Baader Semi-Apo filter 2″

I purchased the Baader Semi-APO filter mainly for photographic usage in combination with the achromatic refractor 120mm Startravel from SkyWatcher. It effectively reduced the purple secondary spectrum around brighter stars. However, for brighter stars this did not work anymore and eventually I gave up trying to create good pictures with an Achromatic refractor. It’s simply not possible. Not only are colors very difficult, but in addition the star size is also very large, creatings little disks instead of pin point images.


For visual use, this filter adds to the fine experience that a short-tube refractor can bring.

The biggest advantage that this filter brought me was the anti-light-pollution effect. I knew it was supposed to do so, because this filter does not only cut the purple part of the spectrum but also removes the Sodium (Natrium) lines of High-Pressure and Low-Pressure Sodium lights. In addition the infrared part is cut off also. Visually this filter is adding more contrast to observations of the brighter Deep-Sky objects.

I experimented with this short tube F5 SkyWatcher Achromat, by stopping down and applying this filter. Also other refractors are included in this overview. All images with an unmodified Nikon D5100 DSLR

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