Focal reducers

[LewisM]

How can we determine if a focal reducer unit will be effective with a particular system? Is there a "formula" for native focal length vs FR reduction factor? Or?

I have acquired a telescope - 100mm, native f/6.4 - and I would like to make it a touch faster - the LONG since discontinued FR for it made it f/4.6, which is in the NICE band.

I have an OLD Vixen FL102S focal reducer (0.7x reduction) that wold bring the native 102mm f/9 instrument to f/6.3 - would this seem feasible to work in a 100mm f/6.4 native scope, or is the geometry going to be off? Just use it and see what happens...

Other than "recycling" this beautiful old Vixen FR, I would need to purchase the Borg variable FR, that actually has an EXACT mark for a 640mm FL scope - and it's $100 cheaper than a Tak TSA FR that might not even be correct for it either.

OR failing all this, just get a TSA flattener (which DOES work well on these) and shoot at f/6.4

[Merlin66 (Ken)]

The matching of reducers is complicated by the optical aberrations in the system. You can fit any reducer to almost any telescope (mechanically) but the image quality may be rubbish.
It comes down to the optical corrections required by the system.

[LewisM]

Thanks Ken, so literally try it and see. I thought as much. I have Tak to Vixen adapters, so this will be the first avenue (it is a beautiful.reducer, fully multicoated)

[pmrid (Peter)]

This may help.
Peter

[Merlin66 (Ken)]

Unfortunately not all reducers are born equal.
Some are just two element achromats - binocular objectives!
Others correct for flatness of field, spherical aberrations, chromatic coma and distortion etc...
Peter,
The positioning of the reducer can change the reduction ratio BUT can also introduce some severe aberrations!!
(One of the secondary benefits of spectroscopy is that almost any reducer can be used - we're only interested in the target on-axis star image)