Friday, May 25, 2012

Venus On Fire

I rolled the refractor out a couple of days ago to look at the Moon and Venus.  It is only a 5" refractor, and you might wonder why that, rather than the 17.5" reflector.  It turns out that fairly bright objects, there is really not that much advantage from the bigger scope.

Yes, resolution improves linearly with diameter of the objective, all else being equal...but when it comes to telescopes, that is seldom the case.  A really good refractor (as the Photon Instruments 5" refractor is, especially with the Aries Chromacor installed) will generally outperform larger (sometimes much larger) reflectors on resolution.  There are a variety of reasons for this: the diagonal mirror and supports in a reflector generally reduces resolution; small refractors can be, and often are quite well-made, while larger reflectors often are somewhat mediocre in their optical figure.  For looking at diffuse objects that cover a bit of sky, such as nebulae or galaxies, the bigger reflector is the better choice--for for planets, an excellent refractor will often perform as well many reflectors that are as much as twice the aperture diameter.

Anyway, I was looking at Venus, which is in a very narrow crescent at the moment.  Keep in mind that in a few weeks, it is going to be transiting the Sun, so it is moving rapidly toward that point.  Because Venus was low in the sky, and atmospheric turbulence was a big issue, the way that extremely bright crescent was bouncing around it looked like Venus was on fire.  Really quite amazing to watch.

1 comment:

Anonymous said...

I caught Venus two nights ago also, using my Kowa spotting scope, using both the 20X LER and a 60X regular eyepiece. It was beautiful, perfectly clear crescent. I am not nearly as serious about astronomy as you are, but since I already had the spotting scope for high power competition, I figured I would give it a try. I have seen Saturn's rings and the bands of Jupiter with all four of the largest moons. The angled eyepiece was a pain in the neck until I got an inexpensive Celestron red dot finder, just for astronomical use.