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Hi Rob
Large achromats with short focal ratios are not recommended for high power work regardless of whether you use a barlow or not. 4x to 8x per inch of aperture or less will yield the best results. Generally they are best suited for low power wide field of view work such as comet finding or milky way sweeping. There will be significant chromatic abberation (colour fringing) on any bright stars or planets. Their advantage over newtonians for this type of low power work is potentially better contrast and better field illumination (less vignetting of the light cone in wide field use).
Having said that they can be used for limited observation of the moon and planets by stopping down the aperture and effectively increasing the focal ratio. I have a nice 6" f5 refractor that is best at 25x - 45x magnification. I have tried it with a 3" aperture stop to give an f10 ratio and it was very good on the moon and planets up to 100x and great for terrestial observation but I have other scopes for that type of thing.
If you want a large achromat for looking at the moon and planets I would recommend an F ratio or 12 or even better 15 or more.
Last edited by anj026; 03-07-2010 at 10:44 AM.
Reason: typo's
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