30 minutes with the Moon

23-May-1999

I've never spent that much time which the moon. Tonight, the transparency is bad but the seeing is good. Nothing else could be another target except the bright moon, which is now nicely placed in my little viewing window out of my window in my sleeping room. It looks fuzzy with naked eyes.

I've used my Ranger to do the observation, and it's the only scope that I've after I sold my C90. I didn't miss my C90, since it could not show me anything that the Ranger cannot. The eyepieces which I've used include a 20mm Tele Vue Plossl and a 32mm Sirius Plossl. I've the 5x Powermate and a 2x barlow with me. Therefore, I have 15x, 24x, 30x, 48x, 75x, 120x, 150x and 240x, quite complete. For the moon, I use 75x, 120x, 150x and 240x. I've a light yellow filter (#12) with me as well.

At both the 150x and 240x, I find that my eyepieces are dirty. At the very beginning, I suspect those were my floaters in my eyes. By rotating the eyepiece, I know that it's not the problem of my eyes. By rotating the barlow or the Powermate, the dirts do not move as well, that means I have to clean my eyepieces.

After cleaning the eyepieces, the views are so good. At 240x, the moon drifts across the field of view quite fast, together with the craters on the moon surface. I've a feeling that I'm traveling slowly in a space ship, looking out from the windows. At 240x, the field is not too big and I could only keep looking at a few craters at a time. I found the view at 150x much more comfortable. Adding a color filter does increase
the contrast.

I find that most of the craters are nearly perfect circles, and they are not very deep when compared with their diameter. I don't have a moon map and so I cannot identify the craters, but I find it interesting just to look at them. I have a feeling that I am very closed to the moon, the craters look so large. (Yes, I know they're actually much larger)

Even with a standard Plossl, the field of view is wide enough and pleasing, and I wonder how it would be if I looked through a wide angle eyepiece. Anyway, it's a matter of emotional feeling rather than it is much more useful.

I find that the thin layer of cloud is a not too bad filter. It softens the image produced, but there are still many surface features visible. It darkens the glare even without my own filter, so it comes for free. I find that the moon is too bright even with a small exit pupul, at 240x, i.e. 0.29mm. And since it is so bright, it is a pretty good target for small scopes.

I switched to 75x finally, and the moon fits nicely in the whole field of view. With the Powermate, the 32mm Sirius Plossl produces sharp image across the whole field of view. The edge performance of the Sirius Plossl is not so good without any barlow. The most suitable magnifications are 120x and 150x for the moon, I think. I prefer to
use the 75x for the eye-relief is slightly more comfortable than that of the 120x. I can see the whole field of view in both setting.

Next time, I should come back with a moon map.