Orpington Astronomical Society

Fay

QuoteI 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

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.