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Deep Sky Objects


Most of the stuff in the sky is faint to the point it can't be seen (ok it can be seen but not with the Mk I naked eye) which is why optical aids such as telescopes help by gathering more light for the eye, and also magnifying the image to make it easier to see the details.

However the eye can only do so much, and the great breakthrough in astronomy was the use of long exposure photography to capture what the eye could not see. A photographic plate or film turns only 1% of light that falls on it into an image - not very efficient. But modern digital cameras turn 99% of light that hits the chip into an image, which means that a) they are very efficient, so much so an 80mm scope can take very deep images b) you can buy one at Jessops and c) you can see when your image is crap straight away instead of waiting 48 hours for the chemists to return your prints.

Most of the following images are taken with a Canon EOS350D SLR on a TMB 114APO scope, using exposures up to three or four minutes long. The trick in this type of photography is to keep the camera pointing exactly at the object all the way through or you get a blurred exposure.....

The subjects are all referred to by their M numbers; this is from the Messier catalogue. Charles Messier was a comet hunter, who made a list of all objects in the sky that could be mistaken for comets. It's a good "Who's who" of DSOs. Here's a bazzing account of the Charles Messier story from another UK astronomer and CloudyNights regular. Take it away Ben Ritchie...



M13, globular cluster in Hercules

Taken with Meade DSI from my old 8" Newt in France. Called globular for fairly obvious reasons.



M29 open cluster

The little square bit in the middle. There might be more to this one with more exposure though. Note the blueness of some stars - these are newer stars.



M42, the great Orion nebula

Quite a complex area of the sky. The brightest point of the image is a cluster of stars that ionize the cloud of gas making it glow red. However there is in front of this glowing cloud a belt of dark particles that block out the left side of what would otherwise be a fairly symmetrical glowing cloud. The fainter object at about 11 o'clock is part of the same glowing nebula, and is called the Running Man by most yanks because of the shape made by the dark dust in front of it. It does serve to give an idea of the width of the dust lane though.







M42 in hydrogen alpha light

Hydrogen alpha is the light given off by excited hydrogen in stars; (the particles get all giddy cos it's hot and sunny - just like most Brits which also tend to get red and excited.) A good Ha filter has the advantage that it will allow in most stellar light but blocks out moonlight and light pollution. However it does produce a red monochrome image, which isn't the end of the world as it can be quite beautiful when converted to black and white. Its other advantage is that it can be added to digital images in the processing stage to augment the red that is otherwise blocked by the aggressive IR filters built into DSLRs, or it can can also be used in black and white to add extra luminance to an image. Hola!


M42 taken with Sigma 170-500 lens

An unusual colouring as an aggressive light pollution filter designed for visual use only was placed at the rear of the lens assembly (with blu tac) to see how it worked. It alos provides a wider view of the nebula complex.



The Flame and Horsehead nebulae

Part of the Orion complex of nebulae, the brightest star in this image is Alnitak, which is the lefthand star in Orion's belt. This is a purely Ha image which accounts for the graininess as the 350D is struggling to capture the light through its aggressive IR filter.




M44, the Beehive

Or Praesaepe if you're posh. A whopper of a cluster in size which is why is doesn't quite come across well on this image. Notable at the moment (Jan 2006) as Saturn is only just out of shot, and with a wider angle can be easily captured in one frame. He said.


and here you go Missus!




M45, the Pleiades

The Seven Sisters; young stars as shown by their blueness, which again excite gas around them making it glow blue this time.




M52 open cluster

That's a bit more like it. No obvious dense centre like M13 so hence an open cluster.



M56 globular cluster

Compare and contrast with M13; this would benefit from more magnification as the object is obviously small in comparison with M13.


M57, the ring nebula

Another object that would benefit from more magnification, the little doughnut in the centre of the frame is a planetary nebula. It has nuffink to do with planets at all but earlier observers thought that these rings of hot gas were spheres or planets. It's the remains of gas shed from a star travelling out and glowing a lot. Think of a ripple from a stone dropped in a pond a seen from above, that sort of thing.



M67 open cluster


M103 open cluster

Note how in this apochromatic image (see below) the red stars stand out - and yes, they really are red! These are older stars as opposed to the blue fellas that make the Pleiades look so frosty. The explanation for all these differences isn't too complex but historically it has involved a) a graph, b) typing "the Herzsprung-Russell sequence" far too often, and c) likewise "the Chandrasekhar limit". All you need to know is that stars are different colours and that Herzsprung-Russell never worked for Audi.



The double cluster in Perseus

Boo yah! The King and Queen of clusters in my humble opinion. Visible with the naked eye when conditions are reasonable. The mad violet halos on this image and others seen above are from using a telescope with an achromatic (as opposed to apochromatic) lens. The basic deal is that white light , being made of up several different colours at differing frequencies, has a naughty tendency to come to focus at slightly different places as the refracting angle of light at the lens is governed by the light's frequency. So in this image the violet light at the fast end of the spectrum is not quite focused, but instead forms a slightly defocused image hence it appears larger than the star proper, and makes a nice little halo. This is exacerbated by short focal ratios (in this case f 5) because the light is even more aggressively bent by the scope lens to get it to a shorter focus point. The cheap way to attack this is to use a minus violet or fringe killer filter which blocks this violet light. Or increase the f ratio by making the scope longer and longer so that the light is barely bent by the lens therefore doesn't split into its spectrum as badly (f12 upwards I am told).The posh way is to use expensive special glass combinations to make sure the light does what it's jolly well told and comes to a lovely unified focus on your focusing plane. As you may see, some of these images don't have the haloes - that's cos they are taken through a dead posh apochromatic triplet thing which costs more than my car. Groovesome.


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