Paulo martin said:
Hello,
Which is best for a newton telescope with small focal lenght(F/6)? Is the Powermate an instrument very similar at barlow lens?
Yes.
Advantages of the Powermate:
-no vignetting at all of some (rare) eyepiece types, like eyepieces with a field stop close to the limit of the barrel or TV Plössls. Mind you, the effect is usually very mild even with a regular barlow.
-keeps eye relief constant - a barlow will increase the eye relief of long focal length eyepieces considerably, rendering them uncomfortable.
-throws the new focal plane out just enough (as much as the height of the part sticking out of the focuser) without requiring a large insertion depth, like e.g. the excellent TV 2x barlow. That means you rarely require extra focuser in- or out-travel; if things come to focus without the Powermate, it will with the Powermate.
-on some models, the magnification does not depend critically on the placement with respect to the focal plane (and thus indirectly on where the focal plane of your eyepieces lies with respect to the end of the eyepiece barrel). That's usually a more important consideration for astrophotography, though.
If none of this matters, then a regular barlow, something like a TV 2x barlow, is usualy more interesting (all these things don't come free: there are more elements and the powermate is more expensive).
But often, one of these things does matter, in particular if you are planning to barlow longer focal length eyepieces (over 20mm) or can't stick the longer TV barlow in the focuser in some conditions.
