Tag Archive: astronomy

Sep 18 2012

The Milky Way

Milky Way

The Milky Way (Canon 600D, 18mm, ISO6400, 30s, f/4.5)

Regular readers will know I recently bought myself a telescope, a Celestron 5SE Schmidt-Cassegrain reflector but this post covers some astrophotography conducted without the aid of a telescope almost the opposite in fact. I’ve been on holiday recently to somewhere with pretty good dark skies, unfortunately I did not have my telescope with me but I did have a tripod, a Canon 600D SLR and a collection of lenses although in this instance I just used the 18-55mm kit lens at the wide end (18mm). I also had my planisphere and a copy of the free planetarium software, Stellarium.

I’ve used my camera with a standard lens to take photos of the night sky before: to make star trails, so far my experiments in this area have been a bit disappointing. The aim with star trail photographs is to have nice bright trails showing the apparent motion of stars around the pole as the earth rotates, against a dark background. In my experiments I used 30s exposures at f/4, ISO200 on a 10mm lens which I then combined using a simple application called StarTrails.

Back to my holiday snaps – I started my evening taking photos as I had done for my star trails, I have to say this was all a bit disappointing – individual photos do show the stars in the sky and with some effort one can trace out the patterns of the constellations – you might just be able to spot Cassiopeia in the image below.

Towards Cassepoia

Cassiopeia (Canon 600D, 18mm, ISO200, 30s, f/4.5)

Getting a bit bored with this, I turned the sensitivity right up so I was imaging at f/4.5 ISO6400 with 30s exposures and suddenly this popped out:

Towards Casseopia

The Milky Way towards Cassiopeia (Canon 600D, 18mm, ISO6400, 30s, f/4.5)

Cassiopeia is in there somewhere but there are just so many more stars (and a few clouds). This opened the flood gates and I indiscriminately fired off shots along the line of the Milky Way, just visible in the image above. At this time of year, in the early evening in the UK the Milky Way goes from horizon to horizon passing almost directly overhead starting a little East of North and finishing a little West of South.

Now this is fun but now I have a bunch of images of parts of the Milky Way. Can I stick them together? It turns out I can, I used Microsoft ICE on the images I had acquired and got this mosaic:

Milk Way Composite

The Milky Way, 5 image mosaic prepared in Microsoft ICE (Canon 600D, 18mm, ISO6400, 30s, f/4.5)

This spans almost horizon to horizon. I was rather pleased with this, however I struggled to work out where I was in the sky, picking out constellations from the huge mess of stars is very tricky. It turns out help is at hand in the form of astrometry.net, this is an online service which takes an image of the night sky and works out which bit of the sky it shows and labels it all nicely. It can’t handle the mosaic image shown above, but can handle the individual images – you can see my images here. One of the formats in which data is provided is Google Earth’s KMZ format, so you can see the images projected onto the celestial sphere in Google Earth – my combined KMZ file is here, it’s 12MB.

There are improvements to be made in the process:

  • if I’d had it with me my 10-22mm lens would have been nice – it would give me more sky in one shot;
  • better familiarity with my planisphere would mean less indiscriminate firing off of shots;
  • ideally I’d have gone for a cloudless night;
  • there’s a bit of optimisation on the exposure settings, ISO6400 is at the limit of my camera and if you zoom right in there is some evidence of colour noise, also towards the zenith the stars are motion smeared – as in star trail pictures so shorter exposures would be nice;
  • compositionally it would be good to include some of the earthly scenery;
  • working out how to turn off the security lights of the holiday cottage we were staying in would have been good.

All in all a rather fun evening!

May 13 2012

The sky at night!

And so after 10 days, I finally had a chance to play with my new telescope on Friday night! Optical astronomy requires at least a few gaps in the clouds but last night at 8pm it was completely clear – I was hopping up and down like an overactive child waiting for the sun to go down (scheduled for about 8:40pm) and simultaneously cursing the slightest wisp of cloud. It should be clear that I’m a bit new to this, so what I write shouldn’t be seen as in the slightest bit authorative.

