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What Determines the Aesthetic Appeal of Astronomical Images? Research & Applications

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What Determines the Aesthetic Appeal of Astronomical Images? Research & Applications Lars Lindberg Christensen Olivier Hainaut Keywords European Southern Observatory European Southern Observatory Astronomical Images, Astrophotography, [email protected] [email protected] Photography, Image Processing Douglas Pierce-Price European Southern Observatory [email protected] Summary In the context of images used for education and outreach purposes, this paper describes a set of parameters that are key in determining the aesthetic appeal, or beauty, of an astronomical image. Rationale Introduction The importance of images in the public The human eye (Figure 1) is one of the photogenic resolution, definition (or struc­ communication of astronomy can hardly most complex creations of nature. With its ture or contrast), colour, composition, sig­ be overstated. Images are not just a means intricate system of sensory cells — light- nal-to-noise ratio, and how well instrumen­ of visual communication. They can inspire sensitive rods and colour­sensitive cones tal artefacts have been removed. awe, wonder and enthusiasm, and portray — we experience the world around us visu­ the Universe as a fascinating place worthy ally. But what determines whether we enjoy In this paper, we do not discuss the details of exploration. Producing engaging astro­ looking at an image or not? Specifically of producing the final colour outreach nomical images with aesthetic appeal or what determines whether we enjoy look­ images from multiple datasets. In essence, beauty is, thus, an important objective for ing at astronomical images like the ones this involves astronomical processing on astronomical communicators. If we can shown in Figure 2? high dynamic range1 FITS files, dynamic determine the parameters that influence range compression of the processed files, how well an image is received by the viewer, The problem of trying to describe beauti­ and final composition and graphical pro­ it becomes easier (and potentially faster) ful images in a logical manner is not iso­ cessing to reach the end result of a low to produce higher quality images and it lated to astronomy. One of the holy grails dynamic range, publication-ready colour becomes possible for a wider range of peo­ of computer graphics science is the algo­ image. An example of a tool for the most ple and observatories to produce them. rithmic description of beauty in self-simi­ sensitive parts of this process is the ESA/ lar life forms, for example, as pioneered by ESO/NASA FITS Liberator software. The Prusinkiewicz & Lindenmayer in their book documentation on this program’s website2 The Algorithmic Beauty of Plants (1990). includes a short introduction to astronomi­ The aim here is to define the algorithmic cal image processing3 and a step-by-step beauty of a plant by reducing it to a series guide to making images4. Other texts on the of interacting components (see Figure 3). production of astronomical colour images are Rector et al. (2007), Christensen (2007), Based on the experience of compos­ and sources referenced therein. ing almost 1000 outreach images from raw data from ESO’s telescopes and the NASA/ESA Hubble Space Telescope, in this paper, we propose that six parame­ Figure 1. The human eye — one of the most complex ters, described in the sections below, are creations of nature. Credit: Petr Novák (under Crea- key in determining the aesthetic appeal of tive Commons via Wikipedia). an astronomical colour image. These are 20 CAPjournal, No. 14, January 2014 CAPjournal, No. 14, January 2014 Figure 2. Collage of beautiful astronomical images from small and large telescopes on the ground and in space — such as the Gemini Observatory, ESO’s Very Large Tel- escope, Chandra X-Ray Observatory, ALMA, the NASA/ESA Hubble Space Telescope, NASA’s Spitzer Space Telescope, ESO’s Visible and Infrared Survey Telescope for Astronomy (VISTA) and ESO’s VLT Survey Telescope (VST). These images serve as inspiration for many and are a stark reminder that our existence here on Earth is just a small cross-section of the many different environments that exist in the Universe. 