Wratten 47

The W47 is the only "old-design" filter that I keep in my collection. This because it offers a very interesting passband rarely found with modern CCD filters : the very deep blue. One could define the "violet band" as the UV band plus the deepest blue wavelenghts (<450 nm). It can represent a interesting compromise between B and UV.
Its particularity however is that it's affected by a huge infrared leakage that will force us to use it along with any IR-blocking filter. Read the photometric test of blue filters

- Imaging the planets in violet light (UV+B)
- Possible use of the filter as an IR-pass filter (use it with a long-pass red filter)

- Important IR leak requiring an IR-blocking solution
- Relatively low quantity of light available
- Sensitive to seeing effect and sky transparency

Jupiter
21 august 2010 - Cassegrain 250, SKYnyx 2-0M
Good violet image of the planet, but with little interest in comparison with B and UV filters. The filter is too sensitive to blue light to reveal the subtil aspect of the planet in UV,and too difficult to use in comparison with the B filter.

Saturn
22 april 2006 - Mewlon 210, Lumenera LU075M
The filter is noticeably more interesting for Saturn, whose albedo varies much more with the wavelenght. Here the image is clearly different than in blue light, and not so far from UV : the globe is dark and the white bright winter haze in the northern hemisphere is well visible.

Mars
16 september 2003 - Newton 180, B&W ToUcam
The filter is highly interesting for Mars. It offers a strong blocking of the surface details, and the white clouds are contrasted. To be considered as the most valuable alternative to the use of the difficult UV filter.

Venus

3 may 2007 - Cassegrain 250, SKYnyx 2-0M
The W47 is extremely interesting on Venus. It shows the same details as the UV filter, but because it's more easier to use, the images can be sharper, despite a lower contrast, as wee see it on the images at right.

The details are due to a still unknown UV/violet absorber.