When selecting an (Ultra-)Narrowband/Highspeed filter, multiple factors must be considered
![](https://www.baader-planetarium.com/blogs/english/wp-content/uploads/sites/2/2022/04/filter-preshift-whitepaper-preview-150x150.jpg)
![](https://www.baader-planetarium.com/blogs/english/wp-content/uploads/sites/2/2022/04/filter-titel_unb-f3-trans-300x78.png)
- Desired Filter Wavelength (H-alpha / OIII / SII)
- Average Working Temperature
- Needed Halfbandwidth (6.5nm or 3.5/4nm)
After selecting the above criteria, a graphic will appear from which you can determine the required filter category based on this information:
- Aperture ratio f/# (x-Axis)
- Central Obstruction in % (y-Axis)