One feature rarely used by the casual photographer actually allows the consumer digicam to function like a CCD camera, the gold standard for astroimaging. Many recent, especially high end, models allow saving images in a RAW format. RAW isn't an acronym, but simply refers to saving the image with little to no processing internal to the camera, that is without level, contrast or gamma adjustments, sharpening or losses due to compression. Actually, quite a bit of image processing goes on inside any digital camera (that's when the light is blinking) without your knowing it before it ends up on the memory card. 

Unfortunately, there are disadvantages to using “raw” images as compared to the much more common JPEG (.jpg) format. RAW requires additional processing steps to become presentable, and not all photo programs will handle these images. In addition, the RAW formats can differ between camera models, and the resulting files are considerably larger. But because the RAW image is a “digital negative” much like that from film cameras, it records far more accurate detail and brightness information that the astrophotographer can use to produce a great image or even extract astronomically important data.

How RAW works at the nuts and bolts level is key to its success. Brightness levels from all three color components are stored as they are recorded on the camera's chip. In reality, each pixel has only one of those colors, determined by a tiny filter, a particular pattern of the reds, blues and mostly greens in what is known as a Bayer mask. The ratio is usually two greens for each red and blue, roughly analogous to the color sensitive cone cells in the human eye. The RAW image stores each color separately, but each plane is missing half or more of the pixels' color information! Importantly, data are stored in in 12- to 14 bits, that is up to 16,384 levels versus the usual 8 bits with just 256 discrete levels. To produce a viewable image, the first step is to interpolate (or “demosaic” as it is known) to compute a red, green and blue value for each pixel. Algorithms can be inside the camera (as described above) or in new photo programs (see below).

RAW imaging provides many opportunities for the astroimager. Here, the data represent actual brightness and color on a linear scale. This means that proper image calibration and processing  can be accomplished, just like that standard with CCD images. Better images result. In high dynamic range (HDR) photography, images taken with widely differing exposure lengths (or extreme “bracketing”) can be digitally combined to produce images with exceptional brightness range, using the extended bit-depth of the RAW format. This allows greater contrast and color fidelity, exceeding that of conventional film and much closer to our own visual impressions. Furthermore, increased bit-depth and linearity allow you to extract precise data from the images for photometry of variable stars or spectroscopy. Real science is thus possible.

Many cameras now offer, though may not advertise, the RAW format, with Nikon and Canon brands seeming to be the most widely supported. Although the RAW format has existed for many years and is an established tool among professional photographers, programs to process these images are now becoming widely available. For processing the images, Picasa, IrfranView and the astro-specialized Iris programs can all handle RAW files, and they are free. Choices among commercial products include iPhoto, Photoshop and Photoshop Elements, ImagesPlus and AIP4WIN. Since file formats are proprietary and a moving target in most cases, be sure to obtain the latest available version of any image processing program. But the results will be worth the effort since RAW imaging affords almost total control over the imaging process with the convenience and low cost of consumer digital imaging. Just one more good thing for amateur astroimaging.

Moondark is written by Doug Miller, published at the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 23 July 2007. Text and images on this web page are free for non-commercial use with attribution under a Creative Commons Attribution-Non-commercial 3.0 License. Ask Doug about other uses.