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Planetary Nebulae: a little guide


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It's a measure of the quality of your posts that you were missed. Many of us could disappear for months and no one would notice! I too hope that everything is settled for you now and look forward to more valuable contributions from you in the future.

The planetary nebulae post is terrific. Not just about PNs but lots of good tips about observing in general. Have you ever considered collecting such articles into a book?



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Just came across this .. ehm thread? article? on planetary nebulae!

What can I say.. it is a fantastic work!!  :smiley:  :smiley:  :smiley:

A big congratulation to Rob for collecting all this information together!

It is very nice to have a place like SGL to share and read all this information!

This astro-community is great!  :smiley:  :smiley:  :smiley:

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Just came across this .. ehm thread? article? on planetary nebulae!

What can I say.. it is a fantastic work!!  :smiley:  :smiley:  :smiley:

A big congratulation to Rob for collecting all this information together!

It is very nice to have a place like SGL to share and read all this information!

This astro-community is great!  :smiley:  :smiley:  :smiley:

Well said ! 

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Another top class guide Rob, thanks for making the effort once again.

I'm sorry to hear about your problems (I haven't been around much myself so I wasn't aware before now) but I hope things are gettimg back to normal for you.

Good job! :D

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On 09/09/2014 at 04:17, Qualia said:




Chapter 2: Observing Practices

When looking at objects on Earth, a microscope or telescope’s main function is to magnify the object. When stepping from Earth into the Solar System, a telescope helps collect light and magnify the distant detail on the Moon and planets. But generally speaking, a telescope looking at deep space objects such as planetary nebulae has a different role to play.

Reaching out to planetary nebulae, hundreds, if not thousands of light years away, the principle function of a telescope isn’t to magnify the object but to collect light from it. When peering into deep space, you are not using a telescope to see objects because they are too small, although they might be, but rather because they are so dim, you need light.   

As a species, we have evolved to see things under the bright light of the Sun and when there is no Sun, we have developed technologies to take its place. To view objects in the dark with a telescope requires a different game plan and a whole range of new techniques to be learnt. This chapter will try to share some insights into those techniques which will probably already be known, but once again, worth going over.

Light Pollution

The single worst enemy for stargazing is light pollution. It wraps itself around the skies and the delicate diamond glint from stars and the luminous glow from deep sky objects are smothered and strangled. A small telescope in the countryside will show planetary nebulae better than a light bucket in the city. In effect, darker the night skies, brighter the deep sky objects

Most of us have to deal with light pollution in some manner or other, and it is possible to do a fair bit of observing even under city skies. The planet and Moon and double stars are not affected and with diligence, you could probably work through most of Messier’s list. With this said, if you’re into your deep sky objects, I feel it is still necessary to plan a number of dark-sky trips during the year. There is nothing quite like a peaceful and clear night where the stars are like fists of silver dust pitched in the wind and those hidden cosmic clouds can at last reveal their mysteries. 

If it isn’t possible to visit dark sites from time to time, don’t beat yourself up about it, but you are robbing yourself of detail and beauty that might have been possible. However, observing in light polluted areas can be extremely useful and shouldn't be used as an excuse not to observe. Not only do you continue to hone your star-hoping skills but you can still make great observations and finds and later compare them with your dark site ones. This is an interesting project in itself, for you'll see the effects LP has on your eye and the given object.

Whether in the city, suburbs or countryside, you will find that the observer’s zenith is where the sky will be darker, so try to plan your sessions accordingly. It’s also a bit of a truism that when folk stop hustling and bustling and start going to bed, light pollution tends to improve. It might also be an idea to note the afternoon skies an hour or two before sunset. The deeper blue they appear, better the chances of a decent dark night. Needless to say, the delightful sight of strong Moonlight is a natural light pollution and nature’s way of telling us to stick to observing double stars, planets and the Moon itself.

Dark Adaptation

It was briefly alluded to above with the mention of red torches and pirate’s eye patches, but you’ll also need to think about adapting your eyes to the dark. The human eye takes time to adjust. After half an hour or so, you will start to notice a significant difference, after an hour, you should be pretty much fully adapted.

If you have planned an evening of planetary nebulae observing, or other deep space objects, try not to observe any bright celestial object such as the Moon or planets with your working eye and throughout the session, it is probably a good idea to keep it covered when not observing. Hence the usefulness of an eye patch. If you’re waiting for the telescope to cool, start out with open clusters, star fields, perhaps some doubles. After half an hour or so, you can start the deep sky observing session with confidence.

