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Old 24-10-2020, 10:14 AM
glend (Glen)
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glend is offline
Join Date: Jun 2013
Location: Lake Macquarie
Posts: 6,146
Focal Length Useability

I am pondering the wisdom of upgrading my 8" Classical Cassegrain ( which I am happy with), to the 10" version, which would bring the f12 focal length from 2400mm to 3000mm. Sure I get more light gathering aperture and resolution as well. However, I am concerned about diminishing useability when native focal length increases. I am acknowledging that this scope is primarily a planetary, and double etc star splitter, but can still poke around very bright nebula like Orion, Tarantula, etc.
Of course focal length useability is impacted by Seeing conditions, and I am concerned that I may not find the addition focal length can be used from my coastal location. Perhaps simply using higher powered quality EPs with the 8" CC could achieve near the same result ( granted that the aperture would not be gathering as much light). As an example, recent Mars observations with the CC8 get hard to push beyond 266X without losing detail, the same 9mm EP would yield 333X with the CC10, assuming Seeing permits. A 6mm EP could return a theoretical 500X on a CC10 but likely never achievable in my environment. I have been using the Scope Calculator to explore the numbers,

But it assumes perfect Seeing.

I am leaving cost considerations out of the discussion.
For all of you long focal length users, do you have opinions on this topic?

Last edited by glend; 24-10-2020 at 10:31 AM.
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