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I have to disagree with Raj concerning barlow lenses.Firstly, "more glass is never a good thing" only applies to the fact that we want all the light we can get, hence most astro scopes invert the image because designers
omit an erecting lens. Regarding optical quality, more glass has no
detrimental effect whatever. This is shown by the fact that good quality
camera lenses will always produce better images with fewer aberrations
than any lens/ mirror/ eyepiece combination, and that includes APO
scopes. The reason for this is that camera lenses contain many
elements in several groups; each group being designed to minimise
a particular aberration. Typically a lens will contain 8-12 pieces of glass.
Barlows don't have to match up to any particular F/L eyepieces.
The eyepiece being used is irrelevant; All the barlow does is cause the light passing through it to diverge and typically doubles or triples the
F/L of the scope. Through a decent quality barlow there will be a
slight loss of quality, but the reason is that you have doubled the magnification, and a 6mm eyepiece will not give the same quality
viewing as a 12mm. As you will be aware, as magnification increases,
image quality deteriorates. So, optically there is not much to choose,
but practically why would I jam my eye up against the tiny lens of
a very short F/L eyepiece when I can view more comfortably through
a medium or long one. Incidentally a barlow does not magnify
without sacrificing eye relief; it actually increases the eye relief of
any eyepiece used with it. A final benefit is the fact that the longer the
F/L of a scope, the less obvious any aberrations in it's optical train
become, which is why scopes many years ago had enormous focal
lengths, such as the Yerkes 40" refractor which is an f/19 scope.
There is also the fact that even if there was a small loss of
image quality, unless you are loaded, and money doesn't matter, it would surely be acceptable in order to effectively double your eyepiece
collection.
raymo.
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