Observing Log 6/3/2020 @ 22:00-7/3/2020 @ 00:30
Andrew Thornett, Alan and Angella Rodgers
- Orion UK 10” Dobsonian Telescope
- Explore Scientific 14mm & 9mm 100-degree AFOV eyepieces
- Tele Vue Ethos 6mm eyepiece
- Tele Vue Big Barlow x2
As a night with predicted cloud cover around 50%, this was a poor choice for astrophotography. A bright 10-day old moon meant that most of the deep sky was washed out, plus cloud quickly passed through obscuring faint objects as we tried to find them. It therefore seemed a poor choice for an evening of astronomy. However, Angella and Alan came around and the three of us spent a wonderful time observing the moon. Usually, we “experienced” amateur astronomers reject the moon but it such a wonderful object with so much detail. Tonight, we took the time to eke out more detail than I have seen before on the moon….not because I couldn’t but rather because I didn’t and after tonight’s experience I feel that this was an error on my behalf – I intend to spend far more time in the future getting to know our close celestial neighbour! I can see now why Patrick Moore used to spend so much time looking at it from his dark skies in Selsey with his 15” telescope when he could have looked at other things.
I also discovered tonight that I still adore my first love of visual observational astronomy – over the years I have done radio astronomy, astrophotography, spectroscopy, amateur telescope making, and all those are great in their individual ways, but getting out there with the telescope and eyepiece under the stars cannot be matched! Wow! Wow! Wow! I also love the Dobsonian telescope – so simple to use and set up, and such great stable views – although does need quite a lot of star-hopping skills but then I enjoy doing that too. No circuit boards to die, limited dependency on batteries (illuminated finders mainly and your torch!)
We also used good old-fashioned printed moon atlases tonight. In particular, we used two books tonight: (I) Thierry Legault and Serge Brunier’s New Atlas of the Moon 2006 which gives large labelled photographs of the moon day by day throughout its cycle and close-ups of particular regions, and (II) Antonin Ruke’s Atlas of the Moon 1990 – this latter very famous tome (mine is the much cheaper first edition) has vast numbers of labelled drawings of different areas of the moon.
We started our journey by identifying Phocylides and Wargentin craters – that make up a Ginger biscuit man shape (2 circles of slightly different sizes close together and easily picked out against other craters). We tried to identify both small craters between these two – but one was over side of moon so could not be seen.
From there we hopped to the ray crate Tycho, and followed the two parallel rays down to crater Bullialdus with its central peak which we could easily see. We then hopped to Gassendi and spent a lot of time on this crater. We could identify a breach in its side wall but had difficulties seeing all the features in its base that were visible on the photographs in the New Atlas of the Moon, even with the 6mm Ethos eyepiece. However, when I added in my 2x Big Barlow to give magnification 400x (1200mm focal length on scope) then they became visible and the view was amazing!
Wikipedia gives a description of the crater which accurately describes the view we saw, “Gassendi is a large lunar impact crater feature located at the northern edge of Mare Humorum. It was named after French astronomer Pierre Gassendi. The formation has been inundated by lava during the formation of the mare, so only the rim and the multiple central peaks remain above the surface. The outer rim is worn and eroded, although it retains a generally circular form. A smaller crater – Gassendi A – intrudes into the northern rim, and joins a rough uplift at the northwest part of the floor. The crater pair bear a curious resemblance to a diamond ring. In the southern part of the crater floor is a semi-circular ridge-like formation that is concentric with the outer rim. It is in the southern part where the rim dips down to its lowest portion, and a gap appears at the most southern point. The rim varies in height from as little as 200 meters to as high as 2.5 kilometers above the surface. The floor has numerous hummocks and rough spots. There is also a system of rilles that criss-crosses the floor, named the Rimae Gassendi. The fresh crater Gassendi A is adjacent to Gassendi to the north.” (https://en.wikipedia.org/wiki/Gassendi_(crater))
The rilles, in particular, could not be seen with 6mm eyepiece but became visible with 3mm eyepiece (6mm+2xBarlow). We weren’t going to stop until we saw those – so were very pleased with ourselves when we got them!
The main issue with 3mm eyepieces (6mm+2xBarlow) was that (even though the Ethos has 100 degree AFOV) the image shot across the field of view, giving little time to take it in before we needed to nudge the scope – the main failing of a Dobsonian telescope in my view.
From Gassendi, we dropped down to Copernicus, and thence to the Sinus Iridum, which stretches from the east at Cape Laplace to Cape Heraclides on the west. This required the 6mm eyepiece before we could start to see detail in the mountains around the edge of the sinus. What a view! Incredible! So beautiful and full of wonder. We could also see ghost craters within it, and this led to a discussion between us on the history of the moon, when one crater forms, gets filled with lava and then new craters occur within the same area without lava filling them.