Digital SLR Image Processing

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The latest developments in IRIS software have been targeted at digital SLR camera usage. The widespread development of detectors in large sizes, with high performance and low cost, is a direct result of the fierce competition in the digital

Figure 6.7. A two-dimensional image of 59 Cygnii 's spectrum (LHIRES2 spectrograph + 60 cm telescope of the Pic du Midi observatory - R = 20.000).

Figure 6.8. The spectral profile extracted from the 2D image in Figure 6.7.

Wavelength (A]

SLR camera marketplace. Digital SLR cameras are simple to employ at the focal plane of our telescope. CCD cameras, optimized for astronomy, are still the best for studies requiring maximum performance, such as deep-sky surveys, but digital cameras have other advantages. These advantages include a single unit, compact size, optical viewfinder and a high-performance, film-sized, color CCD/CMOS sensor. In addition, their cost is falling almost monthly, already becoming much much cheaper than a dedicated CCD camera. The large size of the sensor used can be quite decisive for some scientific studies like nova detection. Overall, digital cameras are not in competition with CCD cameras but rather are complementary.

The image coming out of such digital SLR cameras results from the simultaneous recording of the three-color planes, i.e., red, green and blue. This allows the final restoration of the color image but requires specific processing. To get such results, a tiny color filter covers each pixel in a specific pattern (see Chapter 1). As each color pixel is spatially separated, when combining them, an interpolation algorithm has to be used to recover the final colored image in full resolution. IRIS has alternatives for this but in deep-sky imaging, where noise is predominant, avoiding aggressive algorithms designed to obtain the most resolved image is recommended. A more efficient solution, regarding signal-to-noise ratio optimization that also minimizes artifacts, is usually a simple linear combination of the colored pixels.

The RAW format is virtually mandatory to get the best out of a digital camera. In this format, the camera records the image as it comes straight off the sensor (unfortunately some exceptions exist, like the Nikon D70 where a median filter is systematically applied to the RAW image to eliminate the hot spots, which is nonsense for a RAW image). Images compressed to JPEG format lose precious information in the compression process, which is undesirable for astronomical usage. Every manufacturer has its own RAW format, which greatly complicates the software designer's life! IRIS is able to convert the proprietary RAW format into a standard FITS image for most cameras from the leading manufacturers.

The preprocessing routines applied are then completely identical to that of classical CCD images. Only the specification of the colored filter matrix has to be taken into account and leads to some algorithm adaptation, especially for the flat-field correction or for the white balance function (a solar-type star or a white illuminated screen has to be used).

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