Broadband filters, often referred to as "light pollution reduction" (LPR) or "CLS" (City Light Suppression) filters, are designed to block specific wavelengths of light commonly associated with artificial light sources (like sodium vapor and mercury vapor lamps) while allowing most of the visible spectrum, including the light from broadband celestial objects, to pass through.
Here's a breakdown of their suitability across the Bortle Scale:
Bortle Scale 1-4 (Dark to Rural/Suburban Transition):
* Generally NOT recommended or necessary. If you're lucky enough to be in truly dark or moderately dark skies, a broadband filter will often do more harm than good.
* Loss of Signal: Broadband filters block some light from the desired celestial object, as they are cutting out parts of the spectrum. In dark skies, the benefit of light pollution reduction is minimal, and the loss of natural light from your target can actually reduce the overall signal-to-noise ratio.
* Color Shift: They can introduce a slight color cast, making color calibration more challenging.
* Dimming: They will dim the overall view, which is counterproductive in dark skies where you want to gather as much light as possible.
* Exception: Some astrophotographers might use a mild broadband filter (like an Optolong L-Pro) in Bortle 4 or even 3 if they are trying to specifically combat residual light pollution from a distant city glow on the horizon, or to slightly enhance contrast on some objects. However, for most broadband targets, no filter is often the best choice in these conditions.
Bortle Scale 5-7 (Suburban to Urban Transition):
* Where they are most effective and commonly used. This is the "sweet spot" for broadband filters.
* Light Pollution Reduction: In these areas, there's a significant amount of light pollution from various sources. Broadband filters help to filter out the common culprits (sodium, mercury vapor) making a noticeable difference in reducing sky glow and improving contrast for broadband targets.
* Suitable for Galaxies and Star Clusters: These filters allow enough of the broad spectrum light from galaxies, star clusters, and reflection nebulae to pass through, making them viable targets even from these moderately light-polluted locations.
* More Natural Colors: Compared to narrowband or dual-band filters, broadband filters generally allow for more natural-looking star colors.
* Examples: Many popular broadband filters like the Optolong L-Pro, Astronomik CLS, or similar are designed for these conditions.
Bortle Scale 8-9 (City Sky to Inner-City Sky):
* Limited effectiveness, often less beneficial than dual-band/narrowband filters.
* Newer LED Light Pollution: Modern LED streetlights emit a much broader spectrum of light, which broadband filters struggle to block effectively without also blocking significant amounts of desired light from your celestial target. This makes them less effective against contemporary light pollution.
* Overwhelmed Signal: In extremely light-polluted areas, the sky glow can be so intense that even a broadband filter can't sufficiently reduce it to make fainter broadband targets (like galaxies) stand out. The signal from these objects is simply too overwhelmed by the background.
* Better for Specific Targets: For Bortle 8-9, if you want to image, your best bet for most objects is to focus on emission nebulae using dual-band or narrowband filters. These filters are far more aggressive at isolating specific wavelengths, allowing you to cut through extreme light pollution to capture objects that emit light predominantly in those narrow bands (like H-alpha and O-III).
* Processing is Key: Even with a broadband filter in these conditions, significant post-processing (gradient removal, noise reduction) will be essential to salvage an image. Some astrophotographers even argue that for galaxies in Bortle 8-9, it's sometimes better to shoot without a filter and rely solely on aggressive processing tools like PixInsight's DynamicBackgroundExtraction or Siril's background extraction, as a filter might remove too much valuable signal.
In summary:
* Bortle 1-4: Generally no filter is best for broadband targets.
* Bortle 5-7: Broadband filters are highly recommended and effective for imaging broadband targets (galaxies, star clusters, reflection nebulae) and for general light pollution reduction.
* Bortle 8-9: Broadband filters have limited effectiveness, especially against modern LED light pollution. Dual-band or narrowband filters are usually preferred for imaging emission nebulae from these locations, while broadband targets remain extremely challenging.
If you are in Bangkok (likely Bortle 8 or 9 in most areas), a broadband filter might offer a slight improvement for some targets, but you will likely find dual-band or narrowband filters for nebulae to be much more impactful for imaging. For galaxies, battling the light pollution will be a significant challenge regardless of the filter, and often relies heavily on integration time and advanced post-processing.