DEEP SKY OBSERVING
What do we mean by Deep Sky Observing, everything in space is a long way away compared to distances on planet Earth and even within our own Solar System? Well in this article we mean anything that is outside our own solar system, this includes every star apart from our own Sun, great big clouds of glowing gas called nebulae, double stars, open star clusters, globular star clusters, planetary nebulae, dark nebulae and galaxies. Many astronomers would argue that stars should not be included in this list but there are some very pretty double stars that are well worth observing and some of these can be as challenging as the 'faint fuzzies' which is very descriptive of most Deep Sky Objects or DSOs for short. The DSOs we shall be discussing here are much fainter than the planets like Venus, Mars, Jupiter and Saturn so do not expect to see them in glorious full techni-colour (apart from double stars) unless using long exposure photography, as the eye is too insensitive to colour.
Getting Started
The easiest way to start observing DSOs is with your naked eye or with a pair of binoculars (bins). To start with you need to find your way around the night sky and to do this there is no better way than to take a look at one of the astronomy magazines and look up its star chart for the month or buy a 10inch (254mm) planisphere and then start by finding some of the circumpolar constellations like Cassiopeia and The Plough asterism (informal grouping of stars) which forms only part of the constellation Ursa Major. Then depending on the time of year you are observing, look for some of the brighter more recognisable constellations like Orion, Taurus, Gemini, Cygnus, Aquila, Virgo, Lyra and the faint white band of the Milky Way as it runs across the sky from Cassiopeia through Cygnus and on down to Sagittarius.
Once you start to be able to find your way around the constellations you will find it easier to use the lists of DSOs in the astronomy magazines and on websites as these tell you which constellation the objects lie in as well as giving their celestial coordinates and brightness (magnitude). Using free software like Stellarium will allow you to check the positions of many DSOs but there are so many to observe where should you start? The answer is the brightest! Then as you start to develop your skill as an observer and/or get a larger telescope, you can head for the fainter ones with confidence.
Choice of Telescope
As previously discussed, many DSOs can be seen using the naked eye and binoculars. However, the true splendour of almost all DSOs requires a telescope to be fully appreciated. The main consideration when observing DSOs is their faintness. Therefore the larger the aperture of the telescope the better, a telescope with twice the diameter will actually capture four times more light from the DSO. This will normally mean using a reflecting telescope of some kind, either a large Dobsonian or a smaller GOTO telescope like a Maksutov or Schmidt-Cassegrain. However, if you are intending to start using a camera for long exposure astrophotography then smaller, shorter focal length refractors can be used to great advantage.
Planning your observing
The visibility of all DSOs is greatly impacted by any form of light pollution caused by things such as street lights but also the light of the Moon. This doesn't mean that you have to have perfectly dark skies but try to do your DSO observing when the Moon is between third quarter and first quarter, so that it is less bright and also out of the sky for the majority of the night. From first quarter to third quarter Moon it's best to either observe the Moon itself or any planets that are in the sky. There are however a few notable exceptions to this rule as we shall find out!
Tricks of the Deep Sky Observers’ Trade
Due to the faintness of all DSOs apart from double stars there are a few techniques that seasoned observers use to glimpse the finer details of their targets.
1. High in the Sky:
Probably the number one rule for observing DSOs is to observe them when they are high in the sky. As the Earth spins on its axis, all astronomical targets appear to rise in the East traverse the sky and set in the West. Because of the tilt of the Earth's axis (23.4 degrees), objects appear at their highest above the horizon when they cross the meridian (the line running North-South through your position) and are due South of you. The reason for observing when the object is highest in the sky is because you are looking through less of the Earth's atmosphere. When viewing below 30 degrees you are looking through twice the thickness of atmosphere as when observing at the zenith (the point directly above your head). You can see this for yourself by looking at the sky overhead and then gradually looking downwards towards the horizon, you will notice that there always appears to be a layer of fog down close to the horizon. Therefore when planning any observing it is best to consult tables or astronomy software like SkyTools that tell you what time of year the object will be in the sky (due to the season) and when it will be at its most favourable for observing (time of night).
