This blog post is part of our regular “From the BAA Journal” series. This series features a selection of articles, news, reviews and letters from the Journal of the British Astronomical Association, published six times a year.
The scientific exploration of Mars, by Fredric W. Taylor
Cambridge University Press, 2010. ISBN 978-0-521-82956-4. pp xiii+348, £30.00 (hbk).
The Scientific Exploration of Mars, by Frederick W. Taylor
Despite the broader approach suggested by its title, the focus of this book is firmly on recent and current exploration of Mars by spacecraft. Although it does provide an introductory overview of the history of earth-based observation, this is cursory in the extreme and the reader might well wish for a fuller treatment of the excitement, achievements and misperceptions of the age of telescopic observation of the Red Planet. Perhaps it was felt that such a treatment had already been provided in William Sheehan’s 1996 monograph The planet Mars: a history of observation & discovery, but a more nuanced coverage here would have more effectively contextualised the remarkable achievements of Mars exploration in the space age.
Written for a broad audience of interested scientists, amateur astronomers and general readers, the volume steers a judicious course that carefully combines scientific insight and accessibility. Where it is at its strongest is in the identification and explanation of the key scientific objectives that have driven the exploration of Mars by spacecraft, as well as in the authoritative assessment of the extent to which such objectives have been achieved with regard to our understanding of the Martian surface, atmosphere, climate and potential to support life, either now or in the past. Few are better positioned than Fredric Taylor to write such a book. As well as being Halley Professor of Physics at Oxford, he has been closely involved in solar system exploration by spacecraft, having spent many years in the Space Science division of NASA’s Jet Propulsion Laboratory, as well as having participated in the planning of Europe’s Mars Express mission. As a result, he is in a privileged position to set the science within the larger framework of political and practical considerations, as well as managerial and budgetary constraints. Although much of this will be new to most readers, one nevertheless wonders whether too much space is devoted to the deliberations of working groups and to concept missions that, quite literally, never got off the ground.
Taylor ends his account with a lengthy speculative section on likely plans for Mars exploration, both manned and unmanned, in the near-to-mid future. Here he emphasises the uncertainty of his predictions and their susceptibility to political and economic whims. This is wise indeed, for the cancellation of NASA’s Constellation programme at the very moment of this book’s publication will have done little to advance the cause of the scientific exploration of Mars.
There are a few instances of rushed proof-reading and miscaptioned illustrations, but overall this volume is beautifully produced and engagingly written. It may be read with profit by all interested in the ongoing study of Mars.
Reviewed by Bill Leatherbarrow, Director of the BAA Lunar Section and an active observer of the Moon and planets.
A Question and Answer Guide to Astronomy, by Pierre-Yves Bely, Carol Christian & Jean-René Roy
Cambridge University Press, 2010. ISBN 978-0-521-18066-5. Pp xiv+280, £18.19 (pbk).
A Q&A Guide to Astronomy, by Pierre-Yves Bely, Carol Christian & Jean-René Roy
Here is a very worthy successor to such classic compilations as Asimov’s Please Explain (1973) and Pickering’s 1001 Questions Answered About Astronomy (revised by Moore, 1975), which explored astronomy through the device of question and answer, and fired the imagination of many a budding amateur, this reviewer among them.
In 250 questions, all accurately and readably answered, using non-technical language and bang-up-to-date illustrations, this book guides the reader through the cosmos, from the earliest times and the most basic principles, to the more exotic discoveries and theories of modern times. Its world-wide view includes such topics as Islamic astronomy, the work of famous astronomers of many nationalities, a ‘world tour’ of the largest ground-based optical telescopes, and a visit to the Chicxulub Crater.
Predictably perhaps, there are sections dealing with stars, planets, the Moon and the universe in general, but there are also welcome forays into other fields, for example our Earth itself (a planet not well covered by some ‘general guides’), extraterrestrial life, the history of astronomy, and the ins and outs of the practice of amateur astronomy.
The Question and Answer Guide is a mine of illustrations, mostly in colour, and excellently reproduced. They are reassuringly small, allowing the text to expand around them and speak for itself: it is not always true that a picture is worth a thousand words, if it fills most of a page!
