Nik Szymanek (Univ. of Hertfordshire) and Johan H. Knapen (ING and Univ.
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,
We thank Sharon Stedman, Dan Bramich, Stuart Folkes and Javier Méndez
for their help in the observations and data reduction and handling.