Baader UHC-L deepsky filter (1.25")

Baader UHC-L Nebula filter with highest transmission, realistic star colors, also usable as highly effective LED-optimized luminance filter for LRGB imaging.

The Baader UHC-L filter is the classic allround "light pollution rejection filter", but now also blocked against the main LED emissions in the blue part of the spectrum. The passband curve corresponds to a wide dual-band filter without distorting star colors. The two passbands transmitt all light emitted by the known gas nebulae (the H-alpha and H-beta lines of fluorescent hydrogen as well as the OIII line of oxygen), while both the atmospheric most disturbing stray light from LED lamps and intense bluish car headlights as well as further stray light from classical street lamps and the natural night sky glow (airglow/skyglow) of the atmosphere are blocked to a large extent. At the same time, the UHC-L filter is coated with latest technology (CMOS-optimized) and therefore blocks false light much more effective than all previous UHC filters.

Use of the UHC-L filter

The application of the UHC-L happens mainly in visual and photographic observation of emission nebulae (star forming regions, planetary nebulae, supernova remnants) under medium to highly polluted skies. Its use is also interesting for comet observation: It passes the OIII line at 501nm as well as the two C2 lines at 511nm and 514nm which are characteristic for the gas tail of many comets.

This UHC-L filter is the successor of the well-proven plane-optical polished Baader UHC-S filter. It consequently complements it with the advantages of CMOS optimization (no disturbing halos, blackened filter edges, Life-Coat™-coating) while providing an improved transmission spectrum adapted to also cover modern LED light pollution. It provides a brighter image than many classic LPR (Light Pollution Rejection) filters and is therefore not only suitable for large, fast instruments, but also for smaller telescopes collecting less light, where a large exit pupil cannot be achieved. This applies for long-focal length telescopes as well as for small aperture telescopes that only have an eyepiece socket for 1.25" eyepieces. Unlike the single line or dual.line narrowband filters, the image is also bright enough for high magnification, while not diminishing the number of stars. For deep-sky observers, therefore, this UHC-L filter is the perfect visual complement to narrowband OIII filters.

The UHC-L filter is also excellent for photographic use. It does not just block polluting terrestrial light sources, but at the same time it serves as a luminance filter (L-filter), since it blocks all UV and IR transmission like a UV/IR cut filter. An additional UV/IR-cut filter is therefore not necessary: Cameras without a built-in UV/IR-cut filter thus deliver significantly sharper images on refracting telescopes. Both classical photographers and users of EAA/LiveStacking do benefit from the increased sharpness of stars and better contrast of nebulae, as well as the improved transmittance in H-alpha.

Like all Baader CMOS-optimized filters, Baader UHC-L filters likewise are plano-optically polished, with ultra-hard, sputtered dielectric coatings. Even a one-hour bath in boiling water cannot harm these filters! They are scratch-resistant and can be cleaned as often as desired.

The Baader UHC-L / Ultra-L-Booster Filter offers all the advantages of the CMOS-opitmized Baader filters:

• Increased contrast, matched for typical CMOS quantum efficiency and s/n ratio
• Reflex-Blocker coatings, for largest ever freedom from halos, even under most adverse conditions concerning aux-optics
• Identical filter thickness to existing standards, with utmost care for parfocality
• Blackened edges all around, with filter-lead-side-indicator in the form of a black frontside outer rim, to additionally eliminate any reflection due to light falling onto the edge of a filter
• Each filter coated individually, with sealed coating edge (NOT cut out of a larger plate with coatings left exposed, read more)
• :Life-Coat™ evermore hard coatings to enable a non-aging coating for life – even in a most adverse environment

The UHC filters are general purpose deepsky filters which blocks the light from Hg and Na lamps, but transmits all of important wavelengths, such as H-alpha, H-beta and O-III. UHC filters are available from many different manufacturers, checking the image below might help to choose the right one. As it is evident there is no such thing as UHC "standard". All of the filters transmits most of the light around H-alpha (656nm), Oxigen-III (501nm) and H-beta (485) lines. 

There are wider and narrower filters. But what might be the difference between them in practice? It depends on heavily the selected targets.

A general rule is that the narrowed the transmission is the higher the contrast is, meaning that they work excellently for emission, planetary nebulae and supernova remnants. They don't work that well on objects with wider spectrum, like stellar clusters, galaxies, open- and globular clusters. To photograph these a wider transmission, light pollution reduction filters are recommended.

So: for emission and planetary nebulae, and supernova remnant use a narrowband filter, like Astronomik or Castell UHC.