The fact that the authors are, respectively, an engineer specialising in telescope design, an astrophysicist, and a senior scientist at the Gemini Observatory, ensures that the underlying principles of astronomy are solidly covered, while the contents of the cosmos and our methods of exploring it are not neglected.
Can enthusiasts, perhaps new to astronomy, resist a guide that tells them how to go about finding their own meteorite collection; explains string theory for the uninformed; discusses light pollution and its depredations; and tells them what to do if they discover something ‘new’ in the sky above?
An excellent bibliography and website list complete a very interesting book that ought to have a place on the shelves of astronomers, whatever their level of acquaintance with the noble science.
Reviewed by Bob Mizon, planetarium operator, and co-ordinator of the BAA Campaign for Dark Skies.
This blog post is part of our regular Thursday “From the BAA Journal” series. This series features a selection of articles, news, reviews and letters from the Journal of the British Astronomical Association, published six times a year.
NGC 7008 – a celestial hook, by Stewart Moore, Director, Deep Sky Section
One of the delights of the summer sky for planetary nebula enthusiasts is the number of objects on view. And for visual observers who don’t have the luxury of remote warm-room observing, all available under a pleasant summer sky. Even the lack of truly dark skies is not a serious problem as many planetaries have a high surface brightness and the use of filters can often improve contrast. To many people summer planetaries mean the Ring Nebula or the Dumbbell, but there are numerous other objects which deserve investigation, and one of them is NGC 7008 in Cygnus.
NGC 7008, by Andrea Tasselli
Discovered by William Herschel in 1787, he classified it as a bright nebula rather than a planetary which, considering its shape, was reasonable. To Herschel planetary nebulae were small round objects which often appeared slightly greenish and which reminded him of Uranus, which he had discovered 6 years earlier. The true nature of what we now call planetary nebulae remained a mystery until William Huggins turned his spectroscope on NGC 6543 in 1864 and realised it was a gaseous object. NGC 7008 lies in Cygnus, close to the Cepheus border, at RA 21h 00m 33s and Dec +54o 32′ 35″ (2000.0), almost midway between a line joining mag 1.3 Deneb (a Cygni) and mag 2.4 Alderamin (a Cephei). As with many planetaries its stated magnitude varies widely in the literature and ranges from around 9 to 12, but with a size of only 1.4×1.1 arcmin it appears quite bright and will certainly be visible in a 20cm telescope. It is thought to lie at a distance of around 3,000 light years, with a physical size of around 1 light year.
At first appearance NGC 7008 seems a strange object. It has a hooked shape, with the hook lying north-south, and a double star (h1606, separation 18 arcsec) at its southern end. Imagers intent on capturing detail in the nebula often cause the brighter component of this double star to become bloated, so that it appears to be attached to the nebula enhancing the hook like effect, whereas visually at high power it is clearly separate. High power will also show a variation in intensity across the nebula and what were once thought to be superimposed faint stars are now thought to be knots of material associated with the planetary itself. The mag 13 central star is offset and lies slightly to the west of centre. Spectrographic investigations suggest that the nebula consists of two elongated shells of expanding gas, one inside the other, with the inner shell expanding faster than the outer and causing a ‘snow plough’ effect familiar in planetary nebulae as the inner shell piles in to the outer.
NGC 7008, by Nick Hewitt
A selection of images of NGC 7008 received by the Section are included with this post (depending upon the optical train in the individual telescopes some images are flipped compared to others). Nick Hewitt’s wide field image (field size 38×28 arcmin) was obtained with a TMB 115mm f/7 refractor and SXV-HD CCD camera. The exposure was 10min through an OIII filter. Even at this scale the nebula displays a clear shape. Martin Taylor’s image was taken through his 20cm Cape Newise using a Canon 20D SLR camera. The stacked exposure of 8´4min gives a close approximation to how the nebula appears visually. Fred Stevenson’s image was from a much longer exposure, including an H-alpha component. Taken through his 35cm Meade SCT with a DSI Pro III CCD camera the details were 10×4min Ha, 10×4min Red, 7×4min Green and 13×4min Blue. Andrea Tasselli is one of the leading exponents of planetary nebula imaging in the country and his image here through an Intes Micro M809 (20cm f/10 Mak-Cass) with SXV-H9 CCD shows tremendous detail. The LRGB image consisted of 75min Luminance and 21 min each RGB through Astronomik IIc RGB filters.
