Baader UV/IR-Cut / L-Filter – CMOS-optimized

A brief introduction to the function of CMOS/CCD (L)RGB Filters

CMOS-optimierte Baader LRGB Filter

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 sensors are sensitive, that is – between 300 to 1150 nm at least. Many companies only present a cut-out of the full CCD/CMOS-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.

Baader-Deep-Sky Filters are completely blocked over a spectral range from 300 to 1150 nm for all wavelengths except the desired transmission ranges, which corresponds to the sensitivity range of current CMOS cameras. Thus, they also serve as UV and IR blocking filters, which could cause further reflections. This ensures that there are no leaks outside the target transmission.

Special emphasis was placed on avoiding reflections as best as possible during filter design. Modern CMOS sensors react more sensitively to reflections on glass surfaces in the vicinity of the filters. In developing this modern generation of filters, we have done everything to ensure that they work just as well on CMOS cameras as they do with CCD-cameras.

Transmission curves of the CMOS-optimized Baader LRGB Filter (Luminance, Red, Green, Blue)

Mechanical Properties

• Parfocal and plane polished substrates. Each individual filter is optically fine-polished to 1/4 wave
• Baader CMOS-optimized LRGB filters are hard coated individually. This is the only way to achieve sealed coating edges (Life-Coat™) that make them impermeable against ageing because the penetration of moisture is impossible
• 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)
• 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
• Scratch-resistant Reflex-Blocker™ hard coating, planeoptically polished. Filters can be cleaned repeatedly throughout their entire lifetime as many times as needed – preferably with  Optical Wonder™ Set (Cleaning Fluid and Cloth) (#2905009 , € 21)
• Blackened edges all around, with filter-lead-side-indicator in the form of a telescope-sided black outer rim

Optical Properties

• No filter-induced reflections
• 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.
• Extremely high transmission of the respective color channel, simultaneously reduces stray light and reflections to an unprecedented level. Reflex-Blocker™ coating for maximum insensitivity to retro-reflection from nearest auxiliary optics, even under the most adverse conditions.
• Blackened edges all around, with filter front indicator in the form of a telescope-side black outer rim, to additionally prevent any reflection due to light falling onto the edge of a filter
• 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.

• Life-Coat™: coatings that are even harder to provide an aging-resistant coating over an unlimited service life – even in the most adverse environments

Optimized for modern CMOS cameras, equally well suited for classic CCD camera technologies

For our CMOS-optimized filters we have created the following images showing which side of the filter should face the telescope.

LEGACY FAQ for older unmounted CCD-Filters without black outer rim

Question in Detail:

I just bought LRGB 36mm unmounted filters. I have question: which side of filter should be placed towards telescope? Is it better way of distinguish than "more shiny surface towards telescope"?

Answer:

Always put the more reflective side towards the telescope side. To guide you we already put a small arrow on the filter rim, on those filters were the position matters. This arrow indicates which face of the filter should be directed towards the sky (telescope-sided). All cell-mounted filters are already oriented in a way that the most appropriate filter face is facing the sky when the filter would be mounted directly onto the front end of the nosepiece of a camera.
If you mount your filter the other way, any reflected light would have a short way to the camera sensor, resulting in a higher risk of getting some kind of back-reflections inside the camera field. Many sensors have highly reflective areas near to the light sensitive area, also the area with the bonding contacts is sometimes highly reflective.

But: this is true only for instruments without optical elements near to the focal plane. If you have f.e. a coma corrector, field flattener, focal reducer, focal extender (to a lower degree due to concave surface), or in extreme cases a whole lens group for more complex field corrections a few centimeters in front of the filter it could be useful to flip the filter against the rule from above (thus having the arrow pointing away from the telescope). Cause in such cases the likelihood of reflections from the sensor could be lesser then fort- and back- reflections from such glass-surfaces. If in doubt, it helps to make some test images from a star field with bright stars, using the filter in both ways for comparison.

Should you really have some reflections with both positions it can be more effective to add a spacer between filter and camera, eventually shifting the reflection out of the image field. With focal correctors having curved surfaces changing the filter-lens distance could help also.

Quite often we receive requests for a single filter in an off-standard size. In all cases we are sorry that we must answer as follows:

Sorry (we know it would be so very much cheaper in production - and we would be so much more flexible to fill special requests) - but we have decided long ago to not cut or saw our filters from large plates because this would leave the coating stack open and mutilated (with microscopic cracks) all around, prone to aging and peeling.

Many times we had the chance to inspect our competitors filters after several years of use (due to our 30+years of servicing SBIG-CCD-cameras/and filter wheels) and we realized already 15 years ago how moisture and heat stress can deteriorate even most modern hard coatings, slowly peeling off from the carrying substrate over time, unless the coating stack is sealed all around the filter stack.

As a consequence - in order to create our filters to remain impermeable - we only offer all filter substrates already cut to final shape and run each substrate on an double deck auto-polisher to achieve perfect optical flatness and freedom from cone errors.
Then we do individually coat these substrates in 500 pc per run as minimum to fill a complete coating chamber, in a way that the coating stack (many times 50+ layers) applied onto each filter won't reach to the very edge of the round or square substrate, so that the coating stack remains completely sealed from all sides. In this way we can ensure that our filters will not age at all.

The sad effect is that we cannot offer other sizes unless the inquired production quantity were in the range of 250 to 500 pc (depending on size) and the tooling rings or square holders will be paid for, which serve to precisely center each individual filter substrate within the rotating calotte inside the coating chamber. Sorry - as explained above - we just will not coat onto large plates and cut any shape from them, also because such large plates cannot be polished optically flat in the same way we do it.

For your most urgent need and for single piece solutions we can only recommend to order the next larger size of our respective filter and have that cut to shape by an ophtalmologist locally. We can supply the round filters without the metal cell in such cases; square filters come without cell in any case.