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Another possibility with regards to poor image quality might be to do with heat. If you are bringing your telescope outside from a warm house into a cold back yard it will take an hour or more for the mirror to cool down and give you the best views possible. I find it best to get the telescope outside, collimate while it is still light, then go and have tea and come back in an hour or so...
Dew is another possibility. While observing, use a torch to see if the secondary mirror is fogging up. Also have a look at your eyepiece, as a hot face can cause the eyepiece to fog up very quickly. A quick solution to this is to give the secondary and the eyepiece a short blast with a hairdryer.
Have you tried using the telescope during the day to focus on a distant object? A tree or a light pole on the horizon is a good starting point. This can also be used to help adjust the sighting scope / viewfinder so that what you see in the telescope eyepiece is centred on what you see through the viewfinder. Your telescope instructions should include something on how to adjust the viewfinder. Start with your lowest power eyepiece and work your way up to your highest power, refining the alignment of your viewfinder as you go. You will notice that as the magnification increases the brightness decreases, and any atmospheric turbulence becomes more pronounced, but this sort of testing will at least confirm that you are able to get good views through the telescope, and that what you are seeing through the viewfinder is what you are seeing through the eyepeice.
I'm sure the warning is not necessary, but I'll include it anyway... DO NOT USE THE SUN AS YOUR DISTANT OBJECT, OR TRY THIS IF THE DISTANT OBJECT IS ANYWHERE NEAR THE SUN OR MIGHT REFLECT AN IMAGE OF THE SUN INTO YOUR EYEPIECE.
Just my 2c worth, hope it helps...
Peter
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