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Meeting of the Deep Sky Section, 2011 March 12 (Part I): held at Ashford Village Hall, Ashford Hill, Newbury, Berkshire.

Dr Stewart Moore, Director of the Deep Sky Section, welcomed the audience to the meeting and thanked Newbury Astronomical Society for hosting it. He especially thanked Ann Davies and David Boyd for arranging the refreshments. He explained that this year marked the thirtieth anniversary of the Section's foundation, in 1981 September, and so this year's meeting would be a timely opportunity for several talks reviewing the changes seen over those three decades, but first he presented his annual review of the Section's work.

Annual Review for 2010

44 observers had submitted images to the Section since its previous meeting in 2010 March. Three newsletters had been produced, in May, September and January. For the first time, these had been circulated electronically as pdfs, making it possible to do better justice to the fine colour images that could be obtained with modern CCD cameras than was possible within the Section's printing budget. Black-and-white photocopies remained available on request to those without internet access. Dr Moore added that he was always keen to receive contributions for these newsletters, and that he particularly welcomed images which were accompanied by notes about the equipment used or the story behind the observer's choice of target; this additional context could add considerably to the interest of the images.

Several observing projects had also been publicised to a wider audience by articles in the news section of the BAA Journal. Owen Brazell (1) and Grant Privett (2) had written articles about NGC 40 and Gyulbudaghian's nebula respectively, and the Director encouraged others to consider contributing similar articles.

Describing observations made by Section members over the year, Dr Moore noted that the weather had been particularly unfavourable across the UK for the past few months. This was reflected by a lull in recent supernova discoveries by UK amateurs. Nonetheless on September 1 Ron Arbour had discovered 2010hi in NGC 6621, bringing his tally to 24. Tom Boles made nine discoveries over the year, most recently 2010js in UGC 4924 on November 7, and his total was now 138.

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The Section's project to observe Abell planetary nebulae had received a few new observations, of Abells 12, 33, 36, 37, 39 and 50. The images of Abells 33, 36 and 37 represented the first amateur observations of these objects received by the Section; lying in the magnitude range 11.5-14, these objects presented considerable challenges. A couple of even more difficult planetary nebulae had also been observed by the Section's most dedicated observers. Both Grant Privett and Maurice Gavin had successfully captured images of the Necklace nebula, an object in Sagitta newly discovered (3,4) in 2010 by the IPHAS survey. (5,6) Grant had also captured an image of the Soap Bubble Nebula (PN G75.5+1.7), another relatively recent discovery in Cygnus, this time by American amateur astronomer Dave Jurasevich in 2008 July. Details of these observations could be found in the Section's January newsletter. Within the past few weeks, Grant had turned his attention to faint local group galaxies, submitting images of Leo 2, Leo A and Sextans B.

Turning to variable nebulae, the Director opened with a pair of images of Hubble's variable nebula, taken by Nick James in 2010 March and Maurice Gavin in 2010 November, which showed clear changes to the structure of the nebula over those eight months. Hind's nebula presented a fainter, more challenging target, but Dale Holt and Maurice Gavin had submitted images. Few observations of McNeil's nebula had been received in recent years, since the initial flurry of interest which followed its discovery in 2004; there was an ongoing need for the Section to monitor it, and negative observations would be as useful in this regard as positive detections.

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Gyulbudaghian's nebula had been well observed over the year. At the previous Section meeting in 2010 March, it was reported faint at mag 16.5, but had subsequently brightened rapidly, peaking at mag 15.8 in the early summer. It now seemed to have faded again. David Boyd recorded systematic photometric measurements of both the nebula and the variable star PV Cephei, 11" distant, thought to be the nebula's primary source of illumination. He found that the nebula's lightcurve was a close match to that of PV Cep, but shifted 32 [+ or -] 4 days later. This shift could be explained if the physical separation of the star from the nebula was 32 [+ or -] 4 lightdays, or 0.027 [+ or -] 0.003 parsecs, which was consistent with other estimates of the nebula's distance and angular separation from PV Cep. This work would be written up in full in a future issue of the BAA Journal.

