Focussing short FL eyepieces and the effect of barlows

[John]

If you are using a short focal length eyepiece (say 6mm or less) in a short focal length scope (say F/6 or less) am I right in thinking that the zone of sharp focus will be very shallow and therefore achieving sharp focus becomes a much more precise business ?.

In which case does the addition of a barlow lens into the optical system ease this issue by increasing the apparent focal length of the objective lens / mirror so widening the zone of sharp focus ?.

Is this effect also noticable when imaging, ie: is sharp focus easier to achieve with longer FL scopes ?.

I'm just pondering the implications of a piece I've read about the correct design of barlow lenses and the compromises that the "shorty" barlows inevitably bring as a result.

Thanks,

John

[FLO]

The telescopes focal-ratio (however it is achieved) effects the depth of focus. What this means is that a long focal-ratio (say f10) telescope will feel easier to focus as the focus can be a little in front or behind the optimum point of focus and still be acceptable. Whereas, a short focal-ratio (say f4) will have a very narrow and unforgiving point of acceptable focus, just a little out and the view is unacceptable (this gives the impression that the telescope 'snaps' into focus. In actual fact, regardless of the f-ratio, there is still only one optimum focus position.

HTH

[John]

Thanks for that clarfication Steve

This seems to imply that a focusser with a fine focus option is highly desireable on short focal length scopes.

John

[GazOC]

Focal ratio.

[John]

Focal ratio.

Ahh .... yep - thats what I meant to say

John

[themos]

critical focus zone is given (in microns) as = (focal ratio)² * 2.2

My f/8 gives 140 microns or 0.140 mm. My f/5 gives 55 microns or 0.055 mm