2. Dark adaption:
Dark adaption of the eye happens over a period of 20 minutes or more and is the eye's way of seeing in lower light levels. It is a twofold process that increases the size of the pupil letting more light into the eye and a chemical change that makes the retina more sensitive. Before starting a search for any DSOs it is best to go outside 30 mins before you want to observe and then DO NOT USE any white lights. Use a red torch or head lamp while setting up your scope ready for the night's observing, remember red light will not impact your night vision as much as white light. This is why it is frowned upon to use a white light at Star Parties!
3. Averted Vision:
Using averted vision is a technique of looking just off to one side of the object, either through the eyepiece of a telescope, binoculars or naked eye. This causes the image of the object to form on the eye's retina in an area that is covered with the black and white light receptors called Rods that are much more sensitive and packed more densely than the colour receptors called Cones which cover the central part of the retina. This technique can be a little difficult at first because you suddenly see more of the object and then your eye re-centralises the object, so you lose it again. Persevere and practice because averted vision will enable you to see fainter objects than normal. Three good examples of this, using the naked eye are: The Andromeda Galaxy M31 (late Autumn); The Double Cluster NGC869 and NGC884 (all year); and the Beehive Cluster M44 (late Winter). Look at any of these directly and you will be lucky to see them but look slightly off to one side and they will pop into view!
4. OTA Nudging:
Nudging the telescope's Optical Tube Assembly (OTA) can be a good way to determine whether you can really see something in the eyepiece or not. Sometimes DSOs are barely perceptible against the background sky in the field of view of the eyepiece but if you gently tap the OTA then the target's image wobbles which makes it more discernible.
5. Patience is a Virtue:
Being patient and allowing an image of a centred DSO to build up on your retina is good advice. It is very tempting to rush around the night sky, looking at one DSO after another to see what they all look like in a single evening. This is great fun and will wow all your friends but later go back to each DSO again and spend more time observing it. Certain DSOs like the Hercules Cluster M13 (Summer) that has hundreds of thousands of stars grouped into a globular cluster that really sparkle in larger telescopes and look good even in a smaller scope, really benefit from patience.The longer you look (at least 5 minutes), the more you will see! This is true of all DSOs.
6. Sky Conditions â€" Seeing and Transparency:
The sky conditions are obviously beyond the observer's control but make the most of the nights of really great transparency and seeing. Astronomical seeing refers to the steadiness of the Earth's atmosphere that we are attempting to look through. Unsteady atmosphere causes the stars to twinkle as the refractive index of the atmosphere changes above our heads. The better the seeing the higher magnification that can be used. Often when it has been raining all day and the sky has been washed clean of all the pollution, the sky will be really transparent and will appear crystal clear with jet black background and bright stars. When good seeing and transparency occur at the same time, don't waste a second as it doesn't happen very often in the UK! Make the most of these rare opportunities and you will be glad you did. Take the chance to look at some of the smaller DSOs as you will find that you can push the magnification above the normally recommended maximum of once times the telescope's aperture in millimetres, sometimes up to as high as twice that.
What to Observe
There are all manner of DSOs and double stars that are worth observing, even with the naked eye or bins. Obviously these have to be the brighter ones but also include the furthest object visible to the naked eye! Then there are the fainter ones many of which are listed in famous catalogues the most notable of which are the:
• Messier Catalogue (M numbers) - a catalogue of misty patches that Charles Messier didn't want to confuse with comets;
• Caldwell Catalogue (C numbers) - Compiled by Sir Patrick Caldwell-Moore as a catalogue of some of the night sky's best objects for amateur astronomers to observe;
• New General Catalogue (NGC numbers) - The New General Catalogue is a catalogue of Nebulae and star clusters compiled by John Louis Emil Dreyer in 1888.
