Focal Reducers

[SteveP]

Apologies for getting a bit technical and for some this might be a rather basic question but here goes.

In applying a focal reducer, say f6.3, to an SCT of say f10, the SCT's effective f reduces to f6.3 and its 'speed' therefore increases; thus reducing the imaging time (by about 50%?). But at the same time, does the SCT's focal length also reduce? If so, by how much in this example?

[GazOC]

Yep, just multiply the scopes native focal length by 0.63 for a rough answer.

[Talitha]

Steve, my 8" LX-10 is a 2000mm f/10.

With the focal reducer, it's a 1279mm f/6.3.

[SteveP]

Many thanks to you both

Steve

[Tiny]

But doesn't the focal reduction only take place between the reducer and the eyepiece?

So while the scope might be reduced from an f10 to an F6.3 the focal length is not reduced by that much. If it was the focus would be well up the tube but in fact the focus point is only reduced by a bit.

For example on my F7, 770 scope the focus point gets moved by a bit over an inch with a .6 reducer (While the scope effectively becomes F4ish.)

[GazOC]

The focal length is also reduced by (roughly, depending on where the reducer/ eyepiece/ camera is in the setup) x0.63. The fact that it doesn't physically move that much doesn't matter, a 8" f10 SCT isn't really 2000mm long either...

[Kaptain Klevtsov]

The effective focal length is reduced as Gaz pointed out. With an SCT no part of the optics has a focal length of 10X the aperture, but the combination works as though it has a focal ratio of f/10. Adding another lens creates the effect of reducing the focal length to ~63% of the native focal length.

In exactly the same way, adding a Barlow moves the focus point out by a small (comparatively) amount whilst doubling the focal length.

Kaptain Klevtsov

[GazOC]

Most SCTs have a f2 or f2.5 main mirror, the secondary is minus f5 ish to give f10. The reducer is just doing a similar job to the secondary and altering the focal ratio and length of the system and can be looked at the same way in this example.

[palefire]

I've got a C5 SCT and was wondering about getting a Focal reducer for visual use.

Would using a FR give me a genuinely wider field view or is the field on the C5 limited by the desgin of the scope?

Would I be better off getting a 2" diagonal and a wide field 2" eyepiece?

[GazOC]

I think you'll need an adaptor to use a visual FR in a C5...

[palefire]

http://www.celestron.uk.com/catalogues/view_item.asp?ItemID=31238&CatalogueID=272&CategoryID=3875

seems to indicate that I can just scew the FR onto the back of the C5.

It doesn't mean it's a good idea. . ., just wondered if anyone had done it?

[Macavity]

I kinda wondered about that too... Will it screw into the (one remaining!) thread of a MAK127 and serve any useful purpose. I do have an ATIK (photographic) reducer. This performs the expected function, when screwed onto the barrel of a 1.25" eyepiece. But I don't know if it can be fixed "elsewhere" to EXTEND the functionality of the scope itself...

I must admit I went with the "2-inch back" idea. Most visual experiments seemed merely to accentuate the secondary shadow and/or give an effect of a drunk looking through a drinking straw...