Barlow bits magnification formula?

[Floater]

An appeal to the knowledge pool again:

Is there a formula to work out the magnification provided by using only the bottom part of a 2xbarlow screwed directly into the eyepiece?

Tried it today with the bottom lens of a common or garden Skywatcher 2xBarlow threaded onto a Cemax 12mm EP in a Solarmax 40 (f10). Just curious about the mag that provided and if there are fixed/definite data that I can use.

Almost as an aside, I tried that combination because I didn't seem to be able to find focus using the Barlow in a conventional fashion.

[John]

It's not an exact science because it will depend exactly how far the barlow lens is from the optics of the eyepiece and on the exact focal length of the optical element of the barlow lens (they have focal lengths too). Generally it's around 1.5x I think, for a normal 2x barlow element screwed into the barrel of a standard eyepiece. Move the barlow lens further away from the eyepiece optics and this increases (this is in effect what the barlow body does) and closer to the eyepiece optics and the amplification decreases.

[Floater]

Thanks, John.

Kinda guessed it wouldn't be an 'exact science' but thanks for affirming that.

Nevertheless, your answer gives me a ballpark to work within.

Thanks again.

[YKSE]

I agree with John, for most barlow+eyepiece combinations, we have only approximate barlow effect as designated, partly because the barlow effect may not be exact, partly that it assumes the eyepiece focal plan lies exactly in the eyepiece shoulder which is not true for most eyepieces. If the EP's focal plan lies higher up, barlow will give higher maganification.

To know the more exact barlow effect, we need to know both the barlow lens's focal length(, and the distance of that eyepices focus plane to its shoulder(E), assuming eyepiece barrel length from bottom to shoulder is L, you can caculate your barlow effect (screwing barlow to the bottom of an eyepiece) by this formula

1+(L+E)/B

where E takes minus sign if it's lower than the shoulder, plus sign for higher.

[Floater]

Thanks, YKSE. Time for me to get measuring... !

[YKSE]

To some extend, you know the barlow's focal length   i.e. iIt should be the distance from the barlow lens to its shoulder (if it's a 2x barlow), you can verify with the formula above.