Closing his annual Review, the Director also gave the subsequent talk, reviewing some of the changes in imaging technology over the 30-year history of the Section.

Then and now: thirty years of DSS images

The Deep Sky Section was founded in 1981, initially under the name of the Photographic Section at the 1981 September BAA Council meeting, before changing its name to the Deep Sky Section only a month later at the following Council meeting. (7) Its first Director was Ron Arbour, now known for his work as a discoverer of supernovae, and from the beginning it summarised its work in a triannual newsletter, initially titled the Deep Sky Diary, the first issue of which appeared in 1982 April. The cover of this first issue showed an image of the globular clusters around M31, captured by Geoffrey Johnstone on Kodak Technical Pan 2415 film using a 10.5inch [26.7cm] f/5 Newtonian. On subsequent pages were photographs of M42, NGC 884 (the double cluster) and M31. Producing satisfactory printed reproductions of images like these then presented a considerable challenge, and whilst the speaker noted that each of these original photographs represented a considerable technical achievement at the time, he added with regret that the newsletter had done poor justice to them.

Discussing the equipment used, Dr Moore explained that in the early 1980s, astrophotographers typically worked with Newtonian telescopes of 20-30cm aperture. Also popular were Aero-Ektar lenses, often bought as army spares. Of necessity, the targets chosen were bright objects, usually stellar clusters or galaxies. Exposure times were limited to around 10 minutes by the lack of automated guidance systems.

By contrast, the trend now was that many astrophotographers preferred small aperture refractors on account of their high quality optics, though others still used Schmidt-Cassegrain telescopes. For several years, film photography had been entirely superseded by digital imaging, using either digital SLR cameras or dedicated astronomical CCDs, on account of the more immediate feedback provided by the new devices, together with their improved linearity and quantum efficiency--i.e. sensitivity. Longer exposures, often exceeding eight hours, were made possible by more accurate drive systems and by the advent of automated guiding. The ability of computers to read images directly from modern CCDs allowed astrophotographers to stack huge numbers of short exposures using automated image stacking programs such as Registax. This meant that the need for drift to be completely eliminated over the whole length of the exposure was now much less pressing.

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Dr Moore noted that the increase in length of exposures was at first surprising, given the greatly improved sensitivity of modern detectors. It was apparent that much fainter and more challenging targets were being chosen. There had also been a rise in the use of narrow-band filters, especially among those severely affected by light pollution, and the inherently low transmission of such filters also necessitated longer exposures.

Comparing some of the images recently received with those in the Section's archive, Dr Moore identified galaxies as the main objects that were particularly difficult to photograph in the 1980s, on account of their low surface brightness. The change was less noticeable among images of open star clusters, which were often already accessible in the 1980s, as illustrated by an early 1980s image of M45, recorded by Alan Dowdell on Kodak Tri-X film. The only dramatic change was that modern images could record star colours, whereas the earlier images were invariably black-and-white.

Images of globular star clusters had vastly improved, as a comparison of new and old images of M13 revealed. Here, the difficulty for film photographers was the large dynamic range between the dense cores of clusters and their faint outer members; the reciprocity failure of photographic film had severely limited the available dynamic range.

Assessing which objects had been the most popular photographic targets in the 1980s, Dr Moore commented that the Andromeda Galaxy (M31) had been a particular favourite, likely because its brightness had singled it out as the only galaxy within which significant detail was in easy reach at the time. The Section archive also had plentiful images of M33, M51, and more exotic targets such as Stephan's Quintet, though it was apparent that their low surface brightnesses had posed considerable challenges at the time. Orion's Horsehead nebula seemed almost unique in having retained a constant popularity throughout the past 30 years, probably because it had always been a significant photographic challenge, yet was tantalisingly just within visual reach.

The speaker closed by noting that images of planetary nebulae had also been vastly improved by the advent of CCDs; old film images were almost invariably burnt out, showing little detail and appearing rather like outof-focus stars. By contrast, modern images often revealed so much detail that the objects didn't resemble their visual monikers, such as the Swan, at all. The speaker expressed his thanks to Patricia Wainwright for scanning the archival images used in his talk, and added that she was hoping eventually to scan the entire Section archive, which would then be made available on CD-ROM.

