[JD]

I haven't found anything which explains this so perhaps someone here knows.

I used to work in the film industry and I understand t stops. The difference between t stops and f stop is that f stops are theoretical and t stops (transmission stops) are the actual effective ratio. For example, a 50mm f/1.4 will probably be a t 1.5 or 1.6.

So, if a 250mm Dob has a focal length of 1250mm, it has an f ratio of 5. That's simple. But in terms of transmitted light, when compared with a refractor, it couldn't transmit the same amount of light as a refractor of the same diameter.

Yeah, I know a 250mm refractor...dream on. But seriously, the obstruction created by the secondary mirror must reduce the amount of transmitted light. The mirror on a 250mm Dob is about 50,000 square mm but if the light gathering tube has an obstruction of say 5,000 square mm in the middle of it then surely the instrument is less than ideally effective by about 10%.

So is a 254mm f/5 scope really a t 5.55?

Also, how do designers calculate the necessary size for the secondary mirror?

This is not meant to start a flame war about reflectors vs refractors. Of course, the same applies to a cassegrain too. I'm just curious.