Normal 3 hours of RGB Exposure (G2V)
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3h RGB + 4h of Hydrogen Alpha
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This pair of images should
            satiate both camps: Those purists that insist on genuine G2V
            calibrated colors (Left), and those who would like to see a more
            complete picture of the stellar dynamics in process (Right).
            First, let's give a brief description of the galaxy in the images
            above, and then we will go into some of the details of the pair
            of shots.

            
M33 - Local
            Group Face on Spiral

            
Known also as
            NGC598, this 6th magnitude object is spread over a huge 70 x
            40 arcminute field which we are looking at about half of it here
            in these images. Classed as a SA(s)cd spiral, it is inclined
            only 4 degrees to our line of sight. Its fairly neutral in color
            overall, with a B-V index of .55, which makes it somewhat "tawny".
            The core is very unconcentrated, and in fact we can see the bare
            nucleus near the center, surrounded by a dim yellowish disk with
            dark brownish dust lanes traceable all the way into the core.
            This galaxy has more free hydrogen than any other Local Group
            galaxy in the northern sky, and produces some very enlightening
            comparisons when additional exposures are added that enhances
            the hydrogen lines.

            
Comparing the
            images

            
Images of Local
            Group galaxies tend to share a common characteristic with CCD
            images and my 12.5" - They almost always appear very granular
            or grainy looking. This is because we are resolving the galaxies
            individual stars, and even on the largest images here, the galaxy
            will show tiny blue and yellow stars down to the magnitude limit
            in huge numbers. There is not a piece of this image that does
            not show this granular appearance in fact. On the left standard
            RGB image, the yellow core is surrounded by blue highlights in
            the arms. These are clusters of star clusters, known also as
            OB associations. They consist of dominantly blue supergiant stars
            with a maximum brightness of magnitude 17.5 and down to the images
            estimated limit of 22.5 magnitude. Only hinted at here is the
            amazingly complex interaction between the hydrogen and the OB
            associations. For this, we turn to the right image, that has
            been Halpha/Hbeta hydrogen enhanced.

            
The right image
            has been produced in a special way which can combine every
            shred of hydrogen alpha nebulosity in a deep (4h) dedicated
            set of images of the same field as the RGB frame. The entire
            galaxy is filled with an enormous amount of pink glowing hydrogen.
            From concentrations along the edges of the arms, to small linear
            wisps all the way into the very core. Also huge numbers of very
            dim red filaments are in the spaces between the major regions
            in the arms, yielding a soft general hydrogen glow to the entire
            disk. If you look at the larger image, you can see the fine hair
            like wisps everywhere. Now lets take a look more closely at some
            of the details that can only be seen by this type of image.

            
Below are some
            blowups of small areas of the original frame to illustrate the
            unique processes in this galaxy that can be visualized by hydrogen
            enhancement:



The
            top row shows how the OB associations with their hot fierce stellar
            winds will blow bubbles in the hydrogen, forming dark areas around
            the O/B stars in the cores. These three bubbles show this process
            very well and are all but invisible in the standard RGB image.



            
Some additional
            closeups of OB associations. (O and B are hot blue spectral types
            that emit large amounts of ultraviolet radiation) You can still
            see the areas immediately adjacent to the stars has been cleared
            out, and some of the fantastic shapes and structures of nebulosities
            are seen here. The huge NGC604 on the right is blown out a bit
            in its core, but is the largest bright nebula in M33.

            
Conclusion

            
I must admit
            to you that I belong to both camps - I enjoy the purity of the
            precise G2V calibrated RGB images very much, because it transports
            my mind to that galaxy and I can see it as the eye might if I
            could make it big and bright enough. On the other hand, I want
            all the information and details across the entire spectrum to
            view and contemplate. I love to see the amazing intricate interactions
            between nebulosity and stellar groupings. Mix in swirls of dust,
            a black hole or two in the core, and I'm happy!

            
Processing

            
A standard RGB
            non binned image with 10 minute subframes totaling 3h total was
            produced in the normal way: Calibrate, curves, and cosmic ray
            cleanup. On two other nights 4h of Ha data was taken, combined
            with Gralaks Sigma Reject using standard deviation masking to
            minimize artifacts, and cosmic ray hits. The two were combined
            in Photoshop CS2 by first adjusting the Ha image to range from
            complete black to nearly pure white - but NOT white clipped anywhere.
            The RGB data was split again into its components and the Hydrogen
            data was projected onto first the R channel at 100% using "Screen"
            and then onto the B channel at 15% the same way to fill in the
            Hbeta information. The results were recombined into RGB again.
            The final result is the correct pink coloration, with
            the stars and the remainder of the galaxy very close to G2V in
            coloration.

Instrument:  12.5" f/5 Home made Newtonian
Platform:  Astrophysics 1200 QMD
CCD Camera:  SBIG 10XME NABG with Enhanced Water Cooling
Guider: SBIG ST4
Exposure:  Ha+LRGB = 240:60:60:60 
RGB Combine Ratio:  1: 1.05: 1.11
Filters:  AstroDon RGB Tricolor
Location:  Payson, Arizona
Elevation:  5150 ft.
Sky:  Seeing FWHM = 5 arcsec (Maxim DL - 10min subframe), Transparency 9/10
Outside Temperature:  40F
CCD Temperature:  -20 C
Processing Tools:  Maxim DL, Gralaks Sigma, Photoshop, PixInsight, Starizona Debloomer.
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