How dark is my sky?

[iamjulian]

Hello

I think I have reasonably dark skies, though I know in winter I get the odd night where seem to be able to see a lot more stars than normal. My question is, how can I quantify how dark my skies are? I have heard people refer to their sky as being say, "mag 4". Is this just a case of trying to spot the faintest star and then looking it up on something like stellarium to see what magnitude it is? I have also read people quoting their sky darkness in terms of arc seconds. I know what they are, but how would you measure darkness by that scale?

I'd just like to be able to know how dark my sky is in numbers. Sorry if it is a dumb question.

Thanks

[Aza]

The nights where you see more stars is probably due to it being a moonless night. Either that or its just exceptional conditions.

If you tell me what town you live in, I can look on my Philip's Dark Sky Map and tell you what mag skies that says. Not sure how accurate it is nowadays though??

I use it to find dark sky sites to image from.

[Doc]

My way is not very scientific but it works.

Wait till you are dark adapted and look for the dimmest star you can see in say the East and look on a star chart and note it's magnitude. Now look at your star chart and find a star in the west of the same magnitude and see if you can see it, if you can then thats your limiting magnitude.

[iamjulian]

Thanks Mick, I will give that a try.

Arran, thanks, I am just south of Shrewsbury. So my northern sky is a bit orange, but southern sky is nice and dark. I tried to look at that map online, but they only publish a very tiny version - can't blame them. It would be a nice project for stargazers lounge community to map the UK.

[Aza]

Looks like you are in a great position for dark skies

[JamesK]

Look at steps 1 and 2 of the following. This is a good trick for finding your skies darkness, especially as leo is visable this time of year. Wait till you are fully dark adapted. Mag 4 skies are good, mag 5 is excellent, mag 6 is the best your going to get unless your on some super isolated mountaintop in the himylayas. A large bright moon will reduce your "Naked Eye Limiting Magnitude". Which I assume is what your trying to work out.

Spring Sky Tour: Leo

[narrowbandpaul]

The value of which you speak in arcseconds, is the magnitude per square arcsecond measured from the sky background.

Typically this is done in the filter system used at an observatory such as johnson BVRI or SDSS.

However you could just measure it through RGB or other filters that you use for imaging.

you will need a calibrated camera, ie where you know the conversion from ADU to e-, and the quantum efficiency over the filter.

this way you know the number of photons collected during the exposure.

point a scope and ccd at a featureless part of the sky, record an image of around half to 3/4 signal. Apply bias and flats. Convert to e- and apply QE correction. Sum the corrected counts in a small box, divide by the number of pixels used. Divide by exposure length.

this gives the photons per sec incident on your area......

that would give a good approximation to the true background level.

if ou are a visual observer, just estimate from sky charts as you have been doing. expect around 4-5

[James]

There are some charts out there that mark off areas of sky that you can count the number of stars in - the number you can see will determine the limiting magnitude. I'll try and hunt them out and post a link.

I try and use Ursa Minor - the four stars at the end are of magnitude 2, 3, 4 and 5. If you can see all four stars then you have at least a Mag 5 sky.

James

[iamjulian]

Thanks everyone!

Arran, thanks so much for posting that. Very interesting. A ten minute walk will take me from light green to blue. More reason not to get sucked into buying a monster scope when I invest in a few weeks time.

Paul, sounds interesting. Thanks for the explanation.

James, thanks for the Ursa Minor tip. I shall try that one.

[James]

This page might help :

Limiting Visual Magnitude Determination

James