Here i want to share the experiences i had with astronomical Filters.
Filter to enhance contrast
So far I used the following Filters:
-UHC Astronomik EOS EL Clip filter
-CLS Astronomik EOS XL Clip filter
Both filters will enhance the contrast of your Images. However, especially in light-polluted areas, you will see their full potential. After my Test of them on M42, the Orion Nebula, I came to the conclusion, that they are an extremely helpful help in the area I live in (Bortle 4), especially for Nebulas.
Both filters are working extremely well to eliminate light pollution. The Picture without filter shows clearly the yellowish polluted Sky. The vignetting makes it more visible :). The two Images with filters show almost no sign of light pollution. Again the Vignietting of my OTA helps in detection.
At a closer look, you will see that the remaining light pollution has a blueish color.
The Remaining light pollution is an even white, under my sky conditions.
Contrast and Colors
Both filters are filters. That means they block certain wavelenghts. So some Information is “missing”. This leads to an increased contrast (especially between red and blue/green). but also to somehow shifted colors with my unmodified Canon 6D
the colors are “original”. There is mostly red, grey and some blue.
The Blue gets shifted into green
This shift towards green is more intense. you can also recognize it in the glow of the star on top of the image… it shifted from blue (in the other 2 images) to green.
Contrast is beeing enhanced significantly by any of the 2 filters. The difference between the 2 filters is in my optinion more in the color than in the contrast as you see in the next 3 images
Special thanks to Helmut Franzen for lending me his Filters so i can decide what i need.