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Sh2-132 in Narrowband

November 7, 2014 3 comments

Both of my WO Star 71s are back from China (see previous post), cleaned, and as good as new.  Maybe better.  It took a few days after their return for some clear weather to arrive, but I was so happy to have them back.  William Yang was a good guy to deal with in this whole matter.

Sh2-132 is an odd nebula in that it can be hard to find.  Not so much because it’s not a reasonably bright narrowband target, but because it has no appeal to visual observers, so it’s not well-marked in most atlases.  Not all of the Sharpless objects are worthy targets (in fact the majority are duds), so it can be hard to tell that this one stands out from the emission nebula crowd.

It’s a photogenic target in the Hubble/SHO palette if you can take enough exposures to reveal the details, and several amateur images of it have popped up this season.  The SII is particularly dim, and if I could do it again, I would double the exposure time there.

Sh2-132 in the Hubble/SHO Narrowband Palette

Sh2-132 in the Hubble/SHO Narrowband Palette

Image data:

  • Exposures: 42 x 10 min Ha, 41 x 10 min OIII, 43 x 10 min SII
  • Telescopes: Two William Optics Star 71s (360mm f/5)
  • Cameras: SBIG ST-8300M and QSI 583wsg, 2×2 binned
  • Mount: CGEM
  • Guiding: QHY 5L-II mono, guided using PHD2
  • Conditions:  wind gusts over 25 mph… far from ideal
  • Processing: DeepSkyStacker, PixInsight, Photoshop
  • Dates: Oct 25-27, 2014

Tutorial – Creating contrast in nebula images

November 4, 2012 Leave a comment

Since the weather isn’t cooperating enough for me to capture more images of NGC 7000, let’s use the H-alpha channel image to show a few ways to create contrast in otherwise “flat” looking nebula images.

Here is the image basically right out of calibration, with only a single quick curve applied to stretch it a little.  This is six hours of total integration time (20 minute subexposures).

It’s okay, but nothing stands out.  There nebulosity has no definition.  It’s more like fog than a distant cloud.  Before anything else, the first step is to shrink the stars, because the unsharp mask filter I’ll use in subsequent steps has will exaggerate the stars.  You could move the stars to separate layer, but this is a short example, and that would require another tutorial entirely.  Here, all we do is run the minimum filter (Filter > Other > Minimum) with a radius of 1.  The original image size is about 3100 x 2400, so even the small stars can withstand the Minimum filter at this setting.  Smaller images may need to be resampled to avoid the destruction of smaller stars.

 

A look at the histogram reveals that the image isn’t taking up the full dynamic range, so we’ll use Curves to bring the white point down.  We’ll also subtly shape the bottom end of the curve so we bring up the darkest parts of the nebula at the same time.  This is why I always use Curves instead of Levels, even for simple adjustments.

Now, let’s create contrast between the smaller structures in the image.  The next image shows the results after unsharp mask (Filter > Sharpen > Unsharp mask) with a radius of about 10, a threshold of 3, and a strength of about 150%.  This was done to a layer containing a duplicate of the original image, and the layer was then set to the Soft Light blending mode.  Every image will require different settings, but the beauty of the filter is that you can preview the results as you make adjustments to highlight the details you want.  Because this filter reduced some of the darkest areas around the periphery more than I’d like, I used a layer mask to mask those areas off from the effects of the filter.

 

Things are looking much better.  The edges are better defined now, but I’d still like to create more contrast within the core of the nebula.  So I’ll duplicate the image again to a new layer and apply the unsharp mask again, this time with a radius of 90 pixels.  This creates contrast across larger structures.

 

Finally, it’s time for one more application of curves.  I’m still not happy with the contrast across the body of the nebula.  A subtly shaped curve will allow me to choose the areas I want to differentiate.  By holding down on control while clicking the image, I can create anchor points, shown below as A, B, and C.

 

 

The image is in pretty good shape now, at least for a monochrome image:

Now, let’s hope the skies clear so I can capture some S-II and O-III images to combine with this one!

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