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All You Need to Know About the Dual Band Filter for Vespera: Function, Uses, and Results

06 Aug. 2025

Astrophotography is a demanding and constantly evolving discipline, with a steady stream of innovations aimed at making the work of enthusiasts easier and more rewarding.

Among the more surprising technological leaps, we can highlight the invention of CCD sensors in 1969. Originally designed for computing, they were soon recognized for their photosensitive capabilities. Today, cameras equipped with these sensors are a delight for night photography enthusiasts.

More recently, the emergence of smart telescopes has revolutionized the field, making astrophotography accessible to a much broader audience. No more time-consuming settings, steep learning curves for first images, or hours of image processing on software often out of reach for beginners.

But if there’s one piece of hardware that has evolved significantly in recent decades, it’s the filter. Whether used with a traditional telescope, a refractor, a CCD camera, a standard camera lens, or even a smart telescope, a filter helps get the most out of your equipment.

CLS filters, designed to reduce light pollution, are widely known and used. Their popularity stems from the fact that a large portion of the population is affected by light pollution during observation or imaging sessions. These filters work by blocking specific wavelengths from artificial lighting, thus improving image quality—even in heavily light-polluted urban environments.

Less well-known because they’re used by more advanced users, Dual Band filters are designed to photograph specific objects in the night sky.

What is a Dual Band Filter?

Dual Band filters are narrowband interference filters that isolate two specific emission lines: Hydrogen Alpha (H-alpha) and Oxygen III (O-III). In simple terms, these filters block all wavelengths of light except those two.

To put this into context: all light we see (whether with the naked eye, a camera, or a CCD sensor) is made up of a range of wavelengths. Some fall within the visible spectrum (what the human eye can perceive), others are invisible (ultraviolet, gamma rays, infrared, radio waves, etc.). Some animals, like the mantis shrimp, can even perceive wavelengths far beyond human vision, such as ultraviolet.

Figure 1: Visible and invisible spectrums. Human vision is limited to light between 400 and 700nm. Source: School Mouv.

Hydrogen Alpha and Oxygen III both fall within the visible range, but isolating them from the rest of the spectrum during astrophotography can significantly improve the capture of certain deep-sky objects.

What Is a Dual Band Filter Used For?

The main advantage of a Dual Band filter is that it helps highlight nebulae more effectively. By letting through only H-alpha and O-III emissions, the filter isolates celestial objects, increases contrast against the background sky, and reduces the prominence of stars in the image.

It is especially effective for imaging:

Emission nebulae (e.g., Rosette nebula, Heart Nebula IC 1805)
Dark nebulae surrounded by hydrogen (e.g., Horsehead Nebula, Elephant’s Trunk)
Planetary nebulae (e.g., Owl Nebula M97, Dumbbell Nebula M27)
Supernova remnants (e.g., Cygnus Loop, Crab Nebula M1)

However, it is not recommended for imaging reflection nebulae, galaxies, or star clusters.

Figure 2: Dual Band filters enhance the hydrogen clouds of IC 434. Barnard 33 (Horsehead Nebula, foreground) stands out more clearly.

Another strong advantage of Dual Band filters is their performance in light-polluted areas. By blocking artificial light wavelengths, they significantly improve image quality compared to an unfiltered capture. However, to take full advantage of the filter, it’s essential to choose your targets wisely—regardless of your light pollution level.

Figure 3: This photo of the Rosette Nebula was taken under a heavily light-polluted sky (Bortle 5). The Dual Band filter not only enhanced the nebula’s contrast but also reduced the impact of artificial light.

Dual Band Filter and Smart Telescopes

While there are many Dual Band filters designed for astrophotography cameras, these filters are also available for Vaonis smart telescopes in the Vespera range.

The Dual Band filter for Vespera II and Vespera Pro works just like any other, but with added smart functionality. It consists of an optical element embedded in a plastic ring, paired with an electronic chip. This allows the telescope to detect whether the filter is installed and automatically adjust its internal settings.

This feature is especially handy when using multi-night observations, as the system reminds you to reinsert the filter if it was removed. If the filter is missing, the Singularity app will display a notification prompting you to put it back in place.

From the user’s side, once installed with a simple click, the filter does not alter the usual capture process. Just select your target, control your telescope via the app, and start imaging. As always, you can export your results as FITS files, a stacked TIFF, or a ready-to-share JPEG if you prefer to skip post-processing.

A quick test on a nebula will demonstrate within minutes the undeniable impact this filter has on image quality. While the recommended exposure times remain the same, the target object tends to appear more quickly on screen. That said, it is still advisable to double the suggested exposure duration to capture the maximum amount of detail.

Figure 4: Equipped with the Dual Band filter, Vespera II is ready for all your nighttime sessions—even the most ambitious ones.