Quote:
Originally Posted by AstroJunk
You also miss out on on-axis light too, it's not just the wide angles. This can be quite a lot if your secondary is 'traditionally' sized. I upped my 3.1 secondary to a 3.5 and offset it. The view was much better!
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Hi Jonathan,
If that was on your 20"/F5 the secondary really was undersized (15.5% CO), not traditionally sized and I think most of any gain you noticed would be attributed to up sizing the secondary to 3.5" and not so much by off setting it. Without running the numbers I am guessing the FIF with the 3.1" secondary to have been just less than 6mm. That all having been said I think the offset is important with today's trend at larger and faster newtonians. I would agree that it is probably worthwhile with scopes larger than 16" or 18" and faster than F4. It is also less important if you tend to not use long focal length eyepieces very often, which I don't. Normally the first eyepiece I start with in my 18"/F4.5 is a 17mm Nagler, or a 14mm Pentax XW.
Cheers,
John B