.
@Doug : no problem! thanks anyway!

On 02/10/2016 06:06 PM, Marcio Melendez wrote:
> The problem is that you just changed a variable, dt, and not your
> object hdu (the one that you are writing to). So you need to
> replace dt = dt*1000 with hdu.data = dt*1000. Alternatively you can
> create another object (if you want to keep hdu intact) by:
>
> hdu_new = pyfits.PrimaryHDU(dt) # where dt = hdu.data*1000
>
> hdu_new.writeto('small_mod.fits') 
>
> Remember to add clobber = True to your writeto if you want to
> overwrite the file. 
>
> I hope this help, 
>
> On Wed, Feb 10, 2016 at 11:42 AM, Doug Burke <dburke.gw at gmail.com
> <mailto:dburke.gw at gmail.com>> wrote:
>
>
>     Grigoris,
>
>     If you change 'dt = dt * 1000' to 'dt *= 1000' do you get what you
>     expect?
>
>     Doug
>
>     On Wed, Feb 10, 2016 at 11:30 AM Grigoris Maravelias
>     <gr.maravelias at gmail.com <mailto:gr.maravelias at gmail.com>> wrote:
>
>         Hello!
>
>         I have a problem with a rather simple process, but I cannot
>         understand
>         what I am doing wrong. I have an image in a fits file that i
>         want to
>         open and process its data values, and save it as another file
>         (including
>         some extra information on the header for what I did). So
>         according to
>         the astropy docs [1] I did the following:
>
>         hdulist = fits.open('small.fits')
>         hdu = hdulist[0]
>         hd = hdu.header
>         dt = hdu.data
>
>         hd.append(('HISTORY', 'blabla '), bottom=True)
>
>         print dt            # check data before
>         dt = dt*1000
>         print dt            # and after
>
>         hdulist.writeto('small_mod.fits')
>
>         If I print dt before and after their modification (multiplied
>         by 1000) I
>         can see that they are indeed different (properly multiplied),
>         but when I
>         save the fits file I just see the original data values
>         (however, the
>         HISTORY line is added properly in the primary header).
>
>         What am I missing here?
>
>         Best
>         Grigoris
>
>
>         [1] http://docs.astropy.org/en/stable/io/fits/usage/headers.html
>
>
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>
>
>
> -- 
> Marcio B. Melendez, Ph.D
> JWST ISIM Optics Support Scientist
> Wyle Science, Technology and Engineering Group
> NASA Goddard Space Flight Center  Greenbelt, MD  20771  
> Bldg. 29 Rm. T29-10, phone: 301-286-8517
>
>
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