Annual Meeting of the Deep Sky Section, 2007 March 3
Going Deep
Mr Privett conceded that the term 'deep' was somewhat ambiguous; its meaning depended upon both site and equipment. A Londoner might consider the Crab Nebula (M1) so, meanwhile it had taken on a wholly different meaning in Paul Clark's earlier talk. He hoped presently to give advice which would be relevant irrespective of the definition chosen, and to both visual and CCD observers.
Some, he suspected, might ask the motivation for chasing faint objects, when such fantastic detail was to be found in the likes of M42, M57 and M27. He supposed that he saw them, first and foremost, as a challenge; imaging them required persistence, the refinement of some skills, and the learning of other entirely new ones. If the resulting images were not reward enough, the learning experience might later feed back to allow superior imaging of brighter objects.
The equipment requirements for deep imaging, were, above all, a set-up which was in every way easy to use and maintain. A portable telescope, not too heavy, was ideal. Unwieldy instruments were to be avoided; the work involved in transporting them and setting them up would be a psychological barrier to their use. A driven mount was vital for CCD imaging and very useful for visual observers; a digital setting circle would also save a lot of time in finding objects. Whilst some observers enjoyed DIY work, the speaker viewed telescope maintenance as a separate pastime from that of observing, and was inclined to view any time spent tinkering as time spent not observing.
More mundane items were equally important: a comfortable chair, a chart table at a convenient height, and warm clothing. Anything which made observing uncomfortable would bring on tiredness, and ultimately bring observing sessions to premature closes.
The speaker noted that even his comparatively portable set-up, with which he was well practiced, took him nearly an hour to set up. The advantages of fixed observatories were clear – the speaker admitted that he himself was tempted to set one up – but the drawback was that they didn't allow one to travel to find the best skies, and this was essential in most residential areas.
A dark site was essential, and to aid in finding one, the Dark Skies Map published by Philips was to be highly recommended. Likewise, a moonless night was also near-essential, though First Quarter skies were, at a pinch, usable late in the night, provided that the objects targeted were >90° and not exactly 180° from the Moon, to avoid any scattering into the telescope tube. The speaker used only the darkest area of the sky, which he typically found to be slightly displaced from the zenith, depending upon the directionality of local light pollution. He also only imaged during the hours of astronomical darkness, which meant not at all in June or July.
Mr Privett then turned to give advice specific to CCD imagers. He recommended trying several different eyepieces before imaging, to see which best framed any given target. Having a well-calibrated CCD was vital to minimise image noise. This meant that hot and cold pixel maps needed to be carefully constructed to filter out faulty pixels. Dark subtraction was vital even with the best detectors; he found it necessary to take several dark frames through the night as the detector temperature changed. Flat-fielding could also not be neglected. Finally, vignetting around the edges of frames was an effect to be mindful of; generally, the edges of CCD frames were best discarded. Whilst bright objects could be quite forgiving of these calibrations, they made a huge difference to deeper exposures, and no amount of stacking could average out poor calibration.
As an illustration of the power of stacking, the speaker showed an animation of the improvement in image quality as more and more frames were combined. He generally recommended stacking large numbers of quite short exposures, as an increasing fraction of longer exposures had to be discarded due to cosmic ray hits, etc., though he noted that CCD read-out noise grew with the number of frames stacked, and, as it had non-Gaussian properties, was virtually impossible to remove. An equipment-dependent medium had to be found.
Good focussing needed to be maintained, and the speaker recommended re-checking this at least once an hour. Finally, before post-processing frames, he recommended applying linear gradient removal; the scattering of ambient light around one's observatory into telescope optics invariably produced some linear background gradient.
The speaker then turned to present some of the fruits of his work. Showing first some comparatively well-known objects, he began with M29, remarking how pleasing it was to have an image with not just the bright members of this open cluster, but also much fainter stars. Next, he showed a fine image of local group galaxy Leo I at mag 11.2.
Among some publicity stunts, he had imaged dwarf planet UB313 at mag 19, which, on the grounds of having been an object in the news of late, had impressed his work colleagues. The detection had actually been quite straightforward. In 1996-7 he had set himself a greater challenge of trying to image quasar PC1247+0340 at mag 20.4 in Canes Venatici; at a redshift of z=4.687, this had been the most distant known object at the time, and he had hoped to publish the work as a local paper story. Unfortunately his attempt had been unsuccessful, though with greater experience, he had subsequently managed a positive detection in 2005, stacking 450 frames, each 30-second exposures. The red colour of this object made it difficult: the observation had only been made possible by the good response of modern CCDs into the near-infrared; at 1-μm wavelength, his detector still yielded 10% of its sensitivity at visible wavelengths.
To conclude, Mr Privett remarked that he seemed to have found a niche for himself which he felt was much more rewarding than trying to compete with the fine images which already existed of brighter objects by means of post-processing. Following the applause, a member asked whether a conflict existed between the recommendation of easy-to-use equipment and that of a very portable instrument; as the speaker himself had mentioned, having a fixed observatory saved a lot of time. Mr Privett agreed that there was a conflict, but the majority of observers who lived in light-polluted areas could gain much better images by travelling to a dark site.
After a break for afternoon tea, the meeting resumed with a talk by a professional: Prof. Janet Drew, of Imperial College, London. Prof. Drew was the Principal Investigator (PI) of a project to survey the northern half of the galactic plane in Hα emission – the Isaac Newton Telescope (INT) Photometric Hα Survey of the Northern Galactic Plane (IPHAS) – which would be the subject of her talk.