NGC 7008, by Martin Taylor
Visually in a large telescope (say 30-35cm) and at high power (x200) this planetary shows marvellous detail, appearing around 90 arcmin in size with a brighter area to the north-east end of the hook and a smaller bright area at the southern tip. Careful study will show an interesting textured surface which is much more obvious with an OIII filter, and a darker band which appears to cut the nebula in two. A UHC filter is of minimal use on this object, the view being only marginally better than with no filter at all.
NGC7008, by Fred Stevenson
Eta Carina Nebula
An image of the Eta Carina Nebula taken by Bob Winter at ‘Space Ops’ in the Atacama desert in northern Chile, while on an eclipse tour to Easter Island.
Pentax 75 APO refractor.
QSI 583 Astro camera, LRBG image using L, Ha, Olll, Hb.
33 files at 30secs each, total 16.5 min. Calibrated bias and dark.
Unguided.
Short exposures had to be used as there was star trailing at longer ones due to the great difficulty of polar alignment in the S Hemisphere. In fact the whole thing was very demanding!
Messier Objects M13 and M92, globular clusters in the constellation Hercules, are both currently high overhead at midnight in Summer months. These images are by Bob Garner from London. 350mm Newtonian + SBig 200XM CCD. 10 x 2min exposures.
Globular Cluster M13 in Hercules, by Bob Garner
Globular Cluster M92 in Hercules, by Bob Garner
This blog post is part of our regular “From the BAA Journal” series. This series features a selection of articles, news, reviews and letters from the Journal of the British Astronomical Association, published six times a year.
This article will appear in the August / September 2010 issue, and is written by Nick James and John Mason.
Large prominence approaching third contact. 1/1600s, ISO 800, Megrez 72 refractor, 2× Barlow, Canon EOS 550D. Nick James.
A group of 130 eclipse-chasers travelled to Hao atoll, 920 km east of Tahiti, which boasts the largest lagoon in French Polynesia. The locals on Hao had been busy preparing a camp site with nearly 80 tents and other facilities including a bar, dining area and toilets, and the Mayor and her staff gave us an enthusiastic welcome on arrival at the airport, famous for having the longest runway in the Pacific. The observing site was slightly south of the centre line, but we still expected 3m 33s of totality.
We awoke early on eclipse morning, with the Magellanic Clouds still visible and the Zodiacal Light indicating the imminent sunrise. Everyone set up their equipment in different locations on the site, some on the ocean side of the atoll, facing north-east and others, more sheltered from the wind, nearer the lagoon shore. There were a few drifting clouds but everything looked good leading up to first contact, which occurred at 07:24 local time. The Moon gradually moved downwards across the solar disk; there were two sunspots visible lower right, and these were covered by the Moon a few minutes before second contact. By this time there was an increase in the amount of drifting cloud, but there were still large, clear gaps.
Second contact was due at 08:41:28 and we saw it clearly despite that fact that thin wisps of cloud had by that time moved in front of the Sun. The Baily’s beads at second contact were spectacular and lasted for several seconds; the corona then became visible with the naked eye. It was a classic solar minimum corona with well-defined polar brushes, two prominent coronal streamers on the eastern side of the Sun and a single long streamer on the western side. Numerous small, pinkish prominences were visible in binoculars and these showed up well in images. The sky mid-totality was not particularly dark and only Mercury was visible below and right of the Sun. The horizon colours were not as pronounced as from the Gobi Desert in 2008.
Approaching third contact, a beautiful large prominence appeared upper left on the eclipsed Sun and the reappearance of the chromosphere in this vicinity was clearly visible by naked eye. The third contact diamond ring at 08:45 broke out as a single, then multiple beads around the limb, lingering for several seconds close to the large prominence, before merging. Totality was over and the partial phase would continue for a further 1h 28m, the amount of the cloud cover decreasing rapidly as the Sun rose higher in the sky. Many observers using digital cameras obtained excellent images in spite of the drifting thin cloud and a few of these are included with this report.
Further images and video sequences will be published on a BAA DVD, and these should be e-mailed to Nick James at ndj[at] nickdjames[dot]com. Please contact Nick directly for how to send large files or videos.
