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    Exactly! I forgot that what I was manipulating was just a variable
    and that the object hdu remained in memory unchanged. <br>
    Now it works as it should be!<br>
    <br>
    Thanks Marcio!<br>
    <br>
    @Leticia : I have already tried that but it was not working for the
    above reason.<br>
    @Doug : no problem! thanks anyway!<br>
    <br>
    <div class="moz-cite-prefix">On 02/10/2016 06:06 PM, Marcio Melendez
      wrote:<br>
    </div>
    <blockquote
cite="mid:CAJ0xgiRhvWxDbgu20+=uKWmgy=4QCBhKv8kKfYzJiojrfJ85Cw@mail.gmail.com"
      type="cite">
      <div dir="ltr">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 <span
          style="font-size:12.8000001907349px">dt = dt*1000 with </span><span
          style="font-size:12.8000001907349px">hdu.data = </span><span
          style="font-size:12.8000001907349px">dt*1000. Alternatively
          you can create another object (if you want to keep hdu intact)
          by:<br>
        </span>
        <p class=""><span class="">hdu_new = pyfits.PrimaryHDU(dt) #
            where dt = </span><span style="font-size:12.8000001907349px">hdu.data</span><span
            style="font-size:12.8000001907349px">*1000</span></p>
        <div>hdu_new.writeto('<span style="font-size:12.8000001907349px">small_mod.</span><span
            style="font-size:12.8000001907349px">fits</span>')<span
            style="font-size:12.8000001907349px"> </span></div>
        <div><span style="font-size:12.8000001907349px"><br>
          </span></div>
        <div><span style="font-size:12.8000001907349px">Remember to add </span>clobber
          = True to your writeto if you want to overwrite the file. </div>
        <div><br>
        </div>
        <div>I hope this help, </div>
        <div class="gmail_extra"><br>
          <div class="gmail_quote">On Wed, Feb 10, 2016 at 11:42 AM,
            Doug Burke <span dir="ltr"><<a moz-do-not-send="true"
                href="mailto:dburke.gw@gmail.com" target="_blank">dburke.gw@gmail.com</a>></span>
            wrote:<br>
            <blockquote class="gmail_quote" style="margin:0 0 0
              .8ex;border-left:1px #ccc solid;padding-left:1ex">
              <div dir="ltr"><br>
                <div>Grigoris,</div>
                <div><br>
                </div>
                <div>If you change 'dt = dt * 1000' to 'dt *= 1000' do
                  you get what you expect?</div>
                <div><br>
                </div>
                <div>Doug</div>
              </div>
              <br>
              <div class="gmail_quote">
                <div dir="ltr">On Wed, Feb 10, 2016 at 11:30 AM Grigoris
                  Maravelias <<a moz-do-not-send="true"
                    href="mailto:gr.maravelias@gmail.com"
                    target="_blank">gr.maravelias@gmail.com</a>>
                  wrote:<br>
                </div>
                <blockquote class="gmail_quote" style="margin:0 0 0
                  .8ex;border-left:1px #ccc solid;padding-left:1ex">Hello!<br>
                  <br>
                  I have a problem with a rather simple process, but I
                  cannot understand<br>
                  what I am doing wrong. I have an image in a fits file
                  that i want to<br>
                  open and process its data values, and save it as
                  another file (including<br>
                  some extra information on the header for what I did).
                  So according to<br>
                  the astropy docs [1] I did the following:<br>
                  <br>
                  hdulist = fits.open('small.fits')<br>
                  hdu = hdulist[0]<br>
                  hd = hdu.header<br>
                  dt = hdu.data<br>
                  <br>
                  hd.append(('HISTORY', 'blabla '), bottom=True)<br>
                  <br>
                  print dt            # check data before<br>
                  dt = dt*1000<br>
                  print dt            # and after<br>
                  <br>
                  hdulist.writeto('small_mod.fits')<br>
                  <br>
                  If I print dt before and after their modification
                  (multiplied by 1000) I<br>
                  can see that they are indeed different (properly
                  multiplied), but when I<br>
                  save the fits file I just see the original data values
                  (however, the<br>
                  HISTORY line is added properly in the primary header).<br>
                  <br>
                  What am I missing here?<br>
                  <br>
                  Best<br>
                  Grigoris<br>
                  <br>
                  <br>
                  [1] <a moz-do-not-send="true"
                    href="http://docs.astropy.org/en/stable/io/fits/usage/headers.html"
                    rel="noreferrer" target="_blank">http://docs.astropy.org/en/stable/io/fits/usage/headers.html</a><br>
                  <br>
                  <br>
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                </blockquote>
              </div>
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          <div><br>
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          -- <br>
          <div class="gmail_signature">
            <div dir="ltr">
              <div>
                <div dir="ltr">
                  <div dir="ltr">
                    <div>Marcio B. Melendez, Ph.D</div>
                    <div><span style="font-size:12.8000001907349px">JWST
                        ISIM Optics Support Scientist</span><br>
                      <div style="font-size:12.8000001907349px"><span
                          style="font-size:12.8000001907349px">Wyle </span><span
                          style="font-size:12.8000001907349px">Science,
                          Technology and Engineering Group</span></div>
                      <div style="font-size:12.8000001907349px"><span
                          style="font-size:12.8000001907349px">NASA
                          Goddard Space Flight Center  Greenbelt, MD 
                          20771  </span></div>
                      <div style="font-size:12.8000001907349px"><span
                          style="font-size:12.8000001907349px">Bldg. 29
                          Rm. T29-10, phone: 301-286-8517</span></div>
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