Focal Reducer

[Jammy]

I've been looking on here and on the internet in general to see if I can find a formula that will determine focal ratio when using a focal reducer, but can't find anything.

I know that if I use a Celestron f0.63 focal reducer with my C9.25 with a back focus of 105mm, that will give f6.3 focal ratio, but what if I change the back focus to 90mm or 110mm or some other distance?  How does this affect focal ratio?

Is there a way of calculating the focal ratio for different back focus from a reducer?

[Tinker1947]

Have a look at this.....Use the search option for "Focal Reducer" for more information....

http://www.celestron.com/c3/support3/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=2333

[lensman57]

Hi,

The formula in simple form is D ( distance between the FF/FR and the CCD) = Fr( focal length of the rducer) X (1-R), R being the reduction ratio of the FF/FR. You also have to be aware that with SCTs the focal length varies as focusing is achieved by moving the primary mirror.

Hope this helps.

A.G