Beauty and Astrophysics

Michael S. Bessell, PASA, 17 (2), 179.

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Subsections


Discussion

Are the Colours Real?

The colours that are seen, are 'real' colours, in distinction to the 'false' colour images that one often sees these days. The blue, green and red colours are assigned to those actual observed colours in most cases, in the few cases where they are not they are at least assigned in the order of increasing wavelength. However, because the images utilize at least one narrow band filter centered on an emission line from ionized hydrogen, we 'see' the gas brighter than the star light by a factor that is the ratio of the width of the narrow band filter to the width of the broad band filter that represents the eye's colour band. That is, we increase the brightness of the hot interstellar gas in comparison to the brightness of the stars. We draw attention to the star forming regions by the effect that the hot stars have on the gas. We do not alter the relative colours of stars but we make them fainter in comparison to the brightness of the gas. But we do alter the colour of the gas depending on which emission line we choose to highlight.

For example, in the three images of 30 Doradus (line 5) we see very graphically the effect of changing the combination of filters used. Replacing the narrow green filter with a broad green filter weakens the green light from the gas compared to the star light. Finally replacing the narrow red filter with a broad red filter weakens the red light from the gas. The images shift from being star dominated to gas dominated and the colour of the gas changes while the colour of the stars stays the same.

In most of the images that are shown, by combining narrow band green ([OIII]) and narrow band red ($H\alpha$) light, we produce a yellow colour when the [OIII] line is similar in strength to the $H\alpha$ line; we see this for example in the Lagoon and Trifid Nebulae (line 11) where the ionization is produced by radiation from imbedded hot stars. Colour gradients between red and yellow show the varying strength of these lines as the radiation temperature of the gas changes.

However, in supernova remnants, like the Crab (line 21) and Vela (line 20) some of the ionization results from high energy shocks and in these cases the [OIII] lines can be strong while $H\alpha$ is very weak. In that case the gas will appear green. We also see regions in SN remnants where there are a series of ionization fronts, some green while others are yellow and red illustrating the complex ionization structure and interaction between collisions and radiation. The use of [SII], $H\alpha$ and [NII] filters as seen in the series of Orion images from KPNO, NTT and HST (line 18) shows more subtle ionization differences.

What is reality in an image?

We distort 'reality' in many of our images in several ways. CCDs are linear detectors that faithfully record the relative brightnesses. However, our eyes are logarithmic detectors so is it more 'real' to process the CCD data to be logarithmic? We do this anyway in our data processing as explained below. But we need to do other distortions as well. The range of intensities that the eye can see, or the monitor or hard copy can provide, is limited. There is a great advantage in further compressing the intensity scale so as to keep detail in the bright areas of an image as well as in the fainter part. Photographic plates and prints did this a little by their non-linearity at the bright end but we can achieve it much better digitally. For example, in the images of galaxies (lines 1, 2 and 3) the nuclear regions and bulges of these galaxies tend to be much, much brighter than the outskirts. We have tried as hard as possible to retain the information in those bright regions whilst enhancing the brightness and contrast in the outer spiral arm regions. The nuclear ring in NGC 2997 is just visible and the dust lanes with their imbedded star forming regions are seen in these barred spirals to reach into the central regions. Such details are not obvious in previously published images showing the outer regions.

Another way we distort reality or rather we enhance reality is by emphasizing the hot interstellar regions. Normal broad-band optical images of galaxies and galactic fields mainly show stars. Within a narrow dynamic range these stars are coloured although the brighter ones are invariably white because the images are saturated. However, we know that in most galaxies, in particular in spiral galaxies and disk galaxies like the Milky Way and the Magellanic Clouds the gas content is very high and in fact often dominates the mass distribution in the disk. Is it 'real' then to show a picture that mostly ignores a major physical component? By enhancing the faint light of ionized hydrogen as we have done in our images we are bringing to attention an important mass component. For example, in the wide field view of the Milky Way (line 6) we begin to see that the galactic plane is bathed in a sea of ionized gas and all the bright HII regions are probably connected. In the SMC and LMC (line 3,4) we see galaxies that are dominated by gas, which is what they are. So these images bring a new perspective and in many ways are more 'real' than previous 'reality'. We still cannot see the neutral hydrogen but by combining the Parkes multibeam survey (http://www.atnf.csiro.au/research/multibeam/multibeam.html) with these images, using another colour, we should be able to encompass that as well. So the difficulty and challenge is to present a beautiful image whilst retaining and emphasizing the important physical information, the integrity, of the object.

