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Originally Posted by Rastis
Sorry just thought of this, Where can I get a f8 8 or 10 inch dob from. This sounds like a perfect scope to me. Would be a awfully long scope though. Seems to me that they are all f4 to f6
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Rastis,
Ron Royal used to run a telescope making course through Sydney observatory and they made an 8"/F8 dobsonian from a kit. Ron also used to sell the kit separately to interstate people and then offer telephone support through the building stage. I don't know that he still runs the course as the influx of chinese/taiwanese dobs may have made it difficult to compete.
Discovery telescopes in the USA make an 8"/F7 but they are expensive for what you get. Cave also used to make an 8"/F7 newtonian years ago and very rarely these OTA's are offered on the 2nd hand market. Other than that you would have to commission an ATMer to make you 1 or build it yourself.
However an 8"/F6 is a good compromise between the views it provides and portability , that is why most 8" newts are F6.
In respect to Dennis's comment about hand tracking I agree that you should try both a tracking scope and a dobsonian before you buy. Be aware that it takes a little practice to learn to hand track properly with a dob and it also helps a lot if the dob has been properly optimised for tracking. Some dobs owned by people that don't know what they are doing, or don't care, track horribly and are almost impossible to use. Those owned by people that know or care move like melted butter on toast and are childs play to track at over 500X with a very narrow FOV. Dennis's comment about manually tracking with his SCT is not relevant IMO as a SCT fitted with motor drives is not optimised to be used by hand tracking. Actually some of them are down right impossible to use with the drives switched off. On the other hand a dobsonian is designed to be used by hand tracking and has a much longer moment arm past the pivot axis and is a lot easier to hand track with, if its properly set up.
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Originally Posted by Dennis
An f8 dobsonian would also share the same difficulties in keeping the image in the FOV as at the longer focal length (and higher magnification) the FOV becomes quite small and the planet literally races across the field of the eyepiece.
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AS of NSW have a 16"/F7 dob at Bowen Mountain and it tracks without issue, so I don't see any problem tracking with an 8"/f8 that has about 1/2 the focal length. Likewise I have used a 20"/f5 with a 2.54 metre focal length on numerous occasions at high power without issue. I think your making an assumption there Dennis without ever having tried to track with such a scope.
CS-John B