I am not a comet hunter! But from reading popular astronomy books it seemed to me that the Victorians, in particular, had had more than their fair share of ‘comets of the century’ – great arcing comets that had stretched across large sections of the sky and had amazed the non-astronomers viewing them in their relatively non-light polluted city skies. And nothing like that has occurred in my lifetime. True, that Halley’s Comet in the mid-80’s had returned and I showed friends in London a view of it through my 6” f/8 Newtonian. But it was a bit of a disappointment and I quickly moved on to astounding them with views of Jupiter and the moons.
But then we had some great comets suddenly emerge in the 90’s. Firstly Comet Hyakutake in 1996 – but for some reason I forget I paid little attention to it. But it must have whetted my appetite because I really got excited when Hale-Bopp then burst onto the scene. Like many great comets of the past, high magnification views through the scope were of little benefit apart from curiosity about the nucleus. The real glory came from low mag. binocular views, simple naked eye views from dark sites and it was especially easy to snap with common cameras of the time.
As it lay in the North Sky when most spectacular I remember going a couple of times to sites north of Bath (just pulling into lay-bys or open-gated fields). Photos showed the blue gas tail clearly though this was harder to see with the naked eye. My photo shown here was taken with a 200mm telephoto on a SLR camera piggybacked onto my SCT on one of those trips north of Bath (800ASA colour transparency film). And Hale-Bopp hung around for months, so it seemed, being somewhat distant from the Earth, almost becoming a routine part of the night sky. I do remember looking at the nucleus through my 8” SCT and seeing (and photographing) a spiral pattern around the star-like nucleus. I doubted its reality until I read that it was due to gas jets emanating from a spinning nucleus.
And that was probably meant to be my ‘comet of a lifetime’ but then at the end of 2007 Comet Holmes – an innocuous rarely-seen comet, only known because it had flared up in brightness in 1892, did its brightening trick for a second (recorded) time. Within days of its reported brightening we had a look at it in the eastern sky across Queens Square, after a Herschel Friday meeting, with binoculars. It was bright enough to be seen in the centre of Bath!
I took a lot of ‘wide-field’ digital camera shots, and tried to keep track of its gradually expanding coma over the next 3-4 months.
One of my shots is on the left; on the right is a montage produced by John Kemp, a friend of Dick Phillips.
Combining his and my photos, and measuring the coma diameter I noted the following trend in angular size (left image):
It seems to flatten off with time (29th Oct – the first day John Kemp viewed the outburst is ‘day 0′ on the graph axis!). But over the time of the observations the distance between Holmes and the Earth changed. Using a basic planetarium program I determined the Earth-Holmes distance and converted the angular diameter of the comet to a physical diameter, which included this distance correction (right hand image). There are some ‘error bars’ as the measurement in different directions could be a touch variable.
The flattening off is less clear and seems to indicated that this distant comet’s coma, once it had exploded due to whatever caused its eruption, then expanded pretty constantly for months, presumably because of the weak pressure of the solar wind at its location.
This unusual comet didn’t come and go from the inner solar system as many do but pretty much pottered along in its inter-Mars/Jupiter orbit and faded as its sudden eruption (for unclear reasons – thermal cracking of its icy crust?) slowly dissipated.
And while Holmes was still noticeable, a regular and expected comet – Tuttle – came on the scene. Below is a shot I got off the internet from early 2008 (sorry – unattributable!) showing Tuttle (the green blob towards the bottom of the frame), Holmes (the white blob top left) and the Andromeda Galaxy (top right). Tuttle was only barely naked eye visibility but though not spectacular that is still better than most manage and I was now ‘in the groove’ looking at them!
In the shot below (one of my own) I have tracked the stars during several 30s exposures then combined so the fixed stars are overlaid. But each image of the comet (green) doesn’t overlay as it was moving so fast across the sky during the course of the few minutes it took to take this sequence!
The green glow itself is apparently due to the dominant molecules fluorescing in its coma – mainly various simple organic molecules such as CH.
And my last Comet Tuttle shot – heavily exposed to look for a tail (I couldn’t see one) also shows something else that can divert your attention when star-gazing – man made satellites!
If you look carefully there is one streak almost going through the comet and another above it and another below (fainter). I hope the image reproduces well enough to show these! Again I took several consecutive images and overlaid them – the satellite tracks were present in only a few of them. Curious as to what these relatively slow tracks were I posted a query on a ’satellite specialists’ internet site and was told that these slower tracks obviously weren’t distant geosynchronous satellites, nor fast enough to be in low earth orbit (like the International Space Station) but either unusual elliptical orbit satellites or stages from old rockets in similar orbits. In fact one of my contacts identified one as a 15-year-old Centaur rocket stage! They are apparently well catalogued by these guys interested in satellites!
So a diversion from the great comets we’ve seen in recent years but interesting nonetheless. No bright and expected periodic comets are expected any time soon. But maybe another unexpected one will turn up soon as it did so frequently for the Victorians. But maybe I’m being greedy.
