What is a dark site ?

[acey]

A true dark site is not "pitch black". Once your eyes are fully dark adapted the sky is markedly bright with stars, Milky Way and natural airglow. Moving around without any artificial light is easy. A lamp is only needed for seeing small objects, reading etc. Foreground objects (trees etc) look truly black against the bright sky. In a telescope at high power, the sky background looks truly black (you can't see the eyepiece field stop). After viewing for some time, when you look up at the sky again it's dazzling (you need to shield your eye from it while looking through the eyepiece).

At a light polluted site it's very different. The eye adapts to the ambient light level (dictated by the pervasive glow of streetlights etc, even if not directly visible). Under those conditions a clear sky can look "pitch black", but only because the eye can't adapt fully.

So apparent blackness is not a good test of sky quality, it only tests dark adaptation. The test is limiting magnitude. If you can see stars down to 4 mag then you should manage a few bright DSOs in a telescope (e.g. M31, M42, M57, M13, M81/82). If you can see to 5 mag then you'll see many more, and the Milky Way may be visible. If you can see to 6 mag then the MW will be clearly visible and you'll be able to see all the Messiers above your horizon with a 100mm scope (or smaller), and all (or nearly all) above-horizon NGCs with a 12". If you can see stars fainter than 6 mag then you have very good eyesight.

Light pollution is of three types. One is direct glare from steeetlights etc. You need to shield yourself from that, e.g. by choosing your viewing spot, putting up barriers etc. Two is ground light reflected off walls etc; you can't see lights directly but your garden is indirectly lit up. Shield yourself by putting a hood over your head at the eyepiece and give your eye time to adapt. Three is skyglow caused by ground light reflecting off water vapour in the air. This limits the faintest stars you can see, and there's nothing you can do about it (unless you find that a "light pollution filter" works for you). "Nebula" filters (OIII, UHC etc) are effective on emission nebulae (e.g. M42, M1) but have no effect on other types of objects, i.e. clusters, galaxies (note for pedants: there can be some slight effect on a handful of large galaxies at a dark site, e.g. M33).

At a light polluted site the easiest DSO types are open clusters, bright globular clusters (e.g. M13), bright planetary nebulae (e.g. M57, Eskimo, Cat's Eye). Diffuse nebulae (emission or reflection) are generally more difficult, though with a few bright exceptions (e.g. M42), and galaxies are generally very difficult, again with a few bright exceptions. The reason for all this is that light pollution hurts the limiting surface brightness of a scope more than limiting stellar magnitude. For example see Figure 18 of this paper:

http://arxiv.org/pdf/1405.4209v1.pdf

A telescope can't improve the surface brightness of a target, and most galaxies are of about the same surface brightness as the Milky Way. So if you want to see galaxies well, you need to be able to see the Milky Way with the naked eye.

Edited April 2, 2015 by acey

[Qualia]

This is an amazing post, Acey. Well written and extremely informative. Thank you for taking the time to write it up

[faulksy]

spot on acey. agree with you 100%. thanks for that brilliant post

[ollypenrice]

Agreed, a first class account. We don't often read what you say in your opening paragraph - that a dark site is not apparently dark at all. The more you adapt, the easier it is to move around. We have even debated whether or not Jupiter casts a shadow. What makes moving around impossible is the arrival of mist. Living at 900 metres (3000 feet in old money) we are sometimes cut short by rising valley mist from temperature inversion and at this point finding anything, even at arm's length, becomes near impossible.

When imaging, one problem is assessing the sky, which you cannot usefully do withing ten minutes of looking at a screen. This is whee a sky quality metre comes in. We are usually between 21.6 and 21.9 on clear nights here. In the outback I gather that you can hit the low twenty-twos.

Olly

[RichM63]

Ah 'mist' rolling in, can rapidly reduce unlit visibility to zero. But I still prefer this problem to having a large warm lit town anywhere near by.

[knobby]

Its quite freaky when the mist rolls in or a layer of cloud pops over ... If its really dark you don't notice the clouds at first ... Then out the corner if your eye its like someone's turned the stars off

[damnut]

Great post, one dark site I used to observe was on a hill in Derbyshire. You could watch the fog roll down the hill fill up the valley then creep upwards. Cold in winter but ace! Once had a complete layer of ice over my SCT.

[Paul73]

You know that you are at a dark site when the clouds are black rather than lit up by light reflecting off their undersides.

Great post Acey.

