quick f-stop question

[disturbed13]

when you are figuring out the f-stop on a Cassegrain-Relay scope do you divide the length by 3?
3 as in
end of scope to main mirror > main to secondary > secondary to burned eyeball

im 99% sure on this, at least it makes sense to me
but i would like to hear from someone that really knows
thanks!!!

[KeithBC]

You need to use the manufacturer's published focal length. You can't rely on measurements, because the focal length is altered by the curved secondary. The focal ratio is the focal length divided by the aperture.

[disturbed13]

so when you build your own CASS and you have chosen a f stop you want
your tube length turns out to be 11ft long
should i divide that by 2? or 3? or will it actually have a tube length of 11ft long?

thanks for the reply btw

[KeithBC]

The tube length will be significantly shorter than the focal length. Just how short it will be will depend on the characteristics of the secondary mirror. You will have to design that before you can design the tube. I think, as a general rule, the tube length is around 1/3 or less of the focal length.

Because of the complexity of making the convex secondary, and the difficulty in making an extremely fast primary (on an f/10 SCT, the primary mirror is about f/2), few amateur telescope builders attempt a Cassegrain design. I assume you are talking about an open-tube plain Cassegain, as opposed to a Schmidt-Cassegrain or a Maksutov-Cassegrain. On the latter designs, the complexity is increased by the requirement to grind accurate corrector plates as well. I can't recommend attempting it.

[AustinPSD]

Your question is too vague, and does not relate to a specific relay-Cassegrain design variant, of which there are many.

Designing a relay-cass is not a "back of the napkin" exercise - it relies on use of specific design centers, mirror types (i.e. all-spherical, etc.), and use of a suitable tool, i.e. something like OSLO. A simple divisor rule isn't going to give you a valid approximation of any given parameter of a relay-cass design.

[disturbed13]

the one that got me thinking was Scott Milligan's Version of a Dillworth that some refer to as a Dillworth

[disturbed13]

so the total lenght that is listed would be from the back glass to the front glass
right?
not the main mirror to the front mirror

[AustinPSD]

One can only make some basic inferences from the layout diagram as supplied...

The "Total Length" parameter of 2689.90160 mm is almost certainly the instrument's target focal length (this is 105.9 inches). The less precise figure of merit may refer to the physical length of the folded optic, 30 inches, but there is not enough information contained in the diagram to say for certain.

The former is based on one interpretation of the provided data - i.e. the intent that this is an 8" - 11" aperture relay-cass.

The less likely case is that this is a 30" (762mm) aperture relay-cass, in which case a physical length of 2600mm - 2700mm might be realistic.

Have you tried contacting the designer?