How dark is your sky?

[NTgazer (Chris)]

Returning to what I consider dark skies from my recent trip to the uk and observing in summer light polluted city skies has had me thinking to myself about how to visually represent how dark the sky appears in various locations for us noobs. Say if we wanted to 'compare' darkness relatively, could members who are interested maybe take photos at agreeable, pre-determined settings (thinking dslr wise) that most accurately represent what we can see with dark adapted eyes? So for example we experiment with settings so as to achieve an equivalent iso setting, aperture value and shutter speed for a given focal length that the resulting shot is a good approximation of what can be seen by the naked eye. Once agreed/compromised upon we should be able in theory to take shots at our own locations and compare what is visible. It would also be a good way to monitor seemingly ever increasing light pollution, if for example I took a photo tonight, then with identical settings took another in a year's time (approx-matching same age moon etc) I could compare quite accurately what is visible. As a noob I find relative magnitudes etc quite hard to get my head around (hopefully I'm not the only one!) and an exercise such as this would be very helpful I think.

[el_draco (Rom)]

Hi Chris,
You might find this site useful. http://darksitefinder.com/map/
Its also not quite as simple as "dark". Transparency is also important. Where I observe, the sky in Winter is very dark but also quite still. Transparency is very good, when its not snowing, raining or foggy...

I can see the Sigma Octanis asterism visually on a regular basis even with my 50+ year old eyes. I often get a Milky way over 160 degrees and can easily follow branching dust lanes. Omega Centauri is an easy naked eye object.

Not much to complain about

[Atmos (Colin)]

In Melbourne I am in the red zone, in Heathcote I appear to be one spot off of the dark dark

[AussieTrooper (Ben)]

Originally Posted by Atmos

In Melbourne I am in the red zone, in Heathcote I appear to be one spot off of the dark dark

Same here. Live in inner Melbourne, but my folks' farm in Merrijig is a pretty good site.

[Allan]

One of the easiest and most un-biased ways to measure sky darkness is with a Sky Quality Meter made by Unihedron. I used mine extensively while searching for observing sites and it was very helpful.

An SQM reading around 22.0 is about as dark as the sky can get. Under those conditions Uranus is an easy naked eye object as an example.

[glend (Glen)]

X2 Allan's advice re the Sky Quality Meter. We have dark site readings of 21.77 at Bretti NSW which is the darkest location i have measured personally with the SQM. You can read about how the SQM calculates the darkness value on the Unihedron website. You know your in a truly dark site when the light from the milky way starts affecting the SQM reading, effectively the light pollution from the stars.

[iborg (Philip)]

Hi

I don't doubt that the Unihedron meter is the right tool for the job, but, as very much a noob, I can't justify the money, not when there are other items I still want.

Having just looked up the meter again, I also noticed 'there's an app for that'!!

I doubt it is as good as the meter, but, still better than nothing.

I think my better half has just volunteered to have it on her smart phone!

https://itunes.apple.com/au/app/dark...602989060?mt=8

I expect there are other apps as well.

Philip

[gregbradley]

As a very rough guide I use. My home location has semi rural skies which are reasonably dark to the west and less dark to the Sydney facing east.

I can see the Milky Way easily, I can see the LMC and SMC not so easily and the stars are fairly clear in general.

But when I go to my dark sky site which per the light pollution map is in a dark shadow area so almost as dark as it gets the stars take on a fiery aspect, the Milky Way a 3D almost throbbing aspect and the LMC and SMC are quite easy to see.

So the firey star aspect is my guide. Not just bright points but little fires.

Greg.

[Paul Haese]

Many years ago before lots of tech was available we used the 6th Mag star in the coal sack as guide. When you can see the 6th mag star you are in a fairly dark sky location. I reckon that would be a better start than using cameras as a guide. Though an SQM is about as good as it gets, but you would need to remember that some nights the sodium layer is more agitated than other nights and that can affect the readings of how dark the sky is on any particular night. I saw an interesting lecture my David Malin last year about the colour of the night sky and what causes it to become milky. My site is about 21.6 on average with an SQM and I have seen a night of 22.06 before. On nights when the sky looks a little milky I get readings of around 21.4. Generally though my site is dark with just some minor pollution on the horizon from Adelaide.