This blog post is part of our regular “From the BAA Journal” series. This series features a selection of articles, news, reviews and letters from the Journal of the British Astronomical Association, published six times a year.
This article will appear in the August / September 2010 issue, and is written by Hazel McGee, the Journal Editor.
Setting up on Anaa south, by Hazel McGee
The solar eclipse of 2010 July 11 always promised to be a logistical nightmare to observe. The Moon’s shadow first touched the Earth in the southern Pacific, encountering land at Mangaia in the Cook Islands only after 1450km of open ocean. The narrow track of totality then swung northeast, passing tantalisingly close to the islands of Tahiti and Moorea, which experienced a 98% partial eclipse. Beyond Tahiti the track crossed the Tuamotu archipelago of French Polynesia − thousands of tiny coral atolls, of which very few are inhabited, and even fewer have airstrips that make them accessible to visitors. The track then sped across 3300km of empty Pacific to Easter Island, known as one of the world’s most isolated inhabited islands, and after another 3700km it finally made continental landfall at sun-set in winter on the rugged and inaccessible coast of Chile. Crossing the Andes, the shadow left the Earth after entering Argentina at the tourist resort of El Calafate, where the eclipsed Sun was a mere 1° above the mountainous horizon.
According to NASA, nowhere on the track had weather prospects better than 50% of cloud, but the Tuamotus offered the best chances, with Easter Island notoriously experiencing its rainy season in July. Two major UK-based expeditions both based their operations in Tahiti, with its substantial tourist infrastructure, and chartered aircraft to travel to three inhabited atolls within the eclipse path. For a report by John Mason and Nick James on Hao atoll, see this asssociated blog post.
Astro-Expeditions, led by Brian McGee, Declan Foughnan and Sheridan Williams, carried a total of 160 passengers in a shuttle service of ATR-72 and ATR-42 planes to the tiny Anaa and Hikueru atolls, with a resident population of around 450 people (Anaa) and 150 (Hikueru). With two years to prepare for the event, the locals were looking forward greatly to the party! The population of Anaa took the opportunity to upgrade their home-based visitor accommodation, and each four-person group was housed with a local family for the one or two nights they spent on the island. On Hikueru the visitors stayed in ‘fare potee’ buildings, wood and palm-thatched longhouses specially constructed for the event.
Two observing sites were used on Anaa, which lay N−S across the eclipse track. Most people were based on the north of the island close to the village and airport, and around 20 of us rode in small local fishing boats (with large outboard motors!) to the south of the atoll, where the journey of almost 20km was predicted to provide an additional 44s of totality.
Wading through the cool waters of the lagoon, we joined the boats at first light on a promising morning, with large patches of clear sky interspersed with drifting cloud, and the troublesome wind much reduced from the day before. An exhilarating three-quarter hour speedboat trip brought us to probably the most idyllic eclipse site any of us has experienced: a classic ‘desert island’ beach of firm coral sand and shells, groves of coconut trees providing shade and shelter from the wind, and the quiet waters of the (improbably blue) lagoon lapping gently on the shore. Our local hosts cut coconuts for us to drink, and tiny hermit crabs scurried about, jumping instantly back in their shells should a shadow fall across them, whether of a passing human, or of course the Moon.
First contact was seen at around 07:19 local time (17:19 UT) with the usual mixture of excitement and relief. During the long partial phase the weather was worrying, for the scattered cloud seemed to be increasing, with large patches frequently hiding the Sun for minutes at a time. At second contact, predicted for 18:32:33 UT at this location, the diamond ring shone through a few wisps of cloud but thereafter it remained clear until several minutes after third contact, when cloud covered the Sun again.
Shadow bands rippled across the sand for ten or twenty seconds before second contact, wider, darker and more sharply defined than those in Libya in 2006. (Others also saw them after third contact, but I forgot to look). Then the glorious corona was upon us, two helmet streamers like monkey’s ears at the top of the Sun, and an immensely long streamer below, full of delicate structure like petals in the sky. Through binoculars a string of prominences graced the right-hand limb. With the naked eye only Mercury was visible − the sky was quite bright, and Orion above the Sun could not be seen. I looked briefly with binoculars for Comet C/2009 R1 McNaught) between Mercury and the Sun but as expected, this also was not seen.