Kindly folk at @newburyastro had suggested Venus and Saturn as targets for my first adventure into the night. Useful advice because, as a relative beginner I had little idea what I was going to see, or in fact when I was going to see it. Venus become visible at about 9:20pm towards the now-set sun, it turns out that pointing the ‘scope with the finderscope is much easier than the rather more hazardous enterprise of finding the sun without (something I describe here). In the eyepiece Venus appears as a small, bright crescent.

It was a breezy evening which meant that my view jiggled about a bit, it also jiggled about a bit whenever I touched the telescope. However I did manage a picture of Venus taken on my Canon 400D at prime focus. This is an uncropped view, and it’s upside down.

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Venus (1/50 second, ISO200)

 

Mars made an appearance a little later at about 9:35pm along with a bright star which I believe is Regulus. This enabled me to get my telescope to work out how it was orientated meaning it could track to objects on demand and also tell me what I was looking at (very handy for a novice). My picture of Mars is a little uninspiring, I’ve zoomed in here as far as possible, in Mars’ favour it does look red and it isn’t a simple point.

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Mars (cropped, 1/3 second, ISO200)

 

By now more and more stars were coming out, so I thought I’d try out my piggyback mount. This image is taken with a 10mm lens (i.e. really wide angle) with the telescope simply used as a camera mount pointed at Polaris, it’s a 30s exposure.

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The Northern circumpolar region (Canon 400D, 10mm, 30s ISO200, f/4)

 

 

It took a while to get this because I had auto-focus on and the camera couldn’t find anything to focus on so wouldn’t fire – switching off auto-focus and focusing to infinity manually resolved this. It was at this point I wished I could remember how to switch the display on the back of my camera off because it was really bright, and remember which button was which without being able to see it. The thing that surprised me about this is that there are rather more stars than I could see with my naked eye and some of them are quite strongly coloured. I feel I should go about identifying the stars in my picture.

At this point I thought I’d give Saturn a go, I must admit I thought it was hidden behind buildings and trees from my position in the back garden but I punched it into the telescope handset and it pointed me into the side of the conservatory, so I picked up the telescope and moved it one metre to the right, peered through the finderscope and tweaked my direction a bit and… the planet with ears popped into view!! This was really exciting! I only have one eyepiece for my telescope and it’s quite low magnification but through the eyepiece I could see my target was not a point, and it was not round – it was shaped like a flying saucer and there were slight gaps either side of the central body. Having marvelled at this for a bit I thought I’d try for another photograph:

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Saturn (cropped, 1/4s, ISO200)

 

It’s not the best picture of Saturn taken last night but it is my picture!

The moon hadn’t risen before I went to bed, so when I spotted this morning I rushed out for a photo.

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The Moon 9(1/500s, ISO400)

 

I’ve not done any astrophotography before these (apart from my shots of the sun, and a couple of shots at the moon through a conventional lens). I guess the thing I carried over from that was that the moon is a rock in full sun, so you need to set your exposure times accordingly, the same is true for Mars and Venus so I suspect I should be using shorter exposure times for them to which will also reduce any motion blur.

My first night of viewing has highlighted a need to have a better grip of how to work your camera, plan what you want to look at in advance and, as with an SLR camera, a telescope is simply a gateway drug for further accessory purchase.

May 08 2012

First light–images of the sun

I’ve had my new telescope (a Celestron NexStar 5SE) nearly a week now and so far I have images of miscellaneous chimney pots, arials, pigeons, and… the sun. Only the last of these can be considered fair astronomical game, I’ve had two goes at it so far. I tending to the view that my telescope blog posts shall be like a lab book of what I have done rather than a guide to others, except to perhaps highlight those things that are obvious to experienced astronomers but not to the novice.

