Observers’ Forum

Daytime photometry of and planets

Recently, after a long period of rainy weather and cloudy skies, I began to consider ob- serving variable stars using a CCD camera during the daytime, reckoning that this would increase the number of potential observing opportunities. The problem with daytime observation is the sheer brightness of the sky, which overwhelms the visual observer especially for objects located near the . By contrast, an observer equipped with a CCD camera can take many images of the same patch of sky and then, by adding these together, can stretch the contrast to bring out very faint objects only a percent or two brighter than the sky background. My par- ticular interest is photometry (i.e. the accu- using a modern ‘go-to’ mounting, it is possi- measuring stars of known magnitude at dif- rate measurement of magnitudes) so my task ble to align the scope by pointing it at our ferent altitudes, a value for the extinction co- was not merely to register an image of an nearest . In my case, I temporarily taped a efficient can be determined. So there you have object but also precisely to measure its second filter (optical density 5.0) in front of it: separately image your variable plus one or brightness relative to known stars. the first filter so as to cut down on the amount more reference stars and hey presto, the mag- That all sounds straightforward, but to of light reaching the camera − that way it was nitude of the variable can be obtained. avoid saturating the CCD camera, very short possible to centre the image of the Sun on the There are several potential advantages of (millisecond) exposures would normally be CCD frame before activating the single-star daytime photometry. One is that it becomes required and this would ordinarily lead to alignment required for the polar-aligned equa- feasible to follow a star such as Betelgeuse unacceptably high fluctuations in the appar- torial mount. Thereafter it was possible to for twelve months of the , thereby avoid- ent brightness of objects owing to scintilla- control the telescope pointing by manually ing the usual gaps in the lightcurve when the tion. The trick I used to overcome this limi- entering suitable values of RA and Dec. I usu- star is too close to the Sun. Another advan- tation was to interpose a grey, neutral den- ally picked a bright star such as Capella or tage is avoiding having to get up early to sity (ND) glass filter in the optical train to Vega for the first move of the telescope away observe stars in the morning sky before the cut down the intensity of light reaching the from the Sun, to check the focus and the value Sun rises! Finally, with a bright sky in each CCD camera. For a typical camera, an ND of any positional offset. image, the flat-field issue which normally filter transmitting just 1% of the incident Finally, to maximise the signal-to-noise in bedevils photometry becomes a trivial mat- light (optical density = 2.0) reduces the light the image, it is necessary to take as many ter, as there is normally only a single source sufficiently that the daytime sky can be re- images as possible in a few minutes. With a visible in each image. corded with an exposure of a few tenths of a fast download, for example aided by second. In my setup, I fixed the filter in front windowing down the frame so that of the objective lens of a 60mm refractor the file size is just less than 64Kb, it stopped down to 40mm, thereby avoiding is possible to take several hundred excessive scattered light within the telescope frames and average these to bring out tube. The camera, a Starlight-Xpress SXV- faint objects. H9, was also fitted with a green V filter. To date, I have managed six observ- The next task is to point one’s telescope ing runs during the day using a V filter and camera so as to record the object of inter- and neutral density filter. On three est. Not so easy in the day when stars are occasions I obtained accurate V pho- generally invisible. However, if the sky is tometry of Betelgeuse; for example on clear, one star, the Sun, is always visible. So 2007 July 24 at 09:04UT a measure of V= 0.59 ± 0.03 was obtained. How was this possible without recording a com- Examples illustrating daytime photome- parison star on the same frame as the vari- try are shown in the accompanying images.  fessor von Zahn had been invited, and able? This is the final twist in the tale. Day- For , I measured the planet’s mag- the IAGA meeting. Our last contact was time photometry is only really possible if the nitude on August 5 and August 7 when at during the IUGG meeting of 1999. sky is properly clear − in other words that solar elongations of 11.4° and 9.3°. You can Gadsden was a very kind man and a good the sky is clear blue with no trace of clouds see from the first image (A), where I have friend. He was also an excellent mentor, with visible. With such a sky, the apparent bright- co-added 100 frames, that the raw images clear ideas, and insight into many different ness of the stars and planets depends only on show Mercury superimposed on a strongly- problems. I am thankful that we had a simi- the altitude above the horizon, and so the ex- patterned sky background. Each image cov- lar hobby, the observation of noctilucent tent to which stars brighten or fade as they ers an area of sky some 5 arcmin square and clouds, the legacy of which is many nice pho- rise or set can be accurately measured. In a the background pattern is in effect the flat- tographs and letters. I will never forget him. clear sky this amounts to an extinction of close field for this fraction of the CCD frame. The to 0.20 magnitudes per atmosphere, or trick now is to move the telescope a few Wilfried Schröder airmass. When a star is close to the zenith it is arcminutes and then take a further set of 100 Hechelstrasse 8, D-28777 Bremen, Germany seen through an airmass of about 1.00, whereas images. This second set can then serve as the 1 Obituary, J. Brit. Astron. Assoc., 114(1), stars at an altitude of 30° above the horizon flat-field correction for the first image (and 47 (2004) are seen through an airmass of 2.00 and so vice versa). Doing this results in the second 2 Gadsden M. & Schröder W., Noctilucent would appear about 0.2 magnitudes fainter of the three images from which photometric Clouds, Springer−Verlag, Berlin, 1989 than if located at the zenith. By imaging and measurements can be made. The third image