• Barnard Catalogue (B numbers) - compiled by the astronomer Edward Emerson Barnard in the early 20th century contains a list of 'Dark Markings in the Sky'.
Most GOTO telescopes will have the first three catalogues programmed into their hand controllers.
Below is a list of some suggested DSOs for beginners broken down into three main groups that can be viewed: all year round; during the Winter months; and during the Summer months. Each group is then further broken down for viewing either using naked eye, bins or telescope. The best seasons to observe DSOs are late Autumn, Winter and early Spring as the sky is actually darker at night than during the Summer months which makes the fainter DSOs easier to see during this time of year. This is caused by the tilt of the Earth's axis pointing the northern hemisphere towards the Sun in Summer and away from the Sun in Winter. The Sun therefore drops further below the horizon during Winter months, resulting in darker skies. Obviously some of the DSOs can only be viewed during the Summer months due to the Earth's annual orbit around the Sun, so observing them, is best done near midnight (GMT) as they cross the Meridian. Suggested best observing months are enclosed in [square brackets] but do not have to be stuck to rigidly.
Suggested All Year Round DSOs
The DSOs in this section are circumpolar from northern latitudes of 50 degrees or more. This means that they can be viewed at any time of year but will be found in a different place each month as the Earth progresses around its orbit of the Sun. These give you the chance of getting started straight away with Deep Sky Observing, no matter what the season!
Naked Eye:
Double Cluster (NGC869 and NGC884) - the double cluster is only just visible as a hazy patch to the naked eye and is a good target to practice averted vision on. It lies roughly half way between Perseus and Cassiopeia. [circumpolar]
Binoculars:
Double Cluster (NGC869 and NGC884) - looks its spectacular best viewed through binoculars or a short focal length telescope. Looking through binoculars NGC869 looks like a necklace with glittery pendant hanging down into NGC884 (depends on orientation of the pair when viewed). [circumpolar]
Bodes Galaxy M81 - in Ursa Major can just about be spotted with binoculars and has two very helpful pointer stars in the bowl of the Plough to guide you on your way. Draw a line from Phad diagonally across the bowl to Dubhe and carry on for about the same distance again. Once you have spotted M81 you will find that there is another galaxy M82 very close by. [circumpolar]
Telescope:
ET Cluster (C13 or NGC457) - this cluster of stars lies just under Cassiopeia and is so named as it looks like the guy out of Steven Spielberg's film ET. As Cassiopeia is circumpolar, ET can be viewed at any time of the year. Consequently sometimes ET will appear upside down and sometimes the right way up. The two bright stars at one end of the cluster as supposed to be ET's eyes and the lines of fainter stars to each side are his arms! [circumpolar]
Bodes Galaxy M81 & M82 - Bodes Galaxy, also known as NGC3031 is relatively close to Earth at 12m lys which together with its Super Massive Black Hole producing an active galactic nucleus, makes it a relatively bright DSO for beginners. The nextdoor galaxy M82 (NGC3034) is a starburst galaxy and was the location of a Type 1A supernova on 21st January 2014. This galaxy is many times brighter than the centre of our own Milky Way due to all the star formation taking place. [circumpolar]
Whirlpool Galaxy M51 - One of the brighter face on spiral galaxies that appears to be interacting with a nearby neighbour, producing a bridge of stars between the two. On a dark clear night some of the spiral structure can be seen through 8inch telescopes. [circumpolar]
Suggested Late Autumn - Winter - Spring DSO observing
Naked Eye:
Mira the Wonderful - is an intrinsically variable star, one where the star's actual light output varies. It was the first to be discovered of a special type of variable star now known as Mira Variables. Mira varies in intensity from its brightest of around magnitude 2 right down to magnitude 10 with a total period of 332 days. This means that at maximum brightness it is one of the brightest stars in the constellation of Cetus but at minimum it is totally invisible to the naked eye. Mira's next maxima is predicted to occur between 21-31 May 2015 when unfortunately it won't be in the night-time sky. [Mid-October]
