Re: Re:[APML] orion mosaic

[Wei-Hao Wang <whwang@gmail.com>]

Hi Dave,

About the redness of the Witch Head, I have two explanations.
First, indeed no reflection nebulas are purely blue.  The dust 
clouds reflect light at all wavelengths.  However, the reflection
in blue is stronger than red.  Thus if the nearby star has blue or
average colors, the reflection nebula appears blue.  But it's not 
pure blue.  There is still green and red light reflected.  If the 
nearby star is very red, then it is possible to have red reflection
nebula.  The yellow nebula next to Antares is of this kind.

The above is continuous light.  On the other hand, it is possible
for a reflection nebula to have emission line (like Ha) as well.
In a very intensive UV radiation field (like Orion), molecular hydrogen
will be broken into atomic hydrogen by the powerful UV photon and
the H atom will then be ionized to produce line emission.  However,
if a cloud is so dense that the UV photons cannot penetrate it,
it can stay in the molecular form and remains dark.  If this is the 
case, only the surface layer of that cloud is ionized and emitting
a little Ha emission.  This picture is a good example:
http://www.ifa.hawaii.edu/~wang/gallery/picutres/sco_oph_complex.htm
The dark clouds to the upper left of Antares are covered by thin red skin.  
This is Ha emission coming from the surface of the clouds.  I believe
the Witch Head also has this kind of weak Ha emission.  But since 
itself also reflect blue light, the Ha is not as apparent.

I just used an old picture taken by a 50mm lens as the reference.

Cheers,

Wei-Hao


On Thu, 31 Mar 2005 06:40:38 -0800 (PST), D. Kodama
<kodama@alumni.caltech.edu> wrote:
> Wei-Hao,
> 
> Fantastic mosaics!  It's amazing to me that you can turn out these mosaics
> while pursuing a PhD!
> 
> It's interesting that the Witch Head shows up quite well in your R-channel
> shot.  Most people reproduce that as an almost pure blue object, but for a
> long time I've felt that wasn't quite accurate.   It could be that they
> are trying to color-balance the background and losing the red because of
> so much h-alpha emission in the general area.
> 
> When registering the components of a large mosaic covering a large part of
> the sky, what are you using as an overall registration reference?
> 
> Dave Kodama
> 

-- 
________________________________________________________________
Wei-Hao Wang  :)

Institute for Astronomy at University of Hawaii

Address:                       
2680 Woodlawn Drive         Personal Website:
Honolulu, HI 96822             http://www.ifa.hawaii.edu/~wang
________________________________________________________________
_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo