[Dujon]

I am considering purchasing a barlow for bright planetary observing. Having never used one of these devices I'm wondering what benefits or demerits might attach to their use.

I might add that I've had a bit of a look through the site before asking this question but there doesn't seem to be an answer (or I've missed it).

Firstly: What I'm expecting.

1) A slightly less bright image (which is a positive in some respects, particuarly as I found the moon filter helpul last evening when viewing Jupiter).
2) A slightly larger image.
3) Possible distortion due the extra lens(es) introduced into the light path.

Secondly: What I don't know.

1) When a barlow is described as 2x or 3x (or whatever) is this linear or 'volume'?- I suspect the latter but am unsure.
2) Is chromatic aberration likely to be a problem on brighter objects?
3) Given that I have a 2" Crayford focusser, is it better to invest in a 2" barlow (with adapter) rather than the 1.25"? I ask this because at this point in time I only have a 26mm 2" eyepiece (don't you just love the mixed measurements!) the other two being 1.25" plossls (15 & 9mm).

I am considering purchasing an el cheapo in order to work out whether or not this is the way to go (rather than investing in a dedicated eyepiece), so if anyone has an unused unit that they are happy to part with - cheaply - I'll think about it.

Initially though, your various advice would be much appreciated.