All too soon the pink chromosphere and a huge floating prominence appeared on the
left side of the Sun, binoculars were lowered for a massive, lingering diamond ring, and totality was over. That night the villagers and children of Anaa laid on an unforgettable drumming and dancing pageant in the sports hall, full of joyous local colour, and enjoyed as much by the locals as by ourselves.
Later we heard that our colleagues at both the northern Anaa site and on Hikueru had
shared our experience of ‘scary cloud’ but in both cases it had cleared in time. News from Hao atoll was also good, and then later from Easter Island and even from the cloudy midwinter Argentinian Andes: a remarkable sequence of success for a most memorable eclipse.
This blog post is part of our regular Thursday “From the BAA Journal” series. This series features a selection of articles, news, reviews and letters from the Journal of the British Astronomical Association, published six times a year.
Variable star photometry with a DSLR camera, by Des Loughney
In recent years it has been found that a digital single lens reflex camera is capable of accurate, unfiltered, photometry as well as V-filter photometry.1 Undriven cameras, with appropriate quality lenses, can do photometry down to magnitude 10. Driven cameras, using exposures of up to 30 seconds, can allow photometry to mag 12.
The cameras are not as sensitive or as accurate as CCD cameras primarily because they are not cooled. They do, however, share some of the advantages of a CCD as they have a linear response over a large portion of their range1 and their digital data can be accepted by software programmes such as AIP4WIN.2 They have their own advantages as their fields of view are relatively large. This makes variable stars easy to find and an image can sometimes incorporate several stars of interest. The cameras can be used to analyse bright stars easily.
A V magnitude estimate can be made in fifteen minutes. V is the apparent magnitude of a star, determined by photometry, in a standard wavelength band in the yellow-green region, chosen to correspond with that to which the human eye is most sensitive. In the widely used Johnson photometry system, the filter used to measure V magnitude has a central wavelength of 5450Å and a bandwidth of 880Å. The fifteen minutes includes the taking of twenty 5-second images and the analysis of the images on a computer. An unfiltered estimate can be done in under ten minutes.
With care and in good conditions photometry using a DSLR can be done to an accuracy of 0.02 mag. This can be achieved by studying images with the photometric tools of AIP4WIN and working out the camera settings to achieve the best possible signal to noise ratio (SNR). When the AIP4WIN Single Image Photometry tool is used to make an instrumental magnitude estimate of a star it will state the SNR in the pop-up box. For good quality photometry the reported SNR should be over a hundred.
Eclipsing binaries are observed in order to make estimates of mid-eclipse, which then enable a calculation of the current period of the system. Changes in period can be studied. The greater accuracy of DSLR photometry compared with visual methods enables improved determinations of the time of mid-eclipse. Another advantage of the DSLR approach compared with the visual is that the images can be archived.
DSLR photometry opens a new world of observation to amateur astronomers in the sense that a large number of bright variable stars with variations under 0.5 magnitude can now be studied. I have used the method to create lightcurves of stars with an amplitude of under 0.2 magnitudes.
Equipment
I have found that Canon 350D/450D DSLR cameras are suitable for photometry. My experiments suggest that the cameras should be used with quality lenses of at least 50mm aperture. This allows sufficient light to be gathered over the range of exposures that are possible with an undriven camera. For bright stars (over mag 3) lenses of smaller aperture can be used. I use two excellent Canon lenses of fixed focal length. One is the 85mm f1.8 lens which has an aperture of 52mm. This allows undriven photometry down to mag 8. The second is the 200mm f2.8 lens with an aperture of 72mm, which allows undriven photometry to about mag 10. The camera and lens are mounted on a sturdy tripod. A remote switch (Canon RS60-E3) should be used to prevent camera shake. It is possible for the camera to be remotely controlled from a PC via the connecting cable and the EOS utility software supplied by Canon with the DSLR. The camera mirror can also be set in the locked up position which helps to reduce vibration.
Christian Buil provides useful information on DSLR cameras on his website ‘Observations and Tips’ though some of his articles remain to be translated into English. A translation of his article ‘DSLR High Precision Photometry’ can be found on the Citizen Sky website. It will be noted that Buil’s article was written in 2005.
