Weltevreden SA
26-05-2013, 07:40 AM
The star charts we rely on such as Uranometria, Aladin, and SkySafari are very good, but they don’t show extinction maps of our Galaxy’s midplane and centre. Extinction is caused mainly by dust in the Galactic disc, and maps of it would be a nightmare on top of all the details already on the printed pages or your iWhatever. Extinction is a concern if you’re after objects within 4° of the Galactic midplane. While E. E. Barnard's 'dark nebulae' studies are fascinating and detailed, they are not really full-length maps in the way we think of them. There are three places to download useful extinction maps:
Schlegel et al, 2008 (http://arxiv.org/abs/astro-ph/9809230) is still considered something of a Gold Standard. It was compiled from far IR data, but are nonetheless useful, notably in the bulge region.
Nidever et al, 2012 (http://arxiv.org/abs/1206.5799) has useful colour maps you can use to determine how reddened your area of interest is. Though compiled using near and mid IR data, the maps are reliable in the visual spectrum as well.
Chen et al (http://arxiv.org/abs/1211.3092), 2012 leads you into a very large, detailed database that is probably of more use to astrophotographers than visual fans. If you are interested in yet another set of detailed info about the myriad complexities of our Galaxy, the internal (some might say infernal) links in this paper will keep you on your toes for weeks.
These maps mainly apply to <4° above and below the Galactic disc. If you after an object higher or lower, skip this thread and have a good time.
If you delve into the professional papers, extinction is generally referred to using the terms E(B - V) or E(V - I). This means—roughly—blue band magnitude minus visual band magnitude, and visual minus near infrared. The 'E' means 'extinction' to distinguish extinction values from other (B - V) or (V - I) values are used for other purposes.
It is hard to visualize what exactly this means. To convert these into visual band extinctions by magnitude (i.e., how much fainter the object appears than it would be if well away from the dusty disc), use these conversions:
Multiply E(B - V) times 3.1 to get visual extinction. Example, an object of E(B - V) 1.23 will appear 3.87 magnitudes fainter than it really is.
Multiply E(V - I) times 2.3 to get visual extinction. An object of E(V - I) 1.23 will appear 2.83 magnitudes fainter.
I hope this is a good start to the thread. =Dana in S Africa
Schlegel et al, 2008 (http://arxiv.org/abs/astro-ph/9809230) is still considered something of a Gold Standard. It was compiled from far IR data, but are nonetheless useful, notably in the bulge region.
Nidever et al, 2012 (http://arxiv.org/abs/1206.5799) has useful colour maps you can use to determine how reddened your area of interest is. Though compiled using near and mid IR data, the maps are reliable in the visual spectrum as well.
Chen et al (http://arxiv.org/abs/1211.3092), 2012 leads you into a very large, detailed database that is probably of more use to astrophotographers than visual fans. If you are interested in yet another set of detailed info about the myriad complexities of our Galaxy, the internal (some might say infernal) links in this paper will keep you on your toes for weeks.
These maps mainly apply to <4° above and below the Galactic disc. If you after an object higher or lower, skip this thread and have a good time.
If you delve into the professional papers, extinction is generally referred to using the terms E(B - V) or E(V - I). This means—roughly—blue band magnitude minus visual band magnitude, and visual minus near infrared. The 'E' means 'extinction' to distinguish extinction values from other (B - V) or (V - I) values are used for other purposes.
It is hard to visualize what exactly this means. To convert these into visual band extinctions by magnitude (i.e., how much fainter the object appears than it would be if well away from the dusty disc), use these conversions:
Multiply E(B - V) times 3.1 to get visual extinction. Example, an object of E(B - V) 1.23 will appear 3.87 magnitudes fainter than it really is.
Multiply E(V - I) times 2.3 to get visual extinction. An object of E(V - I) 1.23 will appear 2.83 magnitudes fainter.
I hope this is a good start to the thread. =Dana in S Africa