DRA performs what image processing folks call a “stretch.” A stretch takes some range of pixel values from the file and maps it onto a new range for the screen. To take an example, consider an image that appears over-exposed: much of the information is in the upper range of pixel values and you will have trouble distinquishing any detail. If the over-exposed pixels are not completely saturated (i.e. they don’t all have the maximum value) then a stretch that reduces brightness can reveal this otherwise hidden detail.

HiRISE has a very high signal-to-noise ratio, and our targeting specialists do a very good job choosing camera settings (which they do individually for each and every image) so completely saturated pixels are very rare.

But this also means that a stretch that works well over the entire image (a global stretch) may not be the best, the optimal stretch, for any one sub-image area that you are viewing. This is where the Auto DRA function in IAS becomes critical.

The button (shown below) is located on the right-hand side of the toolbar. A single click will do a stretch based only on the pixels you are viewing. This can bring out detail in shadow–amazingly, there is enough ambient light scattering around in the thin atmosphere to illuminate those scenes (and HiRISE is sensitive enough to pick enough of it up). It can also bring out detail in bright areas of over-exposure. For the color images in particular this can make things look a whole lot better.

Another factor plays a part in this. By default, the IAS viewer performs a global DRA when the image is loaded. As seen in the screenshot below, there are areas in our image that can skew the stretch. The large red rectangle is an area where the red CCDs start imaging before the blue-green. The IRB images often will have a cyan region where one of the IR CCDs was too noisy. We have elected to keep these areas in our images.

When in a sub-area, hit the Auto DRA button and the image should be drastically improved, as you can see in this final screenshot.

DRA early and often!

This is an example of a HiRISE image pre-calibration. Notice the vertical streaks across the image. Harder to see is something known as dark current, which appears with any camera if a black picture is taken with it. There is also variations of pixel sensitivity, all of which must be accounted for. This doesn’t include the special calibrations needed for color images, but there are some as well.

This is the same image in roughly the same spot, taken after the calibrations have been done. Note that there is no longer any vertical lines, at least, none that are obvious. That is one of the things that is corrected during calibration. There are several other things which are done as well, but they aren’t as visible, but the image as a whole has been made more scientifically useful. There is always the risk of cutting into the useful part of an image, but HiRISE has several advantages in this respect. HiRISE has a high SNR, or Signal to Noise Ratio. SNR is exactly what it sounds, the ratio of the signal (In our case the light reflected from the Martian suface) to the noise (Which can be anything from Thermal noise, Cosmic Background Radiation, etc) With HiRISE’s large mirror, and pushbroom CCD arrays, it is able to achieve what hasn’t been achievable in other cameras. What this means is that even relatively dark surfaces are still visible to a degree with HiRISE, which has not been the case with most previous imagers.

Calibration images are taken to assist in this process. They are mostly taken by pointing HiRISE at the dark side of Mars, with various temperatures, recording what image results. There are a variety of what’s known as Stim lamps to give an approximation of white light as well, which also assist with calibration. The results from these images (Which are all very small) are placed into a database to know the correct way to calibrate each image. All of these are done so that everyone can see Mars the way that it was meant to be seen.