When 2x's is 1.5x's

[Regulus]

I read somewhere recently that barlows change there magnification depending where in the imaging or viewing train they are inserted.
So before or after the diagonal I guess.
Why? And is there an equation to calculate the difference?
Trevor

[Shiraz]

this is pretty good. http://www.brayebrookobservatory.org/BrayObsWebSite/BOOKS/BarlowLens.pdf

[raymo]

Trevor, if you have, or can borrow, a graduated reticle eyepiece, you can get a good approximation of the difference before and after the diagonal for your particular scope. This is for people like me who don't need to know the precise difference, and people who can't be bothered with
measurements and formulae.
raymo

[dannat]

the earlier the barlow is placed the glass balrow lens causes a diverging effect on the light cone [without it light rays converge quickly]-the earlier you place the greater the effect,it produces a larger image at the focal plane
think of a > being the normal light cone -then stretch it some for a normal barlow & stretch it some more for a before diag barlow

i have heard it can be up to a factor of 1x extra -but havnt botheed to calculate it myself eg 2x after diag & 3x before diag..

this of course relies on your barlow being lablled right -my GSO ED 1.25" 2.5x barlow is actually about 2.2x

[Renato1]

You can make any Barlow more powerful by putting an extension tube into it, and then the eyepiece into the tube. In fact, you can make a Barlow about 10% more powerful just by not putting the eyepiece all the way down into the Barlow tube (test this out on the moon).

With a 1.25" diagonal, you have to put a 2X Barlow into the diagonal to get 2X. But if you put the diagonal into the Barlow you get approximately 3X (or more if its a 2" Barlow). Erect image diagonals have a longer light path, so that they too would give more than 3X if placed into a 2X Barlow.

I have a short tube 2X Celestron Barlow and a long tube 1.8X Televue Barlow. I have at times swapped the Barlow lenses between the tubes, to get approximately 1.5X and 2.5X Barlows.
Regards,
Renato

[Shiraz]

maybe this chart will help http://www.televue.com/engine/TV3b_page.asp?id=52&Tab=_photo#.VCkS5WeSyNA
Note that the equations still apply and the extra magnification factor will be different for different Barlows, because it depends on the distance that the Barlow is inside the prime focal plane, the distance from the Barlow to the eyepiece focal plane and the (negative) focal length of the Barlow.

[Regulus]

Thanks Ray, those links were very educational. Cheers.
Thanks Raymo - have a illuminated reticule but not graduated. I can see where that might be useful in some other areas too so I might chase that up.
A diverging light cone means significantly duller view though. doesn't it? Inverse square law stuff. Sometimes the trade off in light loss is not worth the extra mag, but on a bright object it is nice to now it's there when wanted.
Very helpful guys, thanks.
Trevor