Focal reducer?

[Astorix]

Morning all, have been browsing some sites this morning looking for bits and bobs and keep seeing focal reducers. What exactly do they do? and would it worth getting one?

[Merlin66]

A focal reducer is a positive lens placed between the objective and the prime focus. It reduces the focal length of the objective and reduces the focal ratio. Most reducers, to work properly, usually have a design distance (between the reducer and the focus)

For SCT's the standard reducer is a x0.63 which changes a f10 system to a f6.3, and requires a distance of 110mm. This gives a wider field of view and brighter images; easier for photography.

Ken

[Astorix]

ok without meaning to sound horrible i am still relatively new to this and not really sure what that means.. could you or some one please explain in more simple terms, for etc i do not know what f10 is or what sct? actually is.

[hunterknox]

SCT = schmidt-cassegrain telescope.

Focal length = how far it is from the first lens or mirror in a telescope to the prime focus point (where you put an eyepiece or camera)

A focal reducer is a lens you put on the eyepiece side of a telescope that effectively makes the image smaller. The same effect could be achieved if you could somehow shorten the focal length, hence "focal reducer". The opposite effect can be achieved with a barlow lens, which is a focal extender: it magnifies the image by effectively increasing the focal length.

People who take astro pics like focal reducers, as Ken says, because they allow you to take shorter exposure images with wider fields of view. Not sure how useful they are for visual work.

[hunterknox]

f10 is an example of a focal ratio.

Focal ratio is the focal length of a telescope divided by the size of the primary lens or mirror.

So, for example, I have a refractor with a 900mm focal length and 100mm lens at the front. The focal ratio (f) is therefore:

900mm/100mm = 9.

Again, as Ken says the shorter the focal ratio the brighter the image. The longer the focal ration the greater the contrast.

Because a focal reducer reduces the effective focal length it also reduces the focal ratio.