MERCURY AND VENUS
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The inferior planets Mercury and Venus both exhibit phases. Because it’s a rocky world, recording Mercury’s phase reveals few surprises, but what’s of more interest are images that show albedo variations on the planet’s surface. This requires apertures larger than 8 inches (200mm) to achieve well. Images of Mercury are best made during daylight hours when the planet is highest above the horizon. Using a red or infrared filter helps to increase the planet’s contrast, darkening the surrounding blue sky.
Venus has a dense atmosphere, and images showing its phase through different filters provide data that records the planet’s phase anomaly. This is most evident around dichotomy when the planet should, mathematically at least, appear exactly half lit. In practice, the phase anomaly means that Venus reaches this phase early when it’s visible in the evening sky and late when it’s in the morning sky. The effect is believed to be related to how sunlight scatters in Venus’s atmosphere.
The Venusian atmosphere can respond well to certain filters. Most variation tends to become evident through shorter wavelengths filters, in particular ultraviolet. Such filters can be expensive, although interesting imaging results have been obtained using visual purple filters (eg, Wratten #23A) fitted with an additional IR-blocking filter.
At the other end of the spectrum, near-infrared filters have been able to reveal the Venusian nighttime hemisphere, typically when the planet appears as a thick crescent. When Venus is a thinner crescent, some people (including William Herschel and Patrick Moore) have claimed you can see a phenomenon known as the Ashen Light, which makes the dark portion of Venus’s disc visible to the eye. Others, however, put these sightings down to observer error or equipment malfunction. To date, there have been no images to support visual reports of this effect. Could you be the first to capture proof of it?
Finally, although the next transit of Venus is not due until 2117, transits of Mercury are more common and timed images of such events are always useful. In particular timings through speciality filters, such as H-alpha, provide yet another dimension to this infrequent event.