Magnitude/ Apparent Magnitude

[gavanco]

I am hoping someone can help me understand the whole concept of magnitude/ Apparent magnitude. I understand the basic of what is about but it is the other factors such as:

* Equipment limitations
* Light Pollution
* etc.

That begins to confuse me.

How does one work out what is achievable to view from a given location keeping in mind the other factors. I have all the star maps and software to know where these objects are in the sky (thereabouts) but need to know if I am dreaming to be able to locate some of these wonders with the factors mentioned above.

Any help will be greatly appreciated......

As dark skies are not available to me all the time anyone who can help me with this would be a great help

[[1ponders] (Paul)]

Hi Gav.

Apparent magnitude measures how bright an object looks from earth and is a function of its absolute magnitude and distance. ie the further away the dimmer yet the bigger/heavier the star the brighter.

Absolute magnitude measures an object's intrinsic brightness and i defined as the apparent magnitude an obect would have if viewed from a distance of 10 parsecs (32.6 light years)

As to working out what mag you can see from your site; you may come across the phrase mag 5 skies or mag 3.5 skies. The phrase is baically telling someone the dimmest stars they can see at any partiular time, with their naked eyes. So if you look at your maps and then at the sky and the faintest star you can see is mag 4 then your sky is a mag 4. Be aware though that the mag value of the sky can change for different parts of the sky. You may have only mag 2 skies towards a serious light pollution source but mag 4.5 at the opposite horizon if there is less LP there. And usually the zenith is different again. It depends on your local viewing conditions, how large the moon is at that time and also the transparency of the sky (how clean and easy to see through is it. Is there high thin cloud, pollutants in the air)


With a telescope you are able to lift your viewing mag because you are collecting more light from the stars and funnelling it to your eyes than your eyes can do on their own. But sooner or later it is going to have an effect, especially when trying to view diffuse deep sky objects like nebula and galaxies which have an intrinsic low surface brightness. A galaxy may be given a mag of say 9 (bright galaxy) but that brightness is spread over the total area of the galaxy. (I think thats right )

So as to locating wonders from Sydney the short answer is yes, but it depends on your sky mag, the magnitude of the object your looking at, how diffuse it is, what your transparency is (and to a certain degree seeing. I've seen seeing so bad over the last few weeks that fairly bright stars are twinkling so much the disappear from view) and the light gathering abilities of you scope.

Hope that helps

[Muddy Diver]

Hi Gavanco

You raise an interesting point, I shall keep a watch on this thread for my own benefit but heres my thoughts.

Most of my viewing takes place from my backyard and recently while searching out M33 (spiral Galaxy) I discovered that a star I was using to guide me towards the target was actually of a dimmer magnitude than the galaxy itself. I could see the star but not find the galaxy and this is purely down to the light pollution as i get lower in the sky. So long as you have this pollution, I don't think you will be able to develop any rule of thumb to determine what you will and wont be able to see.

I am pondering the same issues as you it seems and am setting myself targets to find stars of ever diminishing magnitude to see what the limits are for my equipment. But even then in order for this to be a consistent guide the seeing and transparency must be matched. Sometimes you wont be able to see objects that you have viewed previously or their appearance will differ and its all down to conditions provided your equipment setup hasn't changed.

[rmcpb (Rob)]

For an explanation of Limiting Magnitude have a look at this site

Cheers

[janoskiss (Steve H)]

Muddy, the galaxy's light is spread over a larger area so its "surface brightness" is much less than that of a star of similar magnitude. Apparent magnitude is a measure of the total light reaching the earth from an object. Maximum surface brightness (light intensity per unit solid angle of sky) is the more relevant measure for both visual and photographic astronomy but unfortunately it's rarely included in charts & planetarium software.

[Muddy Diver]

Thanks Steve for the explanation. I tried finding a few other galaxis and can see what you mean. Their brightness is nothing when compared to a star listed with the same magnitude. In trying to determine what my telescope is capable of viewing I should disseminate betweeen galaxy mag and star Mag. Whew... another project!