The first rule about looking at the sun through a telescope is:

Do it carefully with the appropriate solar filter in place

Seriously, be really careful pointing telescopes at the sun – mine is a small one and it concentrates light by a factor of 300, looking near the sun with the naked eye is bad – imagine x300 more light!

I bought a sheet of Baader AstroSolar solar filter film at the same time as I got my telescope, this comes in the form of a thin A4 sheet of material that looks like foil. It has an optical density of 5, meaning it lets 0.001% of the incident light through. There are detailed instructions supplied with the AstroSolar film for constructing your own mount for the material, or you could go and buy a proper mounted filter (here).

The aim of the filter mount is to hold the filter film without stressing it and in a manner convenient to attach it to the front of your telescope. I should, perhaps, have used the “thick card” that the instructions recommended rather than the corrugated cardboard from the box the telescope came in, and it turns out double-sided carpet tape is really exceedingly sticky. However, the result shown in the image below is functional and I have included a built-in “filter shield” of my own invention for storage. Behind the two cardboard rings sandwiching the filter is a cardboard tube which fits neatly over the optical tube.

SolarFilter

Celestron NexStar 5SE with homemade solar filter from Baader AstroSolar

The next challenge is pointing the telescope at the sun, this turns out to be pretty tricky because with a solar filter in place the only thing in the sky that you can see is the sun, the field of view on my telescope is approximately the size of the moon, you can’t navigate by distinctive clouds and you can’t look through your finderscope unless it is also solar filtered. I’d read that you should move the telescope until the shadow of its tube is a circle – I tried doing this on the ground (minimising the area rather than trying to get a circle). At one point I thought I’d found the sun but from later observations I suspect I was staring at an internal reflection. But easier, since my telescope has a non-magnifying StarPointer finderscope I cast the shadow of that onto a piece of card until it looked round (see image below). The second time I tried this, I got a “hole-in-one” – the sun in my field of view at the first attempt! I could improve this by slotting a disk with a small hole in the middle into the finderscope and aligning until a bright spot appeared in the middle.

StarPointer_solarfinding

Shadow of the Celestron Star Pointer, used to align the telescope to the sun

 I have to say that seeing the sun through my telescope for the first time was as exciting as digging up potatoes, that’s to say really exciting!

I then moved to trying to photograph my target, I did this using a Canon 400D SLR. The camera is attached by a T-mount to the back of the telescope, in place of the eyepiece. This means that the telescope is replacing the camera lens,s known as “prime focus photography”. Two configurations are possible: with and without the “Star Diagonal” in place. The field of view through the Star Diagonal is smaller, and dimmer than the direct connection however the viewing position is more comfortable and there is less risk of the camera falling off! The direct connection gives a correctly oriented view through the camera, whilst the Star Diagonal gives an upside-down view. The focus position for the eyepiece and the two different camera configurations are all different. The camera is triggered using a remote release cable.

My first attempt is shown below, this is a 1/640s at ISO200 taken without the StarDiagonal:

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Image of the sun, Canon 400D ISO200, 1/640s exposure

Below are crops to the two visible clusters of sunspots:

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Sunspot detail

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Sunspot detail

These look a little less distinct than they did through the eyepiece which may have been because I forgot to enable “mirror lockup”. The second time around I did a bit better, this is taken at ISO100 with a 1/125s exposure again without the StarDiagonal:

IMG_1837-002

Image of the sun, Canon 400D ISO100, 1/125s exposure

With a detail of the sunspots:

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Sunspot detail

I have a nice set of solar features, sunspots with dark umbra and a paler penumbra, limb darkening (the sun appears less bright towards its edges) and plages (related to faculae) which are bright spots, these are pretty difficult to see. The image below is a crop of the sunspot area to the right hand side of the image above with some contrast enhancement (I boosted the shadows using Picasa) which just about shows the plages:

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Solar photo showing plages

Next time I should probably set the white balance to something other than “auto”, and experiment a bit with exposure times to see if I can get the plages showing up a little better. A Barlow lens would give me some magnification of the sunspots… and so the spending on accessories begins!

