Baader Semi-APO Filter - 2"
The Baader Planetarium Semi-APO filter works like it's name implies, delivering an amazingly neutral view, while cutting most of the annoying and contrast-robbing violet fringe that plagues all achromats. The Semi-APO filter combines a stronger Fringe Killer coating formula with the Moon & Skyglow Neodymium substrate. The result is similar performance to the Fringe Killer plus Moon & Skyglow combination, but with a slightly stronger violet cut and at an even greater value.
The Semi-APO filter benefits from all the host of world-class Baader filter attributes. The Semi-APO filter is truly the world's most advanced 'minus-violet' type filter in every regard. Its high efficiency and neutral coloration make it practically disappear in use, while making most of the violet fringe disappear! The effects on sharpness, contrast, and aesthetics are remarkable, even with small refractors. The RGB enhancement of the Neodymium substrate actually boosts color contrasts, particularly for Jupiter and Mars.
As a side benefit, the Semi-APO filter has also proven to be the best Mars filter we've used, regardless of the telescope used, including large Dobsonians, SCTs, and Apochromats. The unique properties that result from the combination of the Fringe Killer's blue wavelength attenuation (attenuation - not total elimination), plus the selective Neodymium RGB passbands, result in a significant contrast enhancement of Martian features. The image retains its natural coloration and brightness, and the optically polished surfaces maintain every bit of the image sharpness, unlike other manufacturer's filters.
The high quality of Baader Planetarium filters permits their use well ahead of the focal plane with no image degradation (ie, for cameras or binoviewers). The inclusion of Baader's renowned UV/IR Cut coatings on the Semi-APO eliminate UV and IR wavelengths to make it ideal for high-resolution imaging with Webcams, DSLR, and CCD cameras, and the high transmission and fringe reduction work great for both planetary and deep sky imaging.
Baader Planetarium Semi-APO Filter Features
True Optical Quality: Baader glass substrates are of true optical quality. Most other filters are merely made from raw flame-polished, "optically flat", or sometimes a top quality commercially polished filter glass (fine for camera filters), resulting in wavefront deformations that damage image sharpness and contrast when used with telescopes. Baader Planetarium filters are made from striae-free substrates, and actually fine optically polished flat to within 1/4 wave p-v over the entire surface, plane parallel to within 30 seconds of arc! This important and unique custom step adds cost - but the result is a filter that maintains the full wavefront quality of the telescope without double images or ghosting, even at high magnifications. Critical to maintaining this level of flatness are Baader's carefully balanced coating designs, which prevent coating surface stresses from warping the substrate. Such high flatness enables Baader filters to be used far in front of the focal plane, ahead of star diagonals and digital cameras. They are particularly well suited for attachment to the nosepiece of the Maxbright Binocular Viewer (other manufacturer's filters must be used in pairs, attached to the eyepieces).
Filter Durability:Filter durability has always been a hallmark of Baader Planetarium. The multi-layer dielectric coatings are plasma-assisted and ion beam-hardened using the latest technology (the coatings are harder than the glass substrate itself!). Baader filters withstand repeated real-world exposure to the elements and physical cleaning. Users need not leave their precious filters dull and dust encrusted to avoid cleaning - Baader filters may be used and cleaned without fear. We have heard sad stories from owners of other simpler soft-coated dielectric filters, who have seen their coatings degrade from normal exposure to moisture, or wiped away from routine cleaning. Some manufacturers even resort to sandwiching their delicate coatings in between two layers of glass (and risking blurred double images as well as eventual degradation from the edges inward). Baader Filters will truly last a lifetime.
Stackability:Unlike other filters, Baader Planetarium filters are truly designed to be stacked and combined. This is possible due to the high optical quality and highly efficient anti-reflection multicoatings. The ability to combine filters opens up many exciting possibilities, permitting unique spectral characteristics unachievable with any single filter.
Special Filter Cell:Baader's special filter cell design offers unique features that enhance their reliability and usability. Though difficult to produce, the ultra-thin filter cell results in the maximum possible clear aperture, in order to minimize vignetting (1.25" Filter clear aperture is 27mm, 2" Filter aperture is 45mm). The special threading has been designed to fit a wide variety of eyepieces and accessories (there is significant variation between the 1.25" filter threading used by eyepiece and accessory manufacturers). The front 'crown' of the filter incorporates milled notches which make handling and threading the filter a more secure operation in the dark.
Low cost:Despite high quality and advanced technology, Baader Planetarium filters are affordable. By leveraging high volumes across all filters in the Baader family, these filters can be offered at very reasonable prices. 