Averted Vision

As curious as it may sound, looking directly at planetary nebulae or other deep space objects might not be the best method for observing them. When you look directly at something, the image falls on your retina's fovea at the centre of your eye and is extremely good for picking out detail with the aid of good illumination but is relatively blind to dim light. If you look directly at dim planetary nebulae, for example, they may just disappear. As such, try to train yourself to not look directly at the object. By doing so, you are effectively moving the image away from the fovea and onto parts of the retina more sensitive to black, white and grey.

The technique doesn’t take long to master, but will probably seem odd at first. We have evolved to look at things directly, so averted vision is a little counter intuitive. Practice by centering a dim star or DSO in the centre of the eyepiece’s field of view and concentrate your attention on an area just a little off to one side or above. Alternatively, place the object a little to the side or below the centre of vision. Either method should work, but finding your sweet spot will be a matter of trial and error.

Don’t Squint, Jiggle and Breath

Again, try not to squint when observing celestial objects, for by doing so you are not only straining your working eye which can lead to fatigue but also limiting your powers to detect faint objects and tweak detail from them. The solution is to observe with both eyes open, and if it helps, to either cup the non-working eye with the palm of your hand or to use a modified eye patch.

There may also be occasions where you are certain you have the specific area where you think the planetary nebulae or deep space object resides but it appears to be lurking under the limit of visibility. If this is the case, tap the telescope or eyepiece just a little to make the field of view jiggle. It’s not guaranteed but you might find the object revealing itself.

Again, when you are concentrating hard you might find yourself holding your breath without realizing it. Limiting the oxygen to your brain, even for just a few seconds, compromises night vision. So while observing its good practice to breathe steadily and deeply but in a calm and relaxed fashion.

It’s also worth pointing out that night vision is impaired by alcohol, nicotine, and low blood sugar, so try not to smoke or drink too much or go hungry. If you do smoke, when you light up make sure your working eye is completely covered and well protected. You might find you’ll need more than an eye patch to prevent night vision from being damaged by the lighter’s sudden flare up and glare.

Aperture and High Power

When it comes to viewing planetary nebulae and other deep sky objects, aperture rules. As explained in the introduction to this section, the more light gathering capacity your telescope has, the more you are likely to see. Obviously big telescopes have their own disadvantages – transportability, convenience and ease of set up, cool down time, collimation – to name a few, but if you do have a number of scopes, use the biggest one you have for hunting out planetary nebulae.

You’ll recall that in the chapter on possible gear, I mentioned a low power eyepiece (EP) which would be your working horse EP for hunting out planetary nebulae. This advice is based not only on personal experience but also from the fact that lower powers tend to concentrate the object’s light into a smaller area and thus increasing its apparent surface brightness. I feel this advice is sound, but high powers must also be used when it comes to viewing planetary nebulae and other deep space objects in general. As such, try a wide range of powers on any object and don’t limit yourself, for the best magnification is the one that shows you the most amount of detail.

Sketching and Log Book

There’s no correct way for enjoying a stargazing session. Some nights it's a good idea to just run around the heavens, other times to pick an object and tweak as much detail as you can until boredom sets in; there is no right or wrong way.

 But if you want to keep a log book or try your hand at sketching, it is better to be a visitor, rather than a tourist. Many people will go to a museum, for example, and will rush all the paintings, but at the end of the day, they will only remember one or two of them at best and not that well either and by analogy, the same can be used when it comes to stargazing.

As said, there is no right or wrong way to plan your sessions and the following procedure is certainly not one I'd recommend all the time, but I do think it is important to slow down from time to time. I appreciate that this slowing-down exercise can be frustrating at times. In my case, a three hour dark site session involves an operation of around 6 to 7 hours. After all that preparation, packing gear, travelling, setting up and so on, when I get out to the dark site my first instinct is to start buzzing all over the skies, but if a sketch is planned or you really want to get down to details, at sometime or other you will need to give yourself time. 

There are two essential features to visual astronomy. The first is to find the object and the second is to observe it. The former process involves star-hopping and reading star maps, the latter requires you to slow down and to engage yourself with the complexity and beauty of what is being observed. It's been said many times before but anything glanced at will always look like a featureless something or other but the trick is to go beyond this style of looking and practice picking out features and textures.

I find the best way of observing planetary nebulae is to place it in a low power EP and to begin asking some basic questions about it. These questions could include stuff like:

What am I looking at?

What have I read or seen of this object that can inform my observation?

What shape does it seem to have?

How many stars can I see in my field of view? What colour are they? Are there differing intensities of brightness between them?

How does the object's appearance change as I flip from direct to averted vision and back again?

Can the white dwarf be resolved in the object itself?

When I move away from the eyepiece to relax my eye, what can I recall about this object that can inform me further? Was there anything distinct about it that struck me?

I'll then up the magnification again, to something around 90x. And ask a few more questions. Stuff like:

How has the image changed?

How many stars have now been cut away?

Has anything in the image become dimmer or lighter?

Has the object itself changed in any fashion?

Are there any new patterns, shapes or colours to be seen within it?

Are given areas of my new field of view more pronounced than others?

If I close and relax my eye away from the eyepiece can I picture the object there 'within'?

I'll then up the magnification to the optimum power, that which gives the sharpest and clearest image at the highest magnification possible and go through the same questions as above.

In all cases the trick is to ask yourself questions about the object; to not only appreciate the subtle detail and complexity therein but also to have a general picture of how the object is framed within your eyepieces and mind.

If you are going to use a filter which will often be necessary for planetary nebulae, you should go through the same questioning and magnification process as above. The object will look different and you may prefer the look with or without the filter but it's nice to understand why this is so and to appreciate any improvements in the image but also any detrimental affects a filter may have.

Now, all this may look a little long winded and perhaps it is, but I feel it'll pay out dividends. With practice, depending on the complexity of the planetary, such an interrogation shouldn't take more than 15 minutes or so. The questioning process can be exhausting, so I usually dedicate it to one or two objects at most and not on every session. It is also extremely important to have sessions where we just sit back and drink in the beauty around us without eyepiece and telescope and without thought or mind, or to plan other projects like splitting doubles, viewing planets, completing lists, and so on.

If you are going to sketch or record your observation, you are now armed with a better understanding and visual appreciation of the celestial object.

If you’re keeping a log book, try to include the following information:


Date and Time


Observing Conditions (seeing, transparency, temperature, wind etc)

Moon Status (rise, setting time, phase)

Telescope Used (focal ratio and aperture)

Eyepiece (magnification, field of view)


Detailed description of the object

If you want sketch the object, draw yourself a decent sized circle and start by drawing the field stars paying attention to their spacing relative to each other and their relative brightness. It isn't necessary to draw all the stars you see but the ones you do, ought to serve as a guide for the sketch of the object itself.

Sketching is an iterative, mechanical process: you look through the eyepiece, you sketch a little something, you compare, you look again, sketch a little more, compare, and on and on you go. If you find you are getting bored, relax, take a break, and when you are ready, return again.

The point of sketching is not to create some beautiful rendering of the object itself, but rather to train your eye to see better. You keep going back and forth from eyepiece to paper until you feel you have either had enough for the night or that your drawing contains most of the details you have seen. Whatever the tools or techniques used, when sketching be sure to use a dim red light and try to be as comfortable as possible.

Observer’s Morphology

As we have seen, it is widely accepted that the various shell structures of PNs can be explained by the interaction of a fast wind from the central white dwarf and the remnant of a slower wind from the AGB stage of the dying star. Whether these various PN shells are examples of the very same structure viewed from different angles, or are cases of the evolutionary stage of PNs, is still open to debate.

By way of illustration, we can note that the Ring Nebula, M57 and the Helix Nebulae, NGC 7293 are both understood as being seen from where we are looking down onto their poles and as such, the two hemispheres merge to make a ring shape. If seen side on or from the equator, we would see an hourglass shape. One side of the debate contends that this is because all planetary nebulae are formed in like manner and have essentially the same structure, whilst the other side argues that this given stage is part of the evolutionary process of the PN as it slowly disappears into space.

Regardless of the outcome, for the visual observer, PNs are some of the most interesting and beautiful objects in the night sky. A genre full of variation and complex shapes, where morphological classification based on descriptive terms will hold sway based on the limitations of the telescope and optical gear and the conditions under which the observer is viewing.

