A 12mm EP (eyepiece) and a barlow are both optical accessories used in telescopes to increase magnification. The main difference is that a barlow is an additional lens that is placed between the eyepiece and the telescope, while a 6mm EP is simply a different focal length eyepiece. A 12mm EP with a barlow will provide a higher magnification than a 6mm EP alone.
Both options have their advantages. A 12mm EP with a barlow will provide a higher magnification, making it better for viewing smaller and more distant objects such as planets or star clusters. On the other hand, a 6mm EP will provide a wider field of view, making it better for viewing larger objects such as galaxies or nebulae.
Yes, you can use a barlow with any eyepiece, including a 6mm EP. This combination will provide an even higher magnification than using a 6mm EP alone.
One potential downside is that using a barlow can decrease the overall image quality, as it introduces an additional element into the optical path. Additionally, using a barlow can also decrease the amount of light reaching your eye, resulting in a dimmer image.
The best option will depend on the specific object you are viewing and your personal preferences. If you are primarily interested in viewing smaller and more distant objects, the higher magnification of a 12mm EP with a barlow may be better. If you prefer a wider field of view and want to view larger objects, a 6mm EP may be the better choice. It's always a good idea to experiment with both options to see which one works best for your specific telescope and viewing conditions.