Re: Slow telescope focal ratios

Stu Beaber

It's because the critical focus zone (CFZ) is greater with slower telescopes. The CFZ is the distance one can move the focal point without any apparent change in focus, According to Ron Woraski's book "The New CCD Astronomy'', the formula to determine that is CFZ=focal ratio squared times 2.2=answer in microns. He goes on to say that the CFZ for a f/5.5 scope would be one quarter that of a f/11 scope. However real world values for the CFZ are approximately 10-30% greater than theoretical values.


On Thu, Jan 13, 2022 at 11:12 AM Kent Blackwell <kent@...> wrote:

Most of us prefer "fast" telescope ratios. With fast systems, such as f/4 to 4/6 telescope tubes can be shorter, important for transporting our instruments to dark sky locations. There are exceptions, such as folded telescope systems. Back in the late 1960's Celestron offered a Schmidt-Cassegrain 8" f/10 folded system in a very compact tube. I had one years ago and it was compact enough I even took it to a cruise to Africa on an eclipse expedition. 

I currently have a rather rare Meade 10" f/15 Maksutov design. What I've noticed with fast system such as f/4 to f/6 is stars tend to "snap" in and out of focus with just a touch of the focuser. With the above mentioned f/15 system I've noticed I can turn the focusing knob a considerable distance and the stars stay in focus. I also noticed the same with my 4" f/15 Unitron refractor. Focus becomes less critical with "slow" systems such as f/10 to f/15. 

I don't know why I brought this up, but last night I just thought it was amazing how far I could turn the focusing knob on the Meade 10" f/15 system and maintain a sharp focus. 

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