Astrophotography by Keith B. Quattrocchi

          NGC 6888 (Caldwell 27)
         The Crescent Nebula
                              Emission Nebula in Cygnus                          
          Narrow Band Image(Ha-NII-OIII)
         Published in Astronomy's
      "Spectacualr Universe", pg 85, sunner of 2011
          Copyright 2009
          Keith B Quattrocchi


                                      Image Acquisition Information                      
                               Telescope:   16" RCOS Richey Chretien Telescope (ion milled at 6/9) 
                               Camera:       SBIG STL-6303 M
                               Guiding:       SBIG AOL,  Astrodon  MOAG AOG (SBIG 237 with Custom FLR)
                               Filters:        Astrodon Ha, NII, OIII (3 nm filters)
                               Mount:        Software Bisque Paramount ME
                               Acquisition Programs:   The Sky,CCDAutopliot III, CCD Soft. 
                               Processing ProgramsCCDStack, Maxim DL, Photoshop
                               Date: July13, 2009 -August 23, 2009, 2009                              
                               Time: 20 x30 min ( 10 hours) each for each of Ha, NII and OIII:
                               Total of  30 hours imaging time.                              
                               Processing:  CCD Stack and PhotoShop CS/3
Image Infornmation:  NGC 6888, the Crescent Nebula, is a frequently imaged emission nebula in Cygnus.  It is surrounded by a large H-alpha rich region, rich wih a number of interesting objects, including a new Planetary nebula recently recognized (by Dave Jurasevitch, Mel Helm and this author) .  This Crescent Nebula, some 5,000 light years distant,  was formed by two colliding shock waves from the same star, WR 136.  This Wolf-Rayet star initially was a red giant and ejected it's atmosphere, which was then later hit by the rapidly moving "stellar wind" created by the Wolf-Rayet star long after it was a red giant. 
Additional Comments:  This Narrow Band image was a deep image, each subexposure being 30 minutes with a total of 20 subfames for each filter (a total of 30 hours of imaging time).  These filters are 3nm band pass of Ha, NII and OIII from Don Goldman.  This object sheds light in the Ha, NII and OIII wavelengths, making it an interesting object for an NII filter.  In addition, details and extentions of the OIII regions are clearly seen, likely due to both the short band pass of these filters and the longer exposure times.  The Narrow Band image was created by an unequal weighting of Ha/NII/OIII at a ratio of 2:2:5 (OIII signal is relatively weak, but Ha and NII quite similar in strength, thought the pattern was different).   This was essentially a modified Hubble Palette (RGB as Ha/NII/OIII).   This was layered over a modified (enhanced) H-alpha image to bring out some of the details lost in the assembled narrow band image.   Further data stretching, high pass filtering and background smoothing was performed with Adobe Photoshop CS/3.  All data stretching was by careful and repeated use of curves/levels in photoshop.  This step was superior to  automatic DDP stretching and ws responsible for the shock wave details. 

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