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Baader Planetarium U-Filter (Venus Filter)Discontinued 2007, replaced by BPU2.
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). 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
How Are These Constructed?
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?
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.
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.
Right: Baader U (Venus) Filter transmission curve. (80,628 bytes).
NOT A Visual Filter
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:
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 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)
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