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Hi,
Mike at Bintel gave me an interesting article which says for a given aperture, the F ratio makes no difference to the amount of time it takes to take a sub of the same brightness. The article states that for film this is not the case. For CCD's however the brightness of the image is purely dependant on the number of photons hitting the CCD. Since this only changes with aperture the F ratio doesnt matter as far as brightness is concerned.
This means you can wack a 2 by powermate on your F7.5 and go to F15. This will give you a very good image scale and you can continue to take the same number of subs as you took at F7.5 or F6 with the reducer. Of course it will be far harder to guide and you cant increase the resolution.
However it is an interesting experiment for a small scope; and may negate the need for moving up to say a VC200L. When the weather clears this is an experiment I would like to try out.
Guiding at F28 on a Televue 85 with a 4 by powermate will probably be horrible. However a TV85 is a light scope, it's unobstructed and no harder to guide at F28 or 2.4 meters than say a VC200L.
Cheers
paul
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