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John,
Assuming good collimation, coma is less of an issue at higher power and smaller fields of view.
With 1200mm aperture focal length, the 10mm EP gives a magnification of 120 and the 7.5mm EP a magnification of 160.
With the 2x barlow, the magnifications will bump up to 240 and 320. The 240 will require rather good seeing conditions but the 320 will require excellent seeing, which is rather a rarity. If the image is jittery, sometimes in focus but more often not, then seeing is poor.
The barlow is useful but you will need good seeing conditions.
Regards, Rob.
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