Ultra Violet Venus Imagin

The cloud markings in the upper Venusian atmosphere are quite obvious features in the ultraviolet part of the spectrum (see Figure 11.7). Possibly the most popular filters for this type of work are photometric U-Band filters with a pass band centered on 365 nanometers. Such filters allow through light of wavelengths from 300 to 420 nanometers with a peak transmission at about 365. Photometric U-band filters have a great advantage in that they are coated to reject light from the near infrared that leaks through cheaper filters and to which CCD chips are especially sensitive. (Remember, when you remove the lens on a webcam you remove the standard UV and IR blocking filter built into the tiny lens.) A much cheaper alternative is a standard, photographic Wratten 47 filter. However, this filter will require an infra red blocking filter with it as most Wratten 47s will leak infrared through to the CCD. It is important not to get confused here with UV filters, IR filters, UV+IR blocking filters, and IR blocking filters! Blocking filters block, as their name suggests, and many amateurs already own UV+IR blocking filters so that their lensless webcams can be restricted to the same visual band as the human eye. This restriction reduces atmospheric dispersion effects and gives a more natural color. However, combining a Wratten 47 with a UV+IR blocker will be a disaster, as no light will get through! You need a photometric U-band filter or a stacked Wratten 47 and infrared blocking filter.

UV 365nm IR780nm+

Figure 11.7. Venus imaged in the ultraviolet (first two images) and near-infrared (right-hand image) by Damian Peach on September 19, 2004, with a 280-mm Celestron 11.

CCDs are not very sensitive in the ultraviolet. In fact, their sensitivity drops virtually to zero as the wavelength drops to 300 nanometers. However, this is not too much of a problem as, at 365 nanometers (the peak of a U-band filter) CCD response is certainly low, but not zero and, crucially, Venus is a very bright planet. It is close to the Sun and has a high albedo. Venus' bright surface is a godsend when imaging in the UV. Nevertheless, using a color webcam, like a ToUcam Pro, will give far noisier results than using a sensitive, filtered, monochrome webcam like an ATiK 1HS.

If you do not have a UV filter, Venus is still a fascinating object to image in white light, where its brightness can allow very good signal-to-noise images with a short exposure time. Unlike planets like Mars, Jupiter, and even Saturn, the planet's rotation is not a limiting factor in obtaining thousands of frames to stack. The limiting factor is your stacking software and spare hard disk capacity.

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