Venus

Venue is the second planet from the Sun. It is named after the Roman goddess of love and beauty. As the brightest natural object in Earth’s night sky after the Moon, Venus can cast shadows and can be visible to the naked eye in broad daylight. Venus lies within Earth’s orbit, and so never appears to venture far from the Sun, either setting in the west just after dusk or rising in the east a little while before dawn. Venus orbits the Sun every 224.7 Earth days. It has a synodic day length of 117 Earth days and a sidereal rotation period of 243 Earth days. Consequently, it takes longer to rotate about its axis than any other planet in the Solar System and does so in the opposite direction to all but Uranus. This means the Sun rises in the west and sets in the east. Venus does not have any moons, a distinction it shares only with Mercury among the planets in the Solar System.

Venus is a terrestrial planet and is sometimes called Earth’s “sister planet” because of their similar size, mass, proximity to the Sun, and bulk composition. It is radically different from Earth in other respects. It has the densest atmosphere of the four terrestrial planets, consisting of more than 96% carbon dioxide. The atmospheric pressure at the planet’s surface is about 92 times the sea level pressure of Earth, or roughly the pressure at 900 m (3,000 ft) underwater on Earth. Even though Mercury is closer to the Sun, Venus has the hottest surface of any planet in the Solar System, with a mean temperature of 737 K (464 °C; 867 °F). Venus is shrouded by an opaque layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in light. It may have had water oceans in the past, but these would have vaporized as the temperature rose under a runaway greenhouse effect. The water has probably photodissociated, and the free hydrogen has been swept into interplanetary space by the solar wind because of the lack of a planetary magnetic field.

As one of the brightest objects in the sky, Venus has been a major fixture in human culture for as long as records have existed. It has been made sacred to gods of many cultures and has been a prime inspiration for writers and poets as the “morning star” and “evening star”. Venus was the first planet to have its motions plotted across the sky, as early as the second millennium BC.

Its proximity to Earth has made Venus a prime target for early interplanetary exploration. It was the first planet beyond Earth visited by a spacecraft (Venera 1 in 1961), and the first to be successfully landed on (by Venera 7 in 1970). Venusian thick clouds render observation of its surface impossible in visible spectrum, and the first detailed maps did not emerge until the arrival of the Magellan orbiter in 1991. Plans have been proposed for rovers or more complex missions, but they are hindered by Venus’s hostile surface conditions. The possibility of life on Venus has long been a topic of speculation, and in recent years has received active research.

Returning to the Dark Side -or- Recording surface features on Venus

Phil Miles and Anthony Wesley  20th February 2022

This page describes our Venus thermal imaging campaign of January/February 2022. We have previously attempted this in 2017 and documented the results on our report.

Embracing the Dark Side 2017 (shown below)

You should definitely read the report mentioned above for background material on our 2017 campaign as much of the equipment and technique is unchanged in 2022 and is not described here.

Venus 2022 Thermal Imaging Campaign

Our system for the 2022 attempt differed from the 2017 setup in the following ways:

We used a different camera, a Lucid Vision Labs Triton, model TRI051S-MC. This camera uses the recently released Sony Pregius-S sensor imx547 which is superior in many ways to the Pregius gen2 sensor used in 2017. 

Note the protective glass window in the camera was removed to reduce scattering and absorption.

We did not use 2x2 binning. Phils scope was unchanged from the 2017 setup except for the addition of a 1.25x barlow that changed his scope from f/4 to f/5.1 . We used a 1.25x Magic Dakin Barlow for this purpose and recorded all data at an effective focal length of 2590mm (ie 508mm f/5.1). 

f/5.1 seemed to be a good match to the camera and sensor and showed much reduced diffraction spikes or artifacts compared to the Pregius gen2 camera from 2017. Much of the improvement is due to the change in sensor from font side illuminated to back side illuminated.

January 28 2022
The morning of January 28 yielded the best seeing of the entire campaign, and resulted in an image showing a lot of interesting features, both surface features and low clouds. Here is our image and a comparison against the topographic reference. 

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February 15 2022
We had another clear morning with relatively good seeing on this day. Here is our image and a comparison against the topographic map.

