Hi Nino,
First, make sure your scope has had time to cool down to the outside temprature. A 6" Newtonian can take 30min to 1 hour, or more to reach ambient temprature. A Schmidt Cassegrain can take upwards of 2 hours. Cool down is very important, so set up you scope outside as early as possible. Next, make sure you scope is well collimated(optics are aligned). There are several websites that will guide you on how to do this(As soon as I get a few hours, I will devote a page to this in the near futrure).
Now consider the magnification you are using, and the steadiness of the sky. A 6" scope on the best nights of seeing can reach magnifications up to 300x before the image degrades(50x per inch). On average nights, your scope can only reach a max of 180x(30x per inch). If the stars are twinkeling, stick with magnifications of 100-180x. When the image breaks down, you have gone too far on the magnification.
Which size scope is best? The larger the better for pulling out fine details from planets(10-20" is not too much aperture, but a 6" can work fine too). That being said, let me qualify a few points. The better quality the optics, the crisper the views(a top end apochromatic refractor, or a reflector with Zambuto optics are prefered). The steadier the seeing, the crisper the images. Even the largest and the most optically pure telescope cannot beat the seeing conditions. Use a stable mount, with high quality eyepieces. A chain is only as strong as its weakest link. I hope this helps. Let us know how you make out.
Richard Snow
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