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This enigmatic object defies
            what we normally associate with the term "Planetary Nebula"
            in that it is so dim and ill defined, it seems more like an emission
            nebula instead. But the deep blue 16th magnitude central star
            defines its origin, and deep exposures such as this reveal its
            entire envelope. Normally captured as a dim arc in most images,
            my goals here were to expand upon the faint outer extensions
            first suggested to me by Ken Crawfords recent deep image.

            
Abell 21 in
            Gemini is a whopping 12 arcminutes across, and is rated as 11.3
            magnitude. But its light is spread very thin, and most is concentrated
            in the right side of the nebula. Deeper hydrogen alpha images
            show a series of waves or ripples cutting across its face seen
            here well on the left side. In addition, a rippling patch of
            nebulosity on the left suggests an earlier outburst on the less
            denser, and less restrictive side of the object. Also, part of
            a small dim red cloud lies on the bottom left corner of this
            frame. A slight greenish cast can be seen around the central
            "hole" in the nebula. This ancient object does not
            have the energy to produce such colors in abundance, leaving
            a mostly reddish envelope.

            
There are several
            very faint galaxies in this frame, one very yellow distant one
            above the central star within the nebula, and a somewhat brighter
            16th magnitude object to the lower right.

            
Processing:
            A standard G2V RGB image was taken, binned 1x1. Then an hour
            of supplemental H-alpha image set was secured. After creating
            a standard decent looking RGB image, and a strong, but low noise
            H-alpha representation, the images were combined tastefully by
            splitting off the R channel and mixing it 80 percent with the
            H-alpha data. Then the resulting image was combined again with
            the R channel using lighten to restore all of the stars. The
            resulting image was merged with the Green and Blue images to
            form this tri-color image. But we were not done yet. Because
            the H-alpha dominated the image even over the core, it was layered
            over the original RGB image with a layer mask to preserve the
            main arc and core regions, and only add H-alpha data on the periphery
            and outer zones.

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+RGB = 60:20:20:20 
RGB Combine Ratio:  1: 1.05: 1.11
Filters:  AstroDon RGB Tricolor
Location:  Payson, Arizona
Elevation:  5150 ft.
Sky:  Seeing FWHM = 4 arcsec (Maxim DL - 10min subframe), Transparency 9/10
Outside Temperature:  30 F
CCD Temperature:  -30 C
Processing Tools:  Maxim DL, Gralaks Sigma, Photoshop, PixInsight, Starizona Debloomer.
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