Plotting FITS images

This page gives a brief overview of how to make basic plots of FITS files.

Table of contents

1. Basic sky plot
2. Image manipulation
   1. Gamma correction
   2. Contrast limited adaptive histogram equalisation
   3. HDR tonemapping

Basic sky plot

The simplest plotting command is

everystamp plot --image <fits image>

This will create a simple RA-DEC sky plot of the provided image using a linear stretch.

Image manipulation

EveryStamp offers image manipulation that can be used to visualise an image to your liking. The options outlined below are applied in the following order, if used:

1. HDR tonemapping
2. CLAHE
3. Gamma correction

Gamma correction

Gamma correction can be applied as a final step by adding --gamma <gamma> to the plotting arguments. As implemented here, this will gamma stretch an image by

\[I_\mathrm{out} = I_\mathrm{in}^{1 / \gamma}\]

A value of \(\gamma = 2\) results in a square root stretch of the input image.

Contrast limited adaptive histogram equalisation

Contrast limited adaptive histogram equalisation (CLAHE) tries to improve contrast in the image by applying a local histogram equalisation while simultaneously limiting the contrast enhancement in an attempt to reduce the amplification of noise. It is applied as a second-to-last step, before any gamma correction, and can be enabled by using --CLAHE. The --CLAHE-gridsize and --CLAHE-cliplim control the size of the local region and the amount of contrast enhancement, respectively.

HDR tonemapping

When dealing with a high dynamic range (HDR) image, simple tonemapping operators may not be enough. If LuminanceHDR is installed (see installation), several HDR tonemapping algorithms are available through the --hdr-tonemap argument.