Observers' Forum

Observers’ Forum Daytime photometry of stars and planets Recently, after a long period of rainy weather and cloudy skies, I began to consider observing 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 Sun. 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 star. 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 year, 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 Mercury, 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 Venus 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.

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