Dr Moore then welcomed Dr David Arditti to speak.

Using the f/2 HyperStar system for deep sky imaging

Dr Arditti explained that the HyperStar system was a means of reconfiguring widely available Schmidt-Cassegrain telescopes (SCTs), which typically had focal ratios of around f/10, into Schmidt camera configurations with much faster focal ratios of around f/2. This change enlarged the telescope's field of view by a factor of around five, allowing wide-field images to be taken with short exposures. Dr Arditti had been using such a system for around 2% years, but, with the exception of the manufacturer's specifications and some enthusiastic reviews by Greg Parker, (8) he was surprised how little he had seen written about people's experiences of using it. In this talk, he would describe his own experiences of using it from Edgware, London.

The HyperStar system had evolved out of an earlier product, Fastar, which was developed by Celestron in the 1990s but which they discontinued in 2005. Though many Celestron SCTs were still marketed as being Fastar compatible, now the only such kits available were manufactured by an independent Arizona-based company, Starizona, under their new brand name. In practice, conversion kits were now available from Starizona for many models of SCT which weren't marketed as Fastar compatible, including a few Meade telescopes such as the 14" [35.5cm] LX200. As well as offering support for a wider range of telescopes, Starizona also claimed to have reduced the spherical and chromatic aberration of the earlier system.

The purpose of converting an SCT into a Schmidt camera was explained in terms of the length of exposure needed by astrophotographers to record satisfactory images of various objects. For unresolved stars, the amount of light collected from the star scaled simply in proportion to the area of the aperture of the telescope. In other words, the required length of exposure was inversely proportional to the square of the diameter of its aperture. But for extended regions of resolved nebulosity, what mattered was the focal ratio--i.e. the focal length divided by the aperture diameter--of the telescope: the required length of exposure was inversely proportional to the square of this ratio. The reason for the difference was that longer focal lengths equated to higher image scales, and higher image scales meant that the light was more spread out across the sensor, with each pixel receiving less of it.

For effective deep sky imaging, telescopes needed both large apertures and also fast--i.e. small--focal ratios. In other words, it was important for the telescope to have a short focal length. Historically, one way of achieving this had been the Schmidt camera design of telescope, invented by Bernard Schmidt in 1930 and notably adopted by the 48-inch Samuel Oschin Schmidt Telescope used for the Palomar Observatory Sky Survey in 1949-'58.

In common with SCTs, Schmidt cameras brought light to a focus using a catadioptric combination of a refracting corrector plate followed by a spherical short-focal-length primary mirror. But from there onwards, the light path differed. In an SCT, the light went on to a secondary mirror, usually attached to the centre of the corrector plate, which bounced it back down the length of the telescope tube to emerge to the eyepiece through a hole drilled in the centre of the primary mirror. In total, the light traversed the length of the tube of the telescope twice between its reflection from the primary mirror and its reaching a focus, making the Schmidt-Cassegrain a relatively long focal length configuration, with a slow--i.e. large--focal ratio of typically around f/10.

Schmidt cameras achieved much faster focal ratios by removing the secondary mirror and bringing the light to a focus directly from the primary mirror, usually midway along the tube of the telescope. The light only traversed around half the length of the tube before reaching a focus from the primary mirror, equating to a much shorter focal length, and in turn a much faster focal ratio and larger field of view. Historically, a significant disadvantage of Schmidt cameras was that their focal planes were heavily curved, requiring the use of non-flat photographic plates. Also, the placement of the focal plane in the middle of the telescope's optic tube, where it was impossible to mount an eyepiece, made them useless for anything other than photography. The first problem, at least, could now be alleviated by the use of a field-flattening lens immediately in front of the camera, at the slight expense of some chromatic aberration.