May 07 2012

Celestron NexStar 5Se – a 125mm reflecting telescope

CelestronNexStar5SEThis is a brief overview of my shiny new purchase: a Celestron NexStar 5SE telescope. As an experiment I have also embedded a video review (here), I should also point out that so far cloud cover has meant the only celestial object I have observed is the sun (using the appropriate safety measures).

I bought my ‘scope from Sherwood’s, who I am happy to recommend for their good prices, and quick and efficient service. My purchase list was as follows:

  • Celestron NexStar 5SE (with mains adaptor)
  • SLA AstroPower station 12v 7Ah battery pack
  • Piggyback mount for my Canon 400D SLR
  • Universal camera adaptor and T-mount for similar
  • Moon filter
  • Baader solar filter film

The mount is powered, the add-on battery pack seemed like the best option for providing that power conveniently. I have a Canon 400D SLR camera which I wanted to use with the telescope, the piggyback mount lets me put the camera on top of the optical tube and simply use it to point the camera at the sky. The T-mount assembly allows me to use the telescope as a camera lens, albeit without auto-focus and aperture.

The solar filter is essential if you want to look at the sun, and I got the impression a moon filter was useful for dimming the brightness of the moon, photographers will know that when photographing the moon the exposure time is as if for a rock sitting in full sun, which is exactly what it is!

The 5SE is a Schmidt-Cassegrain telescope with a 125mm (5 inch) primary mirror, a focal length of 1250mm and an overall F/ratio of 10. “Schmidt-Cassegrain” means that the open end of the tube has a corrector plate (Schmidt’s contribution) and light is focussed by a large concave primary mirror and a smaller convex secondary mirror in the centre of the corrector plate. The image is viewed through an eyepiece in the back of the optical tube, behind the primary mirror. In practical terms it also means the telescope has a very short tube length making it more portable than similarly specified telescopes. The whole assembly is easy to pick up and carry in its deployed state, and the optical tube in particular was well-packed on delivery forming the basis of a useful carrycase.

The telescope is supplied with a 25mm focal length eyepiece which gives a magnification of x50, the maximum useful magnification of the telescope should be x300 with appropriate eyepiece. Focus is achieved by turning a knob on the back plane of the telescope tube, which moves the primary mirror. The eyepiece is attached to a periscope (Star Diagonal in Celestron’s parlance) to give a more comfortable viewing position. The finderscope is a Celestron Star Pointer, which is a non-magnifying window with an LED spot projected to the middle for guiding, it took me a little while to get the hang of this but I can see the benefit of a low magnification finderscope.

The telescope is on a computerized alt-azimuth mount which also includes an equatorial wedge (like the equatorial platform), meaning that the rotational motion of the mount can be made co-axial with that of the earth – allowing un-rotated tracking of objects through the sky for astrophotographic purposes. The controller is a handset device on a cord, in night time operation the telescope can be aligned to the night sky by pointing it to three different stars, after which it will goto any one of a huge catalogue of celestial objects selected using the handset.

The optical tube feels nice and chunky, although the finderscope is a bit plasticky. The piggyback mount attaches using the same mounting holes as the finderscope, the finderscope then bolts back on top, I did a bit of tweaky of the screws along with adjustments on the finderscope to get it aligned. I have achieved fine views of my neighbours chimney pot!

There is a battery compartment in the mount which takes 8xAA batteries, reading on the internet I understand the lifetime for this set is about 30 minutes in operation, which is why I got both a mains adaptor and a 3rd party battery pack. I suspect I’ll mainly use the add-on battery pack for the convenience of fewer trailing leads. The mount doesn’t operate without power, which is a bit of a drawback, the telescope can be tilted but not rotated. The mount sits on top of a nice chunky tripod, to which it is attached by three screws, so in principle you could make yourself a “manualised” version by sitting the scope on a turntable. I have the slightly spurious desire to see a graduated scale on the mount movements. I’m used to using research grade optical equipment and whilst the optics have that feel about them the mount, although functional, does not.

