Baader Semi-APO Filter - 1.25" # FSAPO-1 2458398

Semi-Apo Filter for Achromatic Refractors

As Baader's newest addition for owners of achromatic refractors, the Semi-APO filter delivers an amazingly neutral color view of bright objects while cutting most of the annoying and contrast-robbing violet fringe that plagues all achromats. The Semi-APO filter is actually two filters in one. It combines a stronger violet “Fringe Killer” coating formula within the Neodymium glass substrate of the Moon & Skyglow filter; you no longer need to buy both filters to achieve the beneficial effects of both! The effects on sharpness, contrast, and aesthetics are remarkable, even with small achromats. The filter also includes the RGB enhancement of the Neodymium substrate boosts color contrasts, particularly for observing Jupiter and Mars.

The Semi-APO filter has proven to be an excellent Mars filter - with ALL Telescopes, including large Dobsonians, SCTs, and Apochromats. The unique properties that result from the combination of the Fringe Killer's blue/violet wavelength attenuation, 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 when used, for example, with 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.  The high transmission and fringe reduction work great for both planetary and deep sky imaging.

Superior Optical Quality, Even at High Magnification

At the heart of the Baader filters are their special fine optical substrate polishing process and advanced hard coating design. The result is a filter that imparts virtually no image degradation. Manufacturers of lesser filters, to prevent deterioration from cleaning or exposure to moisture, sometimes resort to sandwiching their delicate coatings between two layers of filter glass, resulting in potential blurred or double images. Unlike these lesser filters, the superior optical quality of Baader filters allows for use at higher magnifications which are particularly useful for studying fine detail, particularly in brighter diffuse and planetary nebulae. Baader filters can take high magnifications, and stars retain their pinpoint sharpness, even if the filter is used far ahead of the image plane when used for astrophotography or when used ahead of a star diagonal or binoviewer.  Baader filters fulfill the requirements of an ideal optical filter - to perform at highest efficiency without any other detectable effect on the image.

Advanced Optical Manufacturing for Long Life and Superior Performance 

Unlike many budget filters, Baader Planetarium filters are not “biscuit cut” out of larger plates of glass, a process that causes micro-cracks around the filter edges. These cracks can attract moisture between the glass substrate and coating layers via capillary action which causes premature aging and degradation.

By contrast, each Baader filter is individually cut to size (either 1¼", 31mm, 36mm, 2", 50,4mm, 50x50mm, 65x65mm), then polished flat on both sides to a quarter wavelength to ensure maximum image sharpness and distortion over the entire filter, even when the filter is not close to the focal plane. Then each blank is then individually coated with a hard, ion-beam-deposited scratch-resistant coating. However, the coating is not applied all the way to the edge of the substrate to ensure the filter is sealed against the penetration of moisture, even in very humid conditions. This is especially important in multilayer, narrowband filters where moisture can seep in if the coating is not properly deposited. The hard coatings also mean Baader filters are easier and safer to clean without damage.

Baader filters are also tested repeatedly to comply with MIL-specifications. One common process is to boil the test specimen for one hour in salt water to induced damage. Unlike budget filters cut from larger plates, Baader filters remained completely intact without changes in their transmission characteristics after this rigorous environmental testing.

Baader’s careful engineering and manufacturing mean you need to buy a Baader filter just once and it will last you for a lifetime of astronomical observations.

About Baader Filter Cells

Many filter vendors insert their filter blanks into standard tightly-screwed filter cells. Unfortunately, the stress on the filter can result in drastic deformations of the substrate and degradation in optical performance.

That’s why all Baader filters are no longer tightly fixed but are held spring-loaded in the filter cell. The filter glass may ever so slightly “clatter” in the cell when shaken, but that neither affects the image quality nor results in a displacement in the final image. This “clatter” is not a flaw in the filter cell. It is entirely intentional.