October 14, 2017: Spotting Diffuse Objects With Dark Adaptation

One of the problems you might encounter when observing some nebulae and galaxies is that the light that they’re emitting is spread out over a large area – huge, sometimes.  This makes them difficult to find and see.

The Triangulum Galaxy, M33, is a perfect example of this.  Nowadays, it’s risen about 30 degrees up from the horizon at 8pm, 40 degrees by 9pm, just on the other side of Andromeda from, well, Andromeda – M31.  M33 is listed at 5.7 magnitude, making it by far one of the brighter galaxies around.  Should be super easy to find, right? But through a telescope, it’s barely there.  Even at the DAS dark site with my 5-inch Mak, there’s just not much to see.  And more to the point, it’s just very difficult to see through a telescope in the first place.

See Explanation.  Clicking on the picture will download the highest resolution version available.
The Triangulum Galaxy, M33.  Note how the core isn’t much brighter than the rest of the galaxy.  (Image Credit & CopyrightRobert GendlerSubaru Telescope (NAOJ))

Why should this be?  It’s because Triangulum is huge.  It’s 70 minutes long by 40 minutes wide – almost a full square degree.  All of that light, all of that 5.7 magnitude, is spread out over that enormous area. And, unlike Andromeda, M33 doesn’t really have that much of a bright core, so there’s nothing that really stands out about it as opposed to the background.


Integrated Magnitude

The 5.7 magnitude that M33 is listed at is what’s called “integrated magnitude“.  Integrated magnitude is the magnitude an object would be if you shrank all the light that an extended object is emitting down to a single point source – exactly like a star.  “Extended object” means anything that isn’t a star – anything that’s larger than a single point source of light, which includes, of course, nebulae, galaxies, and clusters.

To better understand what this means for yourself, you can do the reverse.  The next time you’re out observing, see if you can find a fifth or sixth magnitude star in an otherwise relatively empty patch of sky. Using a low-powered eyepiece, defocus the image, and keep on defocusing it so that the star, or what’s left of it, fills up the entire field of view of your eyepiece. Then move the telescope a little. You won’t notice much difference between when you were looking at the defocused star vs. looking at empty space. That’s why M33 is so hard to see. And that’s also why most diffuse nebula, and some dim galaxies, spread out over a huge area, are really difficult to spot.

Then there’s the Veil Nebula in Cygnus.  Talk about gargantuan!  It’s divided into two parts, the East Veil and the West Veil, each of which is listed at magnitude 7.0.  The entire complex is listed as being about 3 1/2 by 2 1/2 degrees. although the brighter areas are significantly smaller than that, about  65-70 minutes by 5-10 minutes.

See Explanation.  Clicking on the picture will download the highest resolution version available.
The Veil Nebula in Cygnus.  The East Veil (NGC 6992 and NGC 6995) is on top; the West Veil is at the bottom (NGC 6960).   The dimmer segment in between them is called Pickering’s Triangle (NGC 6970).  Credit & CopyrightMartin Pugh

The Veil Nebula is a supernova remant, where the shockwaves expanding outward from the supernova have collided with interstellar gas, causing them to glow.  An O-III filter is generally needed to see it visually, to increase contrast, as it glows mainly in that wavelength.

Last year, I was able to see the Veil gloriously through a 15-inch Obsession dob equipped with an O-III filter at the DAS dark site.  Some have claimed to have been able to see it just by holding an O-III filter to their eye under truly dark skies, but it is generally considered to be very difficult to see – an 8-inch scope is needed, under those same truly dark skies, to be able to see it well.

Additional examples of this are the Heart and Soul Nebulas.  These are a pair of diffuse nebulae located in Cassiopeia, right next to each other – IC 1805 and IC 1848.  They’re both listed as being magnitude 6.5, which, again, is fairly bright.  But they are huuuuuuge.  The Heart is 150 x 150 minutes; the Soul is 150 x 75.  In other words, the Soul Nebula is roughly the same angular size as the Andromeda galaxy, and the Heart is about twice that size.  But where M31 is magnitude 3.4, these two are both fully three magnitudes dimmer.  This means that, again, spreading that light out over all that amount of sky, they’re both significantly dimmer visually.

Heart (top) and Soul (bottom) Nebulae on the left; that’s the Double Cluster on the right.  From side-to-side, the image spans 10 degrees.  (Image Credit & CopyrightAdrien Klamerius)


Surface Brightness

Another way of expressing this relationship between an object’s angular size on the sky and its brightness is a concept called “surface brightness“.   This is an answer to the question, “How bright is the object per square unit of surface area?”  The area used is one square arc minute – a box measuring an arc minute, one-sixtieth of a degree, on a side, and is also expressed in magnitude.