Paul

[PeterW]

Sky quality meter helps, but poor transparency can still leave you with a dark sky and fewer stars than I see from london. Can someone invent a transparency meter, otherwise you need to look at known objects and Jude how well you can see them.

Cheers

PeterW

[Stub Mandrel]

> most galaxies are of about the same surface brightness as the Milky Way. So if you want to see galaxies well, you need to be able to see the Milky Way with the naked eye.

That comment has saved me an awful lot of chasing my own tail!

[Tzitzis]

This was really helpfull. Thanks a Lot!

Tzitzis

[g33x0r]

> most galaxies are of about the same surface brightness as the Milky Way. So if you want to see galaxies well, you need to be able to see the Milky Way with the naked eye.

That comment has saved me an awful lot of chasing my own tail!

srsly me too.  I keep going outside and trying to find M31 because it seems such an easy target to find and coming up empty.  The sky in my area is so damn bright it's really hard to see much of anything.  I guess I was expecting it to be much brighter and easier to see.  

[laser_jock99]

Another good, easy test for evaluating a dark sky site is to look at the night time clouds. If they are illuminated from underneath by local light pollution sources then the sky will not be as dark as it could be.

In the ideal situation clouds will appear as black shapes against a fairly bright night sky. This video from my observing site sort of illustrates the point- the clouds are noticably darker than the sky behind them, indicating low local light pollution.

From the same site really high clouds (~35,000 feet) actually pick up light pollution from towns 60-80 miles further south.

This contrail drifting through M8 and M20 in Sagitarius is clearly illuminated by distant sodium street lights.

Edited May 8, 2015 by laser_jock99

[Ravi]

Another good, easy test for evaluating a dark sky site is to look at the night time clouds. If they are illuminated from underneath by local light pollution sources then the sky will not be as dark as it could be.

In the ideal situation clouds will appear as black shapes against a fairly bright night sky. This video from my observing site sort of illustrates the point- the clouds are noticably darker than the sky behind them, indicating low local light pollution.

From the same site really high clouds (~35,000 feet) actually pick up light pollution from towns 60-80 miles further south.

This contrail drifting through M8 and M20 in Sagitarius is clearly illuminated by distant sodium street lights.

Thanks for the video it was better than a thousand words!

[Stub Mandrel]

I spent a few nights camping a week ago. It was heavily overcast but there was still enough moonlight that I didn't need a torch.

I remember one night at a true dark site  camping by a lake in snowdonia. The only time I have ever seen the stars reflected in water, and I was able to read teh label of a baked bean can by starlight alone. Didn't need a scope to appreciate the stars that night!

[Pleiades M45]

Very well written, Thank you.

Sent from my iPod touch using Tapatalk

[Golfey]

A really Ace(y) post.

As a beginner simple information helps a lot.

I took a quick look at the reference paper and thought TMI squared.

8/8ths cover is not helping me see anything (10 days new scope 2 days observing! Including day 1) much less the Milky Way.

[clarkpm4242]

Very nicely put.

Am a fan of http://en.wikipedia.org/wiki/Bortle_scale

I also have some favourite 'test' DSOs.

Clear & dark skies

Paul

[Interstellarshaw88]

My best local dark area is Derbyshire the Peak District I've been up several times to capture the Milky Way but nothing like yours laser jock, there's a place called hathersage a little further out but it's been awarded a dark site I've been there 3 times in the last 3 month when the moons not been out.... Twice it started slinging it down on the way and the other was very very cloudy after 5 mins on being there so hopefully I will get to see what a truly dark sky should look like and then can compare it against my local site where I go, so thank you for your information acey.

[Keith Roberts]

Thank you for tips to this beginner!

[Alan64]

Our eyes, as humans, are weak, compared to an animal of the night equipped with tapetum lucida.  For them, the same dark site as described must appear as an overcast day does to ourselves, and at high noon even.

[Gordon73]

Light pollution scales are confusing i think. Im allegedly in an orange zone. So shouldnt see that much. Although with my 130mm reflector i can see stars of the 13th mag and galaxies of 10th mag.

[Tonyhaz]

Hi all

great post, I am new to this, and I want to find a good area to view the skies with the naked eye, I'm in Essex near Southend on sea, looking on the map bradwell seems a good area, would this be ideal for seeing stars, or can u suggest a better area, ? 

 

[antony_courtney1987]

This was very helpful, Thanks!

[SonnyE]

Excellent.

I find myself trapped at what I can find with where I am. Seems where ever I am is what I must work with.

So I filter...

And when an airplane flies through, I shamefully delete the image.

I'm not worthy!