Dumb question on focal length

[No-i-dear]

Most of you will probably slap your hand to your forehead and exclaim 'Doh!' when I ask this, but here goes!

What is the focal length of a reflector?

Is it the distance between the primary and secondary mirrors , or is it from the primary mirror to the eyepiece via the secondary? The reason for asking is that my tube is not marked with the focal length.

Thanks!

[JamesF]

The distance from the primary mirror to the eyepiece, near enough.

James

[Jessun]

Not stupid at all! It's aperture times f-ratio. A 8" or 200 mm aperture telescope at f5 (both usually marked or at least known) will have a fl of a metre.

/Jesper

EDIT got myself cunfused now so removed some woffling...

[JamesF]

Sorry, via the secondary, as you suggest.

James

[JamesF]

It should be distance between the mirrors times two plus distance between secondary and focal plane - such as the eye piece or camera. At least that's my understanding.

Why twice the distance between the secondary and the focal plane? The light only travels that way once?

James

[No-i-dear]

Thanks James, much appreciated as always!

[No-i-dear]

Now I'm confused.

[Jessun]

James, that bit is loooong gone !

I never managed to get 1.6 m to fit inside my RC though... Perhaps it doesnt have to fit?

/Jesper

[No-i-dear]

The Sky at Night Beginners Guide (my bible) says that its the distance between the lens/mirror and the place where the image is brought to a focus. Looks like James is right.

[Jessun]

I agree. Sorry for the confusion!

/Jesper

[cantab]

James, that bit is loooong gone !

I never managed to get 1.6 m to fit inside my RC though... Perhaps it doesnt have to fit?

/Jesper

Ritchy-Cretien's, like SCTs and Maks, have a curved secondary. The focal length only equals the physical primary to focal plane distance when there's a single mirror or lens, not counting flat mirrors, involved.

[damnut]

Hi There,

Glad you are sorted now, no such thing as a dumb question. The fl is important as it allows you to easily calculate the magnification of yor eps.

Mag = Fl scope / Fl ep.

You can have abit of fun (!) on these sites :---

http://www.telescope-simulator.com/index.php?option=com_content&view=article&id=45

http://www.12dstring.me.uk/fov.htm

http://www.stargazing.net/naa/scopemath.htm

Cheers

[Jessun]

Thanks!!!! Never thought about that! It's like having a refractor with a focal reducer in it, light bends twice. I think the penny dropped a my end.

/Jesper

[No-i-dear]

Respect!

[JamesF]

Ritchy-Cretien's, like SCTs and Maks, have a curved secondary. The focal length only equals the physical primary to focal plane distance when there's a single mirror or lens, not counting flat mirrors, involved.

Yes, the "obvious" rules only apply when there's a single optical component involved. When you get more than one optical component in the train then the total effective focal length varies depending on the refractive (reflective?) index of the components and the distance between them. The obvious example of this is a barlow. How could a barlow double the effective focal length when it only adds (say) 40mm to the physical length of the optical train?

The upshot of this is that Maks, SCTs and RCs with multiple optical components can have an effective optical length much longer than the light path. In fact, where focusing involves moving the primary mirror thus changing the distances between the optical components, the act of focusing actually changes the telescope's effective focal length. With a Mak (the only one I've bothered to experiment with thus far), moving the mirror a little doesn't actually stop the telescope being in focus, just moves the position of the focal plane (by more than the about the mirror moved, when I was playing with mine).

James

[John]

Ritchy-Cretien's, like SCTs and Maks, have a curved secondary. The focal length only equals the physical primary to focal plane distance when there's a single mirror or lens, not counting flat mirrors, involved.

Thats my understanding too. With RC's SCT's etc the focal length in the spec is that of the optical system as a whole rather than just the objective or primary mirror. Of course there are refractor designs that use secondary lenses which combine with the objective to produce the working focal length, ie: the petzval design.

[No-i-dear]

Question. How do the 'dual speed' 'scopes work?

[JamesF]

Question. How do the 'dual speed' 'scopes work?

What do you mean by a "dual speed" scope? Can you give an example?

James

[Peter Drew]

Cassegrain/Newtonian?

[Jessun]

The focuser?

/Jesper

[cantab]

"Dual Speed" usually refers to the focuser, ie it has a coarse and a fine control, and not to photographic speed (f/ratio).

[No-i-dear]

Understood. Thanks.