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Dear fellow astronomers, On my lazy journey of OSC astrophotographer, I took my chance and bought the new Askar Colour Magic Sii+Oiii C2 filter, in the hope of adding some Sii signal to my images. I know that the perspective are limited and many poeple advised me that, at this point, I should consider moving to mono. I think that at 93$, adding Sii to all these image was a nice project, allowing to postpone the switch to mono by one year and to continue making the most of my OSC gears. Well, here are some initial results. I tried with little success to integrate my new Sii layer extracted from the OSC data using SIRIL, just like I extract the Ha. Unsatisfied with the result when adding it to my NGC 281, and without much expercience with Sii layers, I ran a comparison between the two Oiii layers. Below, you find a crope of 15300s of Oiii using a l-ultimate on the left. On the right, you find 12000s of Oiii, same very well known region of NGC 281, using the Askar C2. Both images are captured using a TS UNC 200/800 newtonian and an ASI294MC pro. Subs are 300s long in both cases. I just croped the stacks and applied an autostretch in Pixinsight.  The comparison is clear. The C2 delivers very inferior results I am afraid. Note that running FWHM and exentricity tests in Pixinsight, the image capured using the C2 should be slightly better in sharpness. Here are the Sii and the Oiii side by side, where one can immediately see that the Sii suffers from the same problems...  It is clear that the band pass are not comparable. Whereas the l-ultimate has 3nm, making it a high-end filter, the C2 has OIII：35nm±5nm/SⅡ：15nm±3nm I have however difficulty to understand how the difference in spec could account for such a different result. I wanted to share this, in case anyone is tempted. Any thought or hint to improve outcome is much appreciated. Clear skies ! |
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My recommendation is to get a pure SII filter such as the Antlia Edge 4.5nm. Works well to combine with ha/oiii from OSC. https://www.astrobin.com/ndclra/ |
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| I have done a similar comparison between my L-eXtreme and D2 filter (presumably narrower bandwidth than the C2 filter). And i've posted those results here. Since then I realize that the different in the bandwidth can make a very large difference in the contrast of your signal, especially in light polluted skies (or with a bright moon). Also "Cuiv, the lazy geek" on youtube (here) did a video about these filters you might want to check out. He talked about the fact that some energy form the red signals (Ha and Sii) will leak into the green and blue channels (Oiii), so that the Oiii signal always has a little of the Ha or Sii in it (and vice-versa). But that since the Ha is so much stronger than the Sii, Oiii *seems* better in the Ha/Oiii filter than the Sii/Oiii filter, but you are being tricked. Not sure I buy all that, but suffice to say, it's complicated! And i agree that you need to go mono to get the best signals in all the channels. |
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Richard Carande: Exactly, the OIII in an HaOIII filter is being "contaminated" by the strong Ha signal, since it will cross over to the green channel. Which is an inherent characteristic of an OSC camera. So, basically this means that the OIII in an SIIOIII filter is purer, since the SII is not strong enough and more red shifted that it will not be picked up by the green pixels. |
