Is there any difference in the optics of a short FL and a long FL reflector?

[tenbyfifty]

(obviously FL = focal length)

is it just the length of the tube that's different or for example, are the mirrors more/less curved?

[John]

In a conventional newtonian the depth of the parabolic curve in a shorter focal length scope is deeper than that of a longer focal length scope however schmidt-cassegrains and some other designs actually use a very short focal length main mirror and then a specially designed secondary mirror and / or a correcting lens to achieve their focal length so it varies with different optical designs.

So there is more to it than just the length of the tube

John

[barkis]

TBF If you want to calculate the depth of the Sagitta, ie the depth of the curve at the centre of a mirror. the formula is

the radius of the mirror squared, divided it's radius of curvature. Radius of curvature being twice its focal length.

Therefore,if you wished to make a 10" f5 mirror you would need to grind a curve in it .250" deep.

A 10" f8 the curve would be .156".

As you see, the shorter the focal length, the more glass has to be removed. The main reason why a richer field scope, can me more expensive than a longer focus one. The degree of difficulty in putting the correct figure on the mirror is also greater.

Ron.

[radioactive]

my C11 11" primary SCT is a f10 its focal length however is 2800mm

[barkis]

Ah, different set of numbers for that one, the primary mirror has a relatively short focal length, but the light striking your mirror is returned the the secondary, mirror set in the corrector plate, and the focal length is extended by the special curve in the secondary, which is not flat as in a Newtonian. That is why a long focal length can be contained in a short tube. The second mirror acts in a similar way to a Barlow lens.

Ron.

[tenbyfifty]

Thanks for those informative replies.

Should have remembered all this from O level Physics!!!

T'was a long long time ago though

Is it possible to use the same secondary mirror for different FL primaries?

I was wondering whether you could have a dual purpose scope where the primary mirror end of the scope

unlocks from the main OTA and can be replaced with one of a different FL, I guess you'd

have to re-collimate though.

[Kaptain Klevtsov]

There would be little point as the tube is just that, a bit of tube. The expensive bit is the mirror, so the swappable mirror would cost as much as another 'scope when you add in the jiggery pokery needed to make the mirror end easily detachable.

A thing similar to what you describe happens with the faststar and hyperstar systems where the camera goes at the prime focus position of a compound telescope. This uses the native f/ ratio of the primary mirror which is much faster (shorter focal length with the same aperture) system. I imagine that there is some kind of corrector lens in the bit that replaces the secondary mirror, but I've never seen one and so I don't know.

When I built my Skelescope I made provision for loads of focus travel so that I could use focal reducers or Barlow lenses easily. This allows me to use the same 'scope at several focal lengths using these add-on optics.

Kaptain Klevtsov