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Hi Rod,
I have to agree with Mark (Satchmo). Your chances of getting that 6"/F8 achromat to 300X with good image quality are "zero". The physics of the objective lens design simply does not allow it.
The first thing I would be getting is a minus violet filter. This will help enormously at minimising chromatic aberration. It doesn't fix it entirely. Even with a minus violet filter in place that scope is going to run out of choof at about 180X to 200X , which is about 30X to 35X per inch of aperture. A good quality telescope of different design, ie newtonian or an apochromat, will push at least 50X per inch of aperture under good conditions. It's not that these 6" achromats are junk, you're just asking it to do something it was never designed to do, or going to be capable of doing. They are excellent at lower to medium power views of clusters and the like.
I think if you get the 6" achromat, try to limit yourself to a 7mm or 8mm eyepiece as your highest power eyepiece. If you get a minus violet filter the 7mm should work fine for 171X. If you don't get a minus violet filter, go with an 8mm at maximum, for an upper limit of 150X . Above this the image quality just isnt going to be good enough for mine.
Cheers,
John B
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