Ultra-narrowband filters have become extremely popular in astrophotography. A 3nm filter can reject more unwanted background light than a wider filter, making it especially useful under heavy light pollution. But at AstroPeak, where telescopes operate under exceptionally dark southern Utah skies, the narrowest filter is not automatically the best choice.
For most systems hosted at AstroPeak, we recommend 5-7nm narrowband filters as an excellent balance of contrast, performance, and compatibility. There are two main reasons: our dark skies reduce the need for the aggressive background rejection provided by 3nm filters, and a somewhat wider bandpass is more forgiving across different optical systems.
Why 3nm Isn’t Automatically Better
The numbers describe the width of the filter’s transmission band, not its overall quality. A 3nm filter admits a narrower portion of the spectrum than a 6nm filter, which can improve contrast when unwanted background light is significant. That advantage can be valuable from a city or under strong moonlight.
At a dark-sky site, however, there is much less background light to suppress. The goal is not simply to block as much light as possible; it is to capture the desired emission signal while maintaining the contrast your conditions require. At AstroPeak, a 6nm filter generally provides excellent contrast without imposing some of the additional constraints associated with an ultra-narrow bandpass.
This is particularly relevant for the f/5 to f/7 refractors used by many astrophotographers. With these systems, bandpass shift is generally less of a concern, so the choice is driven primarily by imaging conditions, goals, and cost. Under AstroPeak’s dark skies, 6nm is a strong general-purpose choice.
A Note About Fast Optical Systems
Narrowband astrophotography filters are interference filters designed to transmit a small range of wavelengths. An H-alpha filter, for example, is centered near the H-alpha emission line at 656.3nm. The filter’s effective transmission band changes slightly depending on the angle at which light passes through it, shifting toward shorter wavelengths as that angle increases. This is commonly called bandpass shift or blue shift.
The effect becomes more significant in very fast optical systems, which send light through the filter at a wider range of angles. Because a 3nm filter has a narrower transmission window, a shift can move the desired emission line away from the filter’s peak transmission and reduce the amount of useful signal that reaches the camera. A 6nm filter provides more margin around the emission line and is therefore more tolerant of this shift.
Owners of especially fast systems — roughly f/4 and below — should pay closer attention to filter compatibility. The faster the system, the more important it becomes to check the manufacturer’s recommended focal-ratio range. Some manufacturers offer high-speed or pre-shifted filters designed specifically for systems around f/3 or f/2. If you want to use 3nm filters with a very fast telescope, choose a model intended for that light cone.
When 3nm Filters Make Sense
None of this means that 3nm filters are a poor choice. High-quality 3nm filters can produce exceptional images, including at AstroPeak. They may be appropriate when imaging under strong moonlight, pursuing specialized scientific or imaging goals, separating closely spaced emission lines, or using an optical system specifically matched to the filter.
If you already own a quality set of 3nm filters, there is no general reason to replace them. The important point is that 3nm should be selected for a specific benefit, not simply because a smaller number appears more advanced.
Our Recommendation
If you are purchasing narrowband filters for a telescope hosted at AstroPeak, we generally recommend 5-7nm filters. They offer an excellent balance under our dark skies and suit the f/5 to f/7 refractors used by many astrophotographers.
Consider 3nm filters when your imaging goals call for maximum background rejection or another specific capability. If your telescope is especially fast, confirm that any ultra-narrow filter is designed for its focal ratio.
Choose the filter that matches your telescope, imaging goals, and sky — not simply the narrowest filter available.