Narrowband Imaging, Processing Tools and Techniques

ElephantTRunkNebulaSliderSince getting an SBIG ST-10XME and a CFW10 with Narrowband (Ha, SII, and OIII) filters, I could not wait to image an emission nebula.  I received the new goodies on August 9th, and of course, the conditions were not right to image until August 20th.  I had first light with the ST-10XME under a bright full moon so I could only shoot through Hydrogen Alpha filter.  Ten days later on August 30th, I was able to shoot with all 3 filters so I chose the Elephant’s Trunk Nebula (IC 1396) as my target.

Narrowband Imaging

Learning how to image through Narrowband filters was not as difficult as I imagined.  Here are some things I learned along the way:

  • Since these types of filters only let in certain wavelengths of light, one of the main differences was the exposure time.  I typically take 5 minute subs but I had to increase the time to 10 minutes to get a decent looking sub with lots of detail.  Your experience may vary and will depend on the focal length of your telescope and imaging equipment
  • I still focus my telescope manually so focusing follows the same routine of looking at the stars Full Half Width Maximum (FHWM) to see if you are in focus.  I focus with the Ha filter in a 3×3 bin, with a 1 second continuous exposure
  • I have collected data a couple of different ways.  When the moon is in the sky, I will collect Ha data.  If the Moon is not present, I will collect all 3 types of data (Ha, SII, and OIII)
  • With Narrowband imaging, it is all about data data data.  You can’t skimp on subs with these filters.  Plan on taking at least 2 to 3 hours PER filter (if not more).  If you take less than 2 to 3 hours of data, be prepared for some noisy images that are difficult to process
  • Creating flat frames is interesting with Narrowband filters.  Since these filters “filter out” most types of light, the flat frame process became sort of a chore and it was a guessing game as to how long to expose each filter to create a flat frame for the type of saturation I needed.  I created a flat frame light box to facilitate this.
  • Once you have collected the data, calibrating the images with Dark and Flat frames follows the same process as with LRGB filters

Narrowband Processing

I think I got in a little over my head with this but I learned a ton in a short amount of time thanks to the abundance of helpful resources and people that I keep in contact with:

  • The initial stacking of Narrowband images follows the same principles as LRGB stacking so just follow your normal routine (at least that is what I have with stacking my Narrowband images)
  • Here is an example of the 3 stacked FITS files for Ha, SII, and OIII:
Example of what a calibrated set of Narrowband images looks like before processing them in Photoshop (or other programs)

Example of what a calibrated set of Narrowband images looks like before processing them in Photoshop (or other programs)

  • Depending on how “good” your data is, you may want to consider using a program called CCDSharp, which is a free program that is used to sharpen FITS files by applying the Richardson-Lucy algorithm to the image.  Sometimes this is useful when processing Nebula images (or Globular Clusters).  I use this program on as needed basis and it depends on if I like how my individual FITS files turn out.  The following example used the Wizard Nebule (NGC 7380):

Richardson-Lucy Deconvolution Example CCDSharp

  • You may be asking yourself at this point: How do you take these images and map them to the classic Hubble Space Telescope palette?  I asked myself this same question once I finally had images to work with.
  • The classic Huble Space Telescope color palette maps the following way:
    • SII: Red
    • Ha: Green
    • OIII: Blue
  • This is where the internet comes in handy!  I searched and searched to find a website that explained the process so I could easily follow it.  After searching around, I came across Don Goldman’s website and he has explained this entire process very simply and easily.  The only catch is that he uses Photoshop to pull it off so if you have that program you are able to follow his method (below).  One step that you must do (which is left out of the video…but is on his website…is to change the image mode from Grayscale to RGB Color before you follow this tutorial.  You can do this in Photoshop by going to the following menus: Image>Mode>RGB Color (as seen below)

  •   If you have followed the tutorial correctly, here is what the separate images look like when mapped to Red, Green, and Blue:

NarrowbandFilterImagesMappedToHubbleSpaceTelescopePalette

  • After following the video, once you have mapped the Ha, SII, and OIII images to their respective colors in Photoshop, you will have a great start to your image where you can then add any brightness, curves, levels, etc. to each channel.  The best thing about following this method is that you have the flexibility to tweak any aspect of the image to your personal taste.  Here is an example of an image before the tweaking it

IC1396MappedToHubbleSpaceTelescopePalette

  • Here is what your Photoshop Layers Window should look like before you start tweaking it (if you followed the tutorial correctly):

  • Once you have added any other layers to each individual image, it will look something like this (see image below).  I have added additional Brightness/Contrast and Curves layers to bring out the color to my personal taste

  • Sharpening and Removing Noise: The process of adding layers and tweaking colors can sometimes introduce noise into your image.  Once I get the image 95% the way I want it, I then flatten the image and run the color image through a filter that removes noise.  A great Photoshop add on that Michael Caliguiri used and told me about was the Topaz Denoise filter.  To be honest, I just downloaded the free trial of it today and it works wonders for your image!  Give it a try!
  • Once the Color image is where you want it to be, Michael also told me a very cool trick to sharpen up the image and that is to add an Ha layer back into the image as a Luminance layer.  Simply apply a very light Unsharp mask to the Ha image and then copy it as a new layer (select the blending mode to Luminosity).  This does wonders for your the clarity and color of the image.  Here is an example:

  • After the Ha layer is added back in to the image, the last thing to do is to apply any last minute adjustments to colors or any aspect of the image that you would like to enhance.  Personally, I am not a fan of the Magenta stars that come out of the processing of these Narrowband images so I like to remove the Magenta from the image, which can be done in a couple ways:
    1. Apply a Selective Color Layer and change the selection to Magenta (I use some of the techniques outlined in Bob Franke’s Narrowband tutorial)
    2. Add another Hue/Saturation Layer and change the selection to Magenta in the drop down menu.  Then remove the Magenta by decreasing the saturation slider to the left
  • When all is said and done, you have a beautiful image for all of the hard work and effort that was put it into it:

Elephants Trunk Nebula (IC 1396) Narrowband

 

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