Astronomik Oxygen III (OIII) Filter

5.0 Astronomik OIII Filter

- best performing OIII filter available
- excellent quality

- very expensive
- hard to find
- meant for 8" or larger apertures

Before investing in a filter to combat light pollution, I did a lot of research to try and decide which one was the best performer. I live in the middle of a fairly large city (~1million), so performance of the filter is important. Second of course is cost. I compared all the filters I found that were readily available on the market:

- Astronomik (UHC, OIII)
- Baader Planetarium (Moon & Skyglow, UHC-S, OIII)
- Lumicon (Deepsky, UHC, OIII)
- Meade (Broadband, Narrowband, OIII)
- Orion (Skyglow, Ultrablock, OIII)
- Televue (Nebustar, OIII)
- IDAP (LPS)

Note that based on the information I could find, the Celestron brand light pollution filters are made by Baader and so their performance would seem to be the same. As the point of comparison I looked at the spectral response data that was provided for each filter by the manufacturers or by third parties who have actually measured the response of the filters. I digitized and replotted all the data in the same scale on the same graph and compared. The objective of the filter is to reject the largest amount of light pollution wavelengths possible, and have the highest transmission possible in the bands of interest. Light pollution, including skyglow (a natural phenomenon), occupies a band from 400-440nm and 540-640nm. The desirable observation bands for nebulae are: H-beta 486.1, OIII 495.9 & 500.7, H-alpha 656.3, and SII 672.6.

Based on the data I collected, the Astronomik OIII is the best performing OIII filter, having excellent transmission in the band of interest, and very sharp cut-off to nil transmission in the light pollution bands. The Televue and Lumicon are second having very close to the same transmission around OIII as the Astronomik, but they have non-zero transmission below 430nm (Lumicon) and above 650nm (both).

So based on my analysis I searched for and purchased the Astronomik OIII filter. It was hard to find in Canada, and expensive (~$200USD for a 2”). I am however extremely happy with my purchase. The filter is top notch quality, and it has performed for me on my 8” SCT as I expected it to. The filter is made in Germany, and it comes in a nice foam lined soft plastic case that closes securely…no chance of it flying open like other filter cases or cracking if stepped on.

Note that an OIII filter passes only a very small window in the visual band, and so reject the maximum amount of light pollution or star emissions needed to maximize the contrast of nebulae. The result however is that you need a large aperture telescope to use them effectively, 8" or better.