Re: Re:[APML] orion mosaic

["D. Kodama" <kodama@alumni.caltech.edu>]

Wei-Hao,

It would be interesting to see if someone has a CCD image of the Witch
Head taken through a 10 or 20 nm H-alpha filter to see if it is really a
significant emission source.

Is everything in the Orion complex at the same distance?  It may be old
data I'm looking at but Rigel's distance is listed at 250 pc and I'd guess
the Witch Head must also be at about the same distance.  The belt stars
and Orion nebula are listed at distances somewhat farther away than Rigel.
 Meanwhile Betelgeuse is listed as being "nearby" at 150 pc.  I'm just
wondering what is the distance to the veil of H-alpha that seems to run
through the whole area and if Rigel is a contributing ionization source.

At any rate, I think it is the general tint of the H-alpha background that
throws color adjustments off when they are based on assuming black or
dark-gray backgrounds.  One really needs these deep wide field shots to
get close to the correct colors or one needs to use a strictly calibrated
color adjustment procedure which probably will only work with CCD images.

Thanks for the info on what you used for the Registar reference.  I was
wondering if you might have used a computer-generated star map for that
purpose.

Dave



> 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
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>

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