Although it is certainly not necessary to use, perhaps the most common classification system used by the visual observer is the Vorontsov-Velyaminov Scale of Planetary Nebulae, first detailed in 1934. His categories are:

  1. Stellar image (like a star)
  2. Smooth disk (or regular disk) (a) brighter toward center, b ) uniform brightness c) traces of ring structure
  3. Irregular disk (shape not entirely circular) (a) with irregular brightness distribution (varying light and dark areas) b ) with traces of ring structure
  4. Ring structure
  5. Similar to a diffuse nebula
  6. Anomalous form (abnormal form without a regular structure, perhaps like a S or 8)

More complex PNs are characterised by combinations of classes. Accordingly, a PN like M 27 could be described as 3a+4; M 57 would be 3+4, M 76 something like 3+6 and M 97 3a.

Final Thoughts

When planning a session it is a good idea to view sketches by other observers. These are too often overlooked, but they ought to be viewed from time to time. They are typically produced by patient and attentive observers and their drawings should give you an idea of what the object will more or less look like if you were to use similar aperture and magnification

There are so many good books about it's hard to pick any one of them and say, this is the best. There are those which give context and depth to what is being viewed, others a more practical working guide. I feel this post will give you sufficient grounding in planetary nebulae and with the power of the internet and forums like S.G.L, you should be able to hone your enquiry if the need arises.

There are some little jumping tricks you can learn to find yourself about the night sky. For example, find the plough in Ursa Major and look for Merek and Dubhe, the distance and angle between these two is one step. Now count that distance, in that direction another 5 steps and bingo, you'll be with Polaris. Now go back to the Plough and find its end star, Alkaid. Take a jump and dive from it and the next brightest star will be Arcturus - the arc to Arcturus. Learning the big stars and diving quickly between them makes hunting stuff easier.

Without doubt, one of the best guides you’ll find on star-hopping is the guide written Shane Farrell and can be found in the observing section to S.G.L.

My final thought is that if you can master patience you'll be master of yourself and the night sky is a good teacher. She'll teach patience and careful watchfulness; she'll teach industry and care and above all, the night sky teaches trust. Those stars and DSOs are not going anywhere quick. They won't desert you and they're not trying to deceive you. If you don't succeed one night, or you can’t go out for weeks on end due to personal compromises or those set up by clouds and rain, don't be down hearted. In most cases, you've probably discovered something new about yourself and those stars and DSOs will be back to give you another chance, another day.

Stargazing can be a tiresome road and one can suffer for it and be grieved, but the worst you can do is add to this frustration and curse those things beyond your control. Cloudy or uneventful evenings are just that, nothing more and when older they will appear as a singular, non-descript events, yet shining from them like a host of gleaming stars will be those evenings where everything just seemed perfect and the universe at last could murmur to us its secrets.

Useful Bits

Carbon Stars: http://www.astroleague.org/files/obsclubs/CarbonStar/CarbonStar-List.pdf

Planetary Nebulae: http://www.webbdeepsky.com/downloads/The%20brightest%20planetary%20nebulae%20%28white%29%202nd%20ed.pdf (thank you to Mark at Beaufort for pointing this out).

Some of my favourites: M27, M 76, NGC 1535, NGC 2392, NGC 2438, NGC 3242, NGC 6543, NGC 6905, NGC 6826, NGC 7009, NGC 7662.

References Include: Planetary Nebulae, Martin Griffiths. Stardust, John Gribbin. The Hundred Greatest Stars, James Kaler.

I have only just found this strangely enough qualia seeing as I thought I had covered every part of the forums, however and as always reading your notes, thoughts and comments leave my stargazing juices a flowing. The summation of all methodology used when viewing and ways to improve are spot on and I really hope more people take time out to read your stargazing eulogies.

I was discussing PN's a few weeks back with John and some of what you say in here was discussed, some of the other points I have read elsewhere and it is all spot on although I  have never read such an in depth guide before. Well worth the read and I salute you my friend. It was I believe how they say, magnifique! :icon_salut:

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What I find fascinating while reading this article, which by the way is of professional quality, is your dedication to making us understand such a fascinating subject.

very pleasant to read, easy to understand.
I only have congratulations to give you.
it is members like you who make you want to come back to this site.
well done and thank you very much.

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  • 11 months later...

What a fabulous post from Qualia on Planetary Nebula.

I never cease to be surprised and amazed when searching through the archives of this site.

What an absolutely outstanding resource SGL is and its members, past and present are. 👍


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