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Baader Planetarium U-Filter (Venus Filter)

Discontinued 2007, replaced by BPU2.

Baader Trademark Baader 1.25 inch U filter (136,193 bytes) This is a highly specialized, limited purpose filter designed for use by a telescope provided with imaging or other sensor systems that are optimized for uses in the ultraviolet portion of the spectrum. It also may find some uses in laser applications. This is not a filter for visual uses.

The U-Filter is a ultraviolet bandpass filter made so that only light in the 320 to 390 nm portions of the spectrum are transmitted; it will block all else spanning 200 nm to 1.5 microns.

Right: Baader U-filter (101,171 bytes).
Click on image to see enlarged view (205,357 bytes).

This Filter is sold by Company Seven to customers who have a suitable telescope or lens. Suitable telescopes can include reflecting telescopes of 8 inch (20 cm) or larger apertures, or refractive and catadioptric telescopes employing refractive elements made of UV grade fused silica this will include our fused silica Long Distance Microscopes that can provide resolution down to the sub-micron level. The filter can also be employed with specialized camera lenses most notably the famous Nikon UV-105 F4.5 Lens, a limited production conventional and macro lens with high performance spanning from 220nm in the Ultraviolet to 900nm in the Infrared. Please contact Company Seven if you need to buy equipment with these properties.

Filters take away but they can not add - diminishing parts of the spectrum
so that we might reveal what is otherwise obscured...

How Are These Constructed?
The filter is made with a disc of Schott UG-11 optical filter glass that has been ground to be plane parallel and then polished until very smooth. The glass undergoes a vacuum deposition coating process where a twenty layer micron-thin coatings of various elements are applied one upon another in a precise order and thicknesses.

The filter element is housed in a machined aluminum cell, retained by a thread in retaining ring. The filter name, size and part number are in silver, and this mirror like print when combined with its small size make this information difficult to read under dim red lighting. Company Seven recommends our customers keep these filters in well organized accessory cases, so that you know which filter is which. It helps to become familiar with how the elements appear too (read descriptions below).

When viewed parallel to the eye from the female threaded (or field) side, then the filter element appears like a dark gold mirror. When looking from the male threaded (instrument) end then the filter element appears like a deep green colored mirror.

The coatings are calculated to transmit desired wavelengths while simultaneously rejecting all other wavelengths from 200 to 1,500 nm that would otherwise be typical of the UG-11 glass alone.

The Baader U-Filter is made available in a cell diameter of 1.25 (31.7 mm) so that it may be threaded onto most common 1.25 inch diameter female threaded nosepieces of our UV capable CCD or cameras, or sensors. Company Seven can provide adapters that permit the attachment of this filter in line in most other systems too.

What Does This Do For Me?
Before the availability of this filter amateurs interested in UV applications resorted to stacking simpler interference filters, or the use of deep violet colored filters. These techniques attenuate the transmission of the desired wavelengths making these techniques possible only for highly sensitive equipment, ideally employed at higher elevations where there is less atmospheric filtering of UV. And many of these earlier techniques still passed portions of the spectrum beyond that which is desired. And with so many elements in line thereby the filter stacks accumulated errors from less than perfect optical surfaces. These handicaps diminished image brightness, contrast and definition.

The Baader U-Filter makes it possible to obtain photographs, CCD or video images in the deep UV spectral region - a wavelength coverage that up until recently was impractical for most amateur astronomers. This filter provides highly efficient rejection of wavelengths outside the spectral region between 320 nm to 390 nm and this improves contrast.

Venus taken with 1.25 inch U filter (136,193 bytes) The U-Filter has been nicknamed "Venus Filter" because it permits amateurs for the first time to finally break-through prior limitations and obtain meaningful images of the cloud structures of the thick Venutian atmosphere. These are more apparent from Earth with moderate sized instruments when studied in the ultraviolet.

The accompanying image at left is the first test photograph of Venus. This image was taken by Baader Planetarium employee Mr. M. Rietze just two days before the Venus transit of 4 June 2004. The image clearly reveals the cusps enclosing two-thirds of the planet's circumference; a cusp extension is an effect seen at twilight caused by sunlight being diffused by the planet's atmosphere. We generally consider cusps visible to beyond half the disc's circumference to be visible only in very good observing conditions with proper equipment.

