An observation from this afternoon, Sunday 31st May.
I haven't bothered with colour filters at all today. But I have had a couple of other interesting observing aids at my disposal.
Off axis Mask.
Yesterday I noticed the off-axis light cone from the sun creating a long bright oval illuminated up the inside wall of the Newtonian telescope tube, I put my finger into the top end of this oval and felt pretty hot.
So today, because I was worried that this solar image would be nearer to the central axis of the telescope and might shine onto the secondary mirror, I made a simple 125mm off axis mask from cardboard to stop the telescope down, my theory being that if I oriented this correctly I could prevent the solar image reaching the secondary mirror by ensuring it shone onto the inside of the tube instead. It might even improve the tolerance of the telescope to the poor daytime seeing.
The mask excluded stray sunlight really nicely, but the resultant image of Venus was not particularly sharp, nor bright and honestly just looked pretty poor compared to the image I have been studying recently.
So I removed the off axis mask, and used a piece of paper to confirm that the cone of sunlight was exiting the front of the telescope between the secondary mirror and the outer tube wall.without intersecting either of them.
I think if I had held it there for just a few seconds this paper would have caught fire pretty quickly. The light was very intense and it really does illustrate just how damaging the sun can be through a telescope.
Anyway, as long as I didn't look down the front of the telescope, (or accidentally knock it towards the sun) I was safe to observe Venus like this. The image of Venus using the full 300mm aperture was much better than the image through the 125mm off axis mask, bright and sharp and clear.
Cumulus clouds acting as sun shades.
I was reading a report about observing the full atmospheric ring of Venus at inferior conjunction, and it recommended constructing a sun shade to prevent sunlight entering the telescope. The author did warn that at very low elongation angles, the shade does need to be positioned a very long distance in front of the telescope. A quick bit of trigonometry later and I realised that mine would need to be several meters distant, as far as 8 meters away if I want to observe Venus at 2 degrees from the sun. This isnt really a practicable option.
However as luck would have it, the weather has changed a little today. The unbroken hot sunshine in clear blue skies has started to be punctuated with small cumulus clouds passing through on an easterly breeze.
This was actually pretty helpful because periodically a cloud would pass in front of Venus and then a few seconds later it would move away from Venus and pass in front of the Sun, giving me a darker sky to inspect those fine faint cusps against.
The seeing at the boundaries of these clouds was very poor, but once that cleared I would get up to twenty seconds of calm observing conditions with no sun shining into the front of the telescope before the sun shine returned.
In these moments of near-optimum conditions I was afforded pretty good views of the cusps, elongated to a little way short of 270 degrees of the full ring. It's hard to be accurate about this because they fade out gradually at their tips, and, as the seeing and transparency vary, so does the apparent length of these faint extensions.
In poo transparency, and/or poor seeing, the crescent only appears to extend 180 degrees, but when everything lines up, there was definitely something approaching three quarters of a full ring.
So I hope for more little white puffy clouds tomorrow, but I may have to use the 125mm off axis mask if the sun is striking the secondary mirror.
Orion OD 300. Binoviewer. ADC. Apodizer. 25mm Celestron Xcels x166.
Az 240°, Alt 48°, Dia 58", Ph 0.4%, CM1 338°, CM2 189°, Elongation 5°
Hot sunshine punctuated by passing cumulus clouds which could be used to block the sun.
Seeing Ant V to III. Poor immediately after the passage of a cloud, but settling thereafter.
Integrated light (per sketch)