Have any others of you some comet tales of your own? Tell us about them in the chat forum…
This article is intended to describe how the amateur astronomer can start on spectroscopy, a topic often thought of as the province of the professional. I’m concentrating on low resolution spectroscopy here. The beauty of this subject is that plain ordinary stars – that the regular amateur astronomer isn’t usually interested in – become the main targets. Light pollution isn’t an issue either. And once you’re hooked you will want to chase after some pretty exotic objects that are nonetheless easy to observe and maybe eventually try a more specialist high resolution direction.
The simplest start is to buy a specialist grating – one that screws into the eyepiece tube using the standard thread present to take filters. In fact these gratings look just like standard astronomical filters for a 1¼” tube.
The one I use is by Rainbow Optics, an American company, though at the time of writing this article I couldn’t see a UK supplier for it. A similar grating is made by Paton Hawksley in the UK (Keynsham!) and is recommended by many amateurs. It currently retails for less than £90 but as long as you keep it clean it will never wear out!
It is possible to use the grating visually – the Rainbow Optics version came with a ‘cylindrical lens’ that smears out the narrow spectrum of a typical star laterally thus making it easier to see. But only bright stars (1st/2nd magnitude) show the details in their spectrum easily. It is advisable to move quickly onto the photographic course when far more stars and other objects are available.
You will need an SLR camera where you can remove the lens, and replace it with an adapter, one which will slide into the 1¼” eyepiece tube of your telescope and is threaded internally so you can screw the filter into it. So you are using the camera in ‘Prime Focus’ mode with a grating in the light path.
In these pictures of the setup I show the screw-in grating, camera and adapter attached to a small refractor eyepiece tube. However, I normally attach the setup to my 8” SCT telescope – the more light the better!
Find a bright star – Sirius, Vega, Altair (depending on the time of year) are good starters as they show good strong hydrogen lines. Centre it in the camera view finder. You will see one bright and one fainter spectrum either side of the star (the spectrum is ‘blazed’). So offset the star until it is close to the edge of the field of view and the bright spectrum is fully within it. It is often useful to have the star as a reference point in the field of view and subsequent photograph so as to help spectral line measurements later.
But as you get used to identifying features in the spectrum you may prefer to simply have the spectrum only in the field of view with no ‘dead space’ between the spectrum and star. The scale of the spectrum in your field of view can be adjusted by increasing or decreasing the distance between the grating and the camera CCD chip.
If you shoot the spectrum (a couple of seconds at 400-800ASA will do for a bright star but make sure the focus is sorted out at this point!) you will get a bright ‘rainbow’ streak of a spectrum. It is possible to process this ‘as-is’ later – with image processing software and/or simply rotating the spectrum so it is horizontal and then stretching it/resizing it laterally. By doing this you should see the details in the spectrum that you’re after – the dark (or light) spectral lines.
A better photographic option for brighter stars (say >6 mag.) is to smear the image out during the exposure. To do this adjust the grating so that the spectrum’s length is approximately perpendicular to the RA motion direction (this doesn’t have to be accurate); before you start the exposure either turn off your RA motor, or counter its action by using the RA fine speed adjustment control; start an exposure and let it run for 10-15 secs. You should then get a smeared out spectrum showing the spectral lines clearly (if they exist!). You can make the spectrum more symmetrical later, in an upright, less skewed form, by processing the image later.
For interesting spectra, the bright stars (Sirius, Altair, Vega, Deneb) show strong H lines – other lines are more subtle:
‘M’ type stars show a lot of detail, mainly due to molecules such as TiO in their atmospheres – good luck identifying them!
Many bright stars show varying patterns of dark absorption lines. However, there are some exceptions that show bright emission lines:
Note that the red hydrogen line above (α Balmer) is bright – an emission line apparently due to a hot gas disk around this unusual star. And more bright emission lines in β Lyrae:
A really interesting area concerns ‘Wolf-Rayet’ stars – extremely hot massive stars (100000C). There are few known but a whole group seems to be scattered in the Milky Way around Deneb. Tables of their RA and Dec positions are needed with a ‘GOTO scope in order to easily find them – none are brighter than about 7th Mag. In addition they are faint enough with my set-up to not be easily susceptible to the ‘smearing out’ technique (though I try sometimes). But the bright emission lines show up clearly in the narrow spectra, in contrast to adjacent ‘normal’ stars.
And don’t forget to try planetary nebulae, like M57, and other bright nebula like M42. Strictly speaking ‘extended objects’ like these show the most interpretable spectra when the light from the object is passed through a slit before reaching the grating but in this simple set-up interesting results can still be obtained:
Note that I’ve been flashy with lots of colour images (cos I like colour!) but many amateurs stick to intensity plots of the spectra – such as in the corner of one of my Wolf-Rayet spectra above. That after all does show the salient features.
For projects a popular subject seems to be the spectra of variable stars. Even the variability of the bright star Betelguese is unpredictable and the subject of ‘pro-am’ spectral observations.
Links
http://www.astroman.fsnet.co.uk/spectro.htm – Maurice Gavin (ex-BAA President) – source of much
experience
http://astrosurf.com/buil/ – very active French astronomer at an advanced level
http://www.threehillsobservatory.co.uk/astro/spectroscopy.htm – tons of useful info here
IRIS – freeware image processing software is useful – general purpose but also with spectrum specific features
Visual Spec – a dedicated image processing program for spectroscopy (freeware) but can be tricky getting the input images in the right format to be accepted