Image Processing

Finally, some specifics about the observing procedure and image processing. We aim to take three images through each filter with the telescope slightly offset between each exposure in the set of three. The three images through each filter are bias subtracted and flat fielded then superimposed and combined by median filtering to produce a single grey image without cosmic rays and bad columns. The combined images through different filters are registered onto a reference image using the programs GEOMAP and GEOTRAN within IRAF. The 16 bit FITS files are then imported into PHOTOSHOP using a program written by Ralph Sutherland. This program, which generates 8 bit PHOTOSHOP data, samples the image and suggests upper and lower levels for the conversion. It also offers linear or logarithmic scaling. Most of the images shown here have had logarithmic scaling for all three colours (channels). (Sometimes special effects can be produced by combining logarithmic scaling in one or more colours with linear scaling in another.) We often need to reimport images with different levels chosen, especially near the sky to ensure that faint detail is not lost and to maximise the 256 levels in PHOTOSHOP by not wasting them. In PHOTOSHOP the images are copied into the R, G or B channel of a new image and then manipulated by adjusting levels, linearity and contrast. The manipulation of the levels and curves are where the artistic endeavour is required; however, one also needs to address what story is being told in order to determine how best to present the data. This is where the astronomy is needed together with the preconceived ideas of what a coloured astronomical picture should look like. Generally, we feel that the background sky should be dark with no dominant colour; that the bulk of the stars should be white or their colours soft shades of blue-white, white, yellow or orange. Normally, blue stars come out blue-white, most stars white and red giants are generally deep yellow. The glowing interstellar region then produces its own unexpected colours. An image can of course be produced without any a priori astronomical knowledge and this would be an interesting exercise for an artist, but with one exception I will not present any such images here.

The HST images of the Magellanic cloud clusters (line 7,8) show more subtle colours. Because the blue colour is the PHOTOSHOP image represents the far ultraviolet colour of the star. The brightness of the blue image of the hot stars is much brighter than normal optical images. As a consequence, these stars come out very blue. But if those hot stars have strong $H\alpha$ emission (the Be stars), they appear pink in the colour image; you can see the high proportion of Be stars in these Cloud clusters. In addition, the great drop in UV light (compared to blue light) between an A-F star and a KM star makes the AF stars white in the colour images and the KM stars orange.

We have made no attempt to produce real intensities in our manipulations. The intensity mappings are however, monotonic although not linear. The manipulations are done for their impact without undermining the astrophysics. Where exact quantitative data is required this is best done with the individual $\it linear$ monochromatic images in IRAF. We have generated good magnitudes or good stellar free $H\alpha$ images by subtracting or dividing the individual images in IRAF. Michael Murphy (UNSW) has also identified many probable planetary nebulae and Be stars in the Magellanic Clouds by doing photometry on $H\alpha$-[OIII] and [OIII]-continuum images (Murphy & Bessell 1999).

Future Work

Paul Price (Price 1999) is doing an honours project completing the $H\alpha$ survey and comparing it with specific portions of the the Molonglo Radio Survey (Green et al. 1999) and the MSX IR Galactic Plane Survey (Cohen 1999) (http://gibbs1.plh.af.mil/). We are also completing the wide-field Hasselblad survey of the whole galactic plane visible from SSO and exploring with a commercial organisation the possibility of making an interactive CDROM using all the CCD images we have processed. However, generating colour images is very time consuming so it is unlikely that much more can be done without additional resources.


Next Section: Acknowledgements
Title/Abstract Page: Beauty and Astrophysics
Previous Section: Atlas of Images
Contents Page: Volume 17, Number 2

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