C2 Swan-Band Filter (15nm ) – O-III parallel

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 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.

Interferogramm of a Filter from an overseas supplier

A stack of coating layers (up to 60 layers depending on the filter complexity) has a considerable thickness and, above all, a different coefficient of expansion than the carrier glass. When this stack of layers is cut at the edges, the entire stack of layers may separate from the carrier glass over time. Baader Planetarium, on the other hand, allows the coating to gradually run out towards the edge. This means that the outermost filter edge (0.5 mm) cannot be used for filtering, but the coating remains sealed permanently.

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 of a loosely mounted Baader filter glass

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: http://panther-observatory.com/

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.