barlows and focal ratios

[Andrew*]

This question just occurred to me.

We all know that coma is a problem on fast newts.

I bought a 114mm f/8.8 newt as my first telescope. Surely coma would be minimal, and collimation not critical, because it's slow! However, it was really an f/4.4 with a barlow inserted. Now would it be prone to coma or miscollimation?

If not, why not? If I use my f/5 newt with a type 6 Nagler, I should get minimal coma, because T6 Naglers use barlow elements, so actually my telescope is operating slower than f/5. (follow on thought: In fact, I hear Naglers correct coma extremely well. Is this why??)

Okay, the question: Does inserting a barlow element double the focal ratio or only the effective focal length? If the latter, surely it would be more correct to say that barlows half the focal length of your eyepiece (when discussing EPs anyway). Full stop.

All thoughts appreciated!

Andrew

[GazOC]

Barlows don't correct for coma, a scope will show the same mount of coma with a 10mm+ barlow as a straight 5mm because the focal ratio is altered too far up the optical path (near the focus point). Barlows can help with field curvature and astigmatism.

[GazOC]

It really depends how you look at the Naglers. Are the extra elements part of the scope or the eyepiece? But even if you play devils advocate and say the f5 scope is now f10, its the place where the 'adjustment' takes place (near the focus point) that is the factor when you are talking about coma.

[Andrew*]

excellent, thanks for clearing that up.

Anyway, I prefer to think a barlow as dividing the focal length of your eyepiece. Makes for easier calculation... Also, I see a barlow more as a part of the eyepiece than of the telescope.

cheers

Andrew