Pacman Nebula by Prof. Daniel Bertesteanu, Bucharest Astroclub, Romania

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Dec 19

2018

by Astroclubul Bucuresti

At 9,200 light years from us, in the Perseus arm of our Galaxy, there is NGC 281, 
an emission nebulae that can be seen in the direction of the constellation 
Cassiopeia. 
Due to the likeness of a character in some video games, it is also known as 
the Pacman Nebula. 
Inside, a group of young blue and super hot stars clusters are forming  
the open-swarm named  IC 1590.
The intense pressure and radiation ionizing the entire nebula, "sculpting" and 
spreading the gas and dust near it, to the periphery until to 40 light-years away, 
is forming radial columns and compact masses of gas and dust known as
 “Bok globules” that can be easily seen in our image. 
Within these globules there are intense stars formation processes and 
protoplanetary systems.
The photo was shot in “narrow band” by means of three selective filters that 
allowed only the frequencies of the light generated by NGC 281 to pass
 through the various chemical elements in its composition and the temperatures 
in which they are in the nebula. 
Ionized hydrogen atoms, sulfur and nitrogen are emiting  strong in red and the oxygen 
is emitting in green, at aprox. 500nm. 
Due to the abundance of hydrogen in the nebula, intense red emissions of 656.28 nm are 
predominant and are corresponding to the Balmer alpha transitions of hydrogen.

 These real colors are later distributed during the processing in red (R), green (G), 
and blue (B) imagery so as to produce a tricolor image in a false color palette 
because the resulting colors do not match those of the atoms they generated its.

In narrow band astrofotography there are two, well known  colors assignments 
in RGB channels:
- in the Hubble palette for which the master images obtained through the Sulfur (S), 
Hydrogen (H), and Oxygen (O) filters are attributed to the R, G and B channels. 
The resulting colors are false because both the sulfur and hydrogen alpha filters
 let only red to pass. 
However, for a more dynamic and more contrast  aspect, hydrogen is attributed 
to the green channel and oxygen to the blue one. 
Due to the abundance of hydrogen, the images resulting from the combination
 tend to be excessively green and the stars tend to be colored in violet.
Later on, during processing, the green color is fading resulting in a modified 
Hubble palette  dominated by blue and yellow.

- In the Canadian France Haway Telescope (CFHT) palette of the R, G and B channels 
are assigned to the master images obtained through the H, O and S filters. 
In this case, the resulting color is somewhat closer to reality because the hydrogen 
is put into the channel red and oxygen to green. However, the sulfur emissions 
are put into the blue channel which does not correspond to the reality 
because they are more red than those of hydrogen alpha. 
The  resulting image from this color palette is dominated by pink-red (hydrogen) 
and yellow (oxygen).
The pictures  were made in three nights through 6 nm HOS filters with a telescope 
130/650 mm equiped with  a QHY 163 monochrome camera, cooled down to -20 °C, 
totaling 12 hours of integration time and processed in the Hubble palette 
and CFHT colors.

Author: Prof. Daniel Bertesteanu, Bucharest, Romania, 
Member of Astroclubul Bucuresti 
(Bucharest Astroclub, Romania)