Ordinary Meeting, 2005 January 26


New Movements in Imaging Technologies: Telescopic Imaging with Digital SLR

Mr Wise explained that his optical design, which he called Newsie, employed a spherical primary mirror, with two subsequent lenses used to correct for the spherical aberration before rays were brought to focus. The first of these, a negative (diverging) doublet, was placed in front of the focal plane of the primary, and brought the converging rays from it into a near-parallel configuration. As with the Newtonian design, rays were then deflected through 90° by a small flat mirror, leaving the side of the instrument. Here, a positive (converging) doublet brought them to focus.

The speaker saw a number of advantages to this design. The distance between the primary mirror and the secondary optics was shorter than that of most comparable instruments. The result was a compact telescope, which was more readily transportable than an instrument of comparable aperture using more conventional optics. The optics were also comparatively fast – the speaker showed the audience an easily-carriable 200mm f/6 prototype, explaining that he had also made a 400mm f/3 instrument. The usable fields of these were 1.5° and 0.9° respectively, making them ideally suited to wide-field work. He added that he had hopes to make a 24-inch version within the coming year, if the cost was not too great.

In addition, Mr Wise explained that the distance between the last lens of the system and the focal plane was comparatively large, making it straightforward to take images simply by placing a standard camera in front of the eyepiece, without the need for a transfer lens, as was so often the case with other designs. Finally, to reassure the sceptics, he added that the instrument exhibited "no chromatic aberration", despite the use of lenses.

Mr Morgan-Taylor then explained that he would be presenting a series of images, comparing the results from a conventional CCD with those from a consumer digital SLR camera. In the process, he hoped to demonstrate what imaging was possible with some of the newer cameras on the market and, in addition, what it had been possible to achieve using a telescope of Mr Wise's new design. All of the images in his talk had been taken from a location three miles outside Leicester. He had used a so-called "deep sky" filter, about which he regretted he had been unable to find any technical specification, either from the retailer or from his own research. However, in practice it appeared to add a blue tint to colour images, excluding much of the red end of the spectrum, and with it much of the sodium light pollution. Whilst not ideal for colour imaging, the filter seemed a useful tool if the results were grey-scaled.

The advantages of CCDs as imaging sensors were, the speaker was sure, very familiar to many in his audience. They gave a penetrating view of the sky, often less affected by light-pollution than traditional film. In addition, they were typically more sensitive, and free of the reciprocity failure suffered by so many film emulsions when used with long exposure times. Digital SLR cameras themselves brought further advantages: an LCD display on the back, allowing the focussing to be checked quickly prior to taking long exposures, and relatively large chips, the speaker's being 20×16mm. However, the downside was that the CCD arrays in such cameras were usually based on CMOS semiconductors, which astronomers had tended to shy away from in the past on account of their being cheap and invariably rather noisy. Whilst they performed well for daylight shots, long astronomical exposures had historically been rendered impossible by the build-up of noise.

Starting with the Orion Nebula, Mr Morgan-Taylor first showed results obtained directly from the eyepiece, with no flat-fielding or stacking. His first reaction had been that they were exceptionally pleasing images, which perhaps one might normally have expected to have come from a telephoto lens rather than a telescope. In view of this, it seemed that technology had finally arrived which allowed wide-field imaging to be straightforwardly undertaken with an auto-guided instrument. Moving on to the Pleiades, a similar raw image from the eyepiece provided a good view of the cluster, while minimal processing in Adobe® Photoshop® was required to bring the surrounding nebulosity into view.

An animation of Comet Machholz, composed of two-minute exposures taken every three-and-a-half minutes on January 8 revealed another advantage of the use of digital SLR cameras: their ability to be driven by a digital cable release. In this case, the speaker had been able to program such a cable release to take a series of exposures of his chosen length, before retiring into the warm.

In conclusion, the speaker felt that there had been great advances in the quality of CMOS CCD detectors in recent years, to the point where they were now useful tools for astrophotography. In response to a question concerning the temperature-dependence of the noise in the CCD, the speaker replied that he had found images taken at sub-zero temperatures to be greatly superior to those taken when the camera had been warmer. Following applause for Messrs Wise and Morgan-Taylor's presentation, the President welcomed Mr Nick James to speak on a similar theme.




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