The images of the target variable star and comparison are interpreted on a PC using the software package AIP4WIN version 22. This program opens the RAW images produced by a Canon camera which can then be analysed using the program’s multi image/deep sky stacking facilities and its photometric tools.
Methods
Through experience it has been found that accuracy is severely affected by windy conditions which vibrate the camera and tripod, and by atmospheric turbulence and light pollution when the target stars are less than 30° above the horizon. Ideally, if it is a practical option, images are made when the target star is above 40°. With these qualifications reasonably accurate measurements can be made from a suburban setting. A dark sky site is not a necessity. If observations are made from a dark sky site then good estimates can be made when the target star is above 20° altitude but a correction for atmospheric extinction is then required.
Better images result if the camera and lens can reach thermal equilibrium with the outside environment. The camera should be put out for 20 or 30 minutes before imaging. This is more important when using larger lenses (such as a 200 or 400mm) and on a Scottish winter night when the difference in temperature between inside and outside can be over 20°C.
Accompanying the AIP4WIN software is The Handbook of Astronomical Image Processing by Richard Berry & James Burnell 2. A study of this book suggests the right way to go about acquiring quality images. The suggestions have been well confirmed by experiments. The images have to be in RAW format as the JPG format does not retain enough photon information. A reasonable estimate needs at least the average of ten images. If precision is very important then up to 50 images may be necessary.
A master dark frame must be constructed to be subtracted from each image to reduce thermal noise. A master dark frame is made from ten dark images. These are images taken with the camera capped, downloaded to the PC and stacked with AIP4WIN to produce the master dark frame. A separate master dark frame is required for each camera exposure time used. I use sets of dark frames for a week or two and then change them as the outside environment either warms or cools.
In theory it is advisable also to construct master flat field frames in order to remove the effects of the edge distortions of the lens. I have found, however, that the field of view of a DSLR is so wide that a flat field is not required, if you are using high quality lenses, provided the target star and comparison are in or near the centre of the field of view. Stopping down the lens (as part of the process to get the right amount of light) also minimises lens distortion around the edges of the image.
A lot of thought has to go into the selection of a suitable comparison star(s) as only one or two are required. The comparison stars may already be suggested by a standard chart for visual observations. I always check the possible comparisons with the Hipparcos Catalogue to ensure that it really is non-variable or has just a very small variation. It is assumed that the correct magnitude of the comparison is the V magnitude quoted in the Catalogue to one hundredth of a magnitude.
In many studies the comparisons listed on BAAVSS charts can be used with the magnitude listed on the chart. This is because the information required is the change in magnitude rather than the V magnitude. This is usually the case when observing eclipsing binaries.
Camera settings
For satisfactory results it is vital to get a signal to noise ratio (SNR) of over a hundred in the images of the target star and the comparison1. This is achieved by adjusting the camera settings. For studying a star such as r (rho) Cas (magnitude variation 4.1 to 6.2) I used, with an 85mm lens, the settings of ISO 800, f3.5 and an exposure of 5 seconds. These settings result in an SNR of around 220 when the star is mag 4.8, dropping to 100 if the star fades to mag 6. For e (epsilon) Aur (3 to 3.8) I used, with an 85mm lens, settings of ISO 200, f4.5 and an exposure of 5 seconds. These result in an SNR of 140. I changed the ISO for epsilon as otherwise AIP4WIN’s photometry tool recorded the image as being oversaturated at ISO 800. In my experience a higher ISO results in higher SNRs (and saturation if it is too high).
For the eclipsing binary U Cep, which fades to a magnitude of around 9.1, I used, with a 200mm lens, the settings of ISO 800, f2.8 and an exposure of 3.2 seconds. Each setting is usually good over a range of 2.5 magnitudes.
The maximum exposure possible with an undriven camera (to avoid unacceptable blurring), using the 85mm lens, is 6 seconds unless you are working relatively near to the celestial pole when longer exposures are possible. As a general rule it is best to go for the longest possible exposure since this markedly improves the results by reducing the effect of scintillation. The usual maximum exposure with an undriven 200mm lens is 2.5 seconds.
If one is following the fade of an eclipsing binary over a range of more than 2½ magnitudes, it will be necessary to change the settings to catch more or less light as appropriate by altering the aperture stop of the lens. In particular, it is essential to avoid saturating any of the pixels in the stellar images. As a general rule of thumb it is best to keep pixel values to less than 50% saturation.
