The moon is not challenging.
I would like to observe the big planets and their rings.
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Jupiter is pretty easy. A little refractor with a 50 mm aperture (diameter of the primary lens) will show you the disk of Jupiter and the biggest two equatorial belts, and the four galilean satellites. Or Saturn and its rings - that's doable too. The image will be tiny, but you'll see those features - assuming the instrument is not junk and it can take a decent magnification.
However, that's all that you'll see at that level of investment. No small equatorial belts on Jupiter, or inter-belt swirls. With the Great Red Spot shrinking and becoming washed out each year, you may not even see that one. Also, Saturn will be disk + rings, that's it; no Cassini division, no ring shadow, or anything else.
If that's okay, then keep reading.
Availability of instruments depends on where you live. I'm more familiar with the market in North America, but EU can't be too different.
Surprisingly, a very decent cheap telescope is the Galileoscope.
Yes, it's a DIY kit that you assemble yourself. But the primary lens is a surprisingly good achromat doublet; the design makes sense for an instrument like this; it accepts standard 1.25" eyepieces used for bigger telescopes; and the price is within your limits.
As an amateur optician and telescope maker, I was pleasantly surprised by this instrument. It works well up to about 100x, which is the theoretical limit for its aperture.
What will be difficult with this scope - and with any cheap refractor - is the mount. For astronomy you need a rock-solid mount, which is hard to do. The Galileoscope can be plugged into a standard photographic tripod - maybe you can find a good one second-hand. The site also gives suggestions for DIY mounts. Try different things and see what works. I've seen many commercial scopes, more expensive than this, that were basically unusable due to really poorly thought out (or poorly executed) mounts. Beware.
If the DIY route is not appealing, try some ready-made scopes. Here are a couple suggestions of vendors (I'm not affiliated with anyone):
http://www.telescope.com/Telescopes/Best-Telescopes-for-Kids/pc/1/459.uts
http://www.optcorp.com/telescopes/all-telescopes.html
Sort by price and keep digging. The mount will be a weak point, always, in this price range. Those refractors with tall, thin tripods tend to be very wobbly. It's a true hell to push the scope a bit this way, only to have the tripod rebound the other way - very annoying.
You may have better success with one of those short, stubby 76 mm table-top reflectors. Tweak the mount until it moves relatively easy, but not too easy. Hold it with both hands and guide it to track the planet. It may actually work better than one of those tall refractors. OTOH, the focal ratio for this reflector is only f/3.9, which is awfully short (it's hard to control aberrations for such a short instrument, unless you invest in high-end optics).
If you do get a refractor, get one of those with short legs. The nice thing about these cheap refractors is that they have a long focal length (large focal ratio), which helps keep aberrations low.
Frankly, there will be some limitations in this price range no matter what. It would help if you could buy from a store with a good return policy - just in case things don't work well for you.
Finally, you say "the Moon is not challenging"? It can be very challenging, once you get into exploring the tiny craters and so on. Just a suggestion. ;)
answered Jun 21, 2014 at 1:16