Algol or Demon Star - the second brightest star in Perseus is in fact an eclipsing binary star with a dimmer star and a brighter star orbiting each other. The pair are aligned such that as they orbit, they eclipse each other as observed from Earth. This results in the apparent brightness of the star system varying with a marked dip in brightness once every 2 days, 20 hours, and 49 minutes with the eclipse lasting around 10 hours. At its brightest, Algol shines about three times more brightly than at its faintest. Check tables for the timing of this event as you will see it get fainter in comparison to other nearby stars and then brighten again in the course of a full night's winter observing. [November]
The Andromeda Galaxy (M31) - this object is actually the most distant object visible with the naked eye at a distance of 2.5m lys. What you see as a faint patch in the sky, using averted vison, is the centre of our closest neighbour spiral galaxy. If it was bright enough to see the whole galaxy right out to its edges, it would actually appear 6 times the diameter of the Full Moon. [November]
Great Orion Nebula (M42) - this is a large molecular gas cloud that is giving birth to new stars. It is at a distance of 1,300m lys and actually looks like a fuzzy star to the naked eye, appearing as the central star in Orion's sword. [December]
The Pleiades or Seven Sisters (M45) - is a cluster of middle-aged hot bright stars surrounded by very faint nebulosity which is located in the constellation of Taurus. They are clearly discernible on most clear winter nights when it is normal to be able to see seven stars forming a small saucepan shape. [December]
Beehive Cluster or Praesepe (M44) - lies in the constellation of Cancer the crab which is the dimmest of all zodiacal constellations. It can be found just to the north-west of the central star of Cancer but it is normally only just visible and is another good target to try out the averted vision technique on. [Feb]
Coma Berenices - this constellation is supposed to be a truss of hair from Queen Berenices that she dedicated to Aphrodite for her husband's safe return from Syria. It is a faint constellation with its three brightest stars forming a right-angle with fainter stars near 5th magnitude filling the right-angle. On nights of exceptional sky clarity these dimmer stars, the strands of hair, can be seen twinkling. [April]
Binoculars:
Great Orion Nebula (M42) - training binoculars on M42 will start to reveal that this middle 'star' in Orion's sword is much more than appears to the naked eye. [Nov]
Open Star Cluster (M35) - lies just off the tip of the foot of Castor, one of the Gemini twins. This is a pretty compact open cluster which is just visible through binoculars. [Dec]
Pleiades (M45) - all the naked eye objects will appear in more detail in binoculars. However, the Pleiades are at their most spectacular through binoculars even more so than through a larger telescope. [Dec]
Beehive Cluster or Praesepe (M44) - when viewed through binoculars it is easy to see where it got the name Beehive Cluster. The stars in this open star cluster actually appear to be moving like a swarm of bees buzzing around their beehive. [Feb]
Telescopes:
Almaach (Gamma Andromeda) - is the third brightest star in the constellation of Andromeda and the furthest East. It is a stunning coloured binary pair with one brighter golden yellow star next to a blue coloured fainter star. This is the Winter's equivalent of Albireo in Cygnus and some say that Almaach is actually brighter and prettier. [Nov]
Great Orion Nebula (M42) - the jewel in the winter sky lies in Orion. It will appear as a great big swirl of gas and dust in most telescopes. Larger aperture, longer focal length scopes will reveal hidden detail in the centre of the nebula, showing the Trapezium of hot young stars that were only formed about 300,000 years ago. On good clear nights the gas cloud can be seen sweeping around the Trapezium and looking quite like an angel fish. Some people claim to be able to make out a slight colour in the nebula in larger amateur scopes. [Dec]
