Venus nightside: unveiling the surface of Venus

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During mid March 2009, I followed the nightside of Venus at 1 micron wavelengths with my 235 mm Schmidt-Cassegrain telescope. Some good amateur astronomers, like Christophe Pellier (2004, the first amateur to have spot the faint infrared glow of the dark emisphere of Venus) succeeded to image this faint and curious detail, but no one could detect clear surface patterns in the thermal glow of Venus.
Unlike other amateur astronomers, I used a cooled 16 bit CCD camera (SBIG ST-7XME) and acquired images only when the background became enough dark, after the sunset. As expected, the nightside of the planet was quite easy to detect, with just 0.5 seconds exposure (thanks to the high sensitivity of the CCD camera). Exposures bewteen 4 and 10 seconds were the best compromise between the increasing glow of the dayside (about 20000 times brighter) and the signal of the much fainter nightside.


Single raw (uncalibrated) 6 seconds frame. The nightside of the planet is prominent

I acquired about 1700 single exposures in 7 days in a row, 340 each day. Every image was calibrated with master dark frame and flat field. Allignment and processing of the raw images of each observative day was performed in order to show some features in the nightside associated with possible surface structures. In order to avoid the presence of processing artifacts, for each day I obtained 2 processed images by alligning and adding about 170 frames. This data redundancy is useful in the processing step, since artifacts would not be present in the same way in both images, while the true details did.


Raw image. Average of 170 frames calibrated with master dark frame and flat field

Any processing software and filters (walevets, unsharp masks or deconvolution) showed exactly the same small scale and high contrast features on the nightside.
The features rotates following the surface rotational speed, and they don't change significantly over the time. In some days there are also some larger scale features and the contrasts over the disk seems to change, due to the presence of low altitude (30-35 Km) clouds that adsorbed differentially and partially the thermal emission.

In order to have an image with only surface features, I took the images taken on March 16-17-18 and made the median. This mathematical operation tends to erase the contribution of changing details (i.e. fast moving clouds) while the much slower surface features stay unaltered. The resulting image has a better S/N ratio, so it was possible to enphatize some low scale and low temperature variation details.


As a final step, I made a comparison between the features visible in my image and the altimetric radar imagery taken from the Magellan spacecraft in early 90s. the correspondence is nearly perfect, we are observing, for the first time in amateur history, surface features on Venus:
 


Further explanations maybe are due.
The details of the images above are real?

Of course yes, since:
1) they are present in any image:




2) They follow the rotation of the surface (243 days):




3) All the images match perfectly with the altimetric radar image of Magellan spacecraft:

Conclusion: Amateur equipment can produce high quality images of the nightside of Venus and give a good topographic view of the planet. The details are in perfect agrement with altimetric data. A good and new topographic global map is very useful in order to compare with the older Magellan, finding some possible modifications due to volcanism or other large scale phenomena that modify the thermal emission of the soil (modulated by temperature but influenced also by chemical composition). Amateur equipment might spot volcanic eruption as large as 25 Kmē  (as amall bright dots in the nightside) and be a valid support for the knowledge of this still misterious planet.
A continuative imaging campaign will reveal also the behaviour of low layer clouds and how they affect the thermal radiation. Could be strange, but there are no data, then no serious studies about that.

The nightside image analysis is not yet finished. To see the remaining informations contained in the images, click here