278 J. Br. Astron. Assoc. 117, 5, 2007 Observers’ Forum

A daylight occultation of

The daylight lunar occul- of the Moon and Venus. tation of Venus on 2007 Venus was readily seen June 18 (whose circum- in small binoculars a lit- stances are given in the tle to the left of the Handbook) was not Moon, but the rather widely observed in the poor atmospheric trans- UK, with much of the parency made naked eye country suffering from observations very diffi- heavy cloud-cover. The cult. The weather closed Director had no chance at in at around 13:30 UT all to observe, as it was killing off hopes of see- raining throughout most ing the disappearance... of the event in Northamp- Close to the time of re- tonshire. Fortunately sev- appearance the sky be- eral observers have con- The reappearance imaged by John gan to break up.... A Vetterlein, with a 175mm Mak− tributed their results, and × number of high power some are illustrated here. Cass. 175. Image taken through images were obtained the eyepiece using a digital camera The most comprehen- (1/750 sec). using a 175mm sive account came from Maksutov. At 15:30 UT, John Vetterlein, observing from his home in Venus could be seen quite clearly with the Rousay, Orkney (59º 08' 40"N, 2º 58' 48"W): unaided eye a little to the right of the Moon. ‘It was one of those frustrating days we so The weather remained fair for another hour, often experience at this time of year in the after which thick mist returned.’ islands − sea mist (haar) and low cloud com- Mark Kilner (Broadstairs, Kent) also wit- ing and going on a slack easterly airflow. The nessed the reappearance only and sent an morning broke fine but all the conditions just excellent sequence of images. Malcolm Por- described started to plague Rousay from ter (Petts Wood, Kent) also sent an image of around 06:00 UT. Some good clearances oc- the reappearance. Silvia Kowollik Reappearance sequence by Mark Kilner, curred mid-morning, allowing observations (Rosenfeld−Brittheim, Germany) obtained with 102mm OG and digital camera (1/750 sec). The first image in the sequence was a good movie of the reappearance, and Detlev taken at approximately 15:23:57 UT. Niechoy (Göttingen, Germany) also obtained  (B) was made two days later and taken a good series of images of the reappearance. nus against the dull Moon with the ‘diamond shortly before the clouds rolled in. Here Alan Dowdell observed the reappearance ring’ effect witnessed at total solar eclipses. another flatfield was used that was not such with the naked eye from Winchester. Andrew a good match. Paterson observed the disappearance only, Richard McKim, Director, Mercury & It goes without saying that fainter stars comparing the appearance of the brilliant Ve- Venus Section are a more difficult target than planets dur- ing the daytime. On July 31, I was able to measure the brightness of Epsilon Aurigae, Observing the Cat’s Eye which at 08:04 UT, I found to be at V= 3.02±0.05. On August 4−5, I was able to Like so many deep sky follow Beta Lyrae starting a few hours be- objects, NGC 6543 in fore sunset through the night to just after , commonly sunrise. Images C and D have been flat-fielded known as the Cat’s Eye and show the variable during daytime. For Nebula, was first ob- image C, the Sun was more than 10° above served by William Her- the horizon prior to sunset, whilst for image schel. He discovered it on D it was just rising the following day. Un- 1786 February 15 and it fortunately the focus was incorrectly set in the former case and so the star image is became number 37 in his slightly blurred, however this is not prob- class IV list of objects: lematic when it comes to photometry. In the planetary nebulae. Her- latter case, the star was at an altitude of only schel catalogued objects 20° and was imaged through mist, which according to their size made the task more difficult especially when and brightness, and his the star itself was of about magnitude V= 3.4. planetary nebulae class At the end of this exercise, I was pleased was so called because the to find that daytime photometry is defi- small blue/green discs of nitely feasible. It seems quite incredible that many of these objects the brightness of 3rd magnitude stars can reminded him of the be determined from observations made in planet Uranus which he broad daylight! had discovered a few earlier. Although to Richard Miles Detailed image of NGC 6543 by Andrea Tasselli, Lincoln. Intes be fair to other observ- Golden Hill Observatory, Stourton Caundle, Dorset. Micro M809 Mak Cass with Starlight Xpress SXV-H9 CCD camera. ers, Antoine Darquier, [[email protected]] East up. North to left. discoverer in 1779 of

