The Baader L-RGB-C CCD Filter

lrgbc-filterset-beschriftetA brief introduction to the function of CCD Filters

Without modern RGB-filters, the CCD-revolution in astronomy would never have happened. It´s these filters that allow schools and universities do do meaningful imaging in science and education under totally light polluted skies from the midst of major cities.

Unlike terrestrial objects, astronomical objects shine in discrete emission lines. For this reason, any RGB-filter design with gently raising and falling slopes on either side of the transmitted spectral region generally is undesirable. The stars themselves obey to the laws of physics and shine by their stellar temperature colour - with a smooth, wide spectrum. This richness of colours can be covered nicely when adding an L-filter into the imaging process. However - shades and colour hues such as in earthly objects are not available when imaging the sharply defined emission spectra of deep sky objects. For this reason  the slopes on RGB filter curves ought to be produced extremely steep for each colour channel - for maximum energy collection efficiency, while maintaining maximum contrast between the individual spectral emission lines.

Peak transmission of Baader RGB filters are extremely high but at the same time encased tightly within each of the three colour channels - with a very important and deliberate overlap between the B and G spectral region and a calculated wide gap between G and R to exclude a whole family of undesirable terrestrial street light emission lines (just like a UHC nebula filter). Thus colour balance and colour rendition of Baader RGB filters are outstanding, while stray light and reflections are simultaneously reduced to an unprecedented level - causing our filter recipe to having received the worldwide "chinese honour" of being copied by various OEM-marketing companies under a wide variety of trade names. Still our copyists need to prove that they apply the same care onto every aspect of the production process. This also includes selection of highly homogeneous glass-substrates, precision polishing of each individual filter, most expensive evaporation rare earth materials - and a number of other proprietary ideas - to really achieve the same performance on the sky. Given our precision in filter workmanship, the above mentioned B/G overlap does cleanly separate the key emission lines of H-beta and O III but at the same time allows to double the energy in the O III line. Within the spectral region around 580 nm there is no significant celestial emission line, however a whole family of street lights (mostly Mercury and Sodium vapour lamps) emit their devastating energy within that region. Exactly this spectral area is almost completely suppressed by the Baader RGB filter design. All these design features result in substantially improved colour balance and above all, this design transmits the full extent of energy of these important deep sky emission lines better than any other filter recipe we have analysed. The increased contrast and absence of haze and blurriness is recognized repeatedly by experienced users. In this way Baader RGB filters play a major roll in eliminating light pollution when imaging from flawed, light polluted sites.

When comparing different filter offers, always demand to see the full extent of the spectral area were modern CCD-cameras are sensitive, that is - between 300 to 1150 nm at least. Many companies only present a cut-out of the full CCD-sensitive spectral area - mainly to hide off band transmission were their inexpensive design has gaps which causes unwanted light to leak onto the image and spoil the data.

Transmission characteristics of all Baader filters are carefully optimized to only transmit wanted light without off band transmission

 

lrgbc_filterkurve Transmission curves of the Baader LRGBC Filter (Luminance, Red, Green, Blue, Clear) and transmission lines of the bands H-Beta, O III, H-alpha and S II

Mechanical Properties

  • Parfocal and plane polished substrates. Each individual filter is optically fine-polished to 1/4 wave.
  • Baader LRGBC filters are hard coated individually.
  • Baader filters especially are not cut out of large size plate-glass, which is a typical manufacturing process for economy filters (cut-out filters exhibit micro-cracks around all edges. Capillary action between glass and coating layers will lead to premature ageing due to moisture deposition).
  • All Baader filters have individual coating sealed edges, impermeable against ageing because the the penetration of moisture impossible.
  • Baader filters are being tested repeatedly to comply with MIL-specifications. One common process is to boil the test specimen for one hour in salt water. Baader filters remained completely intact as opposed to filters drilled out of large glass plates.
  • All Baader filters sport scratch resistant hard coatings which can be cleaned repeatedly throughout their entire lifetime as many times as needed - preferably with Baader Optical Wonder cleaning fluid.

Optical Properties

  • No reflections when compared to competing RGB-filter recipes.
  • Balanced RGB-design offer 1:1:1 exposure times for most telescope optical systems - an important benefit when imaging in automated mode.
  • Maximized colour contrast for each of the three RGB channels - achieved through steep slopes at all transmission curves combined with science approved placement of spectral window.
  • O III emission line double-weighted in the B and G channel as well, with maximum peak transmission for unparalleled deep sky S/N yield
  • R-Filter provides maximum transmission of H-alpha and S II emission but at the same time completely blocks all NIR and IR from 680 out to 1200 nm
  • Blocking of Mercury and Sodium vapour lamps at 580 nm in the G and R filters blackens the sky background and maximizes colour balance and colour separation.

2 thoughts on “The Baader L-RGB-C CCD Filter”

  • Francesco Megli

    1. Since RGB filters also cut IR and UV fringes, what is the utility of taking an L image?

    2. In case the necessity of an Luminance is maintained, is it feasible to stack an L filter (UV-IR-cut or Neodymium) on top of each RGB filter (that is at the nose of the filter slit) instead of taking and stacking an L image?

    Thank you.

    • Team Baader

      1. The color resolution of the human eye is much less then the luminance resolution.
      So if you take a high res, low noise L image for luminance it is enough to add 3 short exposed color images with low res and bad S/N ratio to achieve a perfect final color image.
      In case of solely RGB, so without L, you would meed much, much more exposure time to achieve a comparable result.
      Furthermore it is possible to mix a Halpha narrowband image into the luminance to further increase image contrast, without ruining the RGB balance.

      2. Stacking an L to RGB filters is completely useless. You would gain nothing, only loosing some light and pronouncing reflections.