Barlow effect on focus

I am thinking about getting a Barlow, but am wondering what effect they have on getting an eyepeice to focus. Do they require more in travel, or out travel than an eyepeice alone?

Thank you,
Robert

Unless I'm mis-remembering...

Standard Barlows used directly ahead of (or "beneath") an eyepiece will require that eyepiece be drawn further inward (towards objective/primary).

When used ahead of *both* a diagonal *and* an eyepiece, focus will be drawn further outward (away from objective/primary).

--At least that's what happens with my Ultima :-)

Best wishes.
-Dan

You almost have to find out by trial and error. Whether it requires more in or out or how much will depend on the amplification factor of the barlow, the length of the barrel its mounted in, the focal ratio of your telescope, and where in the light path you mount it (say before or after the diagonal in a refractor).

Normally, shorty Barlows require some in-travel. However, when inserted to the hilt in a 2" diagonal (be careful!) or a Newtonian, some are nearly parfocal (like the Tele Vue 2x). However, the exact amount of travel required will depend somewhat on the eyepiece used.
--
Mike

Here's another data point for you. On my 120mm refractor the barlow requires over 2" more out-focus travel, compared to the ep alone. It's a standard barlow.

sf