J. Br. Astron. Assoc. 117, 5, 2007 279 Observers’ Forum

but I am reliably informed that it has a shells around the inner portion of the nebula greenish tint to it even in a small in- (see the cover of the 2004 December Jour- strument. 30 or 40cm telescopes show nal). These shells were thought to have been much more detail − a brilliant pin- expelled at roughly 1,500 year intervals as the point central star, a dark inner region outer envelope of the dying star was periodi- and a bright and grainy elongated outer cally ejected into space. envelope. With an OIII filter fitted to Many planetaries contain a large outer halo the eyepiece I find the planetary ap- which is faint and difficult to detect, and the pears brighter, less grainy and also halo around the Cat’s Eye is a definite imaging slightly less elongated. The mag 10.9 challenge. Grant Privett, who enjoys such central star also disappears. Tel- challenges, has imaged this halo with his 25cm escopes of 50cm or larger are neces- f/4.4 Newtonian reflector. The inner region of sary before hints of the structure seen the nebula is, of course, hopelessly overex- in modern digital images can be posed, but detail in the outer halo can be clearly glimpsed with confidence. seen. As Grant commented ‘Who says stu- The Cat’s Eye has always been a pidly faint isn’t pretty?’ Visually this halo is popular target for observers. Images an extreme challenge but it is possible that the and visual observations have recently small knot of material visible in the halo to the NGC 6543 outer halo imaged by Grant Privett, been received from Section members west (right side of image) might be detected in Ledbury, Herefordshire. 250mm f/4.4 Newtonian re- Martin Morgan−Taylor, David large telescopes. This knot, designated IC4677 flector and Starlight Xpress SXV-M7 CCD camera. Wagstaff, Cliff Meredith and Andrea and of photographic magnitude 15.7, was dis- × 201 45 secs. North up. East to left. Tasselli. Andrea’s detailed image, covered by Barnard and was thought for many M57, the Ring Nebula in Lyra, also likened which rivals many professional images and years to be a background − in fact many the appearance of M57 to a fading planet. shows the complexity of the nebula’s internal references and some charting software still Herschel of course did not know the true na- structure, is shown here. It has been suggested refer to it as a galaxy − but it is now known to ture of the ‘nebulae’ he observed and, not sur- by professional astronomers that this com- be a part of the planetary nebula halo. It is a prisingly using a morphologically based sys- plex shell structure and the two small visible challenge, but not an impossible one, and I tem, he made some errors. Some objects cata- equatorial jets on either side could be the re- would be delighted to hear of any successful logued by him as planetaries – which we now sult of a binary central . In 2004 observations. know are the remains of dying stars similar in even more detail was revealed when images mass to the Sun − are distant . NGC from the Hubble Space Telescope were re- Stewart L Moore, Director, Deep Sky 6543 though is indeed a planetary nebula, and leased showing a series of 11 concentric dust Section one of the most beautiful objects of its class in the sky as the many high resolution images – particularly from the Hubble Space Tel- Supernova 2007af in NGC 5584 escope and the Nordic Optical Telescope on La Palma – testify. S. Nakano of Japan announced on IAUC candles but there has been a suggestion that The person who discovered the true na- 8792 the discovery by K. Itagaki of a variation in decline rate may be linked and ture of planetary nebulae was Sir William magnitude 15.4 supernova in NGC 5584, should therefore be studied. Huggins, and the Cat’s Eye nebula featured located at RA 14h 22m 21.03s, Dec –0 23' The difficulty in monitoring extragalactic centrally in the discovery. Huggins, who was 37.6" (2000). The discovery image was supernovae as they fade is that most are born in 1824, two years after William Her- obtained with a 0.60m f/5.7 reflector and found near magnitude 15 or fainter and they schel died, was a pioneer spectroscopist and reached about magnitude 19.0. thus prove a considerable challenge, observed with his wife Margaret from his It is 40" west and 22" south of the centre especially to visual observers. However private observatory at Tulse Hill on the out- of the host galaxy. The discoverer’s recent advancement in imaging now skirts of London. On the evening of 1864 unfiltered CCD magnitudes included in the presents a chance to follow such objects August 29 he pointed his telescope to 37 IAU Circular are: 2007 Feb 24.81 UT, <19.0; down to magnitudes of 19 or fainter, H.IV. ‘...I looked into the spectroscope. No Feb 25.73, <18.5; Mar 1.84, 15.4 although their presence in front of the host spectrum such as I expected! A single bright (discovery); Mar 2.576, 15.3; Mar 7.648, galaxy’s ‘nebulous regions’ can cause line only! At first I suspected some displace- 13.7. The type Ic supernova 1996aq also difficulties in using photometry to arrive at ment of the prism ...then the true interpreta- appeared in NGC 5584. reliable magnitudes. Spectroscopic details separately tion flashed upon me. The light of the nebula announced on CBET 865 by F. Salgado et. al., was monochromatic...the riddle of the nebu- of the Carnegie Supernova Project, indicated lae was solved. The answer, which had come that it was of type Ia and discovered well to us in the light itself, read: Not an aggrega- before maximum light. A spectrum obtained tion of stars, but a luminous gas.’ Reading on March 4.34UT with the European Huggins’ report of his discovery, the excite- Southern Observatory’s 3.5m New ment he must have experienced at this eu- Technology Telescope showed a strong reka moment clearly comes through. resemblance to that of the normal type Ia Lying in Draco at RA 17h 58m 33.4s and supernova 1994D obtained 10 days before Dec +66° 37' 59" (2000.0) – approximately maximum light. 5° east and 1° north of Zeta Draconis − the Supernovae are very important objects Cat’s Eye is still well positioned in the Octo- to study given that certain types, notably ber sky. With a size of around 20 arcsec and a type Ia, are used as prime distance visual magnitude of 8.1 it is an easy object in indicators. Recently the dispersion in Supernova 2007af when first imaged with the absolute maximum magnitude of this type Bradford Robotic Telescope on 2007 April even a small telescope. A 15cm reflector will 18 (04h03mUT), then at magnitude 15.0. show a small bright disc. I am colour blind, has caused concern in their role as distance