The telescope comes with TheSkyX (First Light edition) planetarium software, and also an application called “NexRemote” which seems to allow you to control the telescope using a virtual version of the handset on screen – this seems a bit pointless to me! Other telescope control software is available, and it appears there is an interface standard. The programmer in me is hankering to write my own controller software!

Overall I’m pleased with my new purchase but desperate for a slightly less cloudy night to try it out properly – no doubt more blog posts to follow once I’ve done this! Even at £650 for the telescope it is cheaper than many lenses for my Canon SLR, although it is a little chastening that John Hadley’s 1721 reflecting telescope had a larger primary mirror.

Update:

After a few weeks of twilight use I thought it might be useful to add a couple of further comments which don’t really make a full new blog post:

1. You can get and set the telescope azimuth and altitude directly using the appropriate entries in the Utilities menu, without alignment these values are based on an assumed initial position of 0,0. During the hours of daylight, when only a very limited number of celestial bodies may be visible, you can carry out a “single body” alignment using the “Solar System Align” option in Alignment. This allows you to enable tracking, and to Goto specified absolute coordinates – useful if you want to survey heights of neighbouring obstructions.

2. The 5SE does not support autoguiding whilst the 6SE and 8SE do. The NexStar range does seem a bit confusing in terms of the facilities available across the range, the 5SE for another example is the only one to have a built-in equatorial wedge.

Here is a video tour, which covers much of what I’ve written above but includes the sound of me tripping over the cat’s water bowl:

 

Apr 17 2012

Book Review: Stargazers by Fred Watson

41W3OswkqxL._SS500_This post is a review of “Stargazers:The Life and Times of the Telescope” by Fred Watson. It traces the history, and development of the telescope from a little before its invention in 1608 to the present day.

The book begins its historical path with Tycho Brahe, a Danish astronomer who lived 1546-1601. He built an observatory, Uraniborg, on the Danish island of Hven in view of his patron, King Frederick II of Denmark. Brahe’s contribution to astronomy were the data which were to lead to Johannes Kepler’s laws of planetary motion and ultimately Isaac Newton’s laws of gravitation. On the technical side his observatory represented the best astronomy of pre-telescope days with the use of viewing sights, his Great Armillary with it axis aligned with that of the earth and graduated scales to measure angles. Watson also cites him as a first instance of a research director running a research institute – alongside the observatory he ran a print works to disseminate his results.

The telescope was first recorded in September of 1608, when Hans Lipperhey presented one to Prince Maurice of Nassau in the Netherlands. Clearly it was a device of its time since in very short order several independent inventions appeared, Galileo constructed his own version which led to his publication of “The Starry Messenger” in 1610 which reports his observations using the device. The telescope grew out of the work of spectacle makers; there are some hints of the existence of telescope-like devices in the latter half of the 16th century but these are vague and unsubstantiated. Roger Bacon and Robert Grosseteste both conceived of a telescope-like device in the 13th century, around the time the first spectacles were appearing. Although there are a few lenses from antiquity there is no good evidence that they had been used in telescopes.

The stimulus for the creation of the first telescopes seems to have been a combination of high quality glass becoming available, and skilled lens grinders. The lens making requirements for telescopes are much more taxing than for spectacles. The technology required is not that advanced, if you look around the web you’ll find a community of amateur astronomers grinding their own lenses and mirrors now using fairly simple equipment, typically a turntable with a secondary wheel which produces linear motion for the polishing head back and forward across the turning lens blank. The most technologically advanced bit is probably captured in the first step: “acquire your glass blank”.

