Unfiltered Nebulae and the need for Star Reduction

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Moonless nights are great for imaging without the filter. While I enjoy using my narrowband filter to capture images of faint nebulae, not all objects are really suitable for it and sometimes I just like the unfiltered view.  Therefore I try to take unfiltered pictures whenever the conditions are right. 

The other week I took a number of pictures of nebulae at full C9 focal length of 2350mm. As the objects were within or close to the plane of the Milky Way the pictures had many stars that were quite a distraction from the main object. This hardly a problem when using a narrowband filter, as only a small percentage of the star light is getting through. In unfiltered images number, brightness and size of stars can become an issue. Conditions were probably not perfect, increasing the problem. However, at these long focal lengths the effect is hard to avoid entirely. 


One can address this in image processing by various star reduction algorithms. I took a quick crash course in the most common methods and tried them on my data. While these algorithms appear to have the tendency of introducing some amount of artifacts - I think the resulting images are an improvement.


Here is an example of the Crescent Nebula after Star reduction.




This version has the "original" stars, the nebula itself is comparably hard to see.



And here finally two other objects I captured in the same week. Star reduction has been applied there as well.











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