Aug 7, 2012

SBIG's All Up In Your Grill - First Light

NGC 6888 - Crescent Nebula

NGC 6888 as captured with the new imaging equipment.  SBIG cameras, filter & off axis guider utilizing Explore Scientific optics.
So the Arizona monsoon let up enough over last weekend long enough to allow me to setup the equipment and take some serious first light images.  Once I got the mount polar aligned I left it setup for the next four days - just taking the optics and cameras off each day as the sun rose above the eastern horizon. As you may remember from my recent discussions, I have a lot of hardware and software changes that I've been trying to incorporate all at once.  As a result, this session didn't go completely trouble free, but it was darn close all things considered.  Even since writing those articles I went ahead and pulled the trigger on the SBIG OAG-8300 off-axis guider and the SBIG ST-i mono guide camera.  So far I'm happy with the two of them but there are a couple of minor annoyances that I didn't expect at this price point.  That's another topic for another time though.  Over the course of the four nights I was able to get the entire system integrated and working the way that I want it to work.  All I'm left with now is minor tweaks here and there.  

Gathering the Image

When I saw Manuel Jimenez' image of NGC 6888 last year I knew that it would be my first image if I ever got into narrowband imaging.  It was that simple selecting my first light target.  I didn't expect that I'd get anything near that awesome considering I'm not rocking $100,000 of equipment that includes a 17" Planewave CDK.  I was curious if I could capture some of the same detail with a 5" refractor.  All things considered, I'm really happy with the performance of the Explore Scientific triplet.  I'm working on a full review of it - but I'll just say that it doesn't get enough love in the astronomy community.  It's been worth every penny I paid for it - and it wasn't very many pennies comparatively.

Over the course of the four nights (and days) I managed to pile up 16.5 hours of light frame data using two filters - Hydrogen Alpha and Oxygen III.  Here's the breakdown of all the data that went into the image:
  • 17 x 1800s Light Frames - 1x1 bin - Astrodon 3nm H-Alpha filter
  • 16 x 1800s Light Frames - 1x1 bin - Astrodon 3nm OIII filter
With the exception of a noise reduction plug in (Neat Image) that requires the use of Photoshop CS5, I did everything after the capture using PixInsight 1.7.
  • Created master calibration frames - Bias, Dark, Flat & Flat Darks
  • Calibrated, registered & stacked the light frames with Winsorized Sigma Clipping
  • Using Pixel Math I generated a synthetic green channel that is a mixture of 25% H-Alpha and 75% OIII data.
  • I combined the images as a traditional RGB and processed as normal.

What I Like - and What I Don't

I've come to accept that there will always be things I don't like about one of my images.  This one is no exception.  But overall, I enjoyed putting this one together as much as anything I've ever done in astronomy.  The data was excellent and I couldn't be happier with how it came out in the end.  Okay, yes I could.  But considering the factors involved in gathering the data I'm as happy as can be.  I'm also a firm believer that we don't learn from our successes, we learn from our mistakes.  And thus, a constructive critique is the best way to really improve - so on to what I don't like.

Field Curvature - If you click on the above image and look at the larger size, it's pretty apparent looking toward the edges that there's a good amount of field curvature.  This is self inflicted.  I had hoped to be able to use the ST-8300 without a flattener and my calculations told me that it would be close but acceptable.  I was wrong.  Because the cameras, filter wheel and OAG combined weigh 4.5 lbs (2.1 kg) I don't feel comfortable depending on three thumbscrews and a compression ring to hold the equipment.  Right now the imaging rig is all threaded connections from telescope to camera and there's no flexure.  So I guess I'll start looking for a solution that incorporates a field flattener into the mix while keeping my 100% threaded solution.

Noise Reduction - I overshot it a bit on the noise reduction.  The image is "too smooth" to me.  In my defense, I was shooting in the lower deserts of Arizona from my backyard.  Daytime temperatures were approaching 110º F (43º C) and the lowest night time temperature I saw was in the upper 80's (about 30º C).  This limited the amount of cooling that I was able to get from the camera and the entire image was shot with the TEC sat at 0º C.  The images had huge amounts of thermal noise in a 30 minute sub.  A good set of darks certainly helped but the starting point for post-processing was a pretty noisy image to begin with.  I expect I'll have a much better result when I get the camera in some cooler ambient air - hopefully soon.