NGC 6888 - Crescent Nebula
|NGC 6888 as captured with the new imaging equipment. SBIG cameras, filter & off axis guider utilizing Explore Scientific optics.|
Gathering the ImageWhen 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
- 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'tI'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.