The best example of this is the Ring Nebula, M57.  The Ring is easy to find, located midway between the two bottom stars of Lyra’s parallelogram, Sheliak and Sulafat (I love those names!):

The Ring is a planetary nebula.  It shows us what the death of a star like our sun will look like – in about 5 billion years.  At that time, the sun will expand beyond the orbit of the earth, incinerating it, not to mention Venus and Mercury.  When the sun is no longer able to continue thermonuclear fusion in its core, the outer layers of gas will expand and float away, leaving a white dwarf at the center.

See Explanation.  Clicking on the picture will download  the highest resolution version available.
The Ring Nebula, M57.  The white dwarf just to the left of center is all that remains of the originally sun-sized star that died to give birth to the nebula.  (Hubble photo.)

But, in contrast to the sizes of the Heart and Soul Nebula, M31, and even M33, which are on the order of 2 degrees wide and tall, the Ring is positively TINY.   The distance between Sheliak and Sulafat itself is two degrees.  The Heart or Soul Nebulae wouldn’t even fit in between them.  At the other extreme, the Ring is just over one arc minute across in each direction.  That’s just one-sixtieth of a degree.

But even though the Ring Nebula is magnitude 8.8, it is easily visible in even small scopes, like mine, where the other objects in this post, supposedly much brighter, well, just aren’t.  Surface brightness is measured in magnitude per square arc minute.  Since the Ring is just over a square arc minute each way, it’s less than 2 square arc minutes total, and so its magnitude – and brightness – holds up, making it easy to spot.

On the other end of the size spectrum, the Heart Nebula is about 22,500 square arc minutes in total area.  Spreading out the magnitude 6.5 light over this much area reduces its surface magnitude way down to 15th magnitude, beyond the reach of most scopes under 6 inches aperture, unless observing under truly dark skies.  M33 clocks in at 14th magnitude, making it a tough catch; while M31 is at 13th magnitude, making it that much easier – although that figure averages out the light from the bright core over the entire galaxy as well.  I don’t have a surface brightness for the core itself, but it is far brighter than the spiral arms, and much easier to see, even naked eye.

Interestingly, even though it is much dimmer than M31 itself, M31’s satellite galaxy, M32, is easily visible as well.  It has an integrated magnitude of 8.0, but since it’s so tiny, it’s surface brightness is 12th magnitude – even a little easier to see than M31 itself.


Observing extended objects

Objects like these, tough to spot in the first place, are even tougher to spot in even modest light pollution.  This is because the background sky itself is brighter.  The dim glow from light pollution overwhelms the glow of the diffuse object, almost matching it if your light pollution is bad enough.  Well, not really matching it, but the light pollution can get so bright that your eye will have an extremely difficult time distinguishing between the object and the surrounding skyglow.  You need really, REALLY dark skies to see objects like these. Or you can get a ginormous telescope to collect more light and distinguish the object from the background better. Or, ideally, both.

Large, diffuse objects like these can still be seen from light polluted areas.  They’re best seen with low-powered binoculars, so that the light-emitting area of the object remains small and concentrated – and therefore, easier to spot against the dark sky background.  M31 – Andromeda – is glorious in binoculars, or in a very low-powered, widefield scope, like my Orion ST-80.  But in my 5-inch Mak at 48x with a 32mm Plossl, for example, it’s just a bright core completely surrounded by gray fuzz that eventually melds with the background.

As I’ve written in the past, other extended but even dimmer objects are very difficult as well.  M1, the Crab Nebula, is very difficult to pick out of light-polluted skies because, again, the integrated magnitude (8.4) is so much higher than the surface brightness (12th magnitude) of the object.  M78, a diffuse nebula in Orion, is difficult for this same reason, with basically the same numbers.  Both just seem to fade into the background glow of the sky.  But I saw both of these with my 5-inch scope right from the middle of Denver’s light pollution, which limits me visually to about magnitude 3.8 or 3.9.

There are ways you can improve your chances of seeing objects like these.  If you’re hunting for something that is at the edge of your eyes’ and your scope’s ability to find, one way to see it is to tap the telescope.  When you’re pretty sure you’ve got what you think is the right star field, when you know you’re looking at the right area, but the object still isn’t appearing there, the best way to see if you’re seeing what you want to be seeing is to tap the scope.  The eye is very sensitive to movement, and tapping the scope will shake the view just enough for you to detect the movement of the faint fuzzy.  Of course, using averted vision will help as well for the dimmest of the dim.

But the best way to see them is to make sure you are thoroughly dark adapted.  Steve Case recently posted a TERRIFIC top ten list/guide to dark adaptation on one of the Facebook astropages.  The guide actually comes from Orion’s website.  I can’t recommend this guide enough; and it will improve your observing tremendously.  Please click on the link to read it.

The number one tip from this – and from me! – is to observe from a dark site away from any local light pollution – streetlights, headlights, backyard security lights.  I’m not necessarily talking about travelling out to an actual dark site, miles away from civilization.  Just a site that is actually dark, away from light sources.  If you have light shining anywhere in your view, it will mess up your ability to fully dark adapt.

