I have been observing the stars through binoculars for some time now and finally went out and purchased a telescope. The telescope is a NewStar 4.7 inch, 1000mm EQ refractor and I have 10mm and 25mm Plossl eyepieces. I realize that when I look at stars they will appear to be dots of light, but when I was viewing Venus I was under the impression that I would be able to see more detail. On the 10mm eyepiece all I saw was a circle of light. My question is: is this what I am to expect or do I need a Barlow lens to magnify further? Your input would be much appreciated.
I think that your problem is not the magnification of your eyepiece but the atmospheres of the Earth and Venus...
Let's calculate the angular size of Venus in the sky at its closest approach to Earth. Since Venus is 0.72 AU from the Sun, then the minimum separation between Venus and the Earth is 0.28 AU, or about 40 million kilometres. Venus itself is 12 thousand kilometres across, so its apparent angular size in the sky is about (12 thousand)/(40 million)x(the number of arcminutes in a radian) = ~ 1 arcminute. Now, due to fluctuations in temperature and density in the atmosphere of the Earth, the resolution obtainable on a very clear night from the ground with any size telescope is a few arcseconds (unless you are on a mountain in Hawaii or in the desert in Chile, where things get a little better than this). This "smearing" caused by the atmosphere is called "seeing" by astronomers, and it is the reason why so much money is invested in putting telescopes (like the Hubble Space Telescope) into space. With this reasoning, then, one might expect to be able to resolve features greater than a few arcseconds on the surface of Venus from Earth.
However, Venus itself has an atmosphere that is much thicker than ours, and made mainly of carbon dioxide. This atmosphere completely obscures the surface of Venus in the optical part of the spectrum. Any features in the atmosphere are sufficiently small and transient that they fall below the "few arcsecond" resolution limit set by the Earth's atmosphere; so, no matter how big your optical telescope is, you will not be able to pick out any features on the surface of Venus (this is the main reason why most observations of Venus are made with radio telescopes; radio waves can penetrate the atmosphere of Venus because of their long wavelength).
Now, the eyepieces that you mention are classified by magnification: this means that they will make the image of the sky that you see through the scope look bigger. However, they will not increase the resolution of the image. So, if Venus looks like a blob with a lower magnification, it will just look like a bigger blob with a higher magnification. So increasing the magnification of the eyepiece won't help you see features on Venus.
So I am afraid that you won't see any interesting features on Venus with your telescope, no matter what magnification you choose. However, try looking at Saturn or Jupiter: your nifty new scope will definitely help you pick out the former's rings and the latter's 4 largest moons and Great Red Spot.
This page was last updated on July 18, 2015.