Slightly defocusing the image enables a better photometric analysis of stars. This ensures that light from the star is not modulated by the sensor structure within the camera. Special care has to be taken to achieve the right amount of defocusing. One cannot completely rely on the infinity setting of a camera to judge the amount of defocusing. The point of infinity setting will vary slightly as the lens expands and contracts in response to the ambient temperature. The variation is greater with a larger lens.
Finding the target star
One drawback with these camera models is the viewfinders, which were not designed with astronomy in mind and are not very good for finding the target star. Only bright stars can be seen. The target star and comparison(s) will usually be invisible. When first imaging a target one usually has to take several images of the general area and examine them on the computer to work out how much the camera needs to be adjusted in order to centre the target star. For cameras possessing the LiveView facility, it is very much easier to see and focus on the brighter stars. A description of the LiveView facility for the Canon 40D, which is similar to the 450D, is given on Christian Buil’s website.
Once the target has been imaged several times one soon learns how, by eye, to offset the camera correctly from a bright star. Similarly one learns the right movement of the camera between image sets to compensate for the rotation of the Earth.
The field of view of the 85mm lens is so large that little adjustment is necessary. Two or three sets of images can be taken ten or fifteen minutes apart. The field of view of the 200mm is significantly smaller and more care has to be taken particularly near the celestial equator.
Experience suggests that never more than three sets of ten images should be taken without checking on your PC to ensure that the target stars are centred. It is too easy to make errors as the night draws on.
Results
U Cep
Figure 1: Observations of U Cep in 2009 JanuaryFebruary. The 0.0 point of the phase axis (the predicted time of mid-eclipse) is based on the latest Krakow period of 2.493121 days.
Figure 1 shows recent estimates (2009 January) of the eclipsing binary U Cep using DSLR photometry. The figure presents a light curve of estimates collected on three different nights. The estimates have been combined in a phase diagram. Each point on the figure represents the average magnitude after an analysis of ten images. Each estimate is unfiltered and is not a V estimate. There is no need to use a V filter when the main objective is to time the midpoint of an eclipse, since the shape of the light curve will be similar with and without a filter.
The current stated period of U Cep, which varies in eclipse from mag 6.7 to 9.2, is 2.493121 days. If this period is correct then the midpoint of the eclipse should have coincided with the vertical line. It can be seen that the midpoint is earlier than the predicted time. After a correction to take account of heliocentric time it worked out that the eclipse was about 30 minutes early, which corresponds to a new period of 2.493087 days. This period has now been accepted by the Krakow website4 which provides information on eclipsing binaries.
r Cas
The approach described here is a useful backyard method suitable for amateur astronomers wishing to carry out V photometry, which provides improvements in accuracy compared to visual estimates. It can be used on brighter stars as well as on stars down to mag 10 without having to resort to a telescope with a drive. The set up is robust and easily transported. With experience the time spent in acquiring 10 images and producing a measurement of the magnitude can be less than 10 minutes.
I find the method particularly useful for constructing good lightcurves of eclipsing binaries, Cepheids and irregular variables. The increased accuracy can make a surprising difference to the quality of a lightcurve.
Figure 2: Observations of r Cas by the author over a 300-day period in 2007/2008.
Figure 3: Observations of r Cas by the author over a 300-day period in 2008/2009.
Figures 2 and 3 provide a record of unfiltered observations of the well known variable r Cas over a 600-day period between 2007-2009. Each point on the figures represent the average of ten images. It can be seen that there seems to be a period that approximates to the 320-day period stated in the GCVS.
V photometry
Hoot1 describes how V photometry is possible with a DSLR. The light-sensitive chip within the DSLR is composed of an array of pixels which are individually covered by green, blue and red filters. This is necessary for high quality colour photography. Luckily the software that comes with the camera combined with AIP4WIN enables one to access the data recorded by every individual pixel. An image is produced from data that solely arrives through the green filters. The software produces what is called a ‘green-channel image’. This can be stacked with other ‘green-channel’ images and the stacked image analysed with AIP4WIN’s photometric tools.