The U-Filter passes the Calcium-K line but not in so narrow a bandpass or in the quantity that we recommend alone it in place of a specialized filter such as those made by Calcium K-Line filter. With a large aperture system, and when used in combination with other filters then we can see some applications for studying the atmospheric torch and flare structures across the Sun's disc. Industrial customers of Company Seven may inquire for more information as we do not generally provide this equipment or advice to the amateur community.

U (Venus) Filter Graph (80,628 bytes) Right: Baader U (Venus) Filter transmission curve. (80,628 bytes).
Click on image to see enlarged view (102,631 bytes).

NOT A Visual Filter
The U-Filter is not a visual filter since most telescopes and eyepieces can not pass light in the UV.

The U-Filter is not a visual filter since the human eye can not resolve images clearly in these portions of the spectrum transmitted by this filter.

And particularly if attempting to see the Sun with these filters there are health issues involved. The ultraviolet wavelengths of particular concern to human health are:

  • UV-A = wavlengths between 380 to 315 nm, often termed Long Wave or "blacklight",

  • UV-B = 315 to 280 nm, termed Medium Wave,

  • UV-C = ≤280 nm, known as Short Wave.

There is some debate about the specifics wavelengths and amount of exposure, acute or chronic. But it is generally agreed by the medical community that the human eye should not be exposed to light in these portions of the spectrum since this can cause severe damage of the cornea, lens, and retina.

Lifetime cumulative exposures can also be contributing factors in the development of cataracts, pterygium, or conjuctiva. If you still have any doubts just consider the statistics showing dramatic increases of eye problems of residents living in ozone depleted areas of the southern hemisphere!

Features Of The Baader Filters:

  • The glass elements used to make the Baader Filters are free of striae, optically finely polished plane parallel to within 30 seconds of arc. This eliminates any double images or ghosting due to wedge errors in the glass. These filters tend to far supersede the quality of common color filter elements which are flame polished. The combination of their fine optical craftsmanship and a comparatively thin profile allows these filters to be inserted anywhere into the optical path without introducing astigmatism or other noticeable defects into the image.

  • The U-Filter will thread into conventional slip fit accessories with the common 1.25 inch diameter x 42 tpi (28.5 x 0.6 mm).

  • Stackable filters: each filter cell incorporates both a male and a female thread to allow stacking the filters to obtain even narrower passbands or special effects.

  • The filter cells feature the largest clear aperture of any 1-¼ and 2 inch filters on the market - for complete freedom from vignetting at any focal length. The front crown of the cell incorporates milled notches which makes handling and threading the filter an easy operation.

  • For protection, each filter is provided with its own transparent plastic case. We are astonished however, that as outstanding as the filters are, plastic case latch tend to fail sometimes even when new. Company Seven therefore offer optional third party cases. And for the most discriminating clients, we even offer a custom made wood accessory case described below.

    Optional Wood Case
    optional wood case for Questar Color Filter Set. (52,803 bytes).

    The optional wood case is foam lined with cutaway spaces for filters, eyepieces and other small accessories for your telescope. Please refer to our price list for current prices for the filter set and the optional case or contact Company Seven for more information.

    Right: optional wood box for Color Filter Set (52,803 bytes)
    Click on the image to see enlarged view (190,049 bytes).



    1. It is best to protect the filter from sharp edges or any item that could cause a scratch or abrade the glass.

    2. When not in use, store the filter in a its provided rigid container, and it may help you to keep them organized in a fitted carrying case.

    3. Dew may form on the filters, if this happens do not store a wet filter into its airtight container for long periods of time; whenever possible let the filter "dry out" in order to reduce the potential for problems developing associated with Fungus, etc.

    4. Draw any large, loose bits of foreign matter from the surface of the glass with an air bulb or small vacuum. Stubborn particles can be removed with a soft camel's hair brush. Throughout the year, but especially in cold climates when static become more of a problem we particularly recommend the use of a Staticmaster brush.

    5. Place a few drops of an approved lens cleaning solution (by Carl Zeiss or Kodak) or mild soap solution of clear dish washing liquid diluted with distilled water onto a cotton swab. Then gently wipe in a circular motion with little or no pressure. The filter can be damaged if there are tiny abrasive particles on the surface which are pressed in by rubbing hard.


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