Thanks for your help,
I am purely looking at Newts or Dob's
Since my first post I have located a single supplier, one particular manufacturer - advertising :
Dob with paraboloidal pyrex mirror
and
Dob (collapsible truss) with a parabolic mirror
Both have spec's 254mm, 1200 focal length (f/5 or f/4.7)
So with almost identical spec's from the same manufacturer I still do not understand the difference between "paraboloid" and "parabolic".
I would like to understand the affect it will have on my satisfaction with my investment. I appreciate that being able to visually identify the imperfections may be academic, but I would like to understand what the theoretical implications / limitations are in the differences.
I would like to be able to get into photography with just a mount upgrade.
If anyone can explain how they have different f/ratio with the same aperture and focal length I would appreciate it as the published concensus seems to be :focal ratio = focal length of telescope / aperture of telescope
Dale