30.12.2019 – 23.04.2020
Visualize the impact of seeing while taking images of Venus.
Image acquisition & processing:
I took three different images of Venus during the recent few months. All of the images were taken with the exact same hardware and setting – a ASI ZWO120MC-S Color planetary webcam with a 3x Barlow lens on top of my 750mm Newtonian reflector carried by the Skywatcher EQ3 -Pro mount.
All images were recorded as .avi files (2min @ 60fps) and stacked (20% best) and sharpened in Autostakkert!3.
The first image was the first image taken with the new ASI120MC-S Color camera. The previous images were all taken with a pathetic DIY webcam for 10bucks and were overexposed to the point of being unrecognizable. The ability to be able to control the exposure time and the gain in SharpCap is not to be underestimated but either way: The result was slightly disappointing.
You can clearly see the phase and the shape of Venus – also the color of the planet can easily be seen. But the planet remains a fuzzy blob. Especially with the 3x Barlow lens in use. Also the planet seems to very small. This is due to the position of Venus relative to earth, it’s just „very far“ away.
The second try was three months later in march of 2020. Again 2min .avis with 20% best were chosen and the result was much (!) better this time – with no changes in the hard- or software part what so ever.
The overall shape and the phase (lit and dark side of Venus) is now clearly visible. The planet appears much more sharp and the color evenly distributed. Even the terminator (border dark-bright) is smoothly visible! This was my first „proper“ image of Venus, and I was very proud of it!
The third attempt was now in late April. Again: same settings – same hardware. Nothing changed, but the conditions of the session. This time the planet appears much bigger (position is now nearly closest to earth), the overall appearance is sharp, the edges well defined, the terminator smooth. This is my best image of Venus so far!
Compare the three attempts next to each other:
Implications and conclusions:
What you can see is the effect of seeing (and the approaching Venus). Seeing is the „observation quality“ of your sessions. Air movement, humidity, dust and smog as well as haze are factors that will affect your observation. The reason is simple: The light from the session target needs to pass earths atmosphere in order to reach your camera/eye. And ass every optical component of your rig, the air, too, needs to be of „good quality“.
If the air moves, if you had a thunderstorm hours ago, different air layers of different diffraction properties will swirl and affect the light path. This affected light will no longer be able to travel a straight line – it will bend and twist: move. If you observe under a dusty sky, light will be scattered, resulting in an even more twisted path. As a result the image of a star or a small target like a planet will wobble and bend on your screen like a rubber ball.
Seeing will affect every session – Planetary or DSO – but: The higher the focal length, the smaller the sensor, so the smaller the FOV – or in other words: the smaller the objects you observe, the higher is the impact of seeing.
400mm FL with a full frame sensor going after the North American Nebula? Don’t even think about seeing. But 3000mm FL with a planetary webcam after Mars? Seeing will determine the outcome of your session – no matter how hard you fight.
So take away? Planetary imaging or the imaging of tiny galaxies with high focal lengths need more than clear skies. Choose a night with calm air, dry weather, no recent storm fronts or high altitude winds – no foggy or dusty night. My first mistake was to think: Okay! We have 15km/h winds – the mount will shake slightly. That might be bad for long exposures (it is!) but will do for short planetary videos (it is NOT!).
That was wrong. In some regards, planetary high focal length sessions are more demanding than DSO sessions with short focal lengths. Just keep that in mind.
So as always: Clear skies and greetings!