To my eye, wide-field astrophotography benefits from the use of diffusion filters. These filters spread out the light from the point-source stars and produce a better depiction of relative star brightness. Filters that I use are the Kase Astroblast, Tiffen double fog 3, and Hoya/Kenko Softon-A. The Softon filter is my current favorite. All of these filters produce "round" star images.
I recently acquired a different type of filter, the Hoya Sparkle 6x. This filter adds six diffraction spikes to concentrated light sources. There is also a "Sparkle 4x" variation that adds four diffraction spikes. The original motivation for trying this filter was get more "sparkle" in the photos of the Christmas tree. But it also looks promising for astrophotography.
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| Constellation Auriga. E-M5iii + Sigma 30mm f1/4 + Hoya Sparkle-6x filter. ISO 1600, 15 s. |
The filter does a good job showing relative star brightness and also preserves color quite well.
Auriga is the home of three Messier-catalog open star clusters: M36, M37, and M38.
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| White circles: M38 (top), M36 (Middle), M37 (Bottom). |
It looks like this filter will work best with focal lengths 30 mm and shorter. With longer focal lengths the diffraction effect is way over-done. Here are two examples using a 75mm lens:
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| The Pleiades (M45). Olympus 75mm f/1.8 + Hoya Sparkle-6x filter. |
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| Aldebaran and the Hyades Cluster. Oly 75mm f/1.8 + Sparkle-6x filter. |
And yeah, I think it does a good job on the Christmas tree:
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