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So after trying out the LRGB filters for last post, I wanted to try out the narrowband filters. Those filters are built in a way that only very specific light emission lines mass through. Those filters are used for pictures of nebulae, because stars and galaxies emits light in all wavelengths. The most common narrowband filters, which are the ones I got, are:

  • Hα: Only emissions on a specific wave length given by Hydrogen
  • OIII: Emissions of a kind of ionized Oxygen that can only occur on the vacuum of space
  • SII: Emission lines of Sulfur, also ionized.

This narrowband pictures are used to create fake color pictures, where we assign each of the filters to one of the RGB colors. Which filter go to which color is completely up to you. But there are some well known defaults. For example, the Hubble Palette, used for narrowband pictures from the Hubble telescope.

The Hubble Palette assigns SII to Red, Hα to Green, and OII to Blue. In my first trial with those filters it is easy to see that the Hydrogen Alpha filter got the most details out of the nebula. Since the Bubble Nebula is mostly composed of Hydrogen.

After taking the pictures I notice that they were not very well focused, which is bad. I need to get used to change filters and refocus every time. Anyway I did two editions with the data I got. Initially stacking the pictures with Pixinsight cleaned well the hot pixels, but made the stars have a odd bright halo around it. But stacking with Maxim DL gave better results on this matter, and way worse results on hot pixel removal. I had to remove all the main hot pixels myself to be able to use the Maxim DL stack.

2015-04-25-ngc7635_Ha20x30s_OIII20x30s_SIIl20x30s 2015-04-25-ngc7635_Ha20x30s_OIII20x30s_SIIl20x30s_bBubble Nebula (NGC7635)
Celestron AVX 8″ SCT
Hyperstar 3
Filter: Baader Hyperspeed Ha, OIII, SII
Focal Length: 425 mm
Aperture: f/2.1
Shutter: 30s
Stacking: 20 / 20 / 20
Total exposure: 30 min
Aquisition: Artemis Capture
Post processing: PixInsight, Maxim DL
Taken Apr 27, 2015 around 23:18 CEST

 So, both those pictures are exactly the same, just different post processing. The first one is more smooth, but the second one shows more faint details around the main area of the nebula. I didn’t specially like neither. I believe I need to go way further than 20 exposures to get something smoother.

Something weird happen when i was getting my calibration frames. It seems that Artemis or Maxim DL mirror the saved image. I dont know which. But I took the light frames with Artemis and the calibration frames with Maxim DL, and I could see the hot pixels were mirrored. So all my calibration frames were useless.

Still a lot to learn, and a lot to try =)

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