The Crab Nebula (M1) - lies between the tips of the horns of Taurus and is the remnant of a supernova that was chronicled in 1054 by Chinese observers as the light from the titanic explosion first reached the Earth. M1 is the first entry in Messier's catalogue but it is one of the faintest and hardest to see. In scopes under 8inch aperture in semi-urban skies it is hardly visible. This is where an accurately aligned GOTO telescope can save hours of searching. Even then all of the tricks of the DSO observer's trade will be needed to see much. Larger scopes will reveal more detail but even with a 14inch scope, M1 will still appear very dim in all but the darkest of skies. [Dec]
Horsehead Nebula (B33) - this is an example of a dark nebula of gas and dust which lies in the line of sight of a bright emission nebula in Orion. It is so called as it looks like the head of a seahorse. The Horsehead is located just below the star Alnitak, which is at the East end of Orion's Belt. This can be very tricky to see by eye at a telescope and can require special filters applied to the eyepiece to make it stand out but long exposure photographs under clear skies can really show it up well, as shown below. [Dec]
Flame Nebula (NGC2024) - this nebula lies just to the East end of Orion's belt and can be seen together with the Horsehead in the image below. The Flame is an example of an emission nebula of hydrogen and dust that is being ionised by the intense ultra-violet radiation from Alnitak. This energy then gets re-radiated as light in the red end of the spectrum as the ionised hydrogen gas recombines with the liberated electrons. [Dec]
C30 or NGC7331 - is a relatively bright spiral galaxy in Pegasus in which the core of the galaxy appears to be rotating in the opposite direction to the disk although this can't be discerned visually. (Sept)
Eskimo or Clownface Nebula (C39 & NGC2392) - this is a planetary nebula in Gemini, which appears quite small but which has a striking resemblance to an eskimo's face with a parka hood around it. Due to its small size this is best observed when the seeing is good and steady so that a high magnification of up to two times the aperture in millimetres can be used. [Jan]
Leo Triplet (M65, M66 & NGC3628) - a cluster of three galaxies just below the star Chertan in the body of Leo. This triplet of galaxies can just be fitted into a single 1 degree field of view which can be achieved with a widefield eyepiece at a magnification of 90. M65 and M66 are easily seen but NGC3628 is fainter and is separated by about twice the distance, lying to the north of M65 and M66. [March]
Suggested Late Spring - Summer - Early Autumn DSO Observing
Naked Eye
Delphinus - a small dimmer constellation of 3rd magnitude stars does actually look like its namesake, a leaping dolphin. [Aug]
Double Double or Episilon Lyra - this is a pair of double stars orbiting a common centre of gravity in Lyra the two pairs can just about be split by the naked eye in very dark sky sites under extremely good seeing (apparently!). [Aug]
Milky Way - the band of faintly glowing light arching across the night sky is most easily seen during the Summer months from the UK. It is brightest as it passes through Cygnus on down into Sagittarius which is where the centre of our galaxy lies but much further away than the stars of Sagittarius. As the Milky Way passes through Cygnus it appears to split into two (The Cygnus Rift) which is actually only a visual result of interstellar dust and gas blocking our view of it. [Aug-Sept]
Binoculars
Milky Way - by sweeping across the band of the Milky Way with binoculars it becomes obvious that the white band across the sky, is caused by the light of millions upon millions of stars that are so dense and so far away that the unaided eye can't resolve them as individual stars. Using binoculars allows us to start to resolve them and as you sweep the binoculars across the band you see the field of view start to become completely filled by tiny pinpricks of light from the stars that lie in the disk of our own galaxy. [Aug-Sept]
Globular Cluster (M13 or NGC6205) - is just to the west of the line between the stars eta and zeta Herculis that make up the right side of the Keystone asterism. It lies about 1/3rd the way down from eta to zeta and is considered the best of all globular clusters. In binoculars it can be seen as a misty, fuzzy snowball, just like a comet which is why Messier catalogued it as something to be avoided. [June]
Double Double or Episilon Lyra - the widest pair of this four star system can be easily split using binoculars [Aug]
