Oct 31, 2012

NGC 6914 - Seeing Red


My emphasis on the Hydrogen Alpha data and its use in the luminance
had the intended effect on the detail, but it cost me a lot of the 
reflection nebulosity that's in this nebula complex.
Over the last few months as I've been working to get all of the new equipment from the summer to work together.  I have to say, it wasn't difficult.  I had it all lined out and behaving very quickly. The SBIG imaging setup has performed without a hitch in all that time.  With my implementation of CCDAutoPilot I have been able to massively increase the amount of time that I spend imaging.  Much narrowband data gets collected from my Arizona backyard while I sleep.  During weekends at dark sky observing sites, CCDAutoPilot takes over the equipment and I just forget about it.  It's allowed me to return to visual observing when I'm out on those new moon weekends. 

NGC 6914 is a huge nebular complex in Cygnus.  It's one of those great star filled areas of sky that feature all three types of nebulosity (emission, reflection and dark) in one field of view.  Having seen images of it previously, I wanted to focus specifically on the hydrogen alpha data and the contrast between the emission and dark nebulae in the area.  

Gathering the Image
The imaging rig all setup for the
magnitude 7.5 skies of Portal, AZ.

I started data collection on NGC 6914 in late August.  I cut my teeth on CCDAutoPilot with this object, ironing out the small learning curve and getting all of the software to play nice together.  A handful of subs were lost early on, but it only took a couple of sessions to get everything running like clockwork. 

I live on the far outskirts of the Phoenix metro area, so there is moderate light pollution at my home.  This was the real reason that I wanted narrowband filters - to collect that data from my backyard where it's relatively unaffected by light pollution.  All of the hydrogen alpha data was collected there.  It was this data collection that revealed one shortcoming of my SBIG ST-8300 camera. 

In August and September where I live, temperatures even at sunset are still usually well north of 100ºF (38ºC).  Overnight lows rarely drop below 85ºF (29ºC) during that time.  As a result, it was absolutely impossible to cool the chip of the ST-8300 to anything below 0ºC.  Thermal noise was a huge factor in the 30 minute subexposures that I captured for the hydrogen alpha data. Thankfully, a good set of 30 minute dark frames captured at the same temperature did a good job of minimizing the noise.  Still, I have to believe that it has an impact on the final outcome.

A September trip to a dark sky site in the shadow of the Discovery Channel telescope at 6800' elevation was used to capture the RGB detail.  I began processing the image fully intending that I would use just the H-Alpha data for the luminance channel.  I couldn't achieve an image that I felt was even vaguely good because of the big difference in star size between the H-Alpha and RGB data.  During the October trip to Portal, Arizona I supplemented the H-Alpha with some quality luminance data. 

Over the course of many nights, and three different locations in Arizona I collected just shy of 20 hours of data for this image.  Here's the breakdown:
  • 28 x 1800s - 1x1 bin - H-Alpha 3nm bandpass
  • 10 x 600s   - 1x1 bin - Luminance
  • 16 x 300s   - 2x2 bin - Red, Green and Blue (each) 

Image Processing

I made use of a couple of tools specific to PixInsight to great advantage when blending the H-Alpha data with the rest of the image.  I used the HaRGB combination script to combine the H-Alpha with the rest of the color data.  I wanted also to use the H-Alpha as part of the luminance, so I ran the HaRGB script again with slightly different parameters:
  • I created a new "RGB" image, but used the luminance data for the R channel.  Essentially, I created an LGB.
  • I ran the HaRGB combination script to blend the Ha with the "red" channel of the LGB image.  This accomplished my task of blending the Ha and L channels.
  • Using the Channel Extraction module, I extracted the R channel and threw away the other two.  This gave me a luminance channel that had all of the H-Alpha detail and star sizes from the luminance data.