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ING Newsletter No. 6, October 2002

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Real-Colour Images of Spiral Galaxies

Nik Szymanek (Univ. of Hertfordshire) and Johan H. Knapen (ING and Univ. of Hertfordshire)

The real-colour images of spiral galaxies reproduced in this Newsletter form part of a new series of such pictures, currently under construction. The original images used have all been obtained with the ING telescopes, mostly with the JKT, some with the INT, and an occasional (H-alpha) image with the WHT. We collected most of the images ourselves as part of a large PATT-supported science programme aimed at studying spiral arm structure, and the rest from the ING archive. For each galaxy, we have images in the B and I bands as well as in the near-IR Ks band (see also article on page 3), and in H-alpha. Using a set of newly developed IRAF scripts we can produce series of images for each galaxy which are registered to a high accuracy, i.e., have the same pixel scale, orientation, resolution, and overall image size. This is not trivial, given the use of large amounts of archive data, taken with different cameras, detectors and even telescopes. Whereas the data sets are being produced for scientific work, they serve another purpose which is to show the beauty and the variety of spiral galaxies. Our sample consists of 57 galaxies of all spiral types, from flocculent (multi-armed) to grand-design (two-armed and symmetric), and with and without bars, circumnuclear structure, and rings.

To produce the real-colour images, the image sets, originally in FITS format, are read into an image processing package called Maxim DL. This programme allows manipulation of ING images as well as powerful image co-addition, calibration and colour combination/balancing routines. Images are saved in a variety of formats including FITS, TIFF and JPEG. It is our intent to produce a finished result that extracts detail from the core region of galaxies as well as any other features, such as rings, gravitational tails, interacting arms, etc. A variety of processing routines will do this successfully, such as logarithmic scaling, but in our opinion the best routine is that of “Digital Development” a software algorithm created by the Japanese amateur astronomer Dr. Kunihiko Okano. Digital Development applies a hyperbolic transfer function that sits neatly between the standard gamma curve and a logarithmic curve. The Digital Development curve is successful for a number of reasons. The steeply rising curve withholds the brightness value of the sky background and as the curve levels out the effect is to enhance the middle-grey tones, typically information which is contained in the overexposed (“burnt-out”) core of the galaxy. As the curve finally levels out and flattens this has the effect of compressing the dynamic range of the image. A sharpening routine known as “unsharp masking” is also applied to enhance detail. There are several user-definable parameters within Maxim DL, as well as a preview screen which displays how the image will appear once Digital Development is applied.

Each of the galaxy image components will be processed using the above method. Best (or certainly most spectacular) images are achieved using standard B, R and V images. Additional wavelength data, such as H-alpha components, can be overlaid at any point. Maxim DL allows the colour addition and registration of the individual files but, in our opinion, the best results are obtained using powerful image-manipulation software such as Adobe Photoshop. The FITS files are converted to 8-bit TIFF files after careful scaling, and combined in registration. Further processing such as colour balancing and the removal of unsightly artifacts such as cosmic ray hits, dust-“donuts” and general CCD defects are easily applied in Photoshop.

It is important to realise that the above processing routines are applied to produce aesthetically pleasing images and perhaps not images that are intended for purely scientific research. To be fair, the Digital Development routine extracts more detail than most other applications and performs spectacularly with the nuclei of galaxies and globular clusters. The ING Archive contains many high-quality images taken with the JKT, INT and WHT and these images can be used to demonstrate the quality of ING equipment and, indeed, the quality of the sky at La Palma (see Sky & Telescope, June 2001, pp. 44–45).

We thank Sharon Stedman, Dan Bramich, Stuart Folkes and Javier Méndez for their help in the observations and data reduction and handling.

NGC 3184 [ JPEG | TIFF ] NGC 4321 [ JPEG | TIFF ] NGC 3351 [ JPEG | TIFF ]
NGC 488 [ JPEG | TIFF ] NGC 1169 [ JPEG | TIFF ] NGC 3486 [ JPEG | TIFF ]
NGC 4254 [ JPEG | TIFF ] NGC 4725 [ JPEG | TIFF ] NGC 4579 [ JPEG | TIFF ]
From left to right and from top to bottom: NGC 3184, NGC4321, NGC3351, NGC488, NGC1169, NGC3486, NGC4254, NGC4725 (B-band) and NGC4579.

Email contact: Nik Szymanek (Nik.Szymanek@tesco.net)

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Last modified: 13 December 2010