Exposure: ~9 hrs Total; 4.5 hrs 3 nm Astrodon H-a, 4 hrs 3 nm OIII, 12 min each Astrodon Generation2 RGB, all unbinned

Imaging Camera: Apogee Alta U16M (KAF16803 4k x 4k) at -20C, Apogee AI-FW50-7s filter wheel

Guiding Camera: SBIG ST-402 on Astrodon MonsterMOAG off-axis guider

Telescope: RCOS 16" RC tube, f/8.4

Location: Sierra-Remote Observatories, Shaver Lake, CA

Date Taken: July 17-20, 2008

Moon: Up and nearby in the eastern sky

Data Acquisition: CCDAutoPilot4 running Maxim 4.62.

Processing: CCDStack, Registar, Photoshop CS3

M27, the Dumbbell Planetary Nebula, in Vulpecula is one of the most photographed objects in the night sky. Early images just showed the "bow tie" shape in the core that is readily observed visually. More recent and deeper CCD images show the "lobes", which are pointing toward the lower left and upper right portions of the above image. It is known from other planetary nebulae, such as the Ring Nebula (M57) in Lyra, that these objects commonly have an extended outer "halo" or multiple "halos" that arose from earlier episodes of the proginator star expelling its outer atmosphere. These "halos" are very faint and require long exposures in dark skies. Narrowband imaging is especially useful in improving contrast to bring out details in such faint "halos".

I presented the following image at AIC2007 and NEAIC2008. It shows the improvement in contrast and structural detail in the Crescent Nebula, NGC6888, as the full-width (spectral bandwidth) of the filter is reduced from a wide orange filter, to a deep red filter, and continuing thorugh 9, 6 and 4 nm (full-width at half maximum transmission - FWHM) narrowband filters. The stars become fewer and less pronouned. The object looks open and skeletal with the orange filter and appears full and solid with the narrowband filters. As these filters become narrower, more structural detail appears, as does the more surrounding faint nebulosity.

Thus, faint nebulosity in "halos" can also be brought out using narrower narrowband filters. 3 nm Astrodon H-a and OIII filters were used to bring out the extended "halo" in the above image of M27 for this reason.

I also showed in these talks that the background signal in the narrowband image decreases linearly with the FWHM. Going from a 6 nm to a 3nm narrowband filter halves the background signal. This reduces noise (N) in the signal-to-noise (S/N) ratio. However, an improvement in S/N will only occur if the signal (S) also does not decrease.

The signal depends upon the transmittance of the filter at the specific emission line, such as H-a at 656.3 nm. It is more difficult to manufacture narrower filters. Prior to the introduciton of the Astrodon 3 nm filters, only one other supplier provided 3 nm filters, which were specified with a minimum transmission of 70%. My H-a filter from this supplier was 72%. The highest value reported was 80%. Thus, going from a 6nm 90%T to a 3 nm, 80% filter only showed a small improvement in S/N due to the decrease in both S and N. The benefit of using the narrower filter was not fully realized.

The breakthrough that we have been working on for over a year is to consistently manufacture 3 nm narrwoband filters that exceed 90%T at the emission line. The new Astrodon 3 nm narrowband filters are averaging 92-93%T. The expected improvement in S/N can thus be achieved. We are very pleased with this accomplishment.