1. Photogenic resolution Early marketing for consumer digital cam­ eras often concentrated on the total num­ ber of pixels in the detector, and hence in the resultant photographs. A larger number of “megapixels” is often considered to be an indicator of a better camera. However, in real life there are other limiting techni­ cal factors such as the quality of the cam­ era’s optics. This is also true in the case of astronomical observations: a key factor is the angular resolution of the observa­ tion, which, for a diffraction-limited single- aperture telescope, is improved by increas­ ing the diameter of the telescope’s primary mirror, but not by increasing the number of pixels in the detector. And since astron­ omers use big “zoom lenses”, another limiting factor for astronomical images at visible wavelengths is the atmospheric blurring of images. A phenomenon that manifests itself in the twinkling of stars at Figure 3. Artificial trees generated by biological modelling and visualisation algorithms embedded in the night due to atmospheric scattering or the TreeSketch iPad software. Credit: Steven Longay flickering of distant objects in the daytime due to heat haze. tures that are actually resolved. A many­ real factor that limits the aesthetics of an megapixel image of a blurred object is image is the photogenic resolution, rphoto — So, a large number of pixels alone is not still blurred. Furthermore, an image with the number of effective resolution elements a guarantee of sharpness — the photo excellent sharpness may not be visually (the size of the finest feature that can be may simply be oversampled, i.e., have appealing if a narrow field of view means resolved) across the field of view (FOV): much more finely spaced pixels than that there are not many features in the are needed to display the smallest fea­ picture. Therefore, to be more precise, the rphoto = FOV/θeffective, What Determines the Aesthetic Appeal of Astronomical Images? 21 What Determines the Aesthetic Appeal of Astronomical Images? where θeffective is the effective angular where D is the diameter of the primary In our experience, for an image to look resolution. For an astronomical image, mirror or lens and λ is the wavelength impressive, the photogenic resolution one can view this in simple terms as the observed. As mentioned, however, the real should be greater than of order 1000. For greatest number of stars (considered to resolution — at visible wavelengths at least instance, the MPG/ESO 2.2-metre tele­ be point sources) that can fit side by side — is most often limited by the atmospheric scope’s Wide Field Imager (2.2-metre/ across the field of view. In the ideal case, quality, or seeing. In reality, this usually lim­ WFI) can produce individual images where the optics are perfect and there is no its the effective resolution of any telescope with rphoto > 2000, as can the Wide Field atmospheric distortion, the diffraction-lim­ to that achieved by a 30-centimetre tele­ Channel of Hubble’s Advanced Camera ited angular resolution θdiffraction for a single­ scope, such as those used by advanced for Surveys (HST/ACS-WFC). Images aperture telescope is approximated by: amateur astronomers. with rphoto << 1000 will inevitably look blurred. If an individual observation has θdiffraction = λ/D, a low photogenic resolution (due to low Wavelength range and angular resolution for variousimagers 10−4 ALMA ESR ALMA ESG ALMA full SMA 10−3 CARMA VLBA EVLA Chandra/ACIS-I ALMA full HST/ACS-WFC 0.01 HST/WFC3-UVIS VLT/FORS VLT AO (arcsec) VLT/HAWK-I Hubble VLT/NACO 0.1 ALMA ESG VLT/VIMOS resolution Spitzer/IRAC ALMA ESR Chandra APEX/LABOCA Angular Herschel/PACS 1 Spitzer VLT/VISTA/VLT/2.2-m (Gemini, Keck, etc) Herschel/SPIRE JCMT/SCUBA JCMT/SCUBA-2 10 Herschel VISTA/NIRCAM 2.2-m/WFI ATCA radio mm/submm fir mir nir vis uv X-ray VLBA 100 107 106 105 10000 1000 100 10 1 0.1 Wavelength (microns) Figure 5. Wavelength range and effective angular resolution for a small selection of different astronomical telescopes and imagers. Angular resolution and field of view for various imagers 10000 ALMA ESR ALMA ESG 5000 VISTA ALMA full VST SMA 2.2-m CARMA 2000 EVLA Chandra/ACIS-I Chandra 1000 HST/ACS-WFC HST/WFC3-UVIS 500 8–10-metre class (VLT, Gemini, Keck) VLT/FORS VLT/HAWK-I Hubble 200 VLT/NACO VLT/VIMOS (arcsec) 100 Spitzer/IRAC view of VLT AO APEX/LABOCA Field 50 Herschel/PACS Herschel/SPIRE r JCMT/SCUBA 20 Spitzer phot Figure 4. Comparing the effective angular resolution o = JCMT/SCUBA-2 of Hubble (top) with that of the VLT’s ground-based 10000 8-metre telescope (bottom). As Hubble’s optics are 10 VISTA/NIRCAM 2.2-m/WFI very good, and there is no atmosphere disturbing the Herschel ALMA ESR ALMA ESG ALMA resolution of the image, their picture is limited only by 5 fullarray ATCA the wave nature of light itself and the diameter of the r photo =10 r r photo =100 r photo primary mirror. The VLT image suffers from atmos- r phot 2 photo o = 1 r photo =1000 = = 10 pheric distortion and is oversampled (has fewer 100 000 r photo =10000 effective resolution elements). Credit: NASA & ESA/ 1 Hubble, European Southern Observatory 100 10 1 0.1 0.01 10−3 Angular resolution (arcsec) Figure 6. Effective angular resolution plotted against the field of view for a selection of different imagers. 22 CAPjournal, No. 14, January 2014 CAPjournal, No. 14, January 2014 angular resolution, narrow field of view, or players, such as Chandra, the MPG/ESO both), a mosaic of multiple observations 2.2-metre telescope, the Canada France can improve the resulting photogenic res­ Hawaii Telescope (CFHT), ESO’s VISTA and olution.
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