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Baader Planetarium U-Filter (UV or Venus Filter)

Nicknamed by some the "UG11xx" to differentiate it as a second generation UG-11 based Baader Filter, this is a highly specialized filter designed for use by a camera and lens or telescope provided with imaging or other sensor systems that are optimized for technical and natural studies in the ultraviolet portion of the spectrum. It also may find some uses in laser applications. This filter is not recommended for visual applications by Company Seven.

The BPU2 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 effectively block all else spanning 200 nm to 1120 nm. This U-Filter has improved transmission peaking with 85% transmission at 350nm with a 70nm bandwidth, and with a 5 stop reduction in the IR portion of the spectrum. This replaces the original U-Filter (BPU125) that was discontinued in 2007. Note however that it is only made available in 2 inch diameter.

Right: Baader 2 inch U-filter BPU2 (57,544 bytes).
Click on image to see enlarged view (103,501 bytes).

This Filter is sold by Company Seven only to our customers who have a suitable telescopes; this may include reflectors of 8 inch (20 cm) or larger apertures, refractive or catadioptric telescopes employing refractive elements made of UV grade fused silica, or specialized camera lenses. 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 high grade Schott UG-11 optical filter glass that has been precisely ground to be plane parallel and then polished until very smooth. The glass undergoes a vacuum deposition coating process where a forty layers 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 machined thread-in 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).

The Baader U-Filter is furnished in a machined cell of 2 inch (50.8 mm) diameter so that it may be threaded onto most common 48mm female threaded hardware including 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.

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. For those pursuing UV imaging with digital cameras, then it is possible to unthread the retaining ring and reverse the orientation of the filter element in the cell so that the gold surface faces the camera sensor.

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

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 and amateur photographers. This filter provides highly efficient rejection of wavelengths outside the spectral region between 320 nm to 390 nm (photographically speaking, this is a span of about -3 stops), and this improves contrast.

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.

Left: Venus with U-Filter. Image was taken by Baader Planetarium employee Mr. M. Rietze (9,674 bytes).

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 planet disc 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 to reject excessive heat energy that could otherwise fry a component, then we can see some applications for studying the atmospheric torch and flare structures across the Sun's disc.