2.3.1 Preparing input for extended PSF ¶

We will use an image of the M51 galaxy group in the r (SDSS) band of the Javalambre Photometric Local Universe Survey (J-PLUS) to extract its extended PSF. For more information on J-PLUS, and its unique features visit: http://www.j-plus.es.

First, let’s download the image from the J-PLUS web page using wget. But to have a generalize-able, and easy to read command, we will define some base variables (in all-caps) first. After the download is complete, open the image with SAO DS9 (or any other FITS viewer you prefer!) to have a feeling of the data (and of course, enjoy the beauty of M51 in such a wide field of view):

$ urlend="jplus-dr2/get_fits?id=67510"
$ urlbase="http://archive.cefca.es/catalogues/vo/siap/"
$ mkdir jplus-dr2
$ wget $urlbase$urlend -O jplus-dr2/67510.fits.fz
$ astscript-fits-view jplus-dr2/67510.fits.fz

After enjoying the large field of view, have a closer look at the edges of the image. Please zoom in to the corners. You will see that on the edges, the pixel values are either zero or with significantly different values than the main body of the image. This is due to the dithering pattern that was used to make this image and happens in all imaging surveys⁵⁷. To avoid potential issues or problems that these regions may cause, we will first crop out the main body of the image with the command below. To keep the top-level directory clean, let’s also put the crop in a directory called flat.

$ mkdir flat
$ astcrop jplus-dr2/67510.fits.fz --section=225:9275,150:9350 \
          --mode=img -oflat/67510.fits
$ astscript-fits-view flat/67510.fits

Please zoom into the edges again, you will see that they now have the same noise-level as the rest of the image (the problematic parts are now gone).