astrophotography - Can I expect good quality from a 2X TC on a Nikon 55-300mm lens?

I have a Nikon D3100 with the standard 18-55mm lens, and a Nikon 55-300mm lens. I bought a 2X teleconverter for the 55-300mm lens, but the TC seems to be unusable. At 300mm with the TC everything is blurred and smeared. 'Colourfully cloudy' is how I can best describe it. If I set the 55-300mm lens at about 100 (with the TC) I can get something in focus, but the quality is really bad.

Do I have to use a particular setting for the 2X TC to work well, or is it just not possible to use a 2X TC on a 300mm lens?

I didn't expect much from the TC (at £20 how could I?) but I did expect it to do something! :D

My main use for the 300mm is for astrophotography hence why I wanted the 2X.

Without:

With:

Answer

If I had to guess, the reason for the issue is it being out of focus. I suggest you try to manually focus the lens and see if you can get a good picture. A second guess would be, as others have mentioned, a cheap teleconverter.

Teleconverters (espeically 2x) are not designed to be used on ALL lenses.

The first reason is they will cause the amount of light to be so low that you lose autofocus as the autofocus sensor doesn't have enough light to function properly. As an example, the new Nikon D7100 is claimed to have an upgraded focus sensor that will function all the way down to f/8. Which means an f/4.5 lens with a 2x converter "should" still function. I'm still waiting on reviews to see if it really holds up to this claim.

The second reason has to do with how the piece of glass at the end of the teleconverter that faces the lens may be too long and protrude too far into the back of the lens where the rear lens element resides. This is not always a factor on all lenses, meaning you may be able to physically attach the converter to the lens, but the rear element of some (espeically kit) lenses physically touch the front of the teleconverter.

As far as the Nikon brand of teleconverters, they have a chart of compatibility where your 55-300 is not a member of: http://www.nikonusa.com/en_INC/IMG/Assets/Common-Assets/Images/Teleconverter-Compatibility/en_US_Comp_chart.html

edit:clarity