Albireo or Beta Cygni - is a beautiful double star which forms the beak of the swan Cygnus. It comprises a brighter yellow star contrasting against its slightly fainter blue companion. Albireo is also interesting for the fact that it is not the second brightest star in Cygnus (as would be assumed from its Beta classification) but is in fact only the fifth brightest. [Aug]
Coathanger Asterism - this asterism lies in Vulpecula. Once found it will never be forgotten as it fits within the field of view of a pair of 10 x 50 binoculars and looks exactly like an upside down coathanger. [Aug]
Globular Cluster (M15 or NGC7078) - is to the north-west of the star Enif which is the nostril of the upside-down winged horse Pegasus. It appears as a fuzzy snowball in binoculars. [Sept]
Telescopes
Whale Galaxy (C32 or NGC4631) - this galaxy has a large angular size and contains a central starburst, which is a region of intense star formation. [May]
Ring Nebula (M57) - is a bright planetary nebula that looks like a smoke ring or doughnut set against the blackness of space. Some say that they can see the central white dwarf star that is all that remains of the original star that puffed off its outer layers to form the Ring. This is an excellent target for beginners to image as it is relatively bright and shows beautiful colouring with a central blue region ending at the red and orange outer ring. Such photos definitely reveal the central white dwarf. [Jul]
Double Double or Epsilon Lyrae - splitting all four stars in this double, double star system is considered a good test of both the seeing conditions and the telescope's resolving power. They can be split using a 5inch telescope and show up as two sets of binary stars with the line between the two binary pairs lying at right angles to each other. Amazing when first observed and understood that each close binary pair orbit around themselves and then both binary pairs orbit around a common centre of mass. [Jul]
Albireo or Beta Cygni - is really a beautiful double star when viewed through a telescope. The two stars are bright yellow and blue which make a striking contrast. [Aug]
Dumbbell (M27) - is another planetary nebula which is larger and fainter than the Ring but can still be seen through medium sized telescopes. Some say that it looks more like an apple core than a dumbbell. [Aug]
Globular Cluster (M13 or NGC6205) - This globular cluster consists of over 300,000 stars closely bound together by their own mutual gravity. Moderate sized telescopes reveal a dense ball of stars that on good nights can be resolved almost to the centre. [Aug]
The above can only ever be an introduction to Deep Sky Observing and DSOs. Every astronomer has a different list of favourite DSOs and as you get more practiced you will develop a list of your own which will probably include one of the very first DSOs you ever observed. It is hoped that the above list will give you some ideas and the enthusiasm to get out under the night sky and enjoy Deep Sky Observing, it can be challenging but also very rewarding!
Recommended Further References
With all DSOs, knowing what you are looking at, adds to the enjoyment of the observing. However, try to observe the object first before looking at high resolution photos taken by the likes of the Hubble Space Telescope so as not to 'see' things that you can't possibly be seeing through your own equipment. Then read up about the object you have just observed from reference books and also Wikipedia but remember to always double check the facts from at least two different sources as they can sometimes be wrong!
Deep-Sky Companions: The Caldwell Objects and The Messier Objects by Stephen O'Meara. By using sketches, the author actually attempts to portray how these DSOs will look in amateur telescopes, albeit under pristine skies. These are good books to have with you at the telescope.
Collins Stars & Planets by Ian Ridpath and Wil Tirion is a guide to stars, planets, galaxies and the Solar System. It includes all 88 modern constellations including those in the southern hemisphere.
One very good piece of free software is SkyTools-3 Starter Edition. This software will generate an observing list showing which Messier objects are best visible that night and takes into account the phase of the Moon etc. It will also rank the objects from easy to hard, through both binoculars and small telescopes. Purchase an upgrade to get many more object catalogues and larger telescopes included.
Article by Ian Hargraves
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