Full dark adaptation takes a good half-hour, and should gain you a good half-magnitude or so, so follow the tips in that guide.  And be sure to eat your carrots for the Vitamin A, which helps your night vision, too!

I have two other recommendations that will help with dark adaptation.  As the Orion guide mentions, your eyes dark adapt independently of each other.  Therefore, you can keep one eye dark adapted by wearing an eyepatch over your observing eye for a while – for the 10-15 minutes while you’re setting up, for example.  Make it part of your setup routine to put the eyepatch on first before you do anything else.

(This is the supposedly apocryphal reason why pirates wore eyepatches – because when they attack and board another ship, they’d fight their way on board in daylight.  However, when they went below decks, it was dark.  No problem!  Just switch the eyepatch over to the other eye, and the uncovered eye is already fully dark-adapted.  Take that ye scurvy dogs!  Arrrr!!!)

Since Halloween is almost upon us, ’tis the season to buy eyepatches!  (Fa la la la, la la la.)  I bought a ten-pack of these babies for just two bucks from a drugstore one November 1st:

Not only do eyepatches get and keep your observing eye dark adapted, but they also let you see more because of reduced eye strain.  It’s not natural for you to be looking at objects while winking one eye completely closed.  This causes some compensatory eye strain in the observing eye; plus, it’s just plain uncomfortable after awhile.  Moving the eye patch over to your non-observing eye while observing allows you to keep both eyes open and results in better comfort, better vision, and ultimately, better observing.

Another way to help eliminate the effects of local light pollution –  lights shining anywhere in your area – is to shroud yourself off from them, to physically block them from view.  This can be done by literally building yourself an observing booth – putting up poles with tarps hanging off them, in between you and the offending light sources.  An easier, and just as effective, way is simply to wear the shroud yourself.  Just like old-timey 19th century photographers would duck behind the camera under that black hood to take a picture, you can do the same with a simple dark t-shirt.  Just put it over your head and the eyepiece, and you’re blocked off and in business.

While it’s easier to put a t-shirt over your head than to erect a whole system of tarps to block out ambient light, obviously, there are some drawbacks to the t-shirt method.  One is that as soon as you emerge from under the t-shirt, you’re right back to being in the presence of the local light, ruining your dark adaptation. One way to prevent this is to use both the t-shirt and the eyepatch.  Before your head moves away from the eyepiece, before it emerges out from underneath the t-shirt, put the eyepatch back over your observing eye.

Another drawback is fogging.  If you’re observing in colder temperatures, you have to take care not to breath through your mouth while under the t-shirt, because that hot breath gets trapped underneath the t-shirt, it could hit the eyepiece and fog it over.  If it’s cold enough, this can even happen just from the heat radiating off of your eyeball.  Again, this is because the heat gets trapped under the t-shirt – I’ve had this happen to me.  So you have to take care about that; or use the “build-a-tarp” method instead.

Happy dark adapting!

(Top photo:  Heart Nebula.  Credit & Copyright: Matt Russell)


4 thoughts on “October 14, 2017: Spotting Diffuse Objects With Dark Adaptation

  1. Great post, Jon. You hit the nail on the head with M33, M78 and M1. All three of these objects required multiple attempts before I actually saw them. M33 was the first. I looked for it three or four times and I was getting really frustrated. I knew I was looking in the right place. Finally, one time while scanning the area I noticed one patch of sky was ever so slightly brighter than its surroundings. Not what I was expecting. At that point I learned to use my Sky & Telescope Messier chart, which listed among other things, integrated magnitude and size.

    Recently I observed what I thought was a background galaxy while observing M33. Nothing there on the charts. Turns out it was an active star forming region within M33. It was right on the edge of my 1.1 degree field of view, where the dim surface brightness of the galaxy simply disappeared.

    Getting out of the ‘burbs to dark sky has me spoiled. About the only time I set up my scope in the back yard is for planetary or lunar observations. I took a look at M81/M82 at home one evening. Yes I could find them, but they lacked the fine detail that I can see observing from a dark location.

    I like the eye patch idea. I’ve heard about it before, but this time I think I’ll actually go to the dollar store and pull the trigger. I spend a great deal of time alternating between the eyepiece and the star charts. I use a red light on my charts. This helps, but even red light has a degrading effect on dark adaptation. When you’re hunting deep sky objects, this tiny degradation can mean the difference between seeing and not seeing. The patch might help me out here.

    Love your blog, keep up the good work!

    Liked by 1 person

    1. Hi, Bill! Thanks for the compliment.

      Yup, the eyepatch will definitely help – on, ahem, “both” eyes. Observing with both eyes open helps, too. And combine the patch with the t-shirt idea to block any ambient light – if you have any – and then you’re cooking with gas.

      But getting to a dark site is obviously even better. While you can see these objects under light pollution, you can actually observe them without it.


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