Through the 17th century refracting telescopes were built of ever greater length in an effort to defeat chromatic aberration which arises from the differential refraction of light as a function of wavelength (colour) – long focal length lenses suffered from less chromatic aberration than the shorter focal length ones which would allow a shorter telescope. Johannes Hevelius made telescopes of 46m focal length (physically the telescope would be a little shorter than this), mounted on a 27m mast; Christiaan Huygens dispensed with the “tube” of the telescope entirely and made “aerial telescopes” with even longer focal lengths, up to 64m.

It was known through the work of Alhazen in the 10-11th century, and others, that reflecting, curved-mirrors could be used in place of lenses. A telescope constructed with such mirrors would avoid the problem of chromatic aberration. However, the polishing tolerances for a reflecting telescope are four times higher than that of a lens. Newton built the first model reflecting telescope in 1668 but no-one was to repeat the feat until John Hadley in 1721.

Theoretical understanding of telescopes developed rapidly in the 17th century both for refracting and reflecting telescopes, indeed for reflecting telescopes there were no fundamental advices in the theory between 1672 and 1905. The problem was in successfully implementing theoretical proposals. Newton claimed that chromatic aberration could not be resolved in a refracting telescope, however he was proved wrong by Chester Hall Moor in 1729, and somewhat controversially by John Dollond in 1758 who was able to obtain a patent despite this earlier work (which was defended aggressively by his son) – the trick is to build compound lenses comprised of glass of different optical properties.

Also during the 18th century the construction of reflecting telescopes became more common, William Herschel started building his own reflecting telescopes in 1773 with the aid of Robert Smith’s “Compleat system of opticks”. Ultimately he was to build a 40ft (12m) telescope with a 48 inch (1.2m) mirror in 1789, supported by a grant from George III. During his lifetime Herschel was to discover the planet Uranus (nearly called George in honour of his patron), numerous comets and nebulae. At the time “official” astronomy was more interested in the precise measurement of the positions of stars for the purpose of navigation. Herschel was to be followed by Lord Rosse with his 1.8m diameter mirror telescope built in 1845 at Birr Castle, this has been recently restored (see here). He too was interested in nebula and discovered spiral galaxies.

During the 19th century there were substantial improvements in the telescope mounts, with engineers gaining either an amateur or professional interest (men such as James Nasmyth and Thomas Grubb). Towards the end of the century photography became important, which placed more exacting standards for telescope mounts because to gain maximum benefit from photography it was necessary to accurately track stars as they moved across the sky to enable long exposure times. This is also the century in which stellar spectrography became possible with William Huggins publishing the spectra of 50 stars in 1864. Léon Foucault invented the metal coated glass mirror in 1857 which were lighter and more reflective than the metal mirrors used to that point. As the century ended the largest feasible refracting telescopes with lens diameters of 1m were just around the corner, above this size a lens distorts under its own weight reducing the image quality.

In 1930 Bernhard Schmidt designed a reflecting telescope which avoided the problem of aberrations away from the centre of the field of view making large field of view “survey” telescopes practicable. As a youth in the 1970s I learnt of the 200-inch (5 metre) Hale telescope at Mount Palomar, since then space telescopes able to see in the infra-red and ultra-violet as well as the visible have escaped the distortion the atmosphere brings; adaptive optics are used to counteract atmospheric distortion for earthbound telescopes and there are “distributed” interferometric telescopes which combine signals from several telescopes to create a virtual one of unfeasible size.

Watson mentions briefly radio telescopes and in the final chapters speculates on developments for the future and gravitational lensing – natures own telescopes built from galaxies and spread over light years.

I enjoyed “Stargazers” as a readable account of the history of the telescope which left me with a clear understanding of its principles of operation and the technological developments that enabled its use, it also provides a good jumping off point for further study.

Footnotes

My Evernotes for the book are here, featuring more detailed but slightly cryptic notes and links to related work.

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