Apparition du soir,

The planet Venus has a well deserved reputation of being a featurless object, especially in visual observations, although under good conditions some skillful observers manage to make a few details out of the almost blank disk. It is thought that such lucky observers have eyes sensitive to ultraviolet light.

However, digital imaging can be extremely usefuel here because it allows one to image the planet in some non-visible wavelenghts. I have begun my observing program in january 2004, with the idea of making systematic observations in infrared but above all in ultraviolet, which is the wave band that reveals the details belonging to the upper venusian atmosphere. This has been known for long, but as there was a very weak covering by modern observations, I wasn't sure of what I could expect. The first experiences have brought some surprises, but also many successful results that have turned this featurless disk into one of the most interesting planet to watch !

18, 24, 29 January 2004

These are my very first attempts at imaging Venus. I'm using the 14" SCT of my astro club that is permanently set-up under a dome, and this is a confortable situation to make daylight observations... The 14th and 18th, conditions are bad with a lot of bad seeing. Nonetheless the UV filter already reveals some details ! I have also some doubts about a possible weak band in near-IR (january 14th), near the south pole (beware that the north pole of Venus points to the south).

The images of jan. 29th are obtained under a much steadier sky, but they bring hard discussion about the reality of the "dark arc" seen near the limb of the planet, that closely looks like a processing artefact... apart the fact that it is well visible on every raw frames !! About the artefact thesis, the Pros are offering various explanations all in relation with any optical problem. But they are all not able to explain the fact that the "problem" occured once only.

While I am the only one then to defend the images, I'm more thinking about a canal-like phenomenon (in reference to the so called "martian canals") : a fortuitous alignment of dark structure with a bit of bad luck about perspectives. At this point I don't have the final proof of the reality of the detail, but my arguments are :

- The phenomenon did appear only once ; the imaging technic was completely identical, thus it can't be an optical problem.

- It's identical on every image and is present on the raw frames : it is not a processing artifact.

- Above all, the appearance of the arc varies with the wavelenght, becoming less and less contrasting while the wavelenght is increasing from UV to blue light, as would do any true UV marking.

An example of the debates can be read in french at : http://www.astrosurf.com/ubb/Forum3/HTML/002363.html

Apart of this note some faint details in infrared.

2, 4, 7 February 2004

The observing conditions are improving, so do the images. The UV images show details every time, which are now similar to some classical shapes of the venusian clouds. On the 4th, the image is done with the 16" newtonian of Gino Farroni, the SAF Jupiter coordinator. On the 2nd, two blue light images have been made ; the W47 image, realised with an IR-blocking filter (which is mandatory for the W47 in CCD observations), show the same details than the UV shot but with weaker contrast.

The IR images are still showing very weak details, that are certainly real. On the other hand the advantage of long waves is that they allow me to make very sharp images of the phase...

13 and 14 February

Again some nice details. The images on the 14th are made with my 7" newtonian. With the feb. 13th images I have made a small animation :

It reveal clearly the rotation of the planet in less than two hour. The upper atmosphere, which is the one imaged in UV, rotates in no more than four days.

1st, 8, 16 March 2004

The apparent diameter of Venus is increasing slightly. The first day of the month brought bad conditions but they turned to good later in the month. On the 8th a Psi-shaped feature (?) is identified near the terminator (side-lying). This is one of the classical Venusian structures identified by Charles Boyer a few decades ago.

On march 16th I have made my first images at a wavelenght of 1 micron (with a IR 1000 nm filter). What was expected from such a filter was better penetration of inferiors atmospheric layers than with the IR 780, and possibly a better contrast of the vague details caught in infrared images. But obviously this was a failure, as no additional detail is revealed. The IR1000 filter could be useful for observations of the venusian night side, but this should be tested later.

The most interesting result is this animation of the rotation, probably the first ever made by an amateur at this level. On march 16th, in the afternoon, five UV images have been taken every hour, during four hours. The retrograde rotation of the planet is clearly seen.

27, 28, 29 March 2004

Conditions are still good. My comments here will talk about visual observations of the planet. In visible light Venus seldom shows any details. Nonetheless on the 27th and 29th, I have clearly seen, with the help of a W47 filter (violet), the brighter polar "cusps" with certainty. The visibility of those cusps looks related to their prominance in UV lighy and this is one proof of the reality of those visual observations. For example on the 28th nothing was seen, and the UV image doesn't show anything either.