The process of converting an amateur f/10 SCT into an f/2 Schmidt camera required the secondary mirror and its mounting to be completely removed from the centre of the corrector plate, and replaced with a new assembly which could be clamped in its place. This new assembly comprised a lens--to correct for the spherical aberration normally corrected by the secondary mirror of an SCT, and also to flatten the image plane--and a mounting for a camera--which could be either a dedicated astronomical CCD or a generic DSLR. The diameter of the well-corrected image on the sensor was well suited to the 27mm-diagonal chips used in most DSLRs.

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However, as the camera was placed directly in front of the telescope's aperture, there was a strong incentive for it to be as compact as possible. In addition to the camera itself, cables had to be strung across the aperture to get to the camera; at the very least, a power cable and a USB cable would be needed. Hence, whilst DSLRs were supported, a more compact CCD might be preferred. The speaker used a QHY8 CCD camera that he had bought with his HyperStar system, and found that the obstruction posed by the cables running to it didn't seem to cause serious image degradation on his 11" [28cm] Celestron, except for some diffraction spikes around stars. He supposed that they would probably be barely noticeable on a 14"-aperture telescope, but would pose a very serious problem on telescopes smaller than his own. He added that he had taken especial care to clamp the cables firmly to the telescope's tube at the edge of the aperture before running them on to his computer. If this was not done, any snag in the cables would put force directly onto the camera, and thence onto the thin corrector plate onto which it was clamped. Dew was prone to forming on the camera detector, so a second additional dew heater was needed in addition to the normal heater to keep dew off the corrector plate.

Each model of camera needed its own unique adaptor to interface it to HyperStar in order to achieve a sharp focus. The focusing mechanisms of SCTs could typically only move the primary mirror through a total distance of around 20mm, which was plentiful in the SCT configuration as the secondary mirror acted as a diverging lens, extending the focal length of the telescope by a factor of around five so that the eventual image plane could be shifted back and forth by as much as 100mm. In the Schmidt camera configuration, however, there was no such extension, and the sensor within any given camera needed to be finely positioned within the narrow focal range of the instrument by means of its own specific adaptor.

Describing the process of converting an SCT for use with HyperStar, Dr Arditti explained that whilst a few Celestron SCTs--those marketed as Fastar compatible--needed no modification to receive it, most had to have a conversion kit applied first. These kits were sold by Starizona, and were available for any Celestron SCT of aperture between 6" and 14", as well as the Meade 14" LX200, and comprised a new baffle tube and a new secondary mirror holder. Describing the process of applying such a kit as 'fraught', the speaker said that the telescope needed to be almost completely dismantled. The corrector plate had to be removed from the front of the tube, and the secondary mirror and its holder unscrewed from the plate's centre. Dr Arditti remarked that the secondary mirror holder was glued to the corrector plate, and that significant prising was needed to free it. Furthermore, he had not realised until too late that the corrector plate should not be rotated relative to its original orientation--one of its functions was to correct for the astigmatism of the primary mirror--and hence it was worth making a Tipp-Ex mark on its outer edge and on the tube to show how it should be aligned.

Once the corrector plate was isolated, a new assembly could be clamped in place of the old secondary mirror holder; this new assembly could receive either a HyperStar unit, or a new replacement for the old secondary mirror holder. In addition, a counterweight was added to the telescope's eyepiece holder when in the HyperStar configuration, to balance the added weight of the camera at the front. Once a telescope had been converted for use with HyperStar, it was relatively straightforward to switch it between the SCT and Schmidt camera configurations; the process took perhaps a few minutes in a well-lit room. In the dark it was more fiddly; the speaker had dropped his secondary mirror on a recent attempt, though luckily it had come to no harm.

Describing his experiences of using HyperStar, Dr Arditti identified a few potential limitations of the system. Firstly, it seemed difficult to use it with many autoguider systems, since there was nowhere where the light path could be split. Even SBIG cameras, which combined an autoguider and imaging CCD in the same unit, seemed illsuited since their large packaging would obstruct too much of the telescope's aperture. In practice, he explained he had resorted to using a separate guide telescope on the same mount when taking long exposures. Secondly, use of colour filters needed careful planning. Filters could be inserted into the HyperStar assembly, but the camera had to be removed to do so. This made it impossible to take images in multiple colours whilst preserving the alignment of the camera, and so colour imaging was difficult if not using a colour camera.