It turns out that the standard green filter used in Canon cameras is approximately equivalent to the Johnson V filter. To convert the estimates from green channel analysis to professional level V estimates one has to use a transformation coefficient (TC). The value of the TC has been worked out for my Canon 450D by comparing measurements of non-variable stars, taking into account the difference in their colour expressed as the (B-V) index. I used an ensemble of suitable stars near to lambda Aurigae to calculate the TC. A value of 0.15 was obtained for this particular camera.
V photometry of e Aur, using green channel analysis, was carried out as part of an international campaign to monitor this unusual eclipsing variable. Five measurements of the star were obtained, each based on 20 images taken on the night of 2009 March 28 with the 85mm lens at an aperture of f4.5 using 5 second exposures at ISO 200.
The results were as follows:
20:36 UT V= 3.095
21:09 UT V= 3.097
21:57 UT V= 3.106
22:03 UT V= 3.081
22:30 UT V= 3.097
The average of these five estimates is 3.095 with a standard error of 0.004.
Figure 4: Observations of e Aur over a six-month period between 2009 January - June showing out of eclipse variation.
Figure 4 shows observations of e Aur between 2009 January-April. All the points represent the average of 20 images. The overall picture conveyed by these estimates can be compared with CCD estimates on the website of the International Campaign to study the 2009-2011 eclipse of e Aur. From page 20 of Newsletter 11 which can be downloaded from the website, we see that the estimates record well a pulsation in the target star.
Conclusion
This article demonstrates that photometry of variable stars can be carried out successfully using a DSLR camera, yielding results which are significantly more accurate than visual estimates. The methodology permits the amateur to make accurate observations of a wide range of variables down to mag 10 without the need for a telescope or a drive. It has also been shown that accurate V photometry can be performed using the green channel image produced by the camera. More widespread use of DSLR cameras for photometry should permit more observers to study the variability of relatively bright stars, which until now have not attracted much attention amongst the amateur community mainly because their amplitude of variation, at less than 0.4 magnitudes, is too small to be followed visually.
Acknowledgments
I would like to thank the referees for their helpful comments which have improved the paper. I would also like to thank Richard Miles and Jeff Hopkins for their advice and assistance in developing the methodology for using DSLR cameras to carry out photometry of variable stars.
References
1 J. E. Hoot, Photometry With DSLR Cameras - Proceedings of the 26th Annual Conference of the Society for Astronomical Sciences, held May 22-24, 2007 at Big Bear, California, USA, pp 267-272
2 Richard Berry & James Burnell, The Handbook of Astronomical Image Processing, Willmann-Bell, 2006
These images of the 2010 July 11 Total Solar Eclipse were taken by BAA member Richard Blake-Reed at Hao Atoll using a Nikon D80 + Sigma 500mm zoom lens at infinity, all shots F22 with ISO 400.
Approaching Second Contact, 5 Beads showing, by Richard Blake-Reed
Approaching Second Contact, Two Beads Remaining, by Richard Blake-Reed
Second Contact, 1/125 s, by Richard Blake-Reed
Second Contact, 1/90 s, by Richard Blake-Reed
Approaching Third Contact, 1/125 s, by Richard Blake-Reed
Approaching Third Contact, 1/90 s, Richard Blake-Reed
Thanks to the BAA’s Nick James, John Mason, and Martin Mobberley for sending these images of the 2010 July 11 Total Solar Eclipse:
Partial eclipse and sunspots lower right. 1/320th sec, ISO 800, Megrez 72 refractor, 2X Barlow, Canon EOS 550D, Nick James.
Baily’s Beads at second contact. 1/400th sec, ISO 200, 500mm mirror lens at f/8, Canon EOS 450D, John Mason.
End of Baily’s Beads at second contact. 1/2000th sec, ISO 400, 355mm FS60C refractor at f/5.9, Canon EOS 300D, Martin Mobberley.
Corona mid-totality. 1/40th sec, ISO 400, 355mm FS60C refractor at f/5.9, Canon EOS 300D, Martin Mobberley.
Large prominence approaching third contact. 1/1600th sec, ISO 800, Megrez 72 refractor, 2X Barlow, Canon EOS 550D, Nick James.
Multiple diamond ring near large prominence at third contact. 1/320th sec, ISO 200, 500mm mirror lens at f/8, Canon EOS 450D, John Mason.
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