June 26, 2009: Lunatic in Palo Alto

Jay Reynolds Freeman

I haven't been doing much deep-sky astronomy lately, mostly because I have run out of things that I wanted to do, that I could do with my biggest telescope, Harvey -- a Celestron 14 -- and because upgrading significantly from a C-14 involves a tremendous hit in cost, portability and setup time. Recently I decided to think carefully about things I could do with much smaller and simpler equipment, that could be interesting, and that perhaps could be done from the yard of my home in Palo Alto, California.

I should say in passing that if you had to pick a suburb in a major metropolitan area in which to do astronomy, Palo Alto might run high on your list: The city uses full-cutoff lighting widely in its street lights, and has for a long time been planting decorative trees, and encouraging homeowners to do likewise. Thus, given the clarity of the air that blows in off the Pacific, the sky above is much darker than in many similarly populated areas. Furthermore, the city is close enough to the Pacific coast that the seeing is often excellent -- during the warmer parts of the year, I find that apertures of 10 or 15 cm can run diffraction limited more often than not; that is, I can set up such a telescope and usually count on seeing fairly steady stellar Airy discs at high magnification.

There are some downers. When there is enough wind from the ocean, the fog layer that forms over the cold current off shore moves inland and blocks the heavens. And my own property has no single location from which I can see a good part of the sky. There is a 35-meter redwood tree, plus many roses and a few smaller trees, and plenty of street light glare in front. When I do set up a telescope at home, I am forever having to pick it up and move it around to get a good view.

I have talked about an ecological niche for a "quick-look" telescope in some of my web articles, that being an instrument you keep ready to go inside your dwelling, that is easy to take outside and set up in a hurry. So I decided to make a serious effort to set up a particularly handy example of that kind of equipment, and see what I could do with it. After a bit of fussing with OTAs and mounts, what I ended up with was the optical tube assembly for a 70 mm f/8 Vixen fluorite refractor, mounted on an older (pre dovetail-slot convention) Vixen style altazimuth mounting, in turn on a very light wooden tripod. As I write these words, there are some images of that setup part way down the "Telescopes" page of my main web site:

http://web.mac.com/Jay_reynolds_freeman/My/Telescopes.html

On this tripod, the Vixen 70 is light and compact enough that it is a one-hand carry to get it outside. Since I leave it set up inside, that means that i can go out for an observing run in a single trip, with one hand carrying the telescope and the other perhaps holding the back of a light chair and the strap of a brief case holding a few charts and books and the like. An instrument this small, and a refractor at that, comes to temperature equilibrium almost instantly, so that when I decide to go observing with this setup, I spend more time finding my shoes and putting them on than I do fussing with telescope and accessories.

For an initial observing program, I decided to refamiliarize myself with the Moon. During the lunation that started in late May, 2009, I observed every night for nine days straight, from a few days past new till almost full. The seeing was uniformly excellent: On each night I was able to drop in a 4 mm eyepiece for 140 diameters magnification, and have a rock-steady image.

Why bother observe the Moon with so small a telescope? One way to look at it is to note that the resolving power of a 70 mm instrument is 1.6 arc seconds, whereas the lunar disc is about 1800 arc seconds -- over 1000 resolution elements -- across. Thus in terms of area, a 70 mm aperture show about a million different resolution elements -- pixels, if you will -- across the Lunar disc. That is a lot of detail.

I wasn't trying to make critical observations, really -- mostly just comparing things with the charts in my old copy of Rukl's "Atlas of the Moon", and saying to myself, "Yes, I remember that!" -- or, more honestly and more often, "I had forgotten that!" But I did note a few highlights, that may give you an idea of what a 70 mm can pull in, in good conditions.

Early in the lunation, I could clearly see the Cauchy Wall, but only suspected the Cauchy Rille. Two nights later I had a good view of Rima Ariadaeus and Rima Hyginus. Later, I missed the optimum sun angle for viewing the Triesnecker rille system by half a day, but could still see a portion of the major north/south rille. Several days later, the floor of Gassendi showed many rilles, and as the Moon neared full I may have had a glimpse of part of Rima Darwin.

I spotted a couple of nice sunrise rays. How odd that those pretty phenomena have clearly been around since the (*ahem*) dawn of Lunar astronomy, but that no one seems to have noticed them until the last few decades.

One aspect of Lunar observing that is both vexing and pleasant is that you can't expect to see all the major features just by observing for short sessions on consecutive nights of a lunation, as I did. Much of the surface detail is so subtle that variations in solar elevation of as little as a few degrees make vast differences in its appearance; since the sunrise line on the Moon advances by about fifteen degrees per Earth day, it follows that short sessions a day apart will skip lots of things. I have already mentioned missing a good view of the Triesnecker area, and I also did not get a good view of the Straight Wall, Schroeter's Valley, or the magnificent apparition of sunrise on Sinus Iridum.

Lunar observers tend to focus so much on detail revealed by the pattern of light and shadow near the terminator, that they forget to look at those parts of the Moon where the sun is high. What you are looking at then is a view optimized not for showing topographic features but for distinguishing different kinds of surface material on a planet-wide scale. We have all seen geological maps of the Earth that use different shades of unlikely colors to show Triassic lava flows here and Precambrian granite there; to look at the Moon at high sun angle is to see such a map made manifest by nature itself. The rough ejecta streams from rayed craters and the contrast between the pale anorthosite of the highlands and the darker and more basaltic material of the maria are only the most obvious of these features. What about the light chevron shape that lies selenographic west of Mare Crisium? Or the very dark surface in the Taurus/Littrow region? Apollo 17's landing site was selected in part to investigate that last.

By the way, here is a map of the surface geology of the near side of the Moon, funny colors and all. The link is current as of 26 June 2009:

http://www.lpi.usra.edu/resources/mapcatalog/usgs/I703/150dpi.jpg

So I think I have proved my point, at least to myself, about the prospect of doing interesting astronomy with a very small telescope from a cramped suburban yard ... even if I do have to look at the Moon to do it ...

Jay Reynolds Freeman, Deep-Sky Weasel