Whatever happened to offsetting the diagonal??

Hello,

I've got a sct nowadays but I have to admit that Newtonians are my favorite scopes.

When I was a teenager 35+ years ago I remember that it was common practice to offset the diagonal (lower it if I remember correctly) on Newtonians so it was below the optical axis by a few mm. I believe this was done only on large scopes, which meant 10" and above back then

I even remember reading an article in Sky and Telescope about it and I believe the author included a chart for offsetting the diagonals for different sized primary mirrors and focal lengths.

But nowadays I see nothing about this even though the mirrors available to the public are much, much larger than those available back then.

Was this a bogus practice or perhaps it is not necessary with the faster systems in general use now?

Thanks. Good skies everyone.

Tom B.

Hi Tom,

If you have a slightly larger than necessary diagonal you may be 'ok'. If you do a ray trace it is apparent that an offset is needed away from the focuser to catch the converging light cone.
The 45* flat is closer to the primary (up-stream, into a wider section of the light cone) at the 'away' end of the flat than the front. Without an offset, a portion of the cone is cut-off.

Clean skies!

John W.