The Turtle Nebula in Hercules


The outer halo, or perhaps a collection of jets, is strongest in OIII, but appears very faintly in blue and even in Ha. The OIII and Ha data were used in this image for the halo, although the Ha data is so faint it’s almost invisible compared to OIII. The core was assembled with the RGB data, but it was really impossible to get it to blend well with the rest of the image. a>
Optics/Mount: CPC1100 with f6.3 Celestron reducer on a Milburn Wedge. Dec antibacklash set to 27/27. PEC on.
Camera: SXVR-H694
Exposure: Total exposure time about 28 hours; 49 x 20 mins OIII, 10 x 20 mins Ha, 180x1min RG, 120x1min B. All bin 1x1.
Filters: Astrodon 3nm OIII, 5nm Ha, Type IIi RGB
Date: Data was collected over 12 nights in April and May 2016
Imaging automation and capture: Sequence Generator Pro

Guiding Hardware: ASI120MM, Celestron off axis guider. Guiding scale 0.46 arcsec/pixel. 1 second guide exposures.
Focusing: Rigel Systems GCUSB/nStep on stock C11 focuser
Guiding Software: PHD2. RA aggressiveness 55, hysteresis 0, Minimum Move 1.4 arcsecs, Max RA 1000, Max Dec 600. Guiding unbinned with ZWO connection. Dec set to hysteresis. Dithering set to Extreme with Settle < 1.2.
Guiding performance: FWHM of subs about 1.8 arc seconds, RMS in PHD about 0.75 arcsecs.
Light pollution: Bortle 8 (white zone, NELM about mag 4.5)
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 0.6 arcsecs/pixel
Processing: Bias, darks, and flats. Stacked and processed with PixInsight, with a few minor modifications in Photoshop. The nebula is OIII, colorized to teal blue, and layered onto an RGB star field in Photoshop.