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

This article is a brief primer on how you might make your first attempts at astro-photography.

There are three options:

1. Use a simple ‘point and shoot’ camera
2. Use a webcam
3. Use a DSLR (Digital Single Lens Reflex) camera

Option 1

Your typical astronomical target will be the moon or the planets. If you have a special filter for the sun, or project the sun’s image onto a card from your telescope then the sun is an option too. In general fainter objects in your telescope (star clusters, galaxies, etc) aren’t possible because they will require a longer exposure time than that type of camera normally provides – but check how long an exposure you can take – and control!

Using a ‘point and shoot camera’, firstly set up your telescope as you would for a normal visual observing session – with an eyepiece in place to get a magnified image. You now need to have an arrangement to hold the camera in the place your eye would normally be. This is called  ‘afocal’ photography (http://en.wikipedia.org/wiki/Afocal_photography). Easier said than done. You can make up some sort of tube to fit over the eye piece and lens, or a ‘meccano’ type of framework with a screw (1/4” thread) to fit into the threaded hole most cameras have on their lower body to fit onto a tripod. Easier is to buy a purpose built frame or adapter from an astronomical supplier or on the internet. An item of no more than £20 (in general) will clamp around the eyepiece and also screw into that tripod thread on your camera – there will be a means for moving the camera around until its lens sits over the eyepiece lens.

Then look through the camera viewer and focus the telescope as normal until you focus the planet/moon to your desire. Then play around with exposure times (if you have much control on your automated camera) and see what you get, deleting the images that don’t make your grade – though maybe later after you have a good chance to check them indoors. The shorter but fainter exposures, that can be enhanced later, may give the sharper images when you inspect them carefully.

Option 2

The set up can be very similar to that for option 1 – supporting the webcam over the eyepiece. But another preferable route is to unscrew the plastic lens from the webcam, and support the webcam over the eyepiece tube of your telescope but without the eyepiece (this is called ‘prime focus’ photography). Again there are cheap adaptors available that screw into the webcam lens thread at one end and fit into the eyepiece tube at the other.

You will also need your laptop computer at your side so you can view the webcam image – think about that carefully as it can get pretty damp (dew) as well as cold at night and your laptop may not like it!

You will need to play with your webcam imaging software to get a bright enough image on your laptop using a combination of a long enough ‘frame exposure’ and the gain and brightness. Once you have a focused image (try any planet or the moon – not stars or galaxies) you can take one-off shots but try taking a short video, maybe 30s to 60s to start. You will get several hundred frames in that time – process them with the free software ‘Registax’ that will combine the better exposures to give you an impressive image, free from the ’shimmering’ that you often see with a normal visual observation of planets. For more info see http://www.astro.shoregalaxy.com/webcam_astro.htm.

Option 3

This will use a somewhat more expensive camera than above and will allow you to go beyond the brighter objects in the sky – star clusters, nebula and galaxies become possible. The downside is that to record fainter objects you will need exposures of 30s (-ish) and above so your telescope will need to be motorised to allow it to track the stars automatically. That also requires knowledge of how to ‘polar align’ your telescope. You should mount your camera to your telescope in the ‘prime focus’ mode above (remove the camera lens and attach it with an adapter to your telescope eyepiece tube – again with no eyepiece).

Focus the image by viewing through the camera viewfinder. One tip to help focusing (it is almost impossible to focus on the fainter objects directly) is to first focus on a bright star in the vicinity of your target (which is easier) and then go directly without touching the focus knob, or the camera set up, to your target. Take trial exposures of your bright star to check the focus on the image LCD of the camera. Set your camera to 800ASA; if possible trigger the exposure with a remote control or button on the end of a wire to save on shaking; go for a range of exposures (say 10s upwards) and see what you get! Take multiple exposures of the same object and combine (to simulate an effectively longer exposure) with the freeware ‘Deepskystacker’.

An easy alternative use of a DSLR is to keep a lens on your camera and mount the camera on the side of your telescope tube (‘piggybacking’). In effect your telescope and mount act as a super tripod, especially if your telescope is motorised and ‘polar aligned’. Exposures of 20s plus will show far more stars than you see with the eye. A standard 50mm lens will show a large sky area – typically a full constellation can be seen in one image; a telephoto lens (135mm or 200mm for example) will show much smaller sky areas with some galaxies, nebulae and star clusters showing details. Piggybacking is very simple and can give rewarding and beautiful views of large areas of the sky.

Options 1 and 2, on brighter objects, can easily be done in light polluted towns. However, option 3 will show the light pollution (as an orange ‘cast’ to the picture) in towns. Try shorter exposures when there is light pollution and combine them later. In the darker countryside option 3 will work much better and longer individual exposures can be attempted.

Image processing, in your warm house after your outdoor session, is a topic in itself. But there are ‘freeware’ image processing programs on the internet and one will almost certainly come with your digital camera on a utility CD. Play around with contrast and brightness settings and on fainter objects try ’stretching’ the image. For long exposures with option 3 you should also try to take ‘dark’ frames (at least one image with the telescope mirror/lens covered up; to give the ‘noise’ in the absence of an image – a dark frame is subtracted from the main image when processing) and maybe a ‘flat’ frame (image of a uniformly lit screen or field of view to allow for non-uniform brightness across a field of view produced by most lenses; this image is ‘divided’ into the main image during processing). But these extra steps can wait until you are confident in your abilities in taking shots of the astronomical targets first!

Now give it a go and if you have any queries, or want more detail than I’ve included here, then post on the forum.