Demystifying the Focal Reducer numbers

[pmrid (Peter)]

I recently bought a couple of focal reducers - one from Mr. Mogg and the other from Surplus Shed. .63 and .46 but I have no idea just what those numbers signify. Any explanations forthcoming appreciated.
Peter

[Dennis]

Although you probably know this already, an important characteristic of a telescope is its Focal Ratio (e.g. F10, F6.3, etc), where, FR=Focal Length/Aperture.

So, for a 180mm diameter telescope with an FR of F12, we have a focal length of 2160mm, and, for the ubiquitous 200mm F10 SCT, we have a focal length of 2000mm.

For imaging purposes, the above 2 scopes are “slow”. Modern camera lenses have maximum FR’s of say, F1.8 to F4, which means with their diaphragm (iris) wide open, they can “suck in” a lot of light quickly.

Generally, telescopes are slower, having FR’s from say, F7 to F10 or F12, so they require much longer exposures to record sufficient photons to produce a noise free image.

So, an appropriate Reducer/Corrector or a Reducer/Flattener can be fitted to a ‘scope which effectively reduces the FR of the scope as well as corrects any field curvature.

If you fit a Celestron F6.3 Reducer/Corrector to a Celestron C9.25 F10 scope, then the effective focal length changes from the native 2350mm (at F10) to around 1400mm (at F6.3) giving a wider field of view as well as the capability to record images “faster” due to the “faster” FR of F6.3.

Hope that makes sense!

Cheers

Dennis

[pmrid (Peter)]

Thanks Denis. The light has turned on.
Peter

[Dennis]

Hi Peter

Some Reducers/Correctors and Reducers/Flatteners may be optimised for specific scopes or manufacturers. The spacing between the R/C and the image sensor is usually quite important in order to achieve a flat, corrected field.

Have a look at the bottom of this page for an example, where the author provides an example using the Meade F6.3 R/C.

Cheers

Dennis