280 J. Br. Astron. Assoc. 117, 5, 2007 During 2007 April the author, who had maximum and a clear separation from the previously studied such objects by visual bright nuclear region of the galaxy. As of means only, decided to start experimental 2007 August 14, the supernova has been imaging of novae and supernovae with the imaged on 36 dates, mostly in unfiltered Bradford Robotic Telescope, which operates mode and occasionally using the red filter. in a dome at the Observatorio del Teide 8,000 The graph includes some early results from feet up on the island of Tenerife in the Canary other observers obtained via VSNET and Islands. The telescope is a Celestron 14 of the maximum in ‘green’ (close to visual) was 3910mm focal length and 365mm aperture, magnitude 13.8 on 2007 March 18. The and using the CCD (‘Galaxy mode’) gives a decline from maximum by three magnitudes field of view of 24 arcminutes. Although (t3) took about 90 days and was largely exposures are generally of one minute, limiting linear except for an apparent temporary magnitudes of around 19 have been reached shoulder around April 19. Monitoring is during periods when the Moon is not nearby. continuing to enable a more complete Supernova 2007af was one of ten analysis to be obtained. supernovae chosen for study, but looked favourable given its expected brightening to Guy M. Hurst

Light curve of supernova 2007af showing plots of 76 measures in unfiltered, green (near visual) and red filters.

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