Philosophy, Application and Technique
The planet Venus is surrounded by a closed cloud cover, which moves at high speed around the planet. Here, different cloud bands, which show a steady change of details form similar to the cloud cover of Jupiter. Unfortunately, these clouds can not be seen in visible light, unlike Jupiter, meaning Venus appears completely structureless. However, by using a camera that is sensitive to inaccessible wavelengths unlike the eye, this cloud structure detail can be seen, in particular using ideal wavelengths in UV range between 320nm and 390nm.
Since the early days of amateur astronomy, amateurs have been trying to work in this light. The difficulty is procuring a suitable filter to work precisely in this short wavelength region adequately. A sharp short-pass filter with high transmission is virtually impossible to produce. This also applies, surprisingly, to interference filters. While a theoretically ideal UV bandpass can be produced, there is inevitably at least one second passage window in the visible range. This is fatal because this undesirable area cannot be blocked with coloured glass. In such a case you would need again an ideal short-pass filter, which again is not produced in sufficient quality.
Amateurs have tried with modest success to create a filter for stacking one UV passage without a second passage area. This meant a strongly reduced contrast and modest transmission, which is not ideal. Those who believe you can make do with a less transmission filter are soon disabused. In addition the glass of many telescope optics in the UV have a much lower transmission, which does not help.
The good idea of using an excellent UV-interference filter with a second window in the visible light, and evaluate only the blue channel in the resulting images of the colour camera used of course does not either. The filter on the semiconductor is subject to the same problems mentioned above. Also the blue channel gets light from the other channels added to it. Since the semiconductor is not very UV sensitive, the low leakage of the visual light tends to be shifted to the UV. This double image show significantly the effect of refraction and can make the UV detail inaccessible due to this inefficient rejection of light outside of the 320-390nm spectral range.
With the latest dielectric coating technology, it is possible to achieve an ideal UV characteristic with the highest transmission while blocking other transmission windows. This has made it possible for the first time for an amateur astronomer to photographically observe the changing clouds of Venus and to say goodbye to the vacuous white crescent. All Baader filters are finely polished optically (increases image quality). High magnification during eyepiece projection is also possible, without reducing the sharpness of the optics.
Non-Astronomical Application Areas
These filters can be used in the observation, by technical photography, of nature. For instance they are used to determine the health of plants. Automotive experts can examine if a vehicle has been repaired as the repaired surfaces are very clearly seen due to differing reflectance. Many other applications are still in testing.
The Baader U Filter is only produced in set sizes relevant for use with astronomical telescopes. However, we do produce a family of adaptor rings to mount any of our 2 inch (50.8mm) filters in front of any camera lens which has a filter thread. It is important to note that not all cameras and lenses are suited to UV photography and this could result in a low signal to noise ratio in the image produced. For best results it is advised to use a specialized camera body responsive to UV light (that does not have typical blocking filters that everyday cameras have) and a suitable UV-transmitting photographic lens which has adequate glass substrates for the transmission of UV light.
2 Inch Baader Filter Adaptation for DSLR Camera Lenses
Our Baader DSLR 2" filter holder (#2408166) can be fitted to most DSLR camera lenses either directly or by using lens adapter rings.
Example: For adapting the Baader 2" U-Filter onto a camera lens with 77mm front filter thread you will need:
- Baader DSLR 2" Filter-Holder M48/SP54 (item #2408166)Baader Lens-Adapter-Ring SP 54/ M62 (#2958062)
- Baader Step-up Ring M62/M77(#2968077)