Information about Baader Filters and Filtercells

About Baader Filters

The variety of uses for filters in amateur astronomy has considerably increased during the last decade, enabled by both more accurately manufactured optical accessories, and, above all, by the “digital revolution“.

In the old days, colour filters for visual planetary observations were not screwed in the front part of the eyepiece, but were simply placed between the eyepiece and the eye. Plane-parallelism of these filter glasses was not important, because they were not in the optical path of the telescope.

Today, filters are placed in the optical path of the telescope, even well in front of the focal plane. This definitely requires some degree of plane parallelism and accurate production of the filter glasses.

baader-filterboxen Baader filter boxes can be attached laterally and above the other to create a sturdy unit. Each filter is then neatly in it's own drawer and characterised from the front with name, number and size of filter.

Every single cell mounted filter delivered to our customers is cut as a round or square disc in the according size (1¼", 31mm, 36mm, 2", 50,4mm, 50x50mm, 65x65mm), and then is polished plane to a quarter wavelength on both sides on a computer numerically controlled polisher. After that, the polished blanks are submitted to the costly coating procedures. This sequence is also used for all  unmounted filters.

We deliberately avoid cutting filters from larger sheets, because the coating layers can be damaged at the edges and suffer from microscopic fissures. That lets moisture penetrate and the filters are subject to “ageing”. In particular, this applies to many of the complex dielectric coatings needed for nebular filters, UV/IR blocking filters and emission line filters. Damage to multi-layered filters at the edge results in greater damage than to a single anti-reflection coating. As our filters are not cut-outs, every individual filter disc can then be coated, but not right to the edges. This seals the filter, and no moisture can penetrate into the coating layers. Hence, even the most expensive narrow-band filters are free from ageing, and can be carefully cleaned without hesitation and as often as necessary.

Our UV/IR blocking filters were exposed for 1 hour to boiling water at the company B+W (Schneider Kreuznach, Germany). This accelerated ageing test corresponds to approximately 5 years of filter ageing in actual use. In contrast to cut filters, our edge-sealed filters showed no ageing and above all no changes to the measured transmission at different wavelengths.

The commercial disadvantage of this technology lies in the fact that we cannot produce any filter size by simply cutting it from a sheet. For custom-made filters in a requested size we need a minimum production run of 250 pieces.

High quality optical filters are not cheap. Hence, it is also no surprise if observers complain about „unaccountable“ picture deterioration when using cheap filters in front of binoculars, tele-compressors or Barlow lenses, either visually or photographically. The higher the magnification, the softer and more blurred the picture appears when using a cheap filter, for both visual and photographic observations.

In the manufacturing process great emphasis is placed on guaranteeing that a Baader filter must be bought only once by the customer, because it will be optimally usable for all use kinds of astronomical observations now AND in the future.

nterferogramm of a Filterfrom an overseas supplier Interferogramm of a Filter from an overseas supplier

Ever since we began producing our own filters and series of filters, we have checked the quality of a wide cross-section of „cheap filters“ from different manufacturers (see picture on the right). Many filter manufacturers - mainly in Asia – apparently still take the view that a filter is used only close to the focal plane, and that, hence, a homogeneous glass substrate is not necessary and that it need not be polished fine optically.

They say that if only one cosmetically flawless, smooth glass surface is required, it is not necessary to achieve a high degree of plane-parallel polish. They further believe it is adequate to cut filters from a big stained glass sheet – usually in the format 20x20 cm – and to so-called “raw polish” the filter on both sides. With this process, the glass surface is slightly molten and all saw scratches and surface inaccuracies are invisibly levelled. But so-called „raw-polished“ glass surfaces are completely irregular and deform the wave front of the light significantly!

The “polished“ sheet is coated as the whole, and afterwards the filters are cut out in the desired size. This production method for filters is drastically less expensive than the substantially more sophisticated manufacturing of a Baader filter. In addition, different sized filters can be cut on demand, reducing inventory costs.

Such a “cheap filter“ with irregular glass surfaces MUST always be screwed directly into the eyepiece; otherwise sharpness and definition are reduced, above all when doing observations at high magnification or long-focus photography with an inserted Barlow lens.

About Baader Filter cells

Or: why Baader Filters may "clatter"

interferogramm_baader Interferogramm of a loosely mounted Baader filter glass
interferogramm_baader-first Interferogramm of a tightly locked Baader filter glass

After putting flawlessly performing filter blanks into the standard tightly-screwed filter cells, interferometer measurements revealed drastic deformations caused by the stresses of assembly.

That‘s why all our filters are no longer tightly fixed but held spring-loaded in the filter cell. The filter glass may ever so slightly „clatter“ in the cell, but that neither affects the image quality nor shows a displacement in the final image. A stress-free filter, not tightly fixed in its filter cell, is not a fault and entirely intentional.

Visual observations and astrophotographs obtained with Baader filters are among the best achieved worldwide by amateur astronomers. You may find typical examples using our emission line filters and LRGB filters online, for example at: