Telescope/Diagonal focal length?

[Alfian]

Hi all, this is probably a senior moment in the making -I feel I should know this but can't bring it to mind! I was looking at a new dielectric diagonal and noticed it gave its effective focal length as 85mm. If a telescope OTA sold without diagonal/EP shown for example as 500mm focal length does that mean that with the 85mm diagonal the scope has an effective focal length of 585mm?

This will clearly have a knock on effect in terms of EP magnification/fov. Am I/have I been - missing something very obvious?

[Cornelius Varley]

The focal length of the telescope shouldn't change. The 85mm is probably the distance the diagonal adds to the optical path. If you removed the diagonal and attached a eyepiece directly to the back of the telescope you would need an extension tube of about 85mm to achieve focus.

[ronin]

"Foccl length" to a diagonal makes no sense, as Peter says it will be the optical path length within the diagonal assembly.

Which diagonal say "focal length" out of interest?

Either the retailer has had a moment of forgetfullness or it a Chinese-English translation error.

[Alfian]

Hi Peter & Ronin, thanks for the replies. Make that 2 marbles I've mislaid! One - confusing focal length and optical path and two - misreading "effective optical length" as "effective focal length". Thanks for the clarification guys. Not a good day!

[Peter Drew]

It would be helpfull if all diagonals and similar accessories included light path lengths in their specifications.