Perhaps the most serious difficulty that the speaker experienced was that very fine collimation was required. Having a fast focal ratio meant that the Schmidt camera configuration had a very small depth of focus--i.e. the image moved very quickly out of focus if the sensor was moved back or forth. Any slight misalignment of the sensor from the optic axis of the telescope made it impossible to get the whole field into focus at the same time. The speaker found that his system generally needed re-collimation after every re-pointing. This process was fiddly as the adjustment screws were by now concealed inside the dew shield and behind the camera. A long screwdriver was needed, and when pointing close to the zenith, also a stepladder. Long test exposures were often needed to test the collimation, since clear detections of many stars across the field were needed to ensure that they were all cleanly focused, and the process often took over an hour.

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After finding that he often couldn't achieve collimation at all, the speaker had investigated further. Dismantling his QHY8 camera, he concluded that the sensor itself had been poorly collimated, by mounting it on the chuck of a lathe and turning it slowly by hand whilst reflecting a collimation laser beam off the surface of the CCD. He had, however, been able to improve matters by adjusting the screws used to mount the sensor until the direction of reflection was constant as the camera was rotated.

To close, the speaker showed a few of the fruits of his labours, though he added that he was generally more interested in the technical challenge of acquiring images than in processing them, which was often put off for 2 or 3 years. First, he showed an image of M31 stacked from 31 ten-minute exposures taken in 2009 September. He remarked that he had limited himself to exposures of a maximum of ten minutes as he lived below a flight path into Heathrow, and aircraft often passed through the widened field of his HyperStar setup. Other easy targets had included the close neighbours M42 and NGC 1977 in Orion, galaxies such as M109, and globular clusters such as M13. Some of these appeared surprisingly small in his large field of view.

Lastly, he showed the deepest image that he had ever taken with the system: a 13.3-hour exposure of a field in Cassiopeia, stacked from 84 ten-minute exposures, encompassing M52, NGC 7635, NGC 7538, and a vast number of field stars. The image had been taken over the course of successive nights between 2009 July 25 and 2009 September 12, but the speaker added, to laughter, that he had only got around to processing the data the previous night, for inclusion in his talk.

Following applause for Dr Arditti's outstanding presentation, Nick James remarked that he had always been a little wary of supporting the whole weight of a camera on the corrector plate of an SCT. The speaker agreed, but said that in practice the corrector plate didn't appear to warp under its weight.

An audience member asked whether the speaker felt the effort had been worthwhile, and how his results compared with what could be achieved with small-aperture refractors. The speaker thought in retrospect that his results were broadly comparable to what could be done with a refractor, but that he had nonetheless enjoyed the challenge of installing HyperStar.

[David Arditti's talk was followed by presentations from Owen Brazell, Grant Privett, Geoffrey Johnstone, Nick Hewitt and Prof Derek Ward-Thompson of Cardiff University. Full reports of these will appear in the October Journal.]

References

(1) Brazell O., J. Brit. Astron. Assoc., 120(6), 378 (2010)

(2) Privett G., ibid., 120(6), 379-380 (2010)

(3) Corradi R. L. M., MNRAS, 410, 1439 (2011)

(4) Astronomy Picture of the Day, 2010 Nov 3, http://apod.nasa.gov/apod/ap101103. html (2010)

(5) Drew J. E., et al., MNRAS, 362, 753 (2005)

(6) Ford D. C., J. Brit. Astron. Assoc., 118(1), 53-58 (2007)

(7) McKim R. J. [ed.], 'The British Astronomical Association: The Second Fifty Years', Mem. Brit. Astron. Assoc. 42(2), pp. 104'5 (1990)

(8) see e.g., Parker G., Making beautiful deep-sky images: astrophotography with affordable equipment, Springer, New York, 2007
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Title Annotation:BAA Update
Author:Ford, Dominic
Publication:Journal of the British Astronomical Association
Date:Aug 1, 2011
Words:4024
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