Hyperstar first Light - Part 2 Narrowband Imaging

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One of the main open questions when buying a Hyperstar was how it would work with L-eXtreme dual narrowband filter. At f/2.2 the light will no longer be mostly parallel to the optical axis,but will pass the filter at comparably high angles. This will lead to a downward shift in the filter transmission wavelengths. There are filters that are specifically designed to compensate, others just have wider transmission windows to be less sensitive for that effect. However, as these filters aren’t cheap, I wanted to see what I can do with my L-eXxtreme first.

Results published by other amateur astronomers vary from "the filter is completely useless at f/2" to "among my many filters the L-eXtreme is the best with my Hyperstar". Apparently, there is a bit of variability in the spectral transmission curve between the individual filter specimens.

In my second night of testing the Hyperstar I therefore took a number of images using the filter.






As there was rain forecasted in the 2nd half of that night I went with lower exposure times compared to the unfiltered images on the previous day. However, I still liked the results. Clearly my specimen of the L-eXtreme filter at least isn’t in the "completely useless with the Hyperstar" category.

In order to assess this a bit further I compared a detail of  NGC6992 with the same detail in the unfiltered data (using the same number of frames as a basis). 

Detail of NGC6992 taken with the L-eXtreme filter

The same Detail taken at full spectrum

In direct comparison perhaps the H-alpha could pop out a bit more in the filtered image, we may be getting only part of the signal there. However, the fine details are very similar, if you compare the images no features are missing. So my conclusion for now is that the filter is absolutely worth using with the Hyperstar,  e.g. if Moonlight would limit capturing images otherwise. 








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