Barlows and reducers.

[Mac]

Fay

I would like to get to grips with the implications of adding Barlows on focal lengths &  focal reducers etc etc etc.

Easy as pie.

the multiplication factor of each item, changes your scope by that amount.
for example(simple maths)
a 100mm (aperture) f10 scope has a focal length of 1000mm.
Add a 2* barlow. multiplies the focal length by 2. so now you have a 2000mm focal length scope. BUT your aperture has not changed so your f stop must change as well.
so its now a 2000mm f20 scope.  The area that it covers (view wise) is now 1/4 of the size previously.

Take a 4* barlow. your 1000mm now becomes a 4000mm F40 beast.
The imaging size is now 1/16 of the previous size.

Fit a reducer 0.66 your 100mm f10 1000mm scope now becomes a 660mm f6.6 scope
the field of view has doubled to 2.3 times the size.

and if you have a 0.33 your 100mm f10 scope becomes a 330mm f3.3 scope
with a field of view is now 9 time the size,

My 225mm f10 becomes the following with barlows and reducers..

4500mm f20
9000mm f40
1485mm f6.6
742mm  f3.3

[RobertM]

But always remember if you double the image scale by doubling the focal length the brightness will reduce by a factor of 4.  Conversly halving the focal length quadruples the brightness.  If you want to keep the same brightness but double the focal length then get a scope with double the aperture as well as double the focal length.