Tests of Orion filters
I tested the Orion "Sky glow" and "Ultra block" filters using my instruments last night. My overall impression is that they both work well, with the Ultra block filter being a very close second to a Meade OIII filter. I found that they are really only suited to larger aperture instruments.
I chose a night with extremely poor seeing caused by light scattered from dust in the atmosphere. The objects were at varying altitudes, and were examined using my ETX 4" and LX200 12". Here is a summary of my results
Object |
Common Name |
No Filter |
Sky Glow |
Ultra Block |
| M57 | Ring Nebula (Lyra) | Not visible in 4", barely visible in 12" (very poor contrast caused by light pollution) | Donut visible in 4" with averted vision, good view in 12" | Donut barely visible in 4" with averted vision, superb view in 12" |
| M17 | Swan Nebula (Sagittarius) | Visible in both instruments - contrast was poor. | Easily visible in both instruments - much better contrast | Easily visible in both instruments - excellent contrast, but object is faint in 4" |
| M11 | Wild Duck Cluster (Scutum) | Visible in both instruments - contrast was poor. | Barely visible in 4", improved contrast in 12" | Barely visible in 4" using averted vision, excellent contrast in 12" but the stars appear very green. |
Conclusion
Both filters work well provided enough light is collected. I recommend that the minimum aperture for the Sky Glow filter should be 6", while the Ultra Block needs at least 8" to be really effective. Neither is really useful in a small aperture instrument from the city - partly because it is not possible to get ones eye fully dark adapted.
I found that reflection from the filter was more pronounced than I have seen using filters mounted on the rear cell of my LX 200. This means that an observer needs to be well shielded from ambient light. Draping a dark cloth over one's head works well.
Bruce Dickson