Modifications carried out on the Huygens Raw Images

Huygens Panorama of Titan Shoreline
For this Panorama of the "Shoreline" on Titan, a lot of image processing had to be carried out on some of the DISR Side-Looking and Medium-Resolution Imager Raw Data. Most of the work was done with Adobe Photoshop. At least one, in most cases many of the following modifications have been carried out on individual raw images:

-flatfield correction (Medium-Resolution Imager only)
-histogram correction
-brightness / contrast correction
-horizontal / vertical scaling
-perspective distortion
-parallelogram distortion
-trapezium distortion
-histogram correction with (mainly vertical) gradient
-brightness / contrast correction with (mainly vertical) gradient

For this wide field mosaic of the surface of Titan, the following Modifications were carried out on the individual raw images:
- Flatfield correction
- Histogramm correction
- Trapezium distortion
- size
- brightness/contrast, some images with a gradient

Flatfield corrected DISR MRI frames

Zip-Archive (2.9 MB) for download

All MRI frames have been
- flatfield corrected (flatfield flat_medres_100-350.tif) and
- histogram corrected (Photoshop)
Most Duplicates, obviously broken images and the post-landing images have been removed. 
Flatfield Correction

The images from the Medium-Resolution Imager exhibit some brightness variations that show up on all frames, especially if the original images have low contrast as the ones obtained while Huygens was still high in the hazy atmosphere of Titan. These brightness variations are artifacts by the DISR optical system, in principle every CCD optics has these artifacts, and after 7 Years in space it is to expect that the conditions of the system alter. I used the very early images that show nearly no surface features because Huygens was still high in the hazy clouds (raw image numbers below 200) to make a flatfield of the DISR
Medium-Resolution Imager. All useable images were stacked (Giotto) to mediate out any remaining brightness variations from Titan surface structures.
DISR medium resolution imager flatfield
DISR flatfield contrast enhanced
DISR Medium Resolution Imager Flatfield
DISR Medium Resolution Imager Flatfield, contrast enhanced
Flatfield correction examples
following examples give an impression on the effect of the flatfield correction (Giotto). Small artifacts (dust particles or radiation damages of the CCD) are less effectively corrected; this may be due to the JPEG compression algorythm.
For a flatfield correction do not use the JPG compressed flatfields, but the original flatfield images in TIFF-format (see below).
image 310
image 310 flatfield corrected
image 394
image 394 flatfield corrected
DISR Medium Resolution Imager, frame Nr. 310, histogramm correction applied DISR Medium Resolution Imager, frame Nr. 310, with flatfield correction and histogram correction DISR Medium Resolution Imager, frame Nr. 394, histogramm correction applied
DISR Medium Resolution Imager, frame Nr. 394, with flatfield correction and histogram correction

Here you can download the
DISR MRI, HRI and SLI flatfields
in TIFF format (Zip Datei)

Detailed information on the flatfields (Flatfield_Infos.txt):

DISR Medium Resolution Imager flatfields
February 13, 2005

I stacked the flatfields from the first DISR Medium Resolution Imager raw data
with Huygens still high in the cloudy or hazy atmosphere of Titan, so the structures in the
frames are dominated by optical path / CCD artifacts, and should give a usable flatfield.

The flatfields were stacked, and the flatfield corrections on the individual frames
were carried out, with help of the videoastronomy image processing software
"Giotto" (freeware): http://www.videoastronomy.org
Obviously damaged images in the given intervals were removed before stacking the images.

Images 103-352.
I obtained the best results with this flatfield, although it includes some images
with Titan surface structures in the higher numbers.

Images 13-302.
A flatfield correction with this flatfield results in images with vertical stripes, that
may be due to the fact that the DISR CCD was still relatively warm from the ride through
Titans atmosphere as the first images were obtained.

Images 103-200.
Although the images below 100 are omitted, this flatfield still results in slightly striped
corrected images, but in should contain virtually no surface structures.


Huygens DISR High Resolution Imager (HRI) flatfield
April 8, 2005

filename: flat_HRI_100-319.tif
Flatfield stacked from images 100-319 with obviously damaged images omitted.
The HRI exhibits distinctively less errors than the MRI;
consequently a flatfield correction for the HRI only results in a slight improvement,
mainly a vertical brightness gradient is corrected.


Huygens DISR Side looking imager (SLI) flatfields
February 27, 2005

filename: flat_SLI_103_500.tif
The SLI flatfield are no normal flatfields, because they contain strong vertical
brightness gradients that are due to the brightness gradient of Titans atmosphere
the SLI saw.
Using these flatfields consequently suppresses the natural vertical brightness gradient
of the SLI images what may bring out feeble distant details of Titans landscape, but may
also result in an overcorrection at individual frames.
Any interpretations that base on images corrected this way should always comprise
a critical look at the uncorrected frames!

In these flatfields, the vertical brightness gradient was damped by a brightness
correction (darker) that has the value XX (Photoshop: Brightness/Contrast) at
the top of the image and fades linearly to the bottom (zero). This preserves
more or less the natural brightness gradient of the SLI images when correcting them.


All DISR raw Images are Copyright by ESA, NASA, JPL and University of Arizona