Annual Meeting of the Deep Sky Section, 2007 March 3

 

Two-Colour Imaging of the Deep Sky

Dr Miles explained that his primary interest was in the photometry of comets, asteroids and variable stars. The equipment that he had bought for this deep photometric work also, however, happened to be quite well-suited to deep sky imaging; indeed, whenever he performed photometry on supernovae, deep galaxy images were an immediate by-product. His set-up comprised of a 28-cm Celestron C/11 with two co-mounted 60-mm Takahashi FS60C refractors, one fitted with a V-band (500-700 nm; green) filter and the other an I-band (700-900 nm; infrared) filter, both from Norman Walker. Each Takahashi had a Starlight Xpress SXVF-H9 CCD array attached. In this talk he would be probing the deep-sky potential of these two 60-mm refractors.

The speaker remarked that the separation in wavelength between the V- and I-bands was rather greater than that between the RGB-bands. Consequently, some objects showed I-band magnitudes which were remarkably different from those seen in the V-band. Prototypical variable-star Mira, for example, usually measured around mag 1.5 (I-band) versus mag 6.5 (V); at its recent maximum, it had reached mag –1.4 (I), making it one of the brightest stars in the infrared sky. More extreme still, RYI Andromeda measured mag 8.8 (I) versus mag 15.7 (V); it was 850-times brighter in the infrared than in visible light. The response of his instrumentation was such that for average stars, the V-band filter reduced the amount of light collected from its white-light response by around one magnitude, while the I-band filter reduced it by two.

Dr Miles showed one of his earliest deep-sky images – a 33-minute stacked exposure of the Whirlpool Galaxy (M51), taken as a by-product of his making photometric measurements of supernova 2005cs. Comparing the result with an image by Martin Mobberley from 1992, taken through a 12" aperture on film, showed that long exposures with modern CCDs could now compete, even when taken through small apertures. Comparing the I- and V-band images, there was a slight loss of detail in the I-band; specifically, the star-forming regions in the spiral arms did not stand out so clearly. This loss of detail might be expected; colour images showed these regions to be relatively blue, and so B- or U-band images might have proven a more revealing comparison with the V-band data. A slightly deeper exposure of the Leo Triplet (M65, M66, NGC 3628) revealed a similar comparison between I- and V-band morphologies, as did a 3.5-hour exposure of the Coma Cluster. The speaker reiterated how well these Starlight Xpress images had responded to stacking; he compared the image of Coma with one from his 12" Celestron; the level of discernable detail was comparable. Even a comparison with imaging from the Palomar Sky Survey (POSS I; 1950-7), which had used a 48" Schmidt with 400-times the collecting area of the speaker's Takahashi, was not unfavourable.

The speaker then began to move south, explaining that objects at southerly declinations, which never rose very high in the UK sky, were more accessible in the infrared because atmospheric refraction – the plague of low-altitude imaging – was less severe at these wavelengths. He showed images of open clusters M46 and M47 in Puppis, around 1° apart at δ=–14.5°; at this declination, the level of detail seen in the I- and V-band images were comparable, though the former better differentiated red and blue stars. He pointed out that planetary nebula NGC 2438 – a small smudge tucked within the V-band image of M46 – was completely absent from the I-band image. Evidently the latter filter did not encompass any of the nebula's bright emission lines – Hα (656.3 nm), for example, fell significantly blueward of it.

Moving further south, Dr Miles turned to open clusters NGC 2571, 2580 and 2587, all in Puppis, at around δ=–30°, each of which he had imaged with sixty 30-second frames. He remarked that his infrared imaging was here beginning to show superiority, though the fleeting appearances of these objects severely limited the time available for imaging them. He also showed an image of the Galactic centre, at δ=–29°; he had been somewhat curious to see how it would appear. Whilst nothing had been visible in the V-band, I-band imaging had revealed a source with strange extended morphology, whose position matched that of radio source Sgr A.

A condensed globular cluster in Sagittarius, M69, at δ=–32°20', provided the next step in the southward journey, and a passable I-band image. By contrast, V-band imaging was now virtually impossible. The speaker explained that objects close to the horizon appeared at slightly higher altitudes than suggested by their celestial coordinates, as a result of refraction by the Earth's atmosphere. Moving down in altitude, this phenomenon became rapidly more pronounced in the final few degrees just above the horizon – the regime which his imaging was now entering. At its most extreme, objects which appeared on the nautical horizon had celestial coordinates which actually lay ~0.5° below it. The altitude dependence of this effect was a serious problem when stacking long exposures; it produced a time-varying distortion of the sky along the altitudinal axis, causing stars to appear to set more slowly as they approached the horizon. The effect was also wavelength-dependent: blue light was affected more than red. This led to a chromatic dispersion of these sources; their blue emission was seen at higher altitudes than their red, an illustration of which was the green flash seen at sunset. This rendered it necessary to observe using fairly narrow-band filters to avoid altitudinal smearing, but also meant that the stacking of I-band images was much more feasible than that of their V-band counterparts.

Concluding his southward journey, the speaker showed images of NGC 6723, at δ=–36°37', and NGC 1808, at δ=–37°31'. His observatory's physical southern horizon was at δ=–38°09', but he had yet to identify any deep sky imaging targets in the final 30'.

Returning to where he had begun, he concluded by showing an image which he had acquired as a by-product of photometry of variable star SS Cygni, which he had been studying intensively over the past couple of years. Nearby lay a mag 15.3 (V) galaxy, UGC 11799. By stacking 12-hours-worth of his CCD frames, he had been able to obtain an image of this galaxy which revealed a comparable degree of detail to its POSS image. He added that he had applied no sharpening or other post-processing to his image; these, no doubt, could improve it further.

Dr Miles concluded by arguing that the power of small aperture instruments for imaging was not to be neglected in the age of CCDs and frame stacking. Following the applause, the Director invited Grant Privett to speak.

Cambridge

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