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Embracing the Dark Side -or- Recording features on the night side of Venus

Phil Miles and Anthony Wesley
19th May 2017

During 2016 the topic of recording thermal emissions from the dark side of Venus came up in many of our conversations.
One of us (Anthony) had attempted this in 2013 using an older generation camera and 1000nm longpass filter with some success,
but we thought that much better results might be possible with the new cameras and better equipment available in 2017.
Phil was keen to try this and so we started a project to do this over the 2016/17 conjunction.

In particular we thought that using Phil's 508mm aperture scope in combination with the more modern Point Grey camera should allow for a 
significant increase in signal to noise when looking for the very faint thermal signal at 1010nm.

Christophe Pellier had proved the concept in 2004, and others have since repeated this detection, but in all cases there has been a 
significant problem with the glare (reflected sunlight) from the lit crescent. In amateur scopes the light scatter from this source can 
dominate in the image, making it very difficult to separate from the thermal signal.

After the experience in 2013 Anthony purchased an additional filter - a Semrock bandpass filter covering 850nm-1020nm. 
When used in conjunction with the existing Thorlabs 1000nm longpass filter this gives an effective narrowband filter centred on 1010nm with a 
bandpass of 20nm. He thought this might be better than just the 1000nm longpass on its own as it should block much more of the unwanted 
reflected sunlight; however he had never tried the filter so we didn't know if it would be of any use.

As Phil was going to use his 508mm F/4 Newtonian he set about doing everything he could to reduce unwanted stray light, including making 
a magnetic cover for the primary inspection mirror which is permanently attached opposite the focuser, blocking the area
around the outside of the primary mirror with adhesive foam, and repainting all internal surfaces with black chalkboard paint.

Phil's first attempt using only the Thorlabs FELH1000 on April 11th showed promise, the thermal signal was clearly visible but no discernible 
features were present. This was repeated on the 12th with the same result. The next tests on the 13th, 16th & 17th using 2x2 binning 
looked somewhat better but due to poor seeing no features were seen. These tests were done during the very early stages of the morning 
elongation of Venus, so it was still close to the sun and difficult to image.

Looking at the images we realised that there was still far too much light reaching the camera sensor, Anthony had a Thorlabs 1050/10nm 
bandpass filter which Phil tested on the 18th without success.  It seems there is no thermal signal at 1050nm.

On the 19th Phil added the Semrock 835/170 filter to his system, stacking it onto the Thorlabs 1000nm longpass filter and making an effective 
narrowband filter 1010/20nm. This produced the best image so far and although there were numerous sharp diffraction spikes in the resulting 
data surface features were at last seen. Another test on the 23rd showed similar results. Features were visible but there was still a lot of 
interference from artifacts generated internally.

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This image from  "Detection of Sub-Micron Radiation from the Surface of Venus by Cassini/VIMS"   courtesy  Kevin Baines & others.

We have overlaid the bandpass of the combined thorlabs-semrock filters to show that we're recording close to the centre of the thermal emission 
peak at 1.01 micron.

On the 24th we decided to rotate the camera approximately 45 degrees every 15 mins so that the separate images could be combined to reduce 
the in-camera generated spikes. By chance one of these rotations produced a much smoother image with almost no spikes so the following 
day a test was done using only that alignment with excellent results, all camera orientations are now done in that position as shown 
on the 26th raw image. It seems that some of the internally created artifacts can be reduced to very low levels by choosing the orientation 
for the bright crescent of Venus. While we don't understand the precise mechanism for this improvement it is nonetheless very welcome, 
and may also work for others who are trying to image the same target.

Further stray light reduction was also helped by placing several baffles comprised of black rubber washers of the correct diameter at each 
filter and a fibre washer at the top of the camera extension tube so only the F4 cone is visible to the sensor, the internal filter spacers 
were also flocked to prevent any reflections. The clear glass protective window in the camera was also removed to minimise the number of 
sources of reflections and scattered light.

The barlow was removed as well and the system operated at his native focal length of 2000mm, ie the camera is at prime focus, further reducing 
scattered light.

The final results are very good, with many features clearly visible in the more recent images.

Higher altitude surface features (green/yellow in the elevation map) show as dark features